With mongodb java driver, maxAwaitTime not working on a change stream

Question

I'm using the Java mongodb driver version 3.8.0 and mongodb 3.6.3.

I created a watch on a collection with this:

MongoCursor<ChangeStreamDocument<Document>> cursor = collection.watch().maxAwaitTime(500, TimeUnit.MILLISECONDS).iterator();

The documentation here states about maxAwaitTime:

The maximum amount of time for the server to wait on new documents to satisfy a change stream query.

However, what I'm seeing is that cursor.hasNext() returns only if there is a change on the collection, not when the time passed to maxAwaitTime has elapsed.

When I turn on mongodb's verbose logging I see maxWaitTime set as expected in the getMore command.

How do I cause my watch to timeout when there are no changes? It seems as though maxAwaitTime should do this. Why doesn't it work?

Sorcim

Answer 1

MongoDB drivers implement change stream as an abstraction of a TAILABLE_AWAIT cursor, and maxAwaitTimeMS is to specify a time limit for a getMore command on on TAILABLE_AWAIT cursor.

The way it works, MongoCursor continues to send getMore command to the server in a loop until either:

• A document is found
• The cursor is closed
• An exception occurs

Only when any of the event above happens, the cursor's method next() or hasNext() will return.

While none of the event above happens, the server's getMore command will continue to be called by the Iterator interface. The maxAwaitTime specifies how long to wait before the getMore command timed out while waiting for documents and returns even if there there are no documents found.

How do I cause my watch to timeout when there are no changes?

If your application requires a time out after maxAwaitTime, the mongo-java-driver offers the tryNext() method on MongoCursor. This method will return null after maxAwaitTime if no documents were found, and can be called repeatedly by the application. See also JAVA-2965.

Supervisor