cursor.maxTimeMS()

Session Idle Timeout Overrides maxTimeMS

MongoDB drivers and mongosh associate all operations with a server session, with the exception of unacknowledged write operations. For operations not explicitly associated with a session (i.e. using Mongo.startSession()), MongoDB drivers and mongosh create an implicit session and associate it with the operation.

If a session is idle for longer than 30 minutes, the MongoDB server marks that session as expired and may close it at any time. When the MongoDB server closes the session, it also kills any in-progress operations and open cursors associated with the session. This includes cursors configured with noCursorTimeout() or a maxTimeMS() greater than 30 minutes.

For example, consider a find() operation with the maxTimeMS() configured for a timeout of 31 minutes. The server returns a cursor along with a batch of documents defined by the cursor.batchSize() of the find(). The session refreshes each time the application requests a new batch of documents from the server. However, if the application takes longer than 30 minutes to process the current batch of documents, the session is marked as expired and closed. When the server closes the session, it also kills the cursor despite the cursor being configured with maxTimeMS() greater than 30 minutes. When the application requests the next batch of documents, the server returns an error.

For operations that return a cursor, if the cursor may be idle for longer than 30 minutes, issue the operation within an explicit session using Mongo.startSession() and periodically refresh the session using the refreshSessions command. For example:

var session = db.getMongo().startSession()
var sessionId = session
sessionId  // show the sessionId
var cursor = session.getDatabase("examples").getCollection("data").find().noCursorTimeout()
var refreshTimestamp = new Date() // take note of time at operation start
while (cursor.hasNext()) {
  // Check if more than 5 minutes have passed since the last refresh
  if ( (new Date()-refreshTimestamp)/1000 > 300 ) {
    print("refreshing session")
    db.adminCommand({"refreshSessions" : [sessionId]})
    refreshTimestamp = new Date()
  }
  // process cursor normally
}

In the example operation, the db.collection.find() method is associated with an explicit session. The cursor is configured with cursor.maxTimeMS() to keep the cursor open for at least 31 minutes. The while loop includes a block that uses refreshSessions to refresh the session every 5 minutes. Since the session will never exceed the 30 minute idle timeout, the cursor can remain open up to the configured maxTimeMS().

For MongoDB drivers, defer to the driver documentation for instructions and syntax for creating sessions.

Default Timeout for All Operations

Starting in MongoDB 8.0, you can use the defaultMaxTimeMS cluster parameter to specify a default time limit for individual read operations to complete. If a query specifies a maxTimeMS() option, that value overrides the defaultMaxTimeMS value.