syncrepl.md

Synchronous replication

Since synchronous replication is usually needed to avoid losing some transactions, stolon implements it in a way to avoid any possibility of electing non sync standbys as new masters. When synchronous replication is enabled stolon will always ensure that a master has N synchronous standbys (where N will be between MinSynchronousStandbys and MaxSynchronousStandbys values defined in the cluster specification). If there're not enough available standbys then it will also add a fake standby server in the synchronous_standby_names. Adding a non existing standby server will ensure the master will always block waiting for remote commits.

You can enable/disable synchronous replication at any time and the keepers will reconfigure themselves using stolonctl update to update the cluster specification.

Min and Max number of synchronous replication standbys

In the cluster spec you can set the MinSynchronousStandbys and MaxSynchronousStandbys values (they both defaults to 1). Having multiple synchronous standbys is a feature provided starting from PostgreSQL 9.6. Increasing these value above 1 for postgres versions below 9.6 will lead to errors starting the instance.

Enable synchronous replication.

Assuming that your cluster name is mycluster and using etcd listening on localhost:2379:

stolonctl --cluster-name=mycluster --store-backend=etcd update --patch '{ "synchronousReplication" : true }'

Disable synchronous replication.

stolonctl --cluster-name=mycluster --store-backend=etcd update --patch '{ "synchronousReplication" : false }'

Set min and max number of synchronous replication standbys

Set MinSynchronousStandbys/MaxSynchronousStandbys to a value different than 1 only when using PostgreSQL >= 9.6

stolonctl --cluster-name=mycluster --store-backend=etcd update --patch '{ "synchronousReplication" : true, "minSynchronousStandbys": 2, "maxSynchronousStandbys": 3 }'