mito2/

worker.rs

// Copyright 2023 Greptime Team
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

//! Structs and utilities for writing regions.

mod handle_alter;
mod handle_apply_staging;
mod handle_bulk_insert;
mod handle_catchup;
mod handle_close;
mod handle_compaction;
mod handle_copy_region;
mod handle_create;
mod handle_drop;
mod handle_enter_staging;
mod handle_flush;
mod handle_manifest;
mod handle_open;
mod handle_rebuild_index;
mod handle_remap;
mod handle_truncate;
mod handle_write;

use std::collections::HashMap;
use std::path::Path;
use std::sync::Arc;
use std::sync::atomic::{AtomicBool, AtomicU64, Ordering};
use std::time::Duration;

use common_base::Plugins;
use common_error::ext::BoxedError;
use common_meta::key::SchemaMetadataManagerRef;
use common_runtime::JoinHandle;
use common_stat::get_total_memory_bytes;
use common_telemetry::{error, info, warn};
use futures::future::{join_all, try_join_all};
use object_store::ObjectStore;
use object_store::manager::ObjectStoreManagerRef;
use prometheus::{Histogram, IntGauge};
use rand::{Rng, rng};
use snafu::ResultExt;
use store_api::logstore::LogStore;
use store_api::region_engine::{
    RegionRole, SetRegionRoleStateResponse, SetRegionRoleStateSuccess, SettableRegionRoleState,
};
use store_api::storage::{FileId, RegionId};
use tokio::sync::mpsc::{Receiver, Sender};
use tokio::sync::{Mutex, Semaphore, mpsc, oneshot, watch};

use crate::access_layer::new_fs_cache_store;
use crate::cache::write_cache::{WriteCache, WriteCacheRef};
use crate::cache::{CacheManager, CacheManagerRef, WriteCacheUploadStoreWrapperRef};
use crate::compaction::CompactionScheduler;
use crate::compaction::memory_manager::{CompactionMemoryManager, new_compaction_memory_manager};
use crate::config::MitoConfig;
use crate::error::{self, CreateDirSnafu, JoinSnafu, Result, WorkerStoppedSnafu};
use crate::flush::{FlushScheduler, WriteBufferManagerImpl, WriteBufferManagerRef};
use crate::gc::{GcLimiter, GcLimiterRef};
use crate::memtable::MemtableBuilderProvider;
use crate::metrics::{REGION_COUNT, REQUEST_WAIT_TIME, WRITE_STALLING};
use crate::region::opener::PartitionExprFetcherRef;
use crate::region::{
    CatchupRegions, CatchupRegionsRef, MitoRegionRef, NodeLease, OpeningRegions, OpeningRegionsRef,
    RegionMap, RegionMapRef,
};
use crate::request::{
    BackgroundNotify, BulkInsertRequest, DdlRequest, OptionOutputTx, SenderBulkRequest,
    SenderDdlRequest, SenderWriteRequest, WorkerRequest, WorkerRequestWithTime,
};
use crate::schedule::scheduler::{LocalScheduler, SchedulerRef};
use crate::series_index::{
    IndexFilePurger, SeriesIndexTaskState, series_index_channel, spawn_series_index_tasks,
};
use crate::sst::file::RegionFileId;
use crate::sst::file_ref::FileReferenceManagerRef;
use crate::sst::index::IndexBuildScheduler;
use crate::sst::index::intermediate::IntermediateManager;
use crate::sst::index::puffin_manager::PuffinManagerFactory;
#[cfg(feature = "hdfs-object-store")]
use crate::temp_file_sweeper;
use crate::time_provider::{StdTimeProvider, TimeProviderRef};
use crate::wal::Wal;
use crate::worker::handle_manifest::RegionEditQueues;

/// Identifier for a worker.
pub(crate) type WorkerId = u32;

pub(crate) const DROPPING_MARKER_FILE: &str = ".dropping";

/// Interval to check whether regions should flush.
pub(crate) const CHECK_REGION_INTERVAL: Duration = Duration::from_secs(60);
/// Max delay to check region periodical tasks.
pub(crate) const MAX_INITIAL_CHECK_DELAY_SECS: u64 = 60 * 3;

#[cfg_attr(doc, aquamarine::aquamarine)]
/// A fixed size group of [RegionWorkers](RegionWorker).
///
/// A worker group binds each region to a specific [RegionWorker] and sends
/// requests to region's dedicated worker.
///
/// ```mermaid
/// graph LR
///
/// RegionRequest -- Route by region id --> Worker0 & Worker1
///
/// subgraph MitoEngine
///     subgraph WorkerGroup
///         Worker0["RegionWorker 0"]
///         Worker1["RegionWorker 1"]
///     end
/// end
///
/// Chan0[" Request channel 0"]
/// Chan1[" Request channel 1"]
/// WorkerThread1["RegionWorkerLoop 1"]
///
/// subgraph WorkerThread0["RegionWorkerLoop 0"]
///     subgraph RegionMap["RegionMap (regions bound to worker 0)"]
///         Region0["Region 0"]
///         Region2["Region 2"]
///     end
///     Buffer0["RequestBuffer"]
///
///     Buffer0 -- modify regions --> RegionMap
/// end
///
/// Worker0 --> Chan0
/// Worker1 --> Chan1
/// Chan0 --> Buffer0
/// Chan1 --> WorkerThread1
/// ```
pub(crate) struct WorkerGroup {
    /// Workers of the group.
    workers: Vec<RegionWorker>,
    /// Flush background job pool.
    flush_job_pool: SchedulerRef,
    /// Compaction background job pool.
    compact_job_pool: SchedulerRef,
    /// Scheduler for index build jobs.
    index_build_job_pool: SchedulerRef,
    /// Scheduler for file purgers.
    purge_scheduler: SchedulerRef,
    /// Cache.
    cache_manager: CacheManagerRef,
    /// File reference manager.
    file_ref_manager: FileReferenceManagerRef,
    /// Gc limiter to limit concurrent gc jobs.
    gc_limiter: GcLimiterRef,
    node_lease: Arc<NodeLease>,
    /// Object store manager.
    object_store_manager: ObjectStoreManagerRef,
    /// Puffin manager factory.
    puffin_manager_factory: PuffinManagerFactory,
    /// Intermediate manager.
    intermediate_manager: IntermediateManager,
    /// Schema metadata manager.
    schema_metadata_manager: SchemaMetadataManagerRef,
}

impl WorkerGroup {
    /// Starts a worker group.
    ///
    /// The number of workers should be power of two.
    #[allow(clippy::too_many_arguments)]
    pub(crate) async fn start<S: LogStore>(
        data_home: &str,
        config: Arc<MitoConfig>,
        log_store: Arc<S>,
        object_store_manager: ObjectStoreManagerRef,
        schema_metadata_manager: SchemaMetadataManagerRef,
        file_ref_manager: FileReferenceManagerRef,
        partition_expr_fetcher: PartitionExprFetcherRef,
        plugins: Plugins,
    ) -> Result<WorkerGroup> {
        let (flush_sender, flush_receiver) = watch::channel(());
        let write_buffer_manager = Arc::new(
            WriteBufferManagerImpl::new(config.global_write_buffer_size.as_bytes() as usize)
                .with_notifier(flush_sender.clone()),
        );
        let puffin_manager_factory = PuffinManagerFactory::new(
            &config.index.aux_path,
            config.index.staging_size.as_bytes(),
            Some(config.index.write_buffer_size.as_bytes() as _),
            config.index.staging_ttl,
        )
        .await?;
        let intermediate_manager = IntermediateManager::init_fs(&config.index.aux_path)
            .await?
            .with_buffer_size(Some(config.index.write_buffer_size.as_bytes() as _));
        let index_build_job_pool =
            Arc::new(LocalScheduler::new(config.max_background_index_builds));
        let series_index_store = series_index_store_from_config(&config, data_home).await?;
        let series_index_disk_usage = Arc::new(AtomicU64::new(0));
        let flush_job_pool = Arc::new(LocalScheduler::new(config.max_background_flushes));
        let compact_job_pool = Arc::new(LocalScheduler::new(config.max_background_compactions));
        let flush_semaphore = Arc::new(Semaphore::new(config.max_background_flushes));
        // We use another scheduler to avoid purge jobs blocking other jobs.
        let purge_scheduler = Arc::new(LocalScheduler::new(config.max_background_purges));
        let upload_store_wrapper = plugins.get::<WriteCacheUploadStoreWrapperRef>();
        let write_cache = write_cache_from_config(
            &config,
            puffin_manager_factory.clone(),
            intermediate_manager.clone(),
            upload_store_wrapper,
        )
        .await?;
        let cache_manager = Arc::new(
            CacheManager::builder()
                .sst_meta_cache_size(config.sst_meta_cache_size.as_bytes())
                .vector_cache_size(config.vector_cache_size.as_bytes())
                .page_cache_size(config.page_cache_size.as_bytes())
                .selector_result_cache_size(config.selector_result_cache_size.as_bytes())
                .range_result_cache_size(config.range_result_cache_size.as_bytes())
                .prefilter_result_cache_size(config.prefilter_result_cache_size.as_bytes())
                .index_metadata_size(config.index.metadata_cache_size.as_bytes())
                .index_content_size(config.index.content_cache_size.as_bytes())
                .index_content_page_size(config.index.content_cache_page_size.as_bytes())
                .index_result_cache_size(config.index.result_cache_size.as_bytes())
                .puffin_metadata_size(config.index.metadata_cache_size.as_bytes())
                .write_cache(write_cache)
                .build(),
        );
        let time_provider = Arc::new(StdTimeProvider);
        let total_memory = get_total_memory_bytes();
        let total_memory = if total_memory > 0 {
            total_memory as u64
        } else {
            0
        };
        let compaction_limit_bytes = config
            .experimental_compaction_memory_limit
            .resolve(total_memory);
        let compaction_memory_manager =
            Arc::new(new_compaction_memory_manager(compaction_limit_bytes));
        let gc_limiter = Arc::new(GcLimiter::new(config.gc.max_concurrent_gc_job));
        // A sweep waits for a permit while a GC job takes one without waiting, so
        // sharing the GC limiter would fail GC jobs and report sweeps as GC jobs.
        // A datanode that allows no concurrent GC job runs no sweep either: a
        // limiter of zero would never hand out the permit the task waits for.
        #[cfg(feature = "hdfs-object-store")]
        let sweep_limiter = (config.gc.max_concurrent_gc_job > 0)
            .then(|| Arc::new(Semaphore::new(config.gc.max_concurrent_gc_job)));
        let node_lease = Arc::new(NodeLease::default());

        let workers = (0..config.num_workers)
            .map(|id| {
                WorkerStarter {
                    id: id as WorkerId,
                    config: config.clone(),
                    log_store: log_store.clone(),
                    object_store_manager: object_store_manager.clone(),
                    write_buffer_manager: write_buffer_manager.clone(),
                    index_build_job_pool: index_build_job_pool.clone(),
                    series_index_store: series_index_store.clone(),
                    series_index_disk_usage: series_index_disk_usage.clone(),
                    flush_job_pool: flush_job_pool.clone(),
                    compact_job_pool: compact_job_pool.clone(),
                    purge_scheduler: purge_scheduler.clone(),
                    listener: WorkerListener::default(),
                    cache_manager: cache_manager.clone(),
                    compaction_memory_manager: compaction_memory_manager.clone(),
                    puffin_manager_factory: puffin_manager_factory.clone(),
                    intermediate_manager: intermediate_manager.clone(),
                    time_provider: time_provider.clone(),
                    flush_sender: flush_sender.clone(),
                    flush_receiver: flush_receiver.clone(),
                    plugins: plugins.clone(),
                    schema_metadata_manager: schema_metadata_manager.clone(),
                    file_ref_manager: file_ref_manager.clone(),
                    partition_expr_fetcher: partition_expr_fetcher.clone(),
                    flush_semaphore: flush_semaphore.clone(),
                    #[cfg(feature = "hdfs-object-store")]
                    sweep_limiter: sweep_limiter.clone(),
                    node_lease: node_lease.clone(),
                }
                .start()
            })
            .collect::<Result<Vec<_>>>()?;

        Ok(WorkerGroup {
            workers,
            flush_job_pool,
            compact_job_pool,
            index_build_job_pool,
            purge_scheduler,
            cache_manager,
            file_ref_manager,
            gc_limiter,
            node_lease,
            object_store_manager,
            puffin_manager_factory,
            intermediate_manager,
            schema_metadata_manager,
        })
    }

    /// Stops the worker group.
    pub(crate) async fn stop(&self) -> Result<()> {
        info!("Stop region worker group");

        // TODO(yingwen): Do we need to stop gracefully?
        // Stops the scheduler gracefully.
        self.compact_job_pool.stop(true).await?;
        // Stops the scheduler gracefully.
        self.flush_job_pool.stop(true).await?;
        // Stops the purge scheduler gracefully.
        self.purge_scheduler.stop(true).await?;
        // Stops the index build job pool gracefully.
        self.index_build_job_pool.stop(true).await?;

        try_join_all(self.workers.iter().map(|worker| worker.stop())).await?;

        Ok(())
    }

    /// Submits a request to a worker in the group.
    pub(crate) async fn submit_to_worker(
        &self,
        region_id: RegionId,
        request: WorkerRequest,
    ) -> Result<()> {
        self.worker(region_id).submit_request(request).await
    }

    /// Submits the writes of one request, those of each worker in one message, so that the
    /// worker handles them in one batch. The messages are sent concurrently, so a full worker
    /// queue holds back no other worker. A write whose worker has stopped receives the error a
    /// write submitted on its own gets.
    pub(crate) async fn submit_writes(&self, writes: Vec<SenderWriteRequest>) {
        let mut writes_by_worker: HashMap<usize, Vec<SenderWriteRequest>> = HashMap::new();
        for write in writes {
            let index = region_id_to_index(write.request.region_id, self.workers.len());
            writes_by_worker.entry(index).or_default().push(write);
        }
        join_all(
            writes_by_worker
                .into_iter()
                .map(|(index, writes)| async move {
                    let worker = &self.workers[index];
                    let Err(request) = worker
                        .try_submit_request(WorkerRequest::BatchWrite(writes))
                        .await
                    else {
                        return;
                    };
                    let WorkerRequest::BatchWrite(writes) = request else {
                        unreachable!("the worker returns the request it was given");
                    };
                    for write in writes {
                        write
                            .sender
                            .send(WorkerStoppedSnafu { id: worker.id }.fail());
                    }
                }),
        )
        .await;
    }

    /// Returns true if the specific region exists.
    pub(crate) fn is_region_exists(&self, region_id: RegionId) -> bool {
        self.worker(region_id).is_region_exists(region_id)
    }

    /// Returns true if the specific region is opening.
    pub(crate) fn is_region_opening(&self, region_id: RegionId) -> bool {
        self.worker(region_id).is_region_opening(region_id)
    }

    /// Returns true if the specific region is catching up.
    pub(crate) fn is_region_catching_up(&self, region_id: RegionId) -> bool {
        self.worker(region_id).is_region_catching_up(region_id)
    }

    /// Returns region of specific `region_id`.
    ///
    /// This method should not be public.
    pub(crate) fn get_region(&self, region_id: RegionId) -> Option<MitoRegionRef> {
        self.worker(region_id).get_region(region_id)
    }

    /// Returns cache of the group.
    pub(crate) fn cache_manager(&self) -> CacheManagerRef {
        self.cache_manager.clone()
    }

    pub(crate) fn file_ref_manager(&self) -> FileReferenceManagerRef {
        self.file_ref_manager.clone()
    }

    pub(crate) fn gc_limiter(&self) -> GcLimiterRef {
        self.gc_limiter.clone()
    }

    pub(crate) fn node_lease(&self) -> &NodeLease {
        &self.node_lease
    }

    /// Get worker for specific `region_id`.
    pub(crate) fn worker(&self, region_id: RegionId) -> &RegionWorker {
        let index = region_id_to_index(region_id, self.workers.len());

        &self.workers[index]
    }

    pub(crate) fn all_regions(&self) -> impl Iterator<Item = MitoRegionRef> + use<'_> {
        self.workers
            .iter()
            .flat_map(|worker| worker.regions.list_regions())
    }

    pub(crate) fn object_store_manager(&self) -> &ObjectStoreManagerRef {
        &self.object_store_manager
    }

    pub(crate) fn puffin_manager_factory(&self) -> &PuffinManagerFactory {
        &self.puffin_manager_factory
    }

    pub(crate) fn intermediate_manager(&self) -> &IntermediateManager {
        &self.intermediate_manager
    }

    pub(crate) fn schema_metadata_manager(&self) -> &SchemaMetadataManagerRef {
        &self.schema_metadata_manager
    }
}

// Tests methods.
#[cfg(any(test, feature = "test"))]
impl WorkerGroup {
    /// Starts a worker group with `write_buffer_manager` and `listener` for tests.
    ///
    /// The number of workers should be power of two.
    #[allow(clippy::too_many_arguments)]
    pub(crate) async fn start_for_test<S: LogStore>(
        data_home: &str,
        config: Arc<MitoConfig>,
        log_store: Arc<S>,
        object_store_manager: ObjectStoreManagerRef,
        write_buffer_manager: Option<WriteBufferManagerRef>,
        listener: Option<crate::engine::listener::EventListenerRef>,
        schema_metadata_manager: SchemaMetadataManagerRef,
        file_ref_manager: FileReferenceManagerRef,
        time_provider: TimeProviderRef,
        partition_expr_fetcher: PartitionExprFetcherRef,
    ) -> Result<WorkerGroup> {
        let (flush_sender, flush_receiver) = watch::channel(());
        let write_buffer_manager = write_buffer_manager.unwrap_or_else(|| {
            Arc::new(
                WriteBufferManagerImpl::new(config.global_write_buffer_size.as_bytes() as usize)
                    .with_notifier(flush_sender.clone()),
            )
        });
        let index_build_job_pool =
            Arc::new(LocalScheduler::new(config.max_background_index_builds));
        let series_index_store = series_index_store_from_config(&config, data_home).await?;
        let series_index_disk_usage = Arc::new(AtomicU64::new(0));
        let flush_job_pool = Arc::new(LocalScheduler::new(config.max_background_flushes));
        let compact_job_pool = Arc::new(LocalScheduler::new(config.max_background_compactions));
        let flush_semaphore = Arc::new(Semaphore::new(config.max_background_flushes));
        let purge_scheduler = Arc::new(LocalScheduler::new(config.max_background_flushes));
        let puffin_manager_factory = PuffinManagerFactory::new(
            &config.index.aux_path,
            config.index.staging_size.as_bytes(),
            Some(config.index.write_buffer_size.as_bytes() as _),
            config.index.staging_ttl,
        )
        .await?;
        let intermediate_manager = IntermediateManager::init_fs(&config.index.aux_path)
            .await?
            .with_buffer_size(Some(config.index.write_buffer_size.as_bytes() as _));
        let write_cache = write_cache_from_config(
            &config,
            puffin_manager_factory.clone(),
            intermediate_manager.clone(),
            None,
        )
        .await?;
        let cache_manager = Arc::new(
            CacheManager::builder()
                .sst_meta_cache_size(config.sst_meta_cache_size.as_bytes())
                .vector_cache_size(config.vector_cache_size.as_bytes())
                .page_cache_size(config.page_cache_size.as_bytes())
                .selector_result_cache_size(config.selector_result_cache_size.as_bytes())
                .range_result_cache_size(config.range_result_cache_size.as_bytes())
                .prefilter_result_cache_size(config.prefilter_result_cache_size.as_bytes())
                .write_cache(write_cache)
                .build(),
        );
        let total_memory = get_total_memory_bytes();
        let total_memory = if total_memory > 0 {
            total_memory as u64
        } else {
            0
        };
        let compaction_limit_bytes = config
            .experimental_compaction_memory_limit
            .resolve(total_memory);
        let compaction_memory_manager =
            Arc::new(new_compaction_memory_manager(compaction_limit_bytes));
        let gc_limiter = Arc::new(GcLimiter::new(config.gc.max_concurrent_gc_job));
        // A sweep waits for a permit while a GC job takes one without waiting, so
        // sharing the GC limiter would fail GC jobs and report sweeps as GC jobs.
        // A datanode that allows no concurrent GC job runs no sweep either: a
        // limiter of zero would never hand out the permit the task waits for.
        #[cfg(feature = "hdfs-object-store")]
        let sweep_limiter = (config.gc.max_concurrent_gc_job > 0)
            .then(|| Arc::new(Semaphore::new(config.gc.max_concurrent_gc_job)));
        let node_lease = Arc::new(NodeLease::default());
        let workers = (0..config.num_workers)
            .map(|id| {
                WorkerStarter {
                    id: id as WorkerId,
                    config: config.clone(),
                    log_store: log_store.clone(),
                    object_store_manager: object_store_manager.clone(),
                    write_buffer_manager: write_buffer_manager.clone(),
                    index_build_job_pool: index_build_job_pool.clone(),
                    series_index_store: series_index_store.clone(),
                    series_index_disk_usage: series_index_disk_usage.clone(),
                    flush_job_pool: flush_job_pool.clone(),
                    compact_job_pool: compact_job_pool.clone(),
                    purge_scheduler: purge_scheduler.clone(),
                    listener: WorkerListener::new(listener.clone()),
                    cache_manager: cache_manager.clone(),
                    compaction_memory_manager: compaction_memory_manager.clone(),
                    puffin_manager_factory: puffin_manager_factory.clone(),
                    intermediate_manager: intermediate_manager.clone(),
                    time_provider: time_provider.clone(),
                    flush_sender: flush_sender.clone(),
                    flush_receiver: flush_receiver.clone(),
                    plugins: Plugins::new(),
                    schema_metadata_manager: schema_metadata_manager.clone(),
                    file_ref_manager: file_ref_manager.clone(),
                    partition_expr_fetcher: partition_expr_fetcher.clone(),
                    flush_semaphore: flush_semaphore.clone(),
                    #[cfg(feature = "hdfs-object-store")]
                    sweep_limiter: sweep_limiter.clone(),
                    node_lease: node_lease.clone(),
                }
                .start()
            })
            .collect::<Result<Vec<_>>>()?;

        Ok(WorkerGroup {
            workers,
            flush_job_pool,
            compact_job_pool,
            index_build_job_pool,
            purge_scheduler,
            cache_manager,
            file_ref_manager,
            gc_limiter,
            node_lease,
            object_store_manager,
            puffin_manager_factory,
            intermediate_manager,
            schema_metadata_manager,
        })
    }

    /// Returns the purge scheduler.
    pub(crate) fn purge_scheduler(&self) -> &SchedulerRef {
        &self.purge_scheduler
    }
}

fn region_id_to_index(id: RegionId, num_workers: usize) -> usize {
    ((id.table_id() as usize % num_workers) + (id.region_number() as usize % num_workers))
        % num_workers
}

/// Opens the fixed local series-index store only when the feature is enabled.
async fn series_index_store_from_config(
    config: &MitoConfig,
    data_home: &str,
) -> Result<Option<ObjectStore>> {
    if !config.experimental_enable_series_index {
        return Ok(None);
    }

    let root = Path::new(data_home).join("series_index");
    new_fs_cache_store(&root.to_string_lossy()).await.map(Some)
}

pub async fn write_cache_from_config(
    config: &MitoConfig,
    puffin_manager_factory: PuffinManagerFactory,
    intermediate_manager: IntermediateManager,
    upload_store_wrapper: Option<WriteCacheUploadStoreWrapperRef>,
) -> Result<Option<WriteCacheRef>> {
    if !config.enable_write_cache {
        return Ok(None);
    }

    tokio::fs::create_dir_all(Path::new(&config.write_cache_path))
        .await
        .context(CreateDirSnafu {
            dir: &config.write_cache_path,
        })?;

    let cache = WriteCache::new_fs(
        &config.write_cache_path,
        config.write_cache_size,
        config.write_cache_ttl,
        Some(config.index_cache_percent),
        config.enable_refill_cache_on_read,
        puffin_manager_factory,
        intermediate_manager,
        config.manifest_cache_size,
    )
    .await?
    .with_upload_store_wrapper(upload_store_wrapper);
    Ok(Some(Arc::new(cache)))
}

/// Computes a initial check delay for a worker.
pub(crate) fn worker_init_check_delay() -> Duration {
    let init_check_delay = rng().random_range(0..MAX_INITIAL_CHECK_DELAY_SECS);
    Duration::from_secs(init_check_delay)
}

/// Worker start config.
struct WorkerStarter<S> {
    id: WorkerId,
    config: Arc<MitoConfig>,
    log_store: Arc<S>,
    object_store_manager: ObjectStoreManagerRef,
    write_buffer_manager: WriteBufferManagerRef,
    compact_job_pool: SchedulerRef,
    index_build_job_pool: SchedulerRef,
    series_index_store: Option<ObjectStore>,
    series_index_disk_usage: Arc<AtomicU64>,
    flush_job_pool: SchedulerRef,
    purge_scheduler: SchedulerRef,
    listener: WorkerListener,
    cache_manager: CacheManagerRef,
    compaction_memory_manager: Arc<CompactionMemoryManager>,
    puffin_manager_factory: PuffinManagerFactory,
    intermediate_manager: IntermediateManager,
    time_provider: TimeProviderRef,
    /// Watch channel sender to notify workers to handle stalled requests.
    flush_sender: watch::Sender<()>,
    /// Watch channel receiver to wait for background flush job.
    flush_receiver: watch::Receiver<()>,
    plugins: Plugins,
    schema_metadata_manager: SchemaMetadataManagerRef,
    file_ref_manager: FileReferenceManagerRef,
    partition_expr_fetcher: PartitionExprFetcherRef,
    flush_semaphore: Arc<Semaphore>,
    #[cfg(feature = "hdfs-object-store")]
    /// Limiter for the temporary file sweeps, or `None` if the datanode allows
    /// no concurrent GC job.
    sweep_limiter: Option<Arc<Semaphore>>,
    node_lease: Arc<NodeLease>,
}

impl<S: LogStore> WorkerStarter<S> {
    /// Starts a region worker and its background thread.
    fn start(self) -> Result<RegionWorker> {
        let regions = Arc::new(RegionMap::default());
        let opening_regions = Arc::new(OpeningRegions::default());
        let catchup_regions = Arc::new(CatchupRegions::default());
        let (sender, receiver) = mpsc::channel(self.config.worker_channel_size);

        let running = Arc::new(AtomicBool::new(true));
        let (series_index_task_state, series_index_receiver) = if self.series_index_store.is_some()
        {
            let (state, receiver) =
                SeriesIndexTaskState::new(self.id, self.config.worker_channel_size);
            (Some(Arc::new(state)), Some(receiver))
        } else {
            (None, None)
        };
        let mut series_index_purger = None;
        let series_index_handle = self
            .series_index_store
            .clone()
            .zip(series_index_task_state.clone())
            .zip(series_index_receiver)
            .map(|((store, state), receiver)| {
                let (purger, purge_receiver) = series_index_channel(store.clone());
                series_index_purger = Some(purger.clone());
                spawn_series_index_tasks(
                    self.id,
                    store,
                    regions.clone(),
                    state,
                    receiver,
                    self.config.experimental_series_index_bucket_width,
                    purger,
                    purge_receiver,
                    self.series_index_disk_usage.clone(),
                    self.config.experimental_series_index_max_size.as_bytes(),
                    self.config.experimental_series_index_maintenance_interval,
                    self.time_provider.clone(),
                    self.config.experimental_enable_range_index,
                )
            });
        let now = self.time_provider.current_time_millis();
        let id_string = self.id.to_string();
        let mut worker_thread = RegionWorkerLoop {
            id: self.id,
            config: self.config.clone(),
            regions: regions.clone(),
            catchup_regions: catchup_regions.clone(),
            dropping_regions: Arc::new(RegionMap::default()),
            opening_regions: opening_regions.clone(),
            sender: sender.clone(),
            receiver,
            wal: Wal::new(self.log_store),
            object_store_manager: self.object_store_manager.clone(),
            running: running.clone(),
            memtable_builder_provider: MemtableBuilderProvider::new(
                Some(self.write_buffer_manager.clone()),
                self.config.clone(),
            ),
            purge_scheduler: self.purge_scheduler.clone(),
            write_buffer_manager: self.write_buffer_manager,
            index_build_scheduler: IndexBuildScheduler::new(
                self.index_build_job_pool,
                self.config.max_background_index_builds,
            ),
            series_index_task_state: series_index_task_state.clone(),
            series_index_store: self.series_index_store,
            series_index_purger,
            flush_scheduler: FlushScheduler::new(self.flush_job_pool),
            compaction_scheduler: CompactionScheduler::new(
                self.compact_job_pool,
                sender.clone(),
                self.cache_manager.clone(),
                self.config.clone(),
                self.listener.clone(),
                self.plugins.clone(),
                self.compaction_memory_manager.clone(),
                self.config.experimental_compaction_on_exhausted,
            ),
            stalled_requests: StalledRequests::default(),
            listener: self.listener,
            cache_manager: self.cache_manager,
            puffin_manager_factory: self.puffin_manager_factory,
            intermediate_manager: self.intermediate_manager,
            time_provider: self.time_provider,
            last_periodical_check_millis: now,
            flush_sender: self.flush_sender,
            flush_receiver: self.flush_receiver,
            stalling_count: WRITE_STALLING.with_label_values(&[&id_string]),
            region_count: REGION_COUNT.with_label_values(&[&id_string]),
            request_wait_time: REQUEST_WAIT_TIME.with_label_values(&[&id_string]),
            region_edit_queues: RegionEditQueues::default(),
            schema_metadata_manager: self.schema_metadata_manager,
            file_ref_manager: self.file_ref_manager.clone(),
            partition_expr_fetcher: self.partition_expr_fetcher,
            flush_semaphore: self.flush_semaphore,
            plugins: self.plugins,
            #[cfg(feature = "hdfs-object-store")]
            sweep_limiter: self.sweep_limiter,
            node_lease: self.node_lease,
        };
        let handle = common_runtime::spawn_global(async move {
            worker_thread.run().await;
        });

        Ok(RegionWorker {
            id: self.id,
            regions,
            opening_regions,
            catchup_regions,
            sender,
            handle: Mutex::new(Some(handle)),
            series_index_handle: Mutex::new(series_index_handle),
            series_index_task_state,
            running,
        })
    }
}

/// Worker to write and alter regions bound to it.
pub(crate) struct RegionWorker {
    /// Id of the worker.
    id: WorkerId,
    /// Regions bound to the worker.
    regions: RegionMapRef,
    /// The opening regions.
    opening_regions: OpeningRegionsRef,
    /// The catching up regions.
    catchup_regions: CatchupRegionsRef,
    /// Request sender.
    sender: Sender<WorkerRequestWithTime>,
    /// Handle to the worker thread.
    handle: Mutex<Option<JoinHandle<()>>>,
    /// Handle to the sequential series-index task.
    series_index_handle: Mutex<Option<JoinHandle<()>>>,
    /// Controls the worker-owned series-index task.
    series_index_task_state: Option<Arc<SeriesIndexTaskState>>,
    /// Whether to run the worker thread.
    running: Arc<AtomicBool>,
}

impl RegionWorker {
    /// Submits request to background worker thread.
    async fn submit_request(&self, request: WorkerRequest) -> Result<()> {
        self.try_submit_request(request)
            .await
            .map_err(|_| WorkerStoppedSnafu { id: self.id }.build())
    }

    /// Submits request to background worker thread, or returns it if the worker is stopped.
    async fn try_submit_request(
        &self,
        request: WorkerRequest,
    ) -> std::result::Result<(), WorkerRequest> {
        if !self.is_running() {
            return Err(request);
        }
        let request_with_time = WorkerRequestWithTime::new(request);
        if let Err(mpsc::error::SendError(request_with_time)) =
            self.sender.send(request_with_time).await
        {
            warn!(
                "Worker {} is already exited but the running flag is still true",
                self.id
            );
            // Manually set the running flag to false to avoid printing more warning logs.
            self.set_running(false);
            if let Some(state) = &self.series_index_task_state {
                state.stop();
            }
            return Err(request_with_time.request);
        }

        Ok(())
    }

    /// Stop the worker.
    ///
    /// This method waits until the worker thread exists.
    async fn stop(&self) -> Result<()> {
        let handle = self.handle.lock().await.take();
        self.set_running(false);
        if let Some(state) = &self.series_index_task_state {
            state.stop();
        }

        let mut worker_result = Ok(());
        if let Some(handle) = handle {
            info!("Stop region worker {}", self.id);

            if self
                .sender
                .send(WorkerRequestWithTime::new(WorkerRequest::Stop))
                .await
                .is_err()
            {
                warn!("Worker {} is already exited before stop", self.id);
            }

            worker_result = handle.await.context(JoinSnafu);
        }
        let series_index_result = if let Some(handle) = self.series_index_handle.lock().await.take()
        {
            handle.await.context(JoinSnafu)
        } else {
            Ok(())
        };
        worker_result?;
        series_index_result?;

        Ok(())
    }

    /// Returns true if the worker is still running.
    fn is_running(&self) -> bool {
        self.running.load(Ordering::Relaxed)
    }

    /// Sets whether the worker is still running.
    fn set_running(&self, value: bool) {
        self.running.store(value, Ordering::Relaxed)
    }

    /// Returns true if the worker contains specific region.
    fn is_region_exists(&self, region_id: RegionId) -> bool {
        self.regions.is_region_exists(region_id)
    }

    /// Returns true if the region is opening.
    fn is_region_opening(&self, region_id: RegionId) -> bool {
        self.opening_regions.is_region_exists(region_id)
    }

    /// Returns true if the region is catching up.
    fn is_region_catching_up(&self, region_id: RegionId) -> bool {
        self.catchup_regions.is_region_exists(region_id)
    }

    /// Returns region of specific `region_id`.
    fn get_region(&self, region_id: RegionId) -> Option<MitoRegionRef> {
        self.regions.get_region(region_id)
    }

    #[cfg(test)]
    /// Returns the [OpeningRegionsRef].
    pub(crate) fn opening_regions(&self) -> &OpeningRegionsRef {
        &self.opening_regions
    }

    #[cfg(test)]
    /// Returns the [CatchupRegionsRef].
    pub(crate) fn catchup_regions(&self) -> &CatchupRegionsRef {
        &self.catchup_regions
    }
}

impl Drop for RegionWorker {
    fn drop(&mut self) {
        if let Some(state) = &self.series_index_task_state {
            state.stop();
        }
        if self.is_running() {
            self.set_running(false);
            // Once we drop the sender, the worker thread will receive a disconnected error.
        }
    }
}

type RequestBuffer = Vec<WorkerRequest>;

/// Buffer for stalled write requests.
///
/// Maintains stalled write requests and their estimated size.
#[derive(Default)]
pub(crate) struct StalledRequests {
    /// Stalled requests.
    /// Remember to use `StalledRequests::stalled_count()` to get the total number of stalled requests
    /// instead of `StalledRequests::requests.len()`.
    ///
    /// Key: RegionId
    /// Value: (estimated size, stalled requests)
    pub(crate) requests:
        HashMap<RegionId, (usize, Vec<SenderWriteRequest>, Vec<SenderBulkRequest>)>,
    /// Estimated size of all stalled requests.
    pub(crate) estimated_size: usize,
}

impl StalledRequests {
    /// Returns the estimated size of stalled requests for a region.
    pub(crate) fn estimated_size(&self, region_id: &RegionId) -> usize {
        self.requests
            .get(region_id)
            .map(|(size, _, _)| *size)
            .unwrap_or_default()
    }

    /// Appends stalled requests.
    pub(crate) fn append(
        &mut self,
        requests: &mut Vec<SenderWriteRequest>,
        bulk_requests: &mut Vec<SenderBulkRequest>,
    ) {
        for req in requests.drain(..) {
            self.push(req);
        }
        for req in bulk_requests.drain(..) {
            self.push_bulk(req);
        }
    }

    /// Pushes a stalled request to the buffer.
    pub(crate) fn push(&mut self, req: SenderWriteRequest) {
        let (size, requests, _) = self.requests.entry(req.request.region_id).or_default();
        let req_size = req.request.estimated_size();
        *size += req_size;
        self.estimated_size += req_size;
        requests.push(req);
    }

    pub(crate) fn push_bulk(&mut self, req: SenderBulkRequest) {
        let region_id = req.region_id;
        let (size, _, requests) = self.requests.entry(region_id).or_default();
        let req_size = req.request.estimated_size();
        *size += req_size;
        self.estimated_size += req_size;
        requests.push(req);
    }

    /// Removes stalled requests of specific region.
    pub(crate) fn remove(
        &mut self,
        region_id: &RegionId,
    ) -> (Vec<SenderWriteRequest>, Vec<SenderBulkRequest>) {
        if let Some((size, write_reqs, bulk_reqs)) = self.requests.remove(region_id) {
            self.estimated_size -= size;
            (write_reqs, bulk_reqs)
        } else {
            (vec![], vec![])
        }
    }

    /// Returns the total number of all stalled requests.
    pub(crate) fn stalled_count(&self) -> usize {
        self.requests
            .values()
            .map(|(_, reqs, bulk_reqs)| reqs.len() + bulk_reqs.len())
            .sum()
    }
}

/// Background worker loop to handle requests.
struct RegionWorkerLoop<S> {
    /// Id of the worker.
    id: WorkerId,
    /// Engine config.
    config: Arc<MitoConfig>,
    /// Regions bound to the worker.
    regions: RegionMapRef,
    /// Regions that are not yet fully dropped.
    dropping_regions: RegionMapRef,
    /// Regions that are opening.
    opening_regions: OpeningRegionsRef,
    /// Regions that are catching up.
    catchup_regions: CatchupRegionsRef,
    /// Request sender.
    sender: Sender<WorkerRequestWithTime>,
    /// Request receiver.
    receiver: Receiver<WorkerRequestWithTime>,
    /// WAL of the engine.
    wal: Wal<S>,
    /// Manages object stores for manifest and SSTs.
    object_store_manager: ObjectStoreManagerRef,
    /// Whether the worker thread is still running.
    running: Arc<AtomicBool>,
    /// Memtable builder provider for each region.
    memtable_builder_provider: MemtableBuilderProvider,
    /// Background purge job scheduler.
    purge_scheduler: SchedulerRef,
    /// Engine write buffer manager.
    write_buffer_manager: WriteBufferManagerRef,
    /// Scheduler for index build task.
    index_build_scheduler: IndexBuildScheduler,
    /// Controls the worker-owned series-index task.
    series_index_task_state: Option<Arc<SeriesIndexTaskState>>,
    /// Local store for series and range indexes managed by reconciliation.
    series_index_store: Option<ObjectStore>,
    series_index_purger: Option<IndexFilePurger>,
    /// Schedules background flush requests.
    flush_scheduler: FlushScheduler,
    /// Scheduler for compaction tasks.
    compaction_scheduler: CompactionScheduler,
    /// Stalled write requests.
    stalled_requests: StalledRequests,
    /// Event listener for tests.
    listener: WorkerListener,
    /// Cache.
    cache_manager: CacheManagerRef,
    /// Puffin manager factory for index.
    puffin_manager_factory: PuffinManagerFactory,
    /// Intermediate manager for inverted index.
    intermediate_manager: IntermediateManager,
    /// Provider to get current time.
    time_provider: TimeProviderRef,
    /// Last time to check regions periodically.
    last_periodical_check_millis: i64,
    /// Watch channel sender to notify workers to handle stalled requests.
    flush_sender: watch::Sender<()>,
    /// Watch channel receiver to wait for background flush job.
    flush_receiver: watch::Receiver<()>,
    /// Gauge of stalling request count.
    stalling_count: IntGauge,
    /// Gauge of regions in the worker.
    region_count: IntGauge,
    /// Histogram of request wait time for this worker.
    request_wait_time: Histogram,
    /// Queues for region edit requests.
    region_edit_queues: RegionEditQueues,
    /// Database level metadata manager.
    schema_metadata_manager: SchemaMetadataManagerRef,
    /// Datanode level file references manager.
    file_ref_manager: FileReferenceManagerRef,
    /// Partition expr fetcher used to backfill partition expr on open for compatibility.
    partition_expr_fetcher: PartitionExprFetcherRef,
    /// Semaphore to control flush concurrency.
    flush_semaphore: Arc<Semaphore>,
    /// Plugins for flush hooks.
    plugins: Plugins,
    #[cfg(feature = "hdfs-object-store")]
    /// Limiter for the temporary file sweeps, or `None` if the datanode allows
    /// no concurrent GC job.
    sweep_limiter: Option<Arc<Semaphore>>,
    node_lease: Arc<NodeLease>,
}

impl<S: LogStore> RegionWorkerLoop<S> {
    /// Publishes a loaded region together with its resident compaction state.
    fn register_region(&mut self, region: MitoRegionRef) {
        self.compaction_scheduler
            .register_region(&region.version_control, &region.access_layer);
        self.regions.insert_region(region);
    }

    /// Starts the worker loop.
    async fn run(&mut self) {
        let init_check_delay = worker_init_check_delay();
        info!(
            "Start region worker thread {}, init_check_delay: {:?}",
            self.id, init_check_delay
        );
        self.last_periodical_check_millis += init_check_delay.as_millis() as i64;

        // Buffer to retrieve requests from receiver.
        let mut write_req_buffer: Vec<SenderWriteRequest> =
            Vec::with_capacity(self.config.worker_request_batch_size);
        let mut bulk_req_buffer: Vec<SenderBulkRequest> =
            Vec::with_capacity(self.config.worker_request_batch_size);
        let mut ddl_req_buffer: Vec<SenderDdlRequest> =
            Vec::with_capacity(self.config.worker_request_batch_size);
        let mut general_req_buffer: Vec<WorkerRequest> =
            RequestBuffer::with_capacity(self.config.worker_request_batch_size);

        while self.running.load(Ordering::Relaxed) {
            // Clear the buffer before handling next batch of requests.
            write_req_buffer.clear();
            ddl_req_buffer.clear();
            general_req_buffer.clear();
            let mut bulk_insert_req_num = 0;

            let max_wait_time = self.time_provider.wait_duration(CHECK_REGION_INTERVAL);
            let sleep = tokio::time::sleep(max_wait_time);
            tokio::pin!(sleep);

            tokio::select! {
                request_opt = self.receiver.recv() => {
                    match request_opt {
                        Some(request_with_time) => {
                            // Observe the wait time
                            let wait_time = request_with_time.created_at.elapsed();
                            self.request_wait_time.observe(wait_time.as_secs_f64());

                            match request_with_time.request {
                                WorkerRequest::Write(sender_req) => write_req_buffer.push(sender_req),
                                WorkerRequest::BatchWrite(sender_reqs) => write_req_buffer.extend(sender_reqs),
                                WorkerRequest::Ddl(sender_req) => ddl_req_buffer.push(sender_req),
                                WorkerRequest::BulkInserts(bulk_insert) => {
                                    bulk_insert_req_num += 1;
                                    self.buffer_bulk_insert_request(bulk_insert, &mut bulk_req_buffer)
                                        .await;
                                }
                                req => general_req_buffer.push(req),
                            }
                        },
                        // The channel is disconnected.
                        None => break,
                    }
                }
                recv_res = self.flush_receiver.changed() => {
                    if recv_res.is_err() {
                        // The channel is disconnected.
                        break;
                    } else {
                        // Also flush this worker if other workers trigger flush as this worker may have
                        // a large memtable to flush. We may not have chance to flush that memtable if we
                        // never write to this worker. So only flushing other workers may not release enough
                        // memory.
                        self.maybe_flush_worker();
                        // A flush job is finished, handles stalled requests.
                        self.handle_stalled_requests().await;
                        continue;
                    }
                }
                _ = &mut sleep => {
                    // Timeout. Checks periodical tasks.
                    self.handle_periodical_tasks();
                    continue;
                }
            }

            if self.flush_receiver.has_changed().unwrap_or(false) {
                // Always checks whether we could process stalled requests to avoid a request
                // hangs too long.
                // If the channel is closed, do nothing.
                self.handle_stalled_requests().await;
            }

            // Try to recv more requests from the channel.
            for _ in 1..self.config.worker_request_batch_size {
                // We have received one request so we start from 1.
                match self.receiver.try_recv() {
                    Ok(request_with_time) => {
                        // Observe the wait time
                        let wait_time = request_with_time.created_at.elapsed();
                        self.request_wait_time.observe(wait_time.as_secs_f64());

                        match request_with_time.request {
                            WorkerRequest::Write(sender_req) => write_req_buffer.push(sender_req),
                            WorkerRequest::BatchWrite(sender_reqs) => {
                                write_req_buffer.extend(sender_reqs)
                            }
                            WorkerRequest::Ddl(sender_req) => ddl_req_buffer.push(sender_req),
                            WorkerRequest::BulkInserts(bulk_insert) => {
                                bulk_insert_req_num += 1;
                                self.buffer_bulk_insert_request(bulk_insert, &mut bulk_req_buffer)
                                    .await
                            }
                            req => general_req_buffer.push(req),
                        }
                    }
                    // We still need to handle remaining requests.
                    Err(_) => break,
                }
            }

            self.listener.on_recv_requests(
                write_req_buffer.len()
                    + ddl_req_buffer.len()
                    + general_req_buffer.len()
                    + bulk_insert_req_num,
            );

            self.handle_requests(
                &mut write_req_buffer,
                &mut ddl_req_buffer,
                &mut general_req_buffer,
                &mut bulk_req_buffer,
            )
            .await;

            self.handle_periodical_tasks();
        }

        self.clean().await;

        info!("Exit region worker thread {}", self.id);
    }

    async fn buffer_bulk_insert_request(
        &mut self,
        bulk_insert: BulkInsertRequest,
        bulk_requests: &mut Vec<SenderBulkRequest>,
    ) {
        let BulkInsertRequest {
            metadata,
            request,
            sender,
        } = bulk_insert;

        if let Some(region_metadata) = metadata {
            self.handle_bulk_insert_batch(region_metadata, request, bulk_requests, sender)
                .await;
        } else {
            error!("Cannot find region metadata for {}", request.region_id);
            sender.send(
                error::RegionNotFoundSnafu {
                    region_id: request.region_id,
                }
                .fail(),
            );
        }
    }

    /// Dispatches and processes requests.
    ///
    /// `general_requests` should not contain categorized write, ddl, or bulk insert requests.
    async fn handle_requests(
        &mut self,
        write_requests: &mut Vec<SenderWriteRequest>,
        ddl_requests: &mut Vec<SenderDdlRequest>,
        general_requests: &mut Vec<WorkerRequest>,
        bulk_requests: &mut Vec<SenderBulkRequest>,
    ) {
        for worker_req in general_requests.drain(..) {
            match worker_req {
                WorkerRequest::Write(_) | WorkerRequest::BatchWrite(_) | WorkerRequest::Ddl(_) => {
                    // These requests are categorized before dispatching general requests.
                    continue;
                }
                WorkerRequest::BulkInserts(_) => unreachable!("bulk inserts are buffered"),
                WorkerRequest::Background { region_id, notify } => {
                    if matches!(
                        &notify,
                        BackgroundNotify::RegionEdit(edit_result)
                            if edit_result.update_region_state
                    ) {
                        // Region state must be Editing when reach here.
                        // This call only moves write/bulk write request into stall queue. When region edit result
                        // is processed inside handle_background_notify and region state is switched back to Writable,
                        // stalled request will be processed before the next region edit is dequeued from
                        // RegionEditQueue immediately in handle_region_edit_result. It not only ensured pending writes
                        // are processed in time, but also prevents them from starvation.
                        // TODO(hl): maybe we need to merge those queues for pending requests like pending_ddl,
                        // region edits and stalled request, so we can simplify the coordination between these queues.
                        self.handle_buffered_region_write_requests(
                            &region_id,
                            write_requests,
                            bulk_requests,
                        )
                        .await;
                    }
                    // For background notify, we handle it directly.
                    self.handle_background_notify(region_id, notify).await;
                }
                WorkerRequest::SetRegionRoleStateGracefully {
                    region_id,
                    region_role_state,
                    sender,
                } => {
                    self.set_role_state_gracefully(region_id, region_role_state, sender)
                        .await;
                }
                WorkerRequest::EditRegion(request) => {
                    self.handle_region_edit(request);
                }
                WorkerRequest::Stop => {
                    debug_assert!(!self.running.load(Ordering::Relaxed));
                }
                WorkerRequest::SyncRegion(req) => {
                    self.handle_region_sync(req).await;
                }
                WorkerRequest::RemapManifests(req) => {
                    self.handle_remap_manifests_request(req);
                }
                WorkerRequest::CopyRegionFrom(req) => {
                    self.handle_copy_region_from_request(req);
                }
            }
        }

        // Handles all write requests first. So we can alter regions without
        // considering existing write requests.
        self.handle_write_requests(write_requests, bulk_requests, true)
            .await;

        self.handle_ddl_requests(ddl_requests).await;
    }

    /// Takes and handles all ddl requests.
    async fn handle_ddl_requests(&mut self, ddl_requests: &mut Vec<SenderDdlRequest>) {
        if ddl_requests.is_empty() {
            return;
        }

        for ddl in ddl_requests.drain(..) {
            let res = match ddl.request {
                DdlRequest::Create(req) => self.handle_create_request(ddl.region_id, req).await,
                DdlRequest::Drop(req) => {
                    self.handle_drop_request(ddl.region_id, req, ddl.sender)
                        .await;
                    continue;
                }
                DdlRequest::Open((req, wal_entry_receiver)) => {
                    self.handle_open_request(ddl.region_id, req, wal_entry_receiver, ddl.sender)
                        .await;
                    continue;
                }
                DdlRequest::OfflineCleanup(req) => {
                    self.handle_offline_cleanup_request(ddl.region_id, req)
                        .await
                }
                DdlRequest::Close(req) => {
                    self.handle_close_request(ddl.region_id, req, ddl.sender)
                        .await;
                    continue;
                }
                DdlRequest::Alter(req) => {
                    self.handle_alter_request(ddl.region_id, req, ddl.sender)
                        .await;
                    continue;
                }
                DdlRequest::Flush(req) => {
                    self.handle_flush_request(ddl.region_id, req, ddl.sender);
                    continue;
                }
                DdlRequest::Compact(req) => {
                    self.handle_compaction_request(ddl.region_id, req, ddl.sender)
                        .await;
                    continue;
                }
                DdlRequest::BuildIndex(req) => {
                    self.handle_build_index_request(ddl.region_id, req, ddl.sender)
                        .await;
                    continue;
                }
                DdlRequest::Truncate(req) => {
                    self.handle_truncate_request(ddl.region_id, req, ddl.sender)
                        .await;
                    continue;
                }
                DdlRequest::Catchup((req, wal_entry_receiver)) => {
                    self.handle_catchup_request(ddl.region_id, req, wal_entry_receiver, ddl.sender)
                        .await;
                    continue;
                }
                DdlRequest::EnterStaging(req) => {
                    self.handle_enter_staging_request(
                        ddl.region_id,
                        req.partition_directive,
                        ddl.sender,
                    )
                    .await;
                    continue;
                }
                DdlRequest::ApplyStagingManifest(req) => {
                    self.handle_apply_staging_manifest_request(ddl.region_id, req, ddl.sender)
                        .await;
                    continue;
                }
            };

            ddl.sender.send(res);
        }
    }

    /// Handle periodical tasks such as region auto flush.
    fn handle_periodical_tasks(&mut self) {
        let interval = CHECK_REGION_INTERVAL.as_millis() as i64;
        if self
            .time_provider
            .elapsed_since(self.last_periodical_check_millis)
            < interval
        {
            return;
        }

        self.last_periodical_check_millis = self.time_provider.current_time_millis();

        if let Err(e) = self.flush_periodically() {
            error!(e; "Failed to flush regions periodically");
        }
    }

    /// Handles region background request
    async fn handle_background_notify(&mut self, region_id: RegionId, notify: BackgroundNotify) {
        match notify {
            BackgroundNotify::RegionOpened { region, registered } => {
                self.register_region(region);
                self.region_count.inc();

                // Finish this opening batch before the worker can process a close/reopen,
                // without waiting for the background opener to resume after the ACK.
                let senders = self.opening_regions.remove_sender(region_id);
                if let Some(state) = &self.series_index_task_state {
                    state.wake();
                }
                let _ = registered.send(());
                for sender in senders {
                    sender.send(Ok(0));
                }
            }
            BackgroundNotify::CompactionUnit(req) => {
                self.handle_compaction_unit(region_id, req).await;
            }
            BackgroundNotify::CompactionDdlComplete {
                generation,
                request_index,
            } => {
                self.compaction_scheduler
                    .on_ddl_complete(region_id, generation, request_index);
            }
            BackgroundNotify::CompactionPickFinished(req) => {
                self.handle_compaction_pick_finished(region_id, req).await
            }
            BackgroundNotify::FlushFinished(req) => {
                self.handle_flush_finished(region_id, req).await
            }
            BackgroundNotify::FlushFailed(req) => self.handle_flush_failed(region_id, req).await,
            BackgroundNotify::IndexBuildFinished(req) => {
                self.handle_index_build_finished(region_id, req).await
            }
            BackgroundNotify::IndexBuildStopped(req) => {
                self.handle_index_build_stopped(region_id, req).await
            }
            BackgroundNotify::IndexBuildRetry(req) => {
                self.handle_rebuild_index(req, OptionOutputTx::new(None))
                    .await
            }
            BackgroundNotify::CompactionFinished(req) => {
                self.handle_compaction_finished(region_id, req).await
            }
            BackgroundNotify::CompactionFailed(req) => self.handle_compaction_failure(req).await,
            BackgroundNotify::Truncate(req) => self.handle_truncate_result(req).await,
            BackgroundNotify::DiscardUnflushed(req) => {
                self.handle_discard_unflushed_result(req).await
            }
            BackgroundNotify::RegionChange(req) => {
                self.handle_manifest_region_change_result(req).await
            }
            BackgroundNotify::EnterStaging(req) => self.handle_enter_staging_result(req).await,
            BackgroundNotify::RegionEdit(req) => self.handle_region_edit_result(req).await,
            BackgroundNotify::CopyRegionFromFinished(req) => {
                self.handle_copy_region_from_finished(req)
            }
        }
    }

    /// Handles `set_region_role_gracefully`.
    async fn set_role_state_gracefully(
        &mut self,
        region_id: RegionId,
        region_role_state: SettableRegionRoleState,
        sender: oneshot::Sender<SetRegionRoleStateResponse>,
    ) {
        if let Some(region) = self.regions.get_region(region_id) {
            if matches!(
                region_role_state,
                SettableRegionRoleState::Follower | SettableRegionRoleState::DowngradingLeader
            ) {
                // Fence writes here: writes the worker handles later see
                // Downgrading, and earlier ones have finished. This only
                // changes a writable or staging leader.
                region.set_role(RegionRole::DowngradingLeader);
            }
            #[cfg(feature = "hdfs-object-store")]
            let sweep_older_than = self.config.gc.unknown_file_lingering_time;
            #[cfg(feature = "hdfs-object-store")]
            let sweep_limiter = self.sweep_limiter.clone();
            // We need to do this in background as we need the manifest lock.
            common_runtime::spawn_global(async move {
                match region.set_role_state_gracefully(region_role_state).await {
                    Ok(()) => {
                        let last_entry_id = region.version_control.current().last_entry_id;
                        let _ = sender.send(SetRegionRoleStateResponse::success(
                            SetRegionRoleStateSuccess::mito(last_entry_id),
                        ));
                        // Only a writable leader reclaims what a writer that died
                        // before it renamed its temporary file left behind.
                        #[cfg(feature = "hdfs-object-store")]
                        if region_role_state == SettableRegionRoleState::Leader {
                            temp_file_sweeper::spawn_region_sweep(
                                region.access_layer(),
                                region_id,
                                sweep_older_than,
                                sweep_limiter,
                            );
                        }
                    }
                    Err(e) => {
                        error!(e; "Failed to set region {} role state to {:?}", region_id, region_role_state);
                        let _ = sender.send(SetRegionRoleStateResponse::invalid_transition(
                            BoxedError::new(e),
                        ));
                    }
                }
            });
        } else {
            let _ = sender.send(SetRegionRoleStateResponse::NotFound);
        }
    }
}

impl<S> RegionWorkerLoop<S> {
    /// Cleans up the worker.
    async fn clean(&self) {
        // Closes remaining regions.
        let regions = self.regions.list_regions();
        for region in regions {
            region.stop().await;
        }

        self.regions.clear();
    }

    /// Notifies the whole group that a flush job is finished so other
    /// workers can handle stalled requests.
    fn notify_group(&mut self) {
        // Notifies all receivers.
        let _ = self.flush_sender.send(());
        // Marks the receiver in current worker as seen so the loop won't be waked up immediately.
        self.flush_receiver.borrow_and_update();
    }
}

/// Wrapper that only calls event listener in tests.
#[derive(Default, Clone)]
pub(crate) struct WorkerListener {
    #[cfg(any(test, feature = "test"))]
    listener: Option<crate::engine::listener::EventListenerRef>,
}

impl WorkerListener {
    #[cfg(any(test, feature = "test"))]
    pub(crate) fn new(
        listener: Option<crate::engine::listener::EventListenerRef>,
    ) -> WorkerListener {
        WorkerListener { listener }
    }

    /// Notifies the background opener after the worker acknowledges region registration.
    pub(crate) async fn on_region_open_registered(&self, _region_id: RegionId) {
        #[cfg(any(test, feature = "test"))]
        if let Some(listener) = &self.listener {
            listener.on_region_open_registered(_region_id).await;
        }
    }

    /// Flush is finished successfully.
    pub(crate) fn on_flush_success(&self, region_id: RegionId) {
        #[cfg(any(test, feature = "test"))]
        if let Some(listener) = &self.listener {
            listener.on_flush_success(region_id);
        }
        // Avoid compiler warning.
        let _ = region_id;
    }

    /// Engine is stalled.
    pub(crate) fn on_write_stall(&self) {
        #[cfg(any(test, feature = "test"))]
        if let Some(listener) = &self.listener {
            listener.on_write_stall();
        }
    }

    /// A batch of writes is appended to the WAL.
    pub(crate) async fn on_write_wal_end(&self) {
        #[cfg(any(test, feature = "test"))]
        if let Some(listener) = &self.listener {
            listener.on_write_wal_end().await;
        }
    }

    pub(crate) async fn on_flush_begin(&self, region_id: RegionId) {
        #[cfg(any(test, feature = "test"))]
        if let Some(listener) = &self.listener {
            listener.on_flush_begin(region_id).await;
        }
        // Avoid compiler warning.
        let _ = region_id;
    }

    pub(crate) async fn on_flush_commit_begin(&self, _region_id: RegionId) {
        #[cfg(any(test, feature = "test"))]
        if let Some(listener) = &self.listener {
            listener.on_flush_commit_begin(_region_id).await;
        }
    }

    pub(crate) fn on_flush_cancel_requested(&self, _region_id: RegionId) {
        #[cfg(any(test, feature = "test"))]
        if let Some(listener) = &self.listener {
            listener.on_flush_cancel_requested(_region_id);
        }
    }

    pub(crate) fn on_later_drop_begin(&self, region_id: RegionId) -> Option<Duration> {
        #[cfg(any(test, feature = "test"))]
        if let Some(listener) = &self.listener {
            return listener.on_later_drop_begin(region_id);
        }
        // Avoid compiler warning.
        let _ = region_id;
        None
    }

    /// On later drop task is finished.
    pub(crate) fn on_later_drop_end(&self, region_id: RegionId, removed: bool) {
        #[cfg(any(test, feature = "test"))]
        if let Some(listener) = &self.listener {
            listener.on_later_drop_end(region_id, removed);
        }
        // Avoid compiler warning.
        let _ = region_id;
        let _ = removed;
    }

    pub(crate) async fn on_merge_ssts_finished(&self, region_id: RegionId) {
        #[cfg(any(test, feature = "test"))]
        if let Some(listener) = &self.listener {
            listener.on_merge_ssts_finished(region_id).await;
        }
        // Avoid compiler warning.
        let _ = region_id;
    }

    pub(crate) fn on_recv_requests(&self, request_num: usize) {
        #[cfg(any(test, feature = "test"))]
        if let Some(listener) = &self.listener {
            listener.on_recv_requests(request_num);
        }
        // Avoid compiler warning.
        let _ = request_num;
    }

    pub(crate) fn on_file_cache_filled(&self, _file_id: FileId) {
        #[cfg(any(test, feature = "test"))]
        if let Some(listener) = &self.listener {
            listener.on_file_cache_filled(_file_id);
        }
    }

    pub(crate) fn on_compaction_scheduled(&self, _region_id: RegionId) {
        #[cfg(any(test, feature = "test"))]
        if let Some(listener) = &self.listener {
            listener.on_compaction_scheduled(_region_id);
        }
    }

    pub(crate) async fn on_compaction_pick_begin(&self, _region_id: RegionId) {
        #[cfg(any(test, feature = "test"))]
        if let Some(listener) = &self.listener {
            listener.on_compaction_pick_begin(_region_id).await;
        }
    }

    pub(crate) async fn on_compaction_commit_begin(&self, _region_id: RegionId) {
        #[cfg(any(test, feature = "test"))]
        if let Some(listener) = &self.listener {
            listener.on_compaction_commit_begin(_region_id).await;
        }
    }

    /// Exposes a per-unit test gate before merge starts.
    pub(crate) async fn on_compaction_unit_merge_begin(&self, _region_id: RegionId, _plan_id: u64) {
        #[cfg(any(test, feature = "test"))]
        if let Some(listener) = &self.listener {
            listener
                .on_compaction_unit_merge_begin(_region_id, _plan_id)
                .await;
        }
    }

    /// Exposes a per-unit test gate between manifest commit and worker notification.
    pub(crate) async fn on_compaction_unit_committed(&self, _region_id: RegionId, _plan_id: u64) {
        #[cfg(any(test, feature = "test"))]
        if let Some(listener) = &self.listener {
            listener
                .on_compaction_unit_committed(_region_id, _plan_id)
                .await;
        }
    }

    /// Reports unit visibility to the test listener independently of request completion.
    pub(crate) fn on_compaction_unit_applied(&self, _region_id: RegionId, _plan_id: u64) {
        #[cfg(any(test, feature = "test"))]
        if let Some(listener) = &self.listener {
            listener.on_compaction_unit_applied(_region_id, _plan_id);
        }
    }

    pub(crate) async fn on_compaction_result_notified(&self, _region_id: RegionId) {
        #[cfg(any(test, feature = "test"))]
        if let Some(listener) = &self.listener {
            listener.on_compaction_result_notified(_region_id).await;
        }
    }

    pub(crate) fn on_compaction_cancel_requested(&self, _region_id: RegionId) {
        #[cfg(any(test, feature = "test"))]
        if let Some(listener) = &self.listener {
            listener.on_compaction_cancel_requested(_region_id);
        }
    }

    pub(crate) async fn on_notify_region_change_result_begin(&self, _region_id: RegionId) {
        #[cfg(any(test, feature = "test"))]
        if let Some(listener) = &self.listener {
            listener
                .on_notify_region_change_result_begin(_region_id)
                .await;
        }
    }

    pub(crate) async fn on_enter_staging_result_begin(&self, _region_id: RegionId) {
        #[cfg(any(test, feature = "test"))]
        if let Some(listener) = &self.listener {
            listener.on_enter_staging_result_begin(_region_id).await;
        }
    }

    pub(crate) async fn on_index_build_finish(&self, _region_file_id: RegionFileId) {
        #[cfg(any(test, feature = "test"))]
        if let Some(listener) = &self.listener {
            listener.on_index_build_finish(_region_file_id).await;
        }
    }

    pub(crate) async fn on_index_build_begin(&self, _region_file_id: RegionFileId) {
        #[cfg(any(test, feature = "test"))]
        if let Some(listener) = &self.listener {
            listener.on_index_build_begin(_region_file_id).await;
        }
    }

    pub(crate) async fn on_index_build_abort(&self, _region_file_id: RegionFileId) {
        #[cfg(any(test, feature = "test"))]
        if let Some(listener) = &self.listener {
            listener.on_index_build_abort(_region_file_id).await;
        }
    }

    pub(crate) async fn on_index_build_before_manifest_commit(
        &self,
        _region_file_id: RegionFileId,
    ) {
        #[cfg(any(test, feature = "test"))]
        if let Some(listener) = &self.listener {
            listener
                .on_index_build_before_manifest_commit(_region_file_id)
                .await;
        }
    }

    pub(crate) async fn on_index_build_manifest_committed(&self, _region_file_id: RegionFileId) {
        #[cfg(any(test, feature = "test"))]
        if let Some(listener) = &self.listener {
            listener
                .on_index_build_manifest_committed(_region_file_id)
                .await;
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::test_util::TestEnv;

    #[test]
    fn test_region_id_to_index() {
        let num_workers = 4;

        let region_id = RegionId::new(1, 2);
        let index = region_id_to_index(region_id, num_workers);
        assert_eq!(index, 3);

        let region_id = RegionId::new(2, 3);
        let index = region_id_to_index(region_id, num_workers);
        assert_eq!(index, 1);
    }

    #[tokio::test]
    async fn test_worker_group_start_stop() {
        for (enabled, existing_index) in [(false, false), (true, false), (false, true)] {
            // Use a fresh WAL directory for each case: stopping workers does not stop the log store.
            let env = TestEnv::with_prefix("group-stop").await;
            let root = env.data_home().join("series_index");
            if existing_index {
                tokio::fs::create_dir_all(&root).await.unwrap();
                tokio::fs::write(root.join("retained"), b"index")
                    .await
                    .unwrap();
            }
            let group = env
                .create_worker_group(MitoConfig {
                    num_workers: 4,
                    experimental_enable_series_index: enabled,
                    experimental_series_index_maintenance_interval: Duration::from_secs(3600),
                    ..Default::default()
                })
                .await;

            for worker in &group.workers {
                assert_eq!(worker.series_index_task_state.is_some(), enabled);
                assert_eq!(worker.series_index_handle.lock().await.is_some(), enabled);
            }
            assert_eq!(root.is_dir(), enabled || existing_index);
            if existing_index {
                assert_eq!(
                    tokio::fs::read(root.join("retained")).await.unwrap(),
                    b"index"
                );
            }

            tokio::time::timeout(Duration::from_secs(5), group.stop())
                .await
                .expect("series-index tasks should stop without waiting for their interval")
                .unwrap();
        }
    }
}