MongoDB – Defeating RangeDeleter
Not closing a cursor in MongoDB could hurt you big, so it’s generally not recommended to use no_cursor_timeout=True (pymongo3) or timeout=False (pymongo2). Especially when you run shared MongoDB installation:
PyMongo does “close” cursors when they are garbage collected, but they aren’t closed immediately and closing a cursor in all current versions of MongoDB is asynchronous. Depending on the python implementation, relying on garbage collection to close the cursor is not a great idea. Discarded, not fully iterated cursors can live for some time when using Jython or PyPy which do not do reference counting garbage collection. That’s why the Cursor object has an explicit close() method.
Not using close() method could potentially be the reason behind the following lines in the MongoDB’s log file:
SHARDING [RangeDeleter] waiting for open cursors before removing range
And even if it looks innocuous, it’s actually not quite, since what it means is that a source shard can’t delete its copy of the documents – Step 7 in chunk migration procedure.
At the time of this writing there is still no way (SERVER-3090) to glean more information that pertains to a cursor’s id, so the only way out that I was able to come up with was to kill those dangling cursors using an undocumented (as of this writing) killCursors command:
> use your_db_name
> db.runCommand({killCursors: 'your_collection_name', cursors: [NumberLong(51518759968), NumberLong(51484189302), NumberLong(51451409949), NumberLong(51434938438), NumberLong(51429435438), NumberLong(51383912702)]})
{
"cursorsKilled" : [
NumberLong("51518759968"),
NumberLong("51484189302"),
NumberLong("51451409949"),
NumberLong("51434938438"),
NumberLong("51429435438"),
NumberLong("51383912702")
],
"cursorsNotFound" : [ ],
"cursorsAlive" : [ ],
"cursorsUnknown" : [ ],
"ok" : 1
}
And as a reward look for the following message in the log file:
SHARDING [RangeDeleter] Deleter starting delete for