expire-least-recently-used-data-session-objects.txt

.. default-domain:: mongodb
:title: MongoDB Use Cases - Session Objects

.. http://www.mongodb.org/display/DOCS/Use+Case+-+Session+Objects

===============
Session Objects
===============


MongoDB is a good tool for storing HTTP session objects.

One implementation model is to have a sessions collection, and store the
session object's \_id value in a browser cookie.

With its update-in-place design and general optimization to make updates
fast, MongoDB is efficient at receiving an update to the session
object on every single app server page view.

Purging Old Sessions using TTL Collections
------------------------------------------

The best method to manage and purge old sessions is to use a
:term:`TTL Collection <mongodb:ttl-collections>`.  
A TTL collection is a MongoDB collection which has an ``expireAfterSeconds``
index on an :term:`ISODate <mongodb:isodate>` field.

For example, create a collection :samp:`sessions` with three fields:

   :samp:`_id`
   
      Automatically created by MongoDB
      
   :samp:`lastUsed`
   
      An ISODate field which you will update every time an action occurs
      in the session.
      
   :samp:`sessionId`
   
      An identifier for the session.  This could be an ObjectId from 
      another collection for example.
      
.. code-block:: javascript

   > db.createCollection('sessions');
   {"ok": 1}
   
   > var record = { lastUsed: ISODate(), sessionId: "1cf41e17e21d5697ac7ed84d10caa456"}
   > db.sessions.insert(record)
   > db.sessions.find().pretty()
   {
           "_id" : ObjectId("50fee58e95fa0c941f134114"),
           "lastUsed" : ISODate("2013-01-22T19:16:23.570Z"),
           "sessionId" : "1cf41e17e21d5697ac7ed84d10caa456"
   }
      
You now have a collection with one record.  Now create the TTL index on the 
collection:

.. code-block:: javascript

   > db.sessions.ensureIndex({lastUsed:1}, {expireAfterSeconds:300})
   
This creates a TTL index on the :samp:`lastUsed` field.
Any document whose :samp:`lastUsed` field has a time older than five
minutes (300 seconds) will be purged.  

.. code-block:: javascript

   > db.sessions.getIndexes()
   [
        {
            "v" : 1,
            "key" : {
                "_id" : 1
            },
            "ns" : "writing.sessions",
            "name" : "_id_"
        },
        {
            "v" : 1,
            "key" : {
                "lastUsed" : 1
            },
            "ns" : "writing.sessions",
            "name" : "lastUsed_1",
            "expireAfterSeconds" : 300
        }
    ]

And if we wait a few minutes that solitary document will be purged:

.. code-block:: javascript

    > db.sessions.find()
    >
    
.. note::

   If you insert a document whose timestamp is already ``older`` than
   the expiration window set in :samp:`expireAfterSeconds`, MongoDB 
   will insert the document and then purge it at the next run of the
   TTL scavenger process.


Aging Out Old Sessions using Capped Collections
-----------------------------------------------

One method to age out old sessions is to use the auto-LRU facility of
:term:`capped collections <mongodb:capped collections>`.
A complication is that objects in capped collections may not grow beyond
their initial allocation size.
To handle this, we can "pre-pad" the objects to some maximum size on
initial addition, and then on further updates we are fine if we do not
go above the limit. The following MongoDB shell JavaScript example
demonstrates padding:

.. code-block:: javascript

	> db.createCollection('sessions', { capped: true, size : 1000000 } )
	{"ok" : 1}
	> p = "";
	> for( x = 0; x < 100; x++ ) p += 'x';
	> s1 = { info: 'example', _padding : p };
	{"info" : "example" , "_padding" : "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"}
	> db.sessions.save(s1)
	> s1
	{"info" : "example" , "_padding" : "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx" , "_id" :  ObjectId( "4aafb74a5761d147677233b0") }

	> // when updating later
	> s1 = db.sessions.find( { _id : ObjectId( "4aafb74a5761d147677233b0") } )
	{"_id" :  ObjectId( "4aafb74a5761d147677233b0")  , "info" : "example" , "_padding" : "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"}
	> delete s._padding;
	true
	>  s.x = 3; // add a new field
	3
	> db.sessions.save(s);
	> s
	{"_id" :  ObjectId( "4aafb5a25761d147677233af")  , "info" : "example" , "x" : 3}