Consume change streams from secondary nodes

Ops and Admin Replication
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#1

Hi, reading https://www.mongodb.com/docs/manual/changeStreams/#open-a-change-stream, it mentions:

For a replica set, you can issue the open change stream operation from any of the data-bearing members.

which seems to suggest that we can consume change stream from a replica (secondary node). Some questions around that:

  • Is there any config to only allow read change stream from secondary? If so, how do we handle a secondary being promoted to primary or a secondary node get terminated?
  • Is there a config to only allow read change stream from primary? Or is it actually the default behavior?

Any related example or documentation would also be helpful! Thanks

#2

I believe the default behavior is: “allowing read change stream from any node within a replset”, and there is no special config to enforce read only from primary or secondary.

If that’s the case, do we have any special events being sent out if a node being promoted / demoted at all? Or for change stream API, the “role” of a node (primary vs secondary) is not being exposed to clients at all?

#3

The read preference is at the client driver level. It is not specific to change stream. You specify the read preference when you establish the connection between the client driver and the server. The change stream will use what ever read preference the connection has.

To use other than the default read preference see:

#4

Thanks @steevej , some follow ups:

  • With secondary read preference, will Mongo automatically switch node if the previous secondary node gets promoted to primary?
  • If we create a MongoClient by connecting to a primary node, and start a connection with secondary read preference, would mongo reroute the connection to another node in the replset automatically?
#5

Some more context: when we build connection, we pass in a list of server addresses: debezium/MongoClients.java at main · debezium/debezium · GitHub

One of them is primary and I wonder what would happen if we set readPreference to secondary while we are building a connection with the primary node?

#6

Related ^

After the connection is built and there is a failover (e.g., promote a secondary to primary), would the change stream be re-created automatically to another secondary node?

#7

I have no idea.

Hopefully, someone will jump in the thread.