Authentication Session Persistence

This section explains sessions in the WSO2 Identity Server (IS) and the process of enabling session persistence for these sessions. This is particularly useful when the 'remember me option' is selected when logging into either the service provider or the WSO2 Identity Server.

The Identity Server has multiple local caching usages in different layers to improve the product performance. But most of the caches are ready to persist in the database if required.

There are three types of data objects that are persisted in the database.

1. Session Data

Once the user gets authenticated over WSO2 Identity Server, it will create a session data object which stores the authenticated user and the other authentication flow details. This will be stored in the database to share across the cluster nodes.

2. Operational Data

This covers the same session data as in above but the outdated records. For example, once the user gets authenticated, there will be a record for login status to that session id. When the user logs out from the system, the above record is not removed from the table and instead, a new record is added for the same session id with status called 'logout'. So the valid record is the last one and all the other records under that session id will be outdated. Those outdated records belong to the operational data.

3. Temporary Data

In authentication flow, there are many temporary data objects that will be kept for few seconds only. These data objects are kept in cache. But to make the cluster environment consistent without having the local cache, the data objects are stored in the same database table where the session data is stored. 

When you consider the data persistence which belongs to the critical path and high concurrency situation, you have to further improve the following key aspects as well.

Data Clean-Up

Since the above data are collected over the authentication flow, it will grow the database with that data very frequently. So we had to manage few tasks to clean the data in the database for some given conditions.

Task Pool

We are storing above data within a critical path which controls the authentication. So we have introduced a pooling mechanism to put the data persistence tasks into that pool and continue the critical path without blocking it.

How this will work in the clustered environment.

Even in the clustered environment, we don’t recommend to enable the Hazelcast distributed caching for the Identity data. So in this case also we have to rely only on the local cache. And this case especially we have to disable the pool by ‘PoolSize’ is set to 0. Because otherwise there can be some data inconsistency situation which the one node get late to update the database before the other node try to access the same data from the database. Even we use the local cache here, it will trigger the cache invalidation notification system to consistent the data among the nodes in the cluster when the data get deleted or updated, but not for adding the data.

The following configuration found in the <IS_HOME>/repository/conf/identity/identity.xml file, under the Server and JDBCPersistenceManager elements are used to enable session persistence. 

<SessionDataPersist>
	<Enable>true</Enable>
	<Temporary>true</Temporary>
	<PoolSize>0</PoolSize>
	<SessionDataCleanUp>
		<Enable>true</Enable>
		<CleanUpTimeout>20160</CleanUpTimeout>
		<CleanUpPeriod>1140</CleanUpPeriod>
		<!--Instead of deleting all the records at once, we are deleting the records in chunks to prevent
		the -->
		<!--possible deadlock and lock scenarios. The following property defines the chunk size.-->
		<DeleteChunkSize>50000</DeleteChunkSize>
	</SessionDataCleanUp>
	<OperationDataCleanUp>
		<Enable>true</Enable>
	</OperationDataCleanUp>
</SessionDataPersist>

The following table describes the elements of the configurations mentioned above.

Why do we need to have a proper delete chunk size?

In the world of World Wide Web, Sessions are the simplest way to store data for individual users against a unique session ID. These can be used to persist state information between page requests. When we consider all the requests and responses that come to a page per day, that is really a large amount. Due to this reason, the session data storing in the “IDN_AUTH_SESSION_STORE” table in the WSO2CARBON_DB of WSO2 IS is really high. This table fills up quickly if your system receives too many loads of requests.

After a certain period of time, these session data becomes outdated. In order to stop the exponential table growth, you can run a session clean up task at predefined time intervals via a script. If this storing of session data is huge, the data that needs to be deleted also will come in bulk.

The session clean up task also takes a certain amount of time and consumes a certain amount of resources. Therefore, we always need to improve the performance of session clean up tasks. For this DeleteChunkSize property has introduced in WSO2 Identity Server. This will also help to reduce dead-locks happening during session clean up task. With the use of DeleteChunkSize value, the deletion happens batch wisely. “DeleteChunkSize” of data is deleted at one time.

But if you didn’t select a correct number for the DeleteChunkSize, you will end up with less performance. Therefore selecting suitable chunk size for your database is a must.

This chuck size may differ due to various factors of your environment

• The size of a row of the table. Calculate this by adding bytes needed for each column, and some control information per record.
• The underlying physical storage. e.g Normal SATA spinning Disk, SSD, SCSI, Amazon S3/EBS etc.
• The type of the database engine (e.g. MySql, MSSql, Oracle). Each one has different data organization mechanism in physical storage.
• OS where the DB server runs.

Therefore, it is hard to select a DeleteChunkSize without considering all the above factors.