Added NRT directory kinda working

src/directory/directory.rs

@@ -205,6 +205,16 @@ pub trait Directory: DirectoryClone + fmt::Debug + Send + Sync + 'static {
     /// `OnCommit` `ReloadPolicy`. Not implementing watch in a `Directory` only prevents the
     /// `OnCommit` `ReloadPolicy` to work properly.
     fn watch(&self, watch_callback: WatchCallback) -> WatchHandle;
+
+    /// Ensure that all volatile files reach are persisted (in directory where that makes sense.)
+    ///
+    /// In order to make Near Real Time efficient, tantivy introduced the notion of soft_commit vs
+    /// commit. Commit will call `.flush()`, while softcommit won't.
+    ///
+    /// `meta.json` should be the last file to be flushed.
+    fn flush(&self) -> io::Result<()> {
+        Ok(())
+    }
 }
 
 /// DirectoryClone

src/directory/mmap_directory.rs

@@ -368,7 +368,7 @@ impl Drop for ReleaseLockFile {
 
 /// This Write wraps a File, but has the specificity of
 /// call `sync_all` on flush.
-struct SafeFileWriter(File);
+pub struct SafeFileWriter(File);
 
 impl SafeFileWriter {
     fn new(file: File) -> SafeFileWriter {

src/directory/mod.rs

@@ -13,6 +13,7 @@ mod managed_directory;
 mod ram_directory;
 mod read_only_source;
 mod watch_event_router;
+mod nrt_directory;
 
 /// Errors specific to the directory module.
 pub mod error;

src/directory/nrt_directory.rs

@@ -0,0 +1,195 @@
+use directory::Directory;
+use std::path::{PathBuf, Path};
+use directory::ReadOnlySource;
+use directory::error::OpenReadError;
+use directory::error::DeleteError;
+use std::io::{BufWriter, Cursor};
+use directory::SeekableWrite;
+use directory::error::OpenWriteError;
+use directory::WatchHandle;
+use directory::ram_directory::InnerRamDirectory;
+use std::sync::RwLock;
+use std::sync::Arc;
+use directory::WatchCallback;
+use std::fmt;
+use std::io;
+use std::io::{Seek, Write};
+use directory::DirectoryClone;
+
+
+const BUFFER_LEN: usize = 1_000_000;
+
+
+pub enum NRTWriter {
+    InRam {
+        buffer: Cursor<Vec<u8>>,
+        path: PathBuf,
+        nrt_directory: NRTDirectory
+    },
+    UnderlyingFile(BufWriter<Box<SeekableWrite>>)
+}
+
+impl NRTWriter {
+    pub fn new(path: PathBuf, nrt_directory: NRTDirectory) -> NRTWriter {
+        NRTWriter::InRam {
+            buffer: Cursor::new(Vec::with_capacity(BUFFER_LEN)),
+            path,
+            nrt_directory,
+        }
+    }
+}
+
+impl io::Seek for NRTWriter {
+    fn seek(&mut self, pos: io::SeekFrom) -> io::Result<u64> {
+        match self {
+            NRTWriter::InRam { buffer, path, nrt_directory } => {
+                buffer.seek(pos)
+            }
+            NRTWriter::UnderlyingFile(file) => {
+                file.seek(pos)
+            }
+        }
+    }
+}
+
+impl io::Write for NRTWriter {
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        self.write_all(buf)?;
+        Ok(buf.len())
+    }
+
+    fn flush(&mut self) -> io::Result<()> {
+        match self {
+            NRTWriter::InRam { buffer, path, nrt_directory } => {
+                let mut cache_wlock = nrt_directory.cache.write().unwrap();
+                cache_wlock.write(path.clone(), buffer.get_ref());
+                Ok(())
+            }
+            NRTWriter::UnderlyingFile(file) => {
+                file.flush()
+            }
+        }
+    }
+
+    fn write_all(&mut self, buf: &[u8]) -> io::Result<()> {
+        // Working around the borrow checker.
+        let mut underlying_write_opt: Option<BufWriter<Box<SeekableWrite>>> = None;
+        if let NRTWriter::InRam { buffer, path, nrt_directory } = self {
+            if buffer.get_ref().len() + buf.len() > BUFFER_LEN {
+                // We can't keep this in RAM. Let's move it to the underlying directory.
+                underlying_write_opt = Some(nrt_directory.open_write(path)
+                    .map_err(|open_err| {
+                        io::Error::new(io::ErrorKind::Other, open_err)
+                    })?);
+
+            }
+        }
+        if let Some(underlying_write) = underlying_write_opt {
+            *self = NRTWriter::UnderlyingFile(underlying_write);
+        }
+        match self {
+            NRTWriter::InRam { buffer, path, nrt_directory } => {
+                assert!(buffer.get_ref().len() + buf.len() <= BUFFER_LEN);
+                buffer.write_all(buf)
+            }
+            NRTWriter::UnderlyingFile(file) => {
+                file.write_all(buf)
+            }
+        }
+    }
+}
+
+pub struct NRTDirectory {
+    underlying: Box<Directory>,
+    cache: Arc<RwLock<InnerRamDirectory>>,
+}
+
+
+impl Clone for NRTDirectory {
+    fn clone(&self) -> Self {
+        NRTDirectory {
+            underlying: self.underlying.box_clone(),
+            cache: self.cache.clone()
+        }
+    }
+}
+
+impl NRTDirectory {
+    fn wrap(underlying: Box<Directory>) -> NRTDirectory {
+        NRTDirectory {
+            underlying,
+            cache: Default::default()
+        }
+    }
+}
+
+impl fmt::Debug for NRTDirectory {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "NRTDirectory({:?})", self.underlying)
+    }
+}
+
+impl Directory for NRTDirectory {
+    fn open_read(&self, path: &Path) -> Result<ReadOnlySource, OpenReadError> {
+        unimplemented!()
+    }
+
+    fn delete(&self, path: &Path) -> Result<(), DeleteError> {
+        // We explicitly release the lock, to prevent a panic on the underlying directory
+        // to poison the lock.
+        //
+        // File can only go from cache to underlying so the result does not lead to
+        // any inconsistency.
+        {
+            let mut cache_wlock = self.cache.write().unwrap();
+            if cache_wlock.exists(path) {
+                return cache_wlock.delete(path);
+            }
+        }
+        self.underlying.delete(path)
+    }
+
+    fn exists(&self, path: &Path) -> bool {
+        // We explicitly release the lock, to prevent a panic on the underlying directory
+        // to poison the lock.
+        //
+        // File can only go from cache to underlying so the result does not lead to
+        // any inconsistency.
+        {
+            let rlock_cache = self.cache.read().unwrap();
+            if rlock_cache.exists(path) {
+                return true;
+            }
+        }
+        self.underlying.exists(path)
+    }
+
+    fn open_write(&mut self, path: &Path) -> Result<BufWriter<Box<SeekableWrite>>, OpenWriteError> {
+        let mut cache_wlock = self.cache.write().unwrap();
+        // TODO might poison our lock. I don't know have a sound solution yet.
+        let path_buf = path.to_owned();
+        if self.underlying.exists(path) {
+            return Err(OpenWriteError::FileAlreadyExists(path_buf));
+        }
+        let exists = cache_wlock.write(path_buf.clone(), &[]);
+        // force the creation of the file to mimic the MMap directory.
+        if exists {
+            Err(OpenWriteError::FileAlreadyExists(path_buf))
+        } else {
+            let vec_writer = NRTWriter::new(path_buf.clone(), self.clone());
+            Ok(BufWriter::new(Box::new(vec_writer)))
+        }
+    }
+
+    fn atomic_read(&self, path: &Path) -> Result<Vec<u8>, OpenReadError> {
+        self.underlying.atomic_read(path)
+    }
+
+    fn atomic_write(&mut self, path: &Path, data: &[u8]) -> io::Result<()> {
+        self.underlying.atomic_write(path, data)
+    }
+
+    fn watch(&self, watch_callback: WatchCallback) -> WatchHandle {
+        self.underlying.watch(watch_callback)
+    }
+}

src/directory/ram_directory.rs

@@ -71,36 +71,36 @@ impl Write for VecWriter {
 }
 
 #[derive(Default)]
-struct InnerDirectory {
+pub(crate) struct InnerRamDirectory {
     fs: HashMap<PathBuf, ReadOnlySource>,
     watch_router: WatchCallbackList,
 }
 
-impl InnerDirectory {
-    fn write(&mut self, path: PathBuf, data: &[u8]) -> bool {
+impl InnerRamDirectory {
+    pub fn write(&mut self, path: PathBuf, data: &[u8]) -> bool {
         let data = ReadOnlySource::new(Vec::from(data));
         self.fs.insert(path, data).is_some()
     }
 
-    fn open_read(&self, path: &Path) -> Result<ReadOnlySource, OpenReadError> {
+    pub fn open_read(&self, path: &Path) -> Result<ReadOnlySource, OpenReadError> {
         self.fs
             .get(path)
             .ok_or_else(|| OpenReadError::FileDoesNotExist(PathBuf::from(path)))
             .map(|el| el.clone())
     }
 
-    fn delete(&mut self, path: &Path) -> result::Result<(), DeleteError> {
+    pub fn delete(&mut self, path: &Path) -> result::Result<(), DeleteError> {
         match self.fs.remove(path) {
             Some(_) => Ok(()),
             None => Err(DeleteError::FileDoesNotExist(PathBuf::from(path))),
         }
     }
 
-    fn exists(&self, path: &Path) -> bool {
+    pub fn exists(&self, path: &Path) -> bool {
         self.fs.contains_key(path)
     }
 
-    fn watch(&mut self, watch_handle: WatchCallback) -> WatchHandle {
+    pub fn watch(&mut self, watch_handle: WatchCallback) -> WatchHandle {
         self.watch_router.subscribe(watch_handle)
     }
 }
@@ -118,7 +118,7 @@ impl fmt::Debug for RAMDirectory {
 ///
 #[derive(Clone, Default)]
 pub struct RAMDirectory {
-    fs: Arc<RwLock<InnerDirectory>>,
+    fs: Arc<RwLock<InnerRamDirectory>>,
 }
 
 impl RAMDirectory {

src/indexer/delete_queue.rs

@@ -179,6 +179,11 @@ pub struct DeleteCursor {
 }
 
 impl DeleteCursor {
+
+    pub fn empty() -> DeleteCursor {
+        DeleteQueue::new().cursor()
+    }
+
     /// Skips operations and position it so that
     /// - either all of the delete operation currently in the
     ///   queue are consume and the next get will return None.

src/indexer/index_writer.rs

@@ -259,7 +259,7 @@ pub fn advance_deletes(
             write_delete_bitset(&delete_bitset, &mut delete_file)?;
         }
     }
-    segment_entry.set_meta(segment.meta().clone());
+    segment_entry.set_meta(target_opstamp, segment.meta().clone());
     Ok(())
 }
 
@@ -326,7 +326,12 @@ fn index_documents(
         // to even open the segment.
         None
     };
-    let segment_entry = SegmentEntry::new(segment_meta, delete_cursor, delete_bitset_opt);
+    let segment_entry = SegmentEntry::new(
+        segment_meta,
+        delete_cursor,
+        delete_bitset_opt,
+        last_docstamp,
+    );
     Ok(segment_updater.add_segment(generation, segment_entry))
 }
 
@@ -361,9 +366,9 @@ impl IndexWriter {
     }
 
     #[doc(hidden)]
-    pub fn add_segment(&mut self, segment_meta: SegmentMeta) {
+    pub fn add_segment(&mut self, segment_meta: SegmentMeta, opstamp: u64) {
         let delete_cursor = self.delete_queue.cursor();
-        let segment_entry = SegmentEntry::new(segment_meta, delete_cursor, None);
+        let segment_entry = SegmentEntry::new(segment_meta, delete_cursor, None, opstamp);
         self.segment_updater
             .add_segment(self.generation, segment_entry);
     }
@@ -527,7 +532,7 @@ impl IndexWriter {
         //
         // This will reach an end as the only document_sender
         // was dropped with the index_writer.
-        for _ in document_receiver.clone() {}
+        for _ in document_receiver.iter() {}
 
         Ok(())
     }
@@ -554,6 +559,16 @@ impl IndexWriter {
     /// using this API.
     /// See [`PreparedCommit::set_payload()`](PreparedCommit.html)
     pub fn prepare_commit(&mut self) -> Result<PreparedCommit> {
+        info!("Preparing commit");
+        self.prepare_commit_internal(false)
+    }
+
+    pub fn prepare_commit_soft(&mut self) -> Result<PreparedCommit> {
+        info!("Preparing soft commit");
+        self.prepare_commit_internal(true)
+    }
+
+    pub(crate) fn prepare_commit_internal(&mut self, soft: bool) -> Result<PreparedCommit> {
         // Here, because we join all of the worker threads,
         // all of the segment update for this commit have been
         // sent.
@@ -576,13 +591,13 @@ impl IndexWriter {
             let indexing_worker_result = worker_handle
                 .join()
                 .map_err(|e| TantivyError::ErrorInThread(format!("{:?}", e)))?;
-            indexing_worker_result?;
-            // add a new worker for the next generation.
+            // add a new worker for the next generation, whether the worker failed or not.
             self.add_indexing_worker()?;
+            indexing_worker_result?;
         }
 
         let commit_opstamp = self.stamper.stamp();
-        let prepared_commit = PreparedCommit::new(self, commit_opstamp);
+        let prepared_commit = PreparedCommit::new(self, commit_opstamp, soft);
         info!("Prepared commit {}", commit_opstamp);
         Ok(prepared_commit)
     }
@@ -605,6 +620,11 @@ impl IndexWriter {
         self.prepare_commit()?.commit()
     }
 
+
+    pub fn soft_commit(&mut self) -> Result<u64> {
+        self.prepare_commit_soft()?.commit()
+    }
+
     pub(crate) fn segment_updater(&self) -> &SegmentUpdater {
         &self.segment_updater
     }
@@ -732,6 +752,7 @@ mod tests {
     use Index;
     use ReloadPolicy;
     use Term;
+    use IndexReader;
 
     #[test]
     fn test_operations_group() {
@@ -865,6 +886,13 @@ mod tests {
         let _index_writer_two = index.writer(3_000_000).unwrap();
     }
 
+    fn num_docs_containing_text(reader: &IndexReader, term: &str) -> u64 {
+        let searcher = reader.searcher();
+        let text_field = reader.schema().get_field("text").unwrap();
+        let term = Term::from_field_text(text_field, term);
+        searcher.doc_freq(&term)
+    }
+
     #[test]
     fn test_commit_and_rollback() {
         let mut schema_builder = schema::Schema::builder();
@@ -881,9 +909,12 @@ mod tests {
             searcher.doc_freq(&term)
         };
 
+        let mut index_writer = index.writer(3_000_000).unwrap();
+
+        assert_eq!(index_writer.commit_opstamp(), 0u64);
+        assert_eq!(num_docs_containing_text(&reader, "a"), 0);
         {
             // writing the segment
-            let mut index_writer = index.writer(3_000_000).unwrap();
             index_writer.add_document(doc!(text_field=>"a"));
             index_writer.rollback().unwrap();
             assert_eq!(index_writer.commit_opstamp(), 0u64);
@@ -902,6 +933,35 @@ mod tests {
         reader.searcher();
     }
 
+    #[test]
+    fn test_softcommit_and_rollback() {
+        let mut schema_builder = schema::Schema::builder();
+        let text_field = schema_builder.add_text_field("text", schema::TEXT);
+        let index = Index::create_in_ram(schema_builder.build());
+        let reader = index.reader().unwrap();
+        // writing the segment
+        let mut index_writer = index.writer(3_000_000).unwrap();
+        index_writer.add_document(doc!(text_field=>"a"));
+        index_writer.rollback().unwrap();
+
+        assert_eq!(index_writer.commit_opstamp(), 0u64);
+        assert_eq!(num_docs_containing_text(&reader, "a"), 0u64);
+        {
+            index_writer.add_document(doc!(text_field=>"b"));
+            index_writer.add_document(doc!(text_field=>"c"));
+        }
+        assert!(index_writer.soft_commit().is_ok());
+        reader.reload().unwrap(); // we need to load soft committed stuff.
+        assert_eq!(num_docs_containing_text(&reader, "a"), 0u64);
+        assert_eq!(num_docs_containing_text(&reader, "b"), 1u64);
+        assert_eq!(num_docs_containing_text(&reader, "c"), 1u64);
+        index_writer.rollback().unwrap();
+        reader.reload().unwrap();
+        assert_eq!(num_docs_containing_text(&reader, "a"), 0u64);
+        assert_eq!(num_docs_containing_text(&reader, "b"), 0u64);
+        assert_eq!(num_docs_containing_text(&reader, "c"), 0u64);
+    }
+
     #[test]
     fn test_with_merges() {
         let mut schema_builder = schema::Schema::builder();
@@ -935,7 +995,7 @@ mod tests {
 
             reader.reload().unwrap();
 
-            assert_eq!(num_docs_containing("a"), 200);
+            assert_eq!(num_docs_containing_text(&reader, "a"), 200);
             assert!(index.searchable_segments().unwrap().len() < 8);
         }
     }
@@ -978,7 +1038,7 @@ mod tests {
         let mut schema_builder = schema::Schema::builder();
         let text_field = schema_builder.add_text_field("text", schema::TEXT);
         let index = Index::create_in_ram(schema_builder.build());
-
+        let reader = index.reader();
         {
             // writing the segment
             let mut index_writer = index.writer_with_num_threads(4, 12_000_000).unwrap();

src/indexer/prepared_commit.rs

@@ -6,14 +6,20 @@ pub struct PreparedCommit<'a> {
     index_writer: &'a mut IndexWriter,
     payload: Option<String>,
     opstamp: u64,
+    soft: bool,
 }
 
 impl<'a> PreparedCommit<'a> {
-    pub(crate) fn new(index_writer: &'a mut IndexWriter, opstamp: u64) -> PreparedCommit {
+    pub(crate) fn new(
+        index_writer: &'a mut IndexWriter,
+        opstamp: u64,
+        soft: bool,
+    ) -> PreparedCommit {
         PreparedCommit {
             index_writer,
             payload: None,
             opstamp,
+            soft,
         }
     }
 
@@ -33,7 +39,7 @@ impl<'a> PreparedCommit<'a> {
         info!("committing {}", self.opstamp);
         self.index_writer
             .segment_updater()
-            .commit(self.opstamp, self.payload)?;
+            .commit(self.opstamp, self.payload, self.soft)?;
         Ok(self.opstamp)
     }
 }

src/indexer/segment_entry.rs

@@ -22,6 +22,7 @@ pub struct SegmentEntry {
     meta: SegmentMeta,
     delete_bitset: Option<BitSet>,
     delete_cursor: DeleteCursor,
+    opstamp: u64,
 }
 
 impl SegmentEntry {
@@ -30,14 +31,20 @@ impl SegmentEntry {
         segment_meta: SegmentMeta,
         delete_cursor: DeleteCursor,
         delete_bitset: Option<BitSet>,
+        opstamp: u64,
     ) -> SegmentEntry {
         SegmentEntry {
             meta: segment_meta,
             delete_bitset,
             delete_cursor,
+            opstamp,
         }
     }
 
+    pub fn opstamp(&self) -> u64 {
+        self.opstamp
+    }
+
     /// Return a reference to the segment entry deleted bitset.
     ///
     /// `DocId` in this bitset are flagged as deleted.
@@ -46,7 +53,8 @@ impl SegmentEntry {
     }
 
     /// Set the `SegmentMeta` for this segment.
-    pub fn set_meta(&mut self, segment_meta: SegmentMeta) {
+    pub fn set_meta(&mut self, opstamp: u64, segment_meta: SegmentMeta) {
+        self.opstamp = opstamp;
         self.meta = segment_meta;
     }
 

src/indexer/segment_manager.rs

@@ -11,11 +11,47 @@ use std::path::PathBuf;
 use std::sync::RwLock;
 use std::sync::{RwLockReadGuard, RwLockWriteGuard};
 use Result as TantivyResult;
+use std::sync::Arc;
+use std::collections::HashMap;
+
+/// Provides a read-only view of the available segments.
+#[derive(Clone)]
+pub struct AvailableSegments {
+    registers: Arc<RwLock<SegmentRegisters>>,
+}
+
+impl AvailableSegments {
+    pub fn committed(&self) -> Vec<SegmentMeta> {
+        self.registers
+            .read()
+            .unwrap()
+            .committed
+            .segment_metas()
+    }
+
+    pub fn soft_committed(&self) -> Vec<SegmentMeta> {
+        self.registers
+            .read()
+            .unwrap()
+            .soft_committed
+            .segment_metas()
+    }
+}
+
 
-#[derive(Default)]
 struct SegmentRegisters {
-    uncommitted: SegmentRegister,
+    uncommitted: HashMap<SegmentId, SegmentEntry>,
     committed: SegmentRegister,
+    /// soft commits can advance committed segment to a future delete
+    /// opstamp.
+    ///
+    /// In that case the same `SegmentId` can appear in both `committed`
+    /// and in `committed_in_the_future`.
+    ///
+    /// We do not consider these segments for merges.
+    soft_committed: SegmentRegister,
+    /// `DeleteCursor`, positionned on the soft commit.
+    delete_cursor: DeleteCursor,
 }
 
 /// The segment manager stores the list of segments
@@ -23,9 +59,8 @@ struct SegmentRegisters {
 ///
 /// It guarantees the atomicity of the
 /// changes (merges especially)
-#[derive(Default)]
 pub struct SegmentManager {
-    registers: RwLock<SegmentRegisters>,
+    registers: Arc<RwLock<SegmentRegisters>>
 }
 
 impl Debug for SegmentManager {
@@ -46,11 +81,17 @@ pub fn get_mergeable_segments(
     let registers_lock = segment_manager.read();
     (
         registers_lock
-            .committed
+            .soft_committed
             .get_mergeable_segments(in_merge_segment_ids),
         registers_lock
             .uncommitted
-            .get_mergeable_segments(in_merge_segment_ids),
+            .values()
+            .map(|segment_entry| segment_entry.meta())
+            .filter(|segment_meta| {
+                !in_merge_segment_ids.contains(&segment_meta.id())
+            })
+            .cloned()
+            .collect::<Vec<_>>()
     )
 }
 
@@ -58,21 +99,22 @@ impl SegmentManager {
     pub fn from_segments(
         segment_metas: Vec<SegmentMeta>,
         delete_cursor: &DeleteCursor,
+        opstamp: u64,
     ) -> SegmentManager {
         SegmentManager {
-            registers: RwLock::new(SegmentRegisters {
-                uncommitted: SegmentRegister::default(),
-                committed: SegmentRegister::new(segment_metas, delete_cursor),
-            }),
+            registers: Arc::new(RwLock::new(SegmentRegisters {
+                uncommitted: HashMap::default(),
+                committed: SegmentRegister::new(segment_metas.clone(), opstamp),
+                soft_committed: SegmentRegister::new(segment_metas, opstamp),
+                delete_cursor: delete_cursor.clone(),
+            }))
         }
     }
 
-    /// Returns all of the segment entries (committed or uncommitted)
-    pub fn segment_entries(&self) -> Vec<SegmentEntry> {
-        let registers_lock = self.read();
-        let mut segment_entries = registers_lock.uncommitted.segment_entries();
-        segment_entries.extend(registers_lock.committed.segment_entries());
-        segment_entries
+    pub fn available_segments_view(&self) -> AvailableSegments {
+        AvailableSegments {
+            registers: self.registers.clone()
+        }
     }
 
     /// List the files that are useful to the index.
@@ -108,44 +150,76 @@ impl SegmentManager {
         let mut registers_lock = self.write();
         registers_lock
             .committed
-            .segment_entries()
+            .segment_metas()
             .iter()
-            .filter(|segment| segment.meta().num_docs() == 0)
-            .for_each(|segment| {
+            .filter(|segment_meta| segment_meta.num_docs() == 0)
+            .for_each(|segment_meta| {
                 registers_lock
                     .committed
-                    .remove_segment(&segment.segment_id())
+                    .remove_segment(&segment_meta.id())
+            });
+        registers_lock
+            .soft_committed
+            .segment_metas()
+            .iter()
+            .filter(|segment_meta| segment_meta.num_docs() == 0)
+            .for_each(|segment_meta| {
+                registers_lock
+                    .committed
+                    .remove_segment(&segment_meta.id())
             });
     }
 
-    pub fn commit(&self, segment_entries: Vec<SegmentEntry>) {
-        let mut registers_lock = self.write();
-        registers_lock.committed.clear();
-        registers_lock.uncommitted.clear();
-        for segment_entry in segment_entries {
-            registers_lock.committed.add_segment_entry(segment_entry);
-        }
+    /// Returns all of the segment entries (soft committed or uncommitted)
+    pub fn segment_entries(&self) -> Vec<SegmentEntry> {
+        let registers_lock = self.read();
+        let mut segment_entries: Vec<SegmentEntry   > = registers_lock.uncommitted.values().cloned().collect();
+        segment_entries.extend(registers_lock.soft_committed.segment_entries(&registers_lock.delete_cursor).into_iter());
+        segment_entries
     }
 
-    /// Marks a list of segments as in merge.
+
+    pub fn commit(&self, opstamp: u64, segment_entries: Vec<SegmentEntry>) {
+        let mut registers_lock = self.write();
+        registers_lock.uncommitted.clear();
+        registers_lock
+            .committed
+            .set_commit(opstamp, segment_entries.clone());
+        registers_lock
+            .soft_committed
+            .set_commit(opstamp, segment_entries);
+        registers_lock.delete_cursor.skip_to(opstamp);
+    }
+
+    pub fn soft_commit(&self, opstamp: u64, segment_entries: Vec<SegmentEntry>) {
+        let mut registers_lock = self.write();
+        registers_lock.uncommitted.clear();
+        registers_lock
+            .soft_committed
+            .set_commit(opstamp, segment_entries);
+        registers_lock.delete_cursor.skip_to(opstamp);
+    }
+
+    /// Gets the list of segment_entries associated to a list of `segment_ids`.
+    /// This method is used when starting a merge operations.
     ///
     /// Returns an error if some segments are missing, or if
-    /// the `segment_ids` are not either all committed or all
+    /// the `segment_ids` are not either all soft_committed or all
     /// uncommitted.
     pub fn start_merge(&self, segment_ids: &[SegmentId]) -> TantivyResult<Vec<SegmentEntry>> {
         let registers_lock = self.read();
         let mut segment_entries = vec![];
-        if registers_lock.uncommitted.contains_all(segment_ids) {
+        if segment_ids.iter().all(|segment_id| registers_lock.uncommitted.contains_key(segment_id)) {
             for segment_id in segment_ids {
                 let segment_entry = registers_lock.uncommitted
                     .get(segment_id)
                     .expect("Segment id not found {}. Should never happen because of the contains all if-block.");
-                segment_entries.push(segment_entry);
+                segment_entries.push(segment_entry.clone());
             }
-        } else if registers_lock.committed.contains_all(segment_ids) {
+        } else if registers_lock.soft_committed.contains_all(segment_ids) {
             for segment_id in segment_ids {
-                let segment_entry = registers_lock.committed
-                    .get(segment_id)
+                let segment_entry = registers_lock.soft_committed
+                    .get(segment_id, &registers_lock.delete_cursor)
                     .expect("Segment id not found {}. Should never happen because of the contains all if-block.");
                 segment_entries.push(segment_entry);
             }
@@ -160,35 +234,32 @@ impl SegmentManager {
 
     pub fn add_segment(&self, segment_entry: SegmentEntry) {
         let mut registers_lock = self.write();
-        registers_lock.uncommitted.add_segment_entry(segment_entry);
+        registers_lock
+            .uncommitted
+            .insert(segment_entry.segment_id(), segment_entry);
     }
 
     pub fn end_merge(
         &self,
         before_merge_segment_ids: &[SegmentId],
-        after_merge_segment_entry: SegmentEntry,
+        after_merge_segment_entry: SegmentEntry
     ) {
         let mut registers_lock = self.write();
-        let target_register: &mut SegmentRegister = {
-            if registers_lock
+
+        if before_merge_segment_ids.iter().all(|seg_id|
+            registers_lock
                 .uncommitted
-                .contains_all(before_merge_segment_ids)
-            {
-                &mut registers_lock.uncommitted
-            } else if registers_lock
-                .committed
-                .contains_all(before_merge_segment_ids)
-            {
-                &mut registers_lock.committed
-            } else {
-                warn!("couldn't find segment in SegmentManager");
-                return;
+                .contains_key(seg_id))
+        {
+            for segment_id in before_merge_segment_ids {
+                registers_lock.uncommitted.remove(&segment_id);
             }
-        };
-        for segment_id in before_merge_segment_ids {
-            target_register.remove_segment(segment_id);
+            registers_lock.uncommitted.insert(after_merge_segment_entry.segment_id(),
+                                              after_merge_segment_entry);
+        } else {
+            registers_lock.committed.receive_merge(&before_merge_segment_ids, &after_merge_segment_entry);
+            registers_lock.soft_committed.receive_merge(&before_merge_segment_ids, &after_merge_segment_entry)
         }
-        target_register.add_segment_entry(after_merge_segment_entry);
     }
 
     pub fn committed_segment_metas(&self) -> Vec<SegmentMeta> {

src/indexer/segment_register.rs

@@ -16,7 +16,8 @@ use std::fmt::{self, Debug, Formatter};
 /// merge candidates.
 #[derive(Default)]
 pub struct SegmentRegister {
-    segment_states: HashMap<SegmentId, SegmentEntry>,
+    segment_states: HashMap<SegmentId, SegmentMeta>,
+    opstamp_constraint: u64,
 }
 
 impl Debug for SegmentRegister {
@@ -41,23 +42,28 @@ impl SegmentRegister {
     ) -> Vec<SegmentMeta> {
         self.segment_states
             .values()
-            .filter(|segment_entry| !in_merge_segment_ids.contains(&segment_entry.segment_id()))
-            .map(|segment_entry| segment_entry.meta().clone())
+            .filter(|segment_meta| !in_merge_segment_ids.contains(&segment_meta.id()))
+            .cloned()
             .collect()
     }
 
-    pub fn segment_entries(&self) -> Vec<SegmentEntry> {
-        self.segment_states.values().cloned().collect()
-    }
-
     pub fn segment_metas(&self) -> Vec<SegmentMeta> {
-        let mut segment_ids: Vec<SegmentMeta> = self
+        let mut segment_metas: Vec<SegmentMeta> = self
             .segment_states
             .values()
-            .map(|segment_entry| segment_entry.meta().clone())
+            .cloned()
             .collect();
-        segment_ids.sort_by_key(|meta| meta.id());
-        segment_ids
+        segment_metas.sort_by_key(|meta| meta.id());
+        segment_metas
+    }
+
+    pub fn segment_entries(&self, delete_cursor: &DeleteCursor) -> Vec<SegmentEntry> {
+        self.segment_states
+            .values()
+            .map(|segment_meta| {
+                SegmentEntry::new(segment_meta.clone(), delete_cursor.clone(), None, self.opstamp_constraint)
+            })
+            .collect()
     }
 
     pub fn contains_all(&self, segment_ids: &[SegmentId]) -> bool {
@@ -66,27 +72,77 @@ impl SegmentRegister {
             .all(|segment_id| self.segment_states.contains_key(segment_id))
     }
 
-    pub fn add_segment_entry(&mut self, segment_entry: SegmentEntry) {
+    pub fn receive_merge(&mut self,
+                         before_merge_segment_ids: &[SegmentId],
+                         after_merge_segment_entry: &SegmentEntry) {
+        if after_merge_segment_entry.opstamp() != self.opstamp_constraint {
+            return;
+        }
+        if !self.contains_all(before_merge_segment_ids) {
+            return;
+        }
+        for segment_id in before_merge_segment_ids {
+            self.segment_states.remove(segment_id);
+        }
+        self.register_segment_entry(after_merge_segment_entry.clone());
+    }
+
+    /// Registers a `SegmentEntry`.
+    ///
+    /// If a segment entry associated to this `SegmentId` is already there,
+    /// override it with the new `SegmentEntry`.
+    pub fn register_segment_entry(&mut self, segment_entry: SegmentEntry) {
+        if self.opstamp_constraint != segment_entry.opstamp() {
+            panic!(format!(
+                "Invalid segment. Expect opstamp {}, got {}.",
+                self.opstamp_constraint,
+                segment_entry.opstamp()
+            ));
+        }
+        if segment_entry.meta().num_docs() == 0 {
+            return;
+        }
         let segment_id = segment_entry.segment_id();
-        self.segment_states.insert(segment_id, segment_entry);
+        // Check that we are ok with deletes.
+        self.segment_states.insert(segment_id, segment_entry.meta().clone());
+    }
+
+    pub fn set_commit(&mut self, opstamp: u64, segment_entries: Vec<SegmentEntry>) {
+        self.segment_states.clear();
+        self.opstamp_constraint = opstamp;
+        for segment_entry in segment_entries {
+            self.register_segment_entry(segment_entry);
+        }
     }
 
     pub fn remove_segment(&mut self, segment_id: &SegmentId) {
-        self.segment_states.remove(segment_id);
+        self.segment_states.remove(&segment_id);
     }
 
-    pub fn get(&self, segment_id: &SegmentId) -> Option<SegmentEntry> {
-        self.segment_states.get(segment_id).cloned()
+    pub fn get(&self, segment_id: &SegmentId, delete_cursor: &DeleteCursor) -> Option<SegmentEntry> {
+        self.segment_states
+            .get(&segment_id)
+            .map(|segment_meta|
+                SegmentEntry::new(
+                    segment_meta.clone(),
+                    delete_cursor.clone(),
+                    None,
+                    self.opstamp_constraint
+                ))
     }
 
-    pub fn new(segment_metas: Vec<SegmentMeta>, delete_cursor: &DeleteCursor) -> SegmentRegister {
+    pub fn new(
+        segment_metas: Vec<SegmentMeta>,
+        opstamp: u64,
+    ) -> SegmentRegister {
         let mut segment_states = HashMap::new();
         for segment_meta in segment_metas {
-            let segment_id = segment_meta.id();
-            let segment_entry = SegmentEntry::new(segment_meta, delete_cursor.clone(), None);
-            segment_states.insert(segment_id, segment_entry);
+            segment_states.insert(segment_meta.id(), segment_meta);
+        }
+        SegmentRegister {
+            segment_states,
+            opstamp_constraint: opstamp,
         }
-        SegmentRegister { segment_states }
     }
 }
 
@@ -115,22 +171,22 @@ mod tests {
         let segment_id_merged = SegmentId::generate_random();
 
         {
-            let segment_meta = SegmentMeta::new(segment_id_a, 0u32);
-            let segment_entry = SegmentEntry::new(segment_meta, delete_queue.cursor(), None);
-            segment_register.add_segment_entry(segment_entry);
+            let segment_meta = SegmentMeta::new(segment_id_a, 1u32);
+            let segment_entry = SegmentEntry::new(segment_meta, delete_queue.cursor(), None, 0u64);
+            segment_register.register_segment_entry(segment_entry);
         }
         assert_eq!(segment_ids(&segment_register), vec![segment_id_a]);
         {
-            let segment_meta = SegmentMeta::new(segment_id_b, 0u32);
-            let segment_entry = SegmentEntry::new(segment_meta, delete_queue.cursor(), None);
-            segment_register.add_segment_entry(segment_entry);
+            let segment_meta = SegmentMeta::new(segment_id_b, 2u32);
+            let segment_entry = SegmentEntry::new(segment_meta, delete_queue.cursor(), None, 0u64);
+            segment_register.register_segment_entry(segment_entry);
         }
-        segment_register.remove_segment(&segment_id_a);
-        segment_register.remove_segment(&segment_id_b);
         {
-            let segment_meta_merged = SegmentMeta::new(segment_id_merged, 0u32);
-            let segment_entry = SegmentEntry::new(segment_meta_merged, delete_queue.cursor(), None);
-            segment_register.add_segment_entry(segment_entry);
+            let segment_meta_merged = SegmentMeta::new(segment_id_merged, 3u32);
+            let segment_entry =
+                SegmentEntry::new(segment_meta_merged, delete_queue.cursor(), None, 0u64);
+            segment_register.receive_merge(&[segment_id_a, segment_id_b], &segment_entry);
+            segment_register.register_segment_entry(segment_entry);
         }
         assert_eq!(segment_ids(&segment_register), vec![segment_id_merged]);
     }

src/indexer/segment_updater.rs

@@ -125,7 +125,7 @@ fn perform_merge(
 
     let segment_meta = SegmentMeta::new(merged_segment.id(), num_docs);
 
-    let after_merge_segment_entry = SegmentEntry::new(segment_meta.clone(), delete_cursor, None);
+    let after_merge_segment_entry = SegmentEntry::new(segment_meta.clone(), delete_cursor, None, target_opstamp);
     Ok(after_merge_segment_entry)
 }
 
@@ -155,8 +155,11 @@ impl SegmentUpdater {
         stamper: Stamper,
         delete_cursor: &DeleteCursor,
     ) -> Result<SegmentUpdater> {
+
+        let index_meta = index.load_metas()?;
         let segments = index.searchable_segment_metas()?;
-        let segment_manager = SegmentManager::from_segments(segments, delete_cursor);
+        let opstamp = index_meta.opstamp;
+        let segment_manager = SegmentManager::from_segments(segments, delete_cursor, opstamp);
         let pool = CpuPoolBuilder::new()
             .name_prefix("segment_updater")
             .pool_size(1)
@@ -280,14 +283,30 @@ impl SegmentUpdater {
             .garbage_collect(|| self.0.segment_manager.list_files());
     }
 
-    pub fn commit(&self, opstamp: u64, payload: Option<String>) -> Result<()> {
+    pub fn commit(&self, opstamp: u64, payload: Option<String>, soft: bool) -> Result<()> {
         self.run_async(move |segment_updater| {
             if segment_updater.is_alive() {
                 let segment_entries = segment_updater
                     .purge_deletes(opstamp)
                     .expect("Failed purge deletes");
-                segment_updater.0.segment_manager.commit(segment_entries);
-                segment_updater.save_metas(opstamp, payload);
+                if soft {
+                    // Soft commit.
+                    //
+                    // The list `segment_entries` above is what we might want to use as searchable
+                    // segment. However, we do not want to mark them as committed, and we want
+                    // to keep the current set of committed segment.
+                    segment_updater.0.segment_manager.soft_commit(opstamp, segment_entries);
+                    // ... We do not save the meta file.
+                } else {
+                    // Hard_commit. We register the new segment entries as committed.
+                    segment_updater
+                        .0
+                        .segment_manager
+                        .commit(opstamp, segment_entries);
+                    // TODO error handling.
+                    segment_updater.save_metas(opstamp, payload);
+                    segment_updater.0.index.directory().flush().unwrap();
+                }
                 segment_updater.garbage_collect_files_exec();
                 segment_updater.consider_merge_options();
             }

src/reader/mod.rs

@@ -10,6 +10,7 @@ use Index;
 use Result;
 use Searcher;
 use SegmentReader;
+use schema::Schema;
 
 /// Defines when a new version of the index should be reloaded.
 ///
@@ -158,6 +159,11 @@ pub struct IndexReader {
 }
 
 impl IndexReader {
+
+    pub fn schema(&self) -> Schema {
+        self.inner.index.schema()
+    }
+
     /// Update searchers so that they reflect the state of the last
     /// `.commit()`.
     ///