This page describes changes and new features introduced in MongoDB Mongosync 1.18.
1.18.0 Release
November 12, 2025
New Features
Automatic Natural Scans
Starting in version 1.18, mongosync automatically detects collections that use random _id
values and copies those collections in natural order.
See detectRandomId for more details.
Hot Documents Mitigation
Starting in version 1.18, mongosync introduces the --hotDocIDs option, which allows you
to specify identifiers for documents that update very frequently on source clusters.
Use this option during migrations to help mitigate replication lag caused by hot documents.
To set the --hotDocIDs option from a configuration file, see the hotDocIDs setting.
Fixed Issues
mongosyncwill now retry for up to 30 minutes when an error indicates a network or cluster availability issue.mongosyncwill skip privilege checks when connected to a cluster with no users. If a cluster does not require authorization,mongosyncassumes that it has all the privileges needed.Fixes an issue where deletes from a TTL index during TTL background deletion process cutover could cause
committo fail.Improves logging for /progress endpoint handler
canCommitreporting.Improves handling for
BSONObjectTooLargeerror while applying update events.Improves retry mechanism behavior when a cluster presents network issues or is temporarily unresponsive.
Other Changes
mongosyncnow ignores all databases starting with __mdb_internal. This is a prefix used by MongoDB Atlas for internal purposes, among others. These databases should not be synced between clusters, and there might be activity in them whilemongosyncis syncing.
Live Upgrades
Live upgrades to 1.18 are supported.
Minimum Supported Version
For information on minimum supported versions, see MongoDB Server Version Compatibility.
For best performance, upgrade your source and destination clusters to the most recent MongoDB Server patch release prior to migration. For more information, see Upgrade to the Latest Self-Managed Patch Release of MongoDB.