[MongoDB Storage] Pre-calculate checksums when compacting

.changeset/fifty-socks-fly.md

@@ -0,0 +1,11 @@
+---
+'@powersync/service-module-postgres-storage': minor
+'@powersync/service-module-mongodb-storage': minor
+'@powersync/service-core-tests': minor
+'@powersync/service-module-postgres': minor
+'@powersync/service-module-mongodb': minor
+'@powersync/service-core': minor
+'@powersync/service-module-mysql': minor
+---
+
+Create a persisted checksum cache when compacting buckets.

modules/module-mongodb-storage/src/storage/implementation/MongoBucketBatch.ts

@@ -97,7 +97,7 @@ export class MongoBucketBatch
   private persisted_op: InternalOpId | null = null;
 
   /**
-   * For tests only - not for persistence logic.
+   * Last written op, if any. This may not reflect a consistent checkpoint.
    */
   public last_flushed_op: InternalOpId | null = null;
 

modules/module-mongodb-storage/src/storage/implementation/MongoCompactor.ts

@@ -1,10 +1,11 @@
 import { mongo } from '@powersync/lib-service-mongodb';
-import { logger, ReplicationAssertionError } from '@powersync/lib-services-framework';
-import { InternalOpId, storage, utils } from '@powersync/service-core';
+import { logger, ReplicationAssertionError, ServiceAssertionError } from '@powersync/lib-services-framework';
+import { addChecksums, InternalOpId, storage, utils } from '@powersync/service-core';
 
 import { PowerSyncMongo } from './db.js';
-import { BucketDataDocument, BucketDataKey } from './models.js';
+import { BucketDataDocument, BucketDataKey, BucketStateDocument } from './models.js';
 import { cacheKey } from './OperationBatch.js';
+import { readSingleBatch } from './util.js';
 
 interface CurrentBucketState {
   /** Bucket name */
@@ -27,6 +28,21 @@ interface CurrentBucketState {
    * Number of REMOVE/MOVE operations seen since lastNotPut.
    */
   opsSincePut: number;
+
+  /**
+   * Incrementally-updated checksum, up to maxOpId
+   */
+  checksum: number;
+
+  /**
+   * op count for the checksum
+   */
+  opCount: number;
+
+  /**
+   * Byte size of ops covered by the checksum.
+   */
+  opBytes: number;
 }
 
 /**
@@ -43,13 +59,15 @@ const DEFAULT_MEMORY_LIMIT_MB = 64;
 
 export class MongoCompactor {
   private updates: mongo.AnyBulkWriteOperation<BucketDataDocument>[] = [];
+  private bucketStateUpdates: mongo.AnyBulkWriteOperation<BucketStateDocument>[] = [];
 
   private idLimitBytes: number;
   private moveBatchLimit: number;
   private moveBatchQueryLimit: number;
   private clearBatchLimit: number;
-  private maxOpId: bigint | undefined;
+  private maxOpId: bigint;
   private buckets: string[] | undefined;
+  private signal?: AbortSignal;
 
   constructor(
     private db: PowerSyncMongo,
@@ -60,8 +78,9 @@ export class MongoCompactor {
     this.moveBatchLimit = options?.moveBatchLimit ?? DEFAULT_MOVE_BATCH_LIMIT;
     this.moveBatchQueryLimit = options?.moveBatchQueryLimit ?? DEFAULT_MOVE_BATCH_QUERY_LIMIT;
     this.clearBatchLimit = options?.clearBatchLimit ?? DEFAULT_CLEAR_BATCH_LIMIT;
-    this.maxOpId = options?.maxOpId;
+    this.maxOpId = options?.maxOpId ?? 0n;
     this.buckets = options?.compactBuckets;
+    this.signal = options?.signal;
   }
 
   /**
@@ -117,31 +136,33 @@ export class MongoCompactor {
       o: new mongo.MaxKey() as any
     };
 
-    while (true) {
+    while (!this.signal?.aborted) {
       // Query one batch at a time, to avoid cursor timeouts
-      const batch = await this.db.bucket_data
-        .find(
-          {
+      const cursor = this.db.bucket_data.aggregate<BucketDataDocument & { size: number | bigint }>([
+        {
+          $match: {
             _id: {
               $gte: lowerBound,
               $lt: upperBound
             }
-          },
-          {
-            projection: {
-              _id: 1,
-              op: 1,
-              table: 1,
-              row_id: 1,
-              source_table: 1,
-              source_key: 1
-            },
-            limit: this.moveBatchQueryLimit,
-            sort: { _id: -1 },
-            singleBatch: true
           }
-        )
-        .toArray();
+        },
+        { $sort: { _id: -1 } },
+        { $limit: this.moveBatchQueryLimit },
+        {
+          $project: {
+            _id: 1,
+            op: 1,
+            table: 1,
+            row_id: 1,
+            source_table: 1,
+            source_key: 1,
+            checksum: 1,
+            size: { $bsonSize: '$$ROOT' }
+          }
+        }
+      ]);
+      const { data: batch } = await readSingleBatch(cursor);
 
       if (batch.length == 0) {
         // We've reached the end
@@ -153,34 +174,47 @@ export class MongoCompactor {
 
       for (let doc of batch) {
         if (currentState == null || doc._id.b != currentState.bucket) {
-          if (currentState != null && currentState.lastNotPut != null && currentState.opsSincePut >= 1) {
-            // Important to flush before clearBucket()
-            await this.flush();
-            logger.info(
-              `Inserting CLEAR at ${this.group_id}:${currentState.bucket}:${currentState.lastNotPut} to remove ${currentState.opsSincePut} operations`
-            );
+          if (currentState != null) {
+            if (currentState.lastNotPut != null && currentState.opsSincePut >= 1) {
+              // Important to flush before clearBucket()
+              // Does not have to happen before flushBucketChecksums()
+              await this.flush();
+              logger.info(
+                `Inserting CLEAR at ${this.group_id}:${currentState.bucket}:${currentState.lastNotPut} to remove ${currentState.opsSincePut} operations`
+              );
+
+              // Free memory before clearing bucket
+              currentState!.seen.clear();
+
+              await this.clearBucket(currentState);
+            }
 
-            const bucket = currentState.bucket;
-            const clearOp = currentState.lastNotPut;
-            // Free memory before clearing bucket
-            currentState = null;
-            await this.clearBucket(bucket, clearOp);
+            // Should happen after clearBucket() for accurate stats
+            this.updateBucketChecksums(currentState);
           }
           currentState = {
             bucket: doc._id.b,
             seen: new Map(),
             trackingSize: 0,
             lastNotPut: null,
-            opsSincePut: 0
+            opsSincePut: 0,
+
+            checksum: 0,
+            opCount: 0,
+            opBytes: 0
           };
         }
 
-        if (this.maxOpId != null && doc._id.o > this.maxOpId) {
+        if (doc._id.o > this.maxOpId) {
           continue;
         }
 
+        currentState.checksum = addChecksums(currentState.checksum, Number(doc.checksum));
+        currentState.opCount += 1;
+
         let isPersistentPut = doc.op == 'PUT';
 
+        currentState.opBytes += Number(doc.size);
         if (doc.op == 'REMOVE' || doc.op == 'PUT') {
           const key = `${doc.table}/${doc.row_id}/${cacheKey(doc.source_table!, doc.source_key!)}`;
           const targetOp = currentState.seen.get(key);
@@ -208,6 +242,8 @@ export class MongoCompactor {
                 }
               }
             });
+
+            currentState.opBytes += 200 - Number(doc.size); // TODO: better estimate for this
           } else {
             if (currentState.trackingSize >= idLimitBytes) {
               // Reached memory limit.
@@ -234,24 +270,72 @@ export class MongoCompactor {
           currentState.opsSincePut += 1;
         }
 
-        if (this.updates.length >= this.moveBatchLimit) {
+        if (this.updates.length + this.bucketStateUpdates.length >= this.moveBatchLimit) {
           await this.flush();
         }
       }
     }
 
-    await this.flush();
     currentState?.seen.clear();
     if (currentState?.lastNotPut != null && currentState?.opsSincePut > 1) {
       logger.info(
         `Inserting CLEAR at ${this.group_id}:${currentState.bucket}:${currentState.lastNotPut} to remove ${currentState.opsSincePut} operations`
       );
-      const bucket = currentState.bucket;
-      const clearOp = currentState.lastNotPut;
-      // Free memory before clearing bucket
-      currentState = null;
-      await this.clearBucket(bucket, clearOp);
+      // Need flush() before clear()
+      await this.flush();
+      await this.clearBucket(currentState);
     }
+    if (currentState != null) {
+      // Do this _after_ clearBucket so that we have accurate counts.
+      this.updateBucketChecksums(currentState);
+    }
+    // Need another flush after updateBucketChecksums()
+    await this.flush();
+  }
+
+  /**
+   * Call when done with a bucket.
+   */
+  private updateBucketChecksums(state: CurrentBucketState) {
+    if (state.opCount < 0) {
+      throw new ServiceAssertionError(
+        `Invalid opCount: ${state.opCount} checksum ${state.checksum} opsSincePut: ${state.opsSincePut} maxOpId: ${this.maxOpId}`
+      );
+    }
+    this.bucketStateUpdates.push({
+      updateOne: {
+        filter: {
+          _id: {
+            g: this.group_id,
+            b: state.bucket
+          }
+        },
+        update: {
+          $set: {
+            compacted_state: {
+              op_id: this.maxOpId,
+              count: state.opCount,
+              checksum: BigInt(state.checksum),
+              bytes: state.opBytes
+            },
+            estimate_since_compact: {
+              // Note: There could have been a whole bunch of new operations added to the bucket _while_ compacting,
+              // which we don't currently cater for.
+              // We could potentially query for that, but that could add overhead.
+              count: 0,
+              bytes: 0
+            }
+          },
+          $setOnInsert: {
+            // Only set this if we're creating the document.
+            // In all other cases, the replication process will have a set a more accurate id.
+            last_op: this.maxOpId
+          }
+        },
+        // We generally expect this to have been created before, but do handle cases of old unchanged buckets
+        upsert: true
+      }
+    });
   }
 
   private async flush() {
@@ -266,15 +350,26 @@ export class MongoCompactor {
       });
       this.updates = [];
     }
+    if (this.bucketStateUpdates.length > 0) {
+      logger.info(`Updating ${this.bucketStateUpdates.length} bucket states`);
+      await this.db.bucket_state.bulkWrite(this.bucketStateUpdates, {
+        ordered: false
+      });
+      this.bucketStateUpdates = [];
+    }
   }
 
   /**
    * Perform a CLEAR compact for a bucket.
    *
+   *
    * @param bucket bucket name
    * @param op op_id of the last non-PUT operation, which will be converted to CLEAR.
    */
-  private async clearBucket(bucket: string, op: InternalOpId) {
+  private async clearBucket(currentState: CurrentBucketState) {
+    const bucket = currentState.bucket;
+    const clearOp = currentState.lastNotPut!;
+
     const opFilter = {
       _id: {
         $gte: {
@@ -285,15 +380,16 @@ export class MongoCompactor {
         $lte: {
           g: this.group_id,
           b: bucket,
-          o: op
+          o: clearOp
         }
       }
     };
 
     const session = this.db.client.startSession();
     try {
       let done = false;
-      while (!done) {
+      while (!done && !this.signal?.aborted) {
+        let opCountDiff = 0;
         // Do the CLEAR operation in batches, with each batch a separate transaction.
         // The state after each batch is fully consistent.
         // We need a transaction per batch to make sure checksums stay consistent.
@@ -364,12 +460,16 @@ export class MongoCompactor {
               },
               { session }
             );
+
+            opCountDiff = -numberOfOpsToClear + 1;
           },
           {
             writeConcern: { w: 'majority' },
             readConcern: { level: 'snapshot' }
           }
         );
+        // Update _outside_ the transaction, since the transaction can be retried multiple times.
+        currentState.opCount += opCountDiff;
       }
     } finally {
       await session.endSession();