How to filter on collection in deployment level changestream in mongodb java sdk

Question

I am trying to setup a deployment level change stream with a pipeline filter on collection name using MongoDB java SDK. Here is the code snippet.

   List<Bson> pipeline = Collections.singletonList(Aggregates.match(Filters.or(
        Filters.eq("namespace", "db1.c1"),
        Filters.eq("namespace", "db1.c2"))));
 
    client.watch(pipeline)
        .cursor()
        .forEachRemaining(doc -> {
          System.out.println(doc);
        });

But this query does not match any document. Following variations of the pipeline document does not work either.

    List<Bson> pipeline =
    Collections.singletonList(Aggregates.match(Filters.or(
      Document.parse("{'namespace': 'db1.c1'}"),
      Document.parse("{'namespace': 'db1.c2'}")))); 

Surprisingly pipeline works on other fields of the changestream document. For example, this works:

     List<Bson> pipeline = Collections
     .singletonList(Aggregates.match(Filters.and(
     Document.parse("{'fullDocument.seq': 4}"),
     Filters.in("operationType", Arrays.asList("insert")))));

I am not sure why this would be the case. I would appreciate any help in this regard.

Answer 1

in Order to get changestream events on DB or collection level you need to use filters on below attributes :

ns.db The name of the database.

ns.coll The name of the collection.

In your case if you are interested only for collection C1 & c2 then consider having filter something like below.

List<Bson> pipeline = Collections.singletonList(Aggregates.match(Filters.or(
            Filters.eq("ns.coll", "c1"),
            Filters.eq("ns.coll", "c2"))));
     
        client.watch(pipeline)
            .cursor()
            .forEachRemaining(doc -> {
              System.out.println(doc);
            });

Please Note: above filter is on collections only you can add filter on DB for collection form specific DB like (ns.db = "db1" AND ns.coll : $in :["c1", "c2"] )

MongoDB doc reference: https://docs.mongodb.com/manual/reference/change-events/#change-stream-output