Blog 2021 12 29 Troubleshooting 'MongoDB Cursor xxxxxx not found' Errors
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Troubleshooting 'MongoDB Cursor xxxxxx not found' Errors

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Read operations that return multiple documents do not immediately return all values matching the query. Because a query can potentially match very large sets of documents, these operations rely upon an object called a cursor. A cursor fetches documents in batches to reduce both memory consumption and network bandwidth usage.

One category of issue you may observe occasionally in your application logs is a CursorNotFound entry such as the following (from an application using the MongoDB Java Driver):

com.mongodb.MongoCursorNotFoundException: Query failed with error code -5 and error message 'Cursor 4865637895305205821 not found on server prod-shard-00-00.xxxxx.mongodb.net:27017' on server prod-shard-00-00.xxxxx.mongodb.net:27017

Depending on whether the version of MongoDB your cluster is using is greater than 4.4.7 the “cursor id xxxxxx not found” can refer to two possible timeouts.

(1) cursorTimeoutMillis being exceeded

The cursorTimeoutMillis server parameter sets the expiration threshold (in milliseconds) for idle cursors before MongoDB removes them. The default value for cursorTimeoutMillis is 600000, or 10 minutes. Idle cursors are timed out using the ClientCursorMonitor background job, whose thread is identified in the mongod logs as clientcursormon.

The ClientCursorMonitor identifies and reaps idle cursors every 4 seconds (the default value of clientCursorMonitorFrequencySecs).

When a cursor timeout is identified these can be found in the log with entries similar to the following:

{"t":{"$date":"2021-12-29T09:22:41.937-05:00"},"s":"I","c":"COMMAND","id":51803,"ctx":"conn3","msg":"Slow query","attr":{"type":"command","ns":"admin.foo","appName":"MongoDB Shell","command":{"find":"foo","filter":{},"batchSize":1,"lsid":{"id":{"$uuid":"824bd767-4a7d-4240-a8cd-8f4c83c8cf99"}},"$db":"admin"},"planSummary":"COLLSCAN","cursorid":4225966264683133400,"keysExamined":0,"docsExamined":1,"numYields":0,"nreturned":1,"reslen":123,"locks":{"ReplicationStateTransition":{"acquireCount":{"w":1}},"Global":{"acquireCount":{"r":1}},"Database":{"acquireCount":{"r":1}},"Collection":{"acquireCount":{"r":1}},"Mutex":{"acquireCount":{"r":1}}},"storage":{},"protocol":"op_msg","durationMillis":0}}
{"t":{"$date":"2021-12-29T09:22:44.096-05:00"},"s":"I","c":"QUERY","id":20529,"ctx":"clientcursormon","msg":"Cursor timed out","attr":{"cursorId":4225966264683133400,"idleSince":{"$date":"2021-12-29T14:22:41.937Z"}}}
{"t":{"$date":"2021-12-29T09:22:48.031-05:00"},"s":"I","c":"COMMAND","id":51803,"ctx":"conn3","msg":"Slow query","attr":{"type":"command","ns":"admin.$cmd","appName":"MongoDB Shell","command":{"getMore":4225966264683133400,"collection":"foo","batchSize":1,"lsid":{"id":{"$uuid":"824bd767-4a7d-4240-a8cd-8f4c83c8cf99"}},"$db":"admin"},"cursorid":4225966264683133400,"numYields":0,"ok":0,"errMsg":"cursor id 4225966264683133185 not found","errName":"CursorNotFound","errCode":43,"reslen":129,"locks":{},"protocol":"op_msg","durationMillis":0}}

The log messages above have had their log component verbosity increased and were generated using the following script:

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# bash
rm -rf data && mkdir data
$(m bin 4.4.7-ent)/mongod --dbpath data --bind_ip_all
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// mongo shell
db.runCommand({ setParameter: 1, cursorTimeoutMillis: 1000 });
db.runCommand({ setParameter: 1, clientCursorMonitorFrequencySecs: 2 });
db.foo.drop();
db.foo.insertMany([ {}, {} ]);
db.setLogLevel(4, 'command')
db.foo.find({}).batchSize(1).forEach(function(d) {
  printjson(d);
  sleep(1000 * 6);
});
db.setLogLevel(-1, 'command')

Note that the log contains a …”s”:”I”,”c”:”QUERY”,”id”:20529,”ctx”:”clientcursormon”,”msg”:”Cursor timed out…“ entry, which is at the default log verbosity. If this message is present the cursor timed out as a result of being idle longer than cursorTimeoutMillis and would have returned an error such as:

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Error: command failed: {
	"ok" : 0,
	"errmsg" : "cursor id 4225966264683133185 not found",
	"code" : 43,
	"codeName" : "CursorNotFound"
} with original command request: {
	"getMore" : NumberLong("4225966264683133185"),
	"collection" : "foo",
	"batchSize" : 1,
	"lsid" : {
		"id" : UUID("824bd767-4a7d-4240-a8cd-8f4c83c8cf99")
	}
}

(2) localLogicalSessionTimeoutMinutes being exceeded

Starting with MongoDB 3.6 server sessions, or logical sessions, are the underlying framework used by client sessions to support Causal Consistency and retryable writes. When using a MongoDB Driver that is 3.6+ compatible implicit sessions are used (per the Drivers Sessions specification). As such, if your application is using a 3.6+ compatible driver, you are using sessions.

The default value of localLogicalSessionTimeoutMinutes is 30 minutes and controls the time (in minutes) that a session remains active after its most recent use. Sessions that have not received a new read/write operation from the client or been refreshed with refreshSessions within this threshold are cleared from the cache.

Starting with MongoDB 4.4.8 (via SERVER-6036) when a cursor is opened as part of a session, its lifetime will be tied to that session and as a result closing or timing out of a session will kill all associated cursors. This results in cursorTimeoutMillis/clientcursormon not being used to control cursor timeouts for any cursor with a session id.

When a cursor times out as a result of the session being reaped the error appears as follows:

{"t":{"$date":"2021-12-29T08:02:04.942-05:00"},"s":"I","c":"COMMAND","id":51803,"ctx":"conn1","msg":"Slow query","attr":{"type":"command","ns":"admin.foo","appName":"MongoDB Shell","command":{"find":"foo","filter":{},"batchSize":1,"lsid":{"id":{"$uuid":"824bd767-4a7d-4240-a8cd-8f4c83c8cf99"}},"$db":"admin"},"planSummary":"COLLSCAN","cursorid":63169428846689080,"keysExamined":0,"docsExamined":1,"numYields":0,"nreturned":1,"reslen":123,"locks":{"ReplicationStateTransition":{"acquireCount":{"w":1}},"Global":{"acquireCount":{"r":1}},"Database":{"acquireCount":{"r":1}},"Collection":{"acquireCount":{"r":1}},"Mutex":{"acquireCount":{"r":1}}},"storage":{},"protocol":"op_msg","durationMillis":0}}
{"t":{"$date":"2021-12-29T08:03:53.030-05:00"},"s":"I","c":"QUERY","id":20528,"ctx":"LogicalSessionCacheRefresh","msg":"Killing cursor as part of killing session(s)","attr":{"cursorId":63169428846689080}}
{"t":{"$date":"2021-12-29T08:04:04.968-05:00"},"s":"I","c":"COMMAND","id":51803,"ctx":"conn1","msg":"Slow query","attr":{"type":"command","ns":"admin.$cmd","appName":"MongoDB Shell","command":{"getMore":63169428846689080,"collection":"foo","batchSize":1,"lsid":{"id":{"$uuid":"824bd767-4a7d-4240-a8cd-8f4c83c8cf99"}},"$db":"admin"},"cursorid":63169428846689080,"numYields":0,"ok":0,"errMsg":"cursor id 63169428846689082 not found","errName":"CursorNotFound","errCode":43,"reslen":127,"locks":{},"protocol":"op_msg","durationMillis":0}}

These log entries were generated by adjusting our previous example as seen below:

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# bash
rm -rf data && mkdir data
$(m bin 4.4.11-ent)/mongod --dbpath data --bind_ip_all --setParameter logicalSessionRefreshMillis=1000 --setParameter localLogicalSessionTimeoutMinutes=1
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db.foo.drop();
db.foo.insertMany([ {}, {} ]);
db.setLogLevel(4, 'command')
db.foo.find({}).batchSize(1).forEach(function(d) {
  printjson(d);
  sleep(1000 * 120);
});
db.setLogLevel(-1, 'command')

Now the log entry that controls timing out the cursor is generated by the LogicalSessionCacheRefresh thread. Note that as the end result is the same (idle cursor is timed out) the error returned to the application would appear to be the same as well:

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uncaught exception: Error: command failed: {
	"ok" : 0,
	"errmsg" : "cursor id 63169428846689082 not found",
	"code" : 43,
	"codeName" : "CursorNotFound"
} with original command request: {
	"getMore" : NumberLong("63169428846689082"),
	"collection" : "foo",
	"batchSize" : 1,
	"lsid" : {
		"id" : UUID("824bd767-4a7d-4240-a8cd-8f4c83c8cf99")
	}
}

Summary

Regardless of which MongoDB 3.6+ version is being used an idle cursor can time out with the failure bubbling up to the application with an error message such as “cursor id 63169428846689082 not found”, which is the same as if the cursor were timed out.

Note that setting the noCursorTimeout cursor option in a MongoDB 3.6+ cluster can still result in a cursor being closed as Session Idle Timeout Overrides noCursorTimeout.

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

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var session = db.getMongo().startSession()
var sessionId = session.getSessionId().id

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

}

Refreshing explicit sessions is one way to work around these timeouts, however this is only one approach. If there is a need to keep cursors idle for long periods of time these should be evaluated on a case by case basis to ensure refactoring isn’t a better solution.

All examples in this article use the mongo shell however the logic could be adapted to your preferred language and used with the appropriate MongoDB Driver.

Have any questions or comments? Post them below ;)

This post is licensed under CC BY 4.0 by the author.