Redis-backed non-sticky session store for Apache Tomcat
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README.markdown

Redis Session Manager for Apache Tomcat

Overview

An session manager implementation that stores sessions in Redis for easy distribution of requests across a cluster of Tomcat servers. Sessions are implemented as as non-sticky--that is, each request is able to go to any server in the cluster (unlike the Apache provided Tomcat clustering setup.)

Sessions are stored into Redis immediately upon creation for use by other servers. Sessions are loaded as requested directly from Redis (but subsequent requests for the session during the same request context will return a ThreadLocal cache rather than hitting Redis multiple times.) In order to prevent collisions (and lost writes) as much as possible, session data is only updated in Redis if the session has been modified.

The manager relies on the native expiration capability of Redis to expire keys for automatic session expiration to avoid the overhead of constantly searching the entire list of sessions for expired sessions.

Data stored in the session must be Serializable.

Tomcat Versions

This project supports both Tomcat 6 and Tomcat 7. Starting at project version 1.1, precompiled JAR downloads are available for either version of Tomcat while project versions before 1.1 are only available for Tomcat 6.

The official release branches in Git are as follows:

  • tomcat-6: Continuing work for Tomcat 6 releases. Compatible with Java 6.
  • master: Continuing work for Tomcat 7 releases. Compatible with Java 6 or 7.
  • tomcat-7: Finalized; has now been merged into master. Compatible with Java 6 or 7.
  • java-7: All of the work from master for Tomcat 7 but taking advantage of new features in Java 7. Compatible with Java 7 only.

Architecture

  • RedisSessionManager: provides the session creation, saving, and loading functionality.
  • RedisSessionHandlerValve: ensures that sessions are saved after a request is finished processing.

Note: this architecture differs from the Apache PersistentManager implementation which implements persistent sticky sessions. Because that implementation expects all requests from a specific session to be routed to the same server, the timing persistence of sessions is non-deterministic since it is primarily for failover capabilities.

Usage

Add the following into your Tomcat context.xml (or the context block of the server.xml if applicable.)

<Valve className="com.radiadesign.catalina.session.RedisSessionHandlerValve" />
<Manager className="com.radiadesign.catalina.session.RedisSessionManager"
         host="localhost" <!-- optional: defaults to "localhost" -->
         port="6379" <!-- optional: defaults to "6379" -->
         database="0" <!-- optional: defaults to "0" -->
         maxInactiveInterval="60" <!-- optional: defaults to "60" (in seconds) --> />

The Valve must be declared before the Manager.

Copy the tomcat-redis-session-manager.jar and jedis-2.0.0.jar files into the lib directory of your Tomcat installation.

Reboot the server, and sessions should now be stored in Redis.

Session Change Tracking

As noted in the "Overview" section above, in order to prevent colliding writes, the Redis Session Manager only serializes the session object into Redis if the session object has changed (it always updates the expiration separately however.) This dirty tracking marks the session as needing serialization according to the following rules:

  • Calling session.removeAttribute(key) always marks the session as dirty (needing serialization.)
  • Calling session.setAttribute(key, newAttributeValue) marks the session as dirty if any of the following are true:
    • previousAttributeValue == null && newAttributeValue != null
    • previousAttributeValue != null && newAttributeValue == null
    • !newAttributeValue.getClass().isInstance(previousAttributeValue)
    • !newAttributeValue.equals(previousAttributeValue)

This feature can have the unintended consequence of hiding writes if you implicitly change a key in the session or if the object's equality does not change even though the key is updated. For example, assuming the session already contains the key "myArray" with an Array instance as its corresponding value, and has been previously serialized, the following code would not cause the session to be serialized again:

List myArray = session.getAttribute("myArray");
myArray.add(additionalArrayValue);

If your code makes these kind of changes, then the RedisSession provides a mechanism by which you can mark the session as dirty in order to guarantee serialization at the end of the request. For example:

List myArray = session.getAttribute("myArray");
myArray.add(additionalArrayValue);
session.setAttribute("__changed__");

In order to not cause issues with an application that may already use the key "__changed__", this feature is disabled by default. To enable this feature, simple call the following code in your application's initialization:

RedisSession.setManualDirtyTrackingSupportEnabled(true);

This feature also allows the attribute key used to mark the session as dirty to be changed. For example, if you executed the following:

RedisSession.setManualDirtyTrackingAttributeKey("customDirtyFlag");

Then the example above would look like this:

List myArray = session.getAttribute("myArray");
myArray.add(additionalArrayValue);
session.setAttribute("customDirtyFlag");

Possible Issues

There is the possibility of a race condition that would cause seeming invisibility of the session immediately after your web application logs in a user: if the response has finished streaming and the client requests a new page before the valve has been able to complete saving the session into Redis, then the new request will not see the session.

This condition will be detected by the session manager and a java.lang.IllegalStateException with the message Race condition encountered: attempted to load session[SESSION_ID] which has been created but not yet serialized. will be thrown.

Normally this should be incredibly unlikely (insert joke about programmers and "this should never happen" statements here) since the connection to save the session into Redis is almost guaranteed to be faster than the latency between a client receiving the response, processing it, and starting a new request.

If you encounter errors, then you can force save the session early (before sending a response to the client) then you can retrieve the current session, and call currentSession.manager.save(currentSession) to synchronously eliminate the race condition. Note: this will only work directly if your application has the actual session object directly exposed. Many frameworks (and often even Tomcat) will expose the session in their own wrapper HttpSession implementing class. You may be able to dig through these layers to expose the actual underlying RedisSession instance--if so, then using that instance will allow you to implement the workaround.

Acknowledgements

The architecture of this project was based on the Mongo-Tomcat-Sessions project found at https://github.com/dawsonsystems/Mongo-Tomcat-Sessions