mongodb TTL not working - mongodb

Question

I had executed this command to set a TTL Index on mongodb,
db.sessions.ensureIndex({'expiration':1},{"expireAfterSeconds" : 30})
but after 4 days,I found these documents were not removed.
I had confirmed command and document's field was correct.
I don't know how to fix it.
after executed db.serverStatus(), I got
localTime is 2015-01-16 11:03:05.554+08:00
and the following is some info of my collection
db.sessions.getIndexes()
{
"0" : {
"v" : 1,
"key" : {
"_id" : 1
},
"name" : "_id_",
"ns" : "meta.sessions"
},
"1" : {
"v" : 1,
"key" : {
"expiration" : 1
},
"name" : "expiration_1",
"ns" : "meta.sessions",
**"expireAfterSeconds" : 30**
}
}
db.sessions.find()
/* 0 */
{
"_id" : ObjectId("54b4c2e0f840238ca1436788"),
"data" : ...,
"expiration" : **ISODate("2015-01-13T16:02:33.947+08:00"),**
"sid" : "..."
}
/* 1 */
{
"_id" : ObjectId("54b4c333f840238ca1436789"),
"data" : ...,
"expiration" : ISODate("2015-01-13T16:06:56.942+08:00"),
"sid" : ".."
}
/* ... */

Answer 1

To expire data from a collection (Tested in version 3.2) you must create indexes:
db.my_collection.createIndex( { "createdAt": 1 }, { expireAfterSeconds: 3600 } )
After that, every document that you insert in this collection must have the "createdAt" with the current date:
db.my_collection.insert( {
"createdAt": new Date(),
"dataExample": 2,
"Message": "Success!"
} )
The document will be removed when the date createdAt value + expireAfterSeconds value arrive.
Note: This background task in MongoDB, by default, happens once every 60 seconds.

Answer 2

When you create TTL index in the foreground (like you did), MongoDB begins removing expired documents as soon as the index finishes building. Best to tail -f mongod.log during index creation to track the progress. You may wish to remove & recreate index if something went wrong.
If index was created in the background, the TTL thread can begin deleting documents while the index is building.
TTL thread that removes expired documents runs every 60 seconds.
If you created index on the replica that was taken out of the replica set and is running in standalone mode index WILL be created but documents will NOT be removed until you rejoin (or remove replica set) configuration. If this is the case you may get something similar to this in the mongod.log
** WARNING: mongod started without --replSet yet 1 documents are
** present in local.system.replset
** Restart with --replSet unless you are doing maintenance and no other
** clients are connected.
** The TTL collection monitor will not start because of this.
** For more info see http://dochub.mongodb.org/core/ttlcollections