neon/libs/posthog_client_lite/src/background_loop.rs

//! A background loop that fetches feature flags from PostHog and updates the feature store.

use std::{sync::Arc, time::Duration};

use arc_swap::ArcSwap;
use tokio_util::sync::CancellationToken;
use tracing::{Instrument, info_span};

use crate::{CaptureEvent, FeatureStore, PostHogClient, PostHogClientConfig};

/// A background loop that fetches feature flags from PostHog and updates the feature store.
pub struct FeatureResolverBackgroundLoop {
    posthog_client: PostHogClient,
    feature_store: ArcSwap<FeatureStore>,
    cancel: CancellationToken,
}

impl FeatureResolverBackgroundLoop {
    pub fn new(config: PostHogClientConfig, shutdown_pageserver: CancellationToken) -> Self {
        Self {
            posthog_client: PostHogClient::new(config),
            feature_store: ArcSwap::new(Arc::new(FeatureStore::new())),
            cancel: shutdown_pageserver,
        }
    }

    pub fn spawn(
        self: Arc<Self>,
        handle: &tokio::runtime::Handle,
        refresh_period: Duration,
        fake_tenants: Vec<CaptureEvent>,
    ) {
        let this = self.clone();
        let cancel = self.cancel.clone();

        // Main loop of updating the feature flags.
        handle.spawn(
            async move {
                tracing::info!("Starting PostHog feature resolver");
                let mut ticker = tokio::time::interval(refresh_period);
                ticker.set_missed_tick_behavior(tokio::time::MissedTickBehavior::Skip);
                loop {
                    tokio::select! {
                        _ = ticker.tick() => {}
                        _ = cancel.cancelled() => break
                    }
                    let resp = match this
                        .posthog_client
                        .get_feature_flags_local_evaluation()
                        .await
                    {
                        Ok(resp) => resp,
                        Err(e) => {
                            tracing::warn!("Cannot get feature flags: {}", e);
                            continue;
                        }
                    };
                    let project_id = this.posthog_client.config.project_id.parse::<u64>().ok();
                    match FeatureStore::new_with_flags(resp.flags, project_id) {
                        Ok(feature_store) => {
                            this.feature_store.store(Arc::new(feature_store));
                            tracing::info!("Feature flag updated");
                        }
                        Err(e) => {
                            tracing::warn!("Cannot process feature flag spec: {}", e);
                        }
                    }
                }
                tracing::info!("PostHog feature resolver stopped");
            }
            .instrument(info_span!("posthog_feature_resolver")),
        );

        // Report fake tenants to PostHog so that we have the combination of all the properties in the UI.
        // Do one report per pageserver restart.
        let this = self.clone();
        handle.spawn(
            async move {
                tracing::info!("Starting PostHog feature reporter");
                for tenant in &fake_tenants {
                    tracing::info!("Reporting fake tenant: {:?}", tenant);
                }
                if let Err(e) = this.posthog_client.capture_event_batch(&fake_tenants).await {
                    tracing::warn!("Cannot report fake tenants: {}", e);
                }
            }
            .instrument(info_span!("posthog_feature_reporter")),
        );
    }

    pub fn feature_store(&self) -> Arc<FeatureStore> {
        self.feature_store.load_full()
    }
}