Table of contents
This page describes what to do in case of an Icinga alert. For more information you could search the govuk-puppet repo for the source of the alert

MongoDB rollback

What is a rollback

Explanation from the MongoDB documentation:

In some failover situations primaries will have accepted write operations that have not replicated to the secondaries after a failover occurs. This case is rare and typically occurs as a result of a network partition with replication lag. When this member (the former primary) rejoins the replica set and attempts to continue replication as a secondary the former primary must revert these operations or “roll back” these operations to maintain database consistency across the replica set.

How to investigate

The rolled-back records can be viewed with bsondump which will output human readable JSON.

Some judgement is required to determine whether it is necessary to recover the rolled-back data. In one example, we found that the user had resubmitted the affected record(s) a few minutes later, presumably in response to a UI error. Therefore recovery was not necessary.

How to recover

If you are happy that the rolled-back data should be restored and doesn’t conflict with any existing records, then you can use mongorestore.

The following arguments are necessary:

  • -h: Hostname of the current PRIMARY node, from rs.status().
  • -d: Name of the database, from the BSON filename.
  • -c: Name of the collection, from the BSON filename.
  • --objcheck: Validate BSON. Not the default in MongoDB 2.2

For example:

dcarley@production-licensing-mongo-1:/var/lib/mongodb/rollback$ mongorestore -h licensing-mongo-2 -d licensify-audit -c audit --objcheck licensify-audit.audit.2013-05-30T12-45-42.1.bson
connected to: licensing-mongo-2
Thu May 30 13:56:52 licensify-audit.audit.2013-05-30T12-45-42.1.bson
Thu May 30 13:56:52 going into namespace [licensify-audit.audit]
1 objects found

Unfortunately mongorestore may not give any feedback about whether the restore was successful. It is advisable to check the logs on the host you referenced, and query MongoDB for the _id of the records you expect.

When satisfied, the BSON files and rollback directory can be deleted. This will resolve the Nagios alert.

More about Icinga alerts

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