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MongoDB Cluster-to-Cluster Sync

Release Notes for mongosync 1.6

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  • Patch Releases
  • 1.6.1 Release
  • 1.6.0 Release
  • Exclusion Filters
  • Regular Expression Filters
  • 7.0 Support
  • Other Notes
  • Minimum Supported Version

October 3, 2023

Issues Fixed:

  • Fixed a bug that can cause long running mongosync operations to prematurely timeout. Now, mongosync produces fewer MaxTimeMSExpired errors during the collection copy phase.

September 22, 2023

Starting in 1.6.0, the start API endpoint now supports the use of exclusion filters through the excludeNamespaces parameter.

For more information, see Filtered Sync.

Both inclusion and exclusion filters in Filtered Sync now support matching databases and collections using Regular Expressions with the databaseRegex and collectionsRegex fields.

For more information, see Regular Expressions in Filters.

Starting in 1.6.0, mongosync supports sync with MongoDB 7.0.

New Features:

  • Sped up the commit process by parallelizing metadata cleanup.

  • Reduced memory allocation during change event application by approximately 30% for pre-6.0 releases of MongoDB Server versions.

  • Changed telemetry to also send error messages to Segment when mongosync exits with a fatal error.

  • Added support for mongosync to sync between source and destination clusters that have the same major and minor versions, but different patch versions.

  • Upgraded build to use Go 1.20.

  • mongosync now periodically logs when server operations take a longer than expected time.

Issues Fixed:

  • Fixed a bug introduced in mongosync v1.5.0 and discovered by our internal testing where in an unlikely edge case mongosync can miss a change event to be applied during the Collection Copy phase.

    In order for this bug to occur, the change event must fall between a specific _id range that mongosync is in the process of copying, must occur and be processed while that specific _id range is being copied, and must not be followed by another change event for the same _id throughout the rest of the migration.

    This bug only occurs when mongosync is processing change events with near zero replication lag during Collection Copy.

  • Fixed a bug where mongosync could crash due to the source cluster containing legacy indexes with unknown index options.

  • Fixed a bug where mongosync could crash with an error during change event application, if the application fails to read 500 documents within 5 minutes or reaches the end of the oplog when reading from change streams on the source cluster.

  • Fixed a bug where mongosync could crash when indexes apply constraints to collection documents.

    This crash occured when such an index was added or dropped at nearly the same time as an index that violated these constrains was deleted or inserted. It would manifest with an error like:

    Ambiguous field name found in array (do not use numeric field names in embedded elements in an array)

    or a similar error. This fix makes this crash less likely, but it does not elimiate the possibility that it will occur.

  • Fixed a bug where mongosync can fail to copy certain documents if the insertion of documents causes duplicate key errors at the same time as a write concern error. This bug has been present in mongosync since its first release and may have arisen if the destination cluster logs reported a write concern error alongside a duplicate key error.

  • Fixed a bug introduced in mongosync 1.5 where change events may be skipped during the Collection Copy phase when there is little write activity on the source cluster.

  • Fixed a rare bug introduced in mongosync v1.5.0 and discovered by our internal testing where there may be a small continuity gap between Oplog Rollover Resilience (ORR) cycles in a specific situation, causing the ORR mechanism to potentially miss a change event to be applied during the Collection Copy phase.

    This is an extreme case as it requires consistently little write activity on the source cluster (i.e. mongosync replication lag is consistently near zero during Collection Copy) as well as the change event to fall in specific _id ranges between ORR cycles.

  • Fixed a bug introduced in v1.5.0 where mongosync could cause data consistency errors in collections with non-default collation.

    This issue only affects collections with non-simple collation that are present during mongosync initialization and contain documents with string _id values. Any collections with non-simple collation created after mongosync initialization or whose documents have all non-string _id values are unaffected.


    In order to mitigate this issue, mongosync v1.6.0 automatically disables Oplog Rollover Resilience if it finds at least one collection with non-simple collation during Mongosync initialization.

    We will investigate a more permanent solution for future releases.

In 1.6, the minimum supported versions of MongoDB are 6.0.8 and 7.0.0.

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