From 485bb670baa2425f8b6d7da7698dc677666f09da Mon Sep 17 00:00:00 2001
From: Ralf Kistner <ralf@journeyapps.com>
Date: Wed, 16 Jul 2025 16:46:59 +0200
Subject: [PATCH 1/3] Add check for resumeTokens going back in time.

---
 modules/module-mongodb/src/replication/ChangeStream.ts | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/modules/module-mongodb/src/replication/ChangeStream.ts b/modules/module-mongodb/src/replication/ChangeStream.ts
index f85de233b..9d0b19821 100644
--- a/modules/module-mongodb/src/replication/ChangeStream.ts
+++ b/modules/module-mongodb/src/replication/ChangeStream.ts
@@ -947,6 +947,16 @@ export class ChangeStream {
               timestamp: changeDocument.clusterTime!,
               resume_token: changeDocument._id
             });
+            if (batch.lastCheckpointLsn != null && lsn < batch.lastCheckpointLsn) {
+              // Checkpoint out of order - should never happen with MongoDB.
+              // If it does happen, we throw an error to stop the replication - restarting should recover.
+              // Since we use batch.lastCheckpointLsn for the next resumeAfter, this should not result in an infinite loop.
+              // This is a workaround for the issue below, but we can keep this as a safety-check even if the issue is fixed.
+              // Driver issue report: https://jira.mongodb.org/browse/NODE-7042
+              throw new ReplicationAssertionError(
+                `Change resumeToken ${(changeDocument._id as any)._data} (${timestampToDate(changeDocument.clusterTime!).toISOString()}) is less than last checkpoint LSN ${batch.lastCheckpointLsn}. Restarting replication.`
+              );
+            }
 
             if (waitForCheckpointLsn != null && lsn >= waitForCheckpointLsn) {
               waitForCheckpointLsn = null;

From d057e37faf7c41f393a8297712cb24b2f66551cc Mon Sep 17 00:00:00 2001
From: Ralf Kistner <ralf@journeyapps.com>
Date: Wed, 16 Jul 2025 16:49:05 +0200
Subject: [PATCH 2/3] Reduce retry delay on general MongoDB change stream
 errors.

---
 modules/module-mongodb/src/replication/MongoErrorRateLimiter.ts | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/modules/module-mongodb/src/replication/MongoErrorRateLimiter.ts b/modules/module-mongodb/src/replication/MongoErrorRateLimiter.ts
index 17b65c66c..3c770fb9b 100644
--- a/modules/module-mongodb/src/replication/MongoErrorRateLimiter.ts
+++ b/modules/module-mongodb/src/replication/MongoErrorRateLimiter.ts
@@ -28,7 +28,7 @@ export class MongoErrorRateLimiter implements ErrorRateLimiter {
       // Could be fail2ban or similar
       this.setDelay(120_000);
     } else {
-      this.setDelay(30_000);
+      this.setDelay(5_000);
     }
   }
 

From 5c3e6edbe6e249b26249616920d8595788d09cd0 Mon Sep 17 00:00:00 2001
From: Ralf Kistner <ralf@journeyapps.com>
Date: Wed, 16 Jul 2025 16:49:57 +0200
Subject: [PATCH 3/3] Add changeset.

---
 .changeset/sweet-years-tickle.md | 8 ++++++++
 1 file changed, 8 insertions(+)
 create mode 100644 .changeset/sweet-years-tickle.md

diff --git a/.changeset/sweet-years-tickle.md b/.changeset/sweet-years-tickle.md
new file mode 100644
index 000000000..02cc3f6ef
--- /dev/null
+++ b/.changeset/sweet-years-tickle.md
@@ -0,0 +1,8 @@
+---
+'@powersync/service-module-mongodb': patch
+'@powersync/lib-service-mongodb': patch
+'@powersync/service-core': patch
+'@powersync/service-image': patch
+---
+
+[MongoDB Replication] Fix resumeTokens going back in time on busy change streams.