Downgrade 4.2 to 4.0

The following page summarizes various considerations for downgrading to4.0. For specific instructions for your deployment type, see:

Downgrade Path

Once upgraded to 4.2, if you need to downgrade, we recommend downgrading to the latest patch release of 4.0.

If you downgrade, you can only downgrade to a 4.0.12 or later version.You cannot downgrade to a 4.0.11 or earlier version.

Considerations for Replica Sets and Sharded Clusters

Starting in MongoDB 4.2, change streams areavailable regardless of the "majority" read concernsupport; that is, read concern majority support can be eitherenabled (default) or disabledto use change streams.

In MongoDB 4.0 and earlier, change streams areavailable only if "majority" read concern support isenabled (default).

If you have disabled read concern "majority", change streams will bedisabled once you downgrade to 4.0-series.

Create Backup

Optional but Recommended. Create a backup of your database.

Prequisites

To downgrade from 4.2 to 4.0, you must remove incompatible featuresthat are persisted and/or update incompatible configuration settings.These include:

Downgrade Feature Compatibility Version (fCV)

Before downgrading the binaries, you must downgrade thefeatureCompatibilityVersion (fCV) to “4.0” and remove all persistedfCV 4.2 features that are incompatible with 4.0, such as:

For specific instructions to downgrade the fCV and remove thesefeatures, see:

zstd Compression

Using zstd for data compression, journal compression, ornetwork messages requires additional considerations for downgrades.

For detailed instructions appropriate for your deployment type, see:

tls-Prefixed Configurations

Before downgrading the binaries, update tls-prefixed configurationoptions for ssl-prefixed aliases.

For detailed instructions appropriate for your deployment type, see:

Procedures