Replication in MongoDB usually happens from the primary to the secondaries. In some cases, we may want to replicate from another secondary instead of the primary. Chained replication helps alleviate the primary from read load but at the same time increases average replication lag for the secondary that chooses to replicate from a secondary. This makes sense as replication first has to go from the primary to the secondary (1) and then from this server to another secondary (2).

Chained replication can be enabled (and disabled respectively) with the following cfg command:

> cfg.settings.chainingAllowed = true

In cases where printjson(cfg) doesn't reveal a settings subdocument, then we need to create an empty one first:

> cfg.settings = { }