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    Documentation of design discussion at our October 2008 Brno clustering dev meetings.


    JBoss Cache is considering moving to a data partitioning approach based on consistent hashing.  Idea is a particular key (i.e. Fqn) under which cached data is stored would be hashed using an algorithm that incorporates the cluster view.  Which nodes in the cluster the data is stored upon would be derived from the hash function.  This approach is a logical alternative to buddy replication; both seek to improve memory and network utilization by not requiring data to be redundantly stored on every node in the cluster.


    Data partitioning differs from buddy replication in not requiring data ownership, i.e. that access to the data should only occur from one node.  However, the more access to the data can be confined to a node where that data is locally cached, the more performant the overall system would be.


    Storing session data in a consistent-hash based data partition can theoretically be highly performant, because sessions are meant to be sticky -- i.e. accessed only via one node.  The trick is determining how to ensure that the node to which web requests are sent is the one where the data is locally stored.  What I outline below is how an enhanced mod_cluster could help ensure this occurs.




    The mod_cluster code on the httpd side has available to it the same consistent hashing algorithm as is used on the JBoss side, as well as the same inputs (i.e. the JBoss-side cluster topology).  Based on this, mod_cluster can always determine the primary node for a session (session id being the value being hashed) as well as all backup nodes.  With this information, mod_cluster can properly route requests both before and after failover.


    Request Handling


    This is the approach discussed in Brno:


    1) Initial request comes to node 1. This is for a new session, so node 1 has the freedom to specify the session id. It generates an id that hashes to node 1.  Effect is the session data will be cached locally.

    2) Second request arrives, mod_cluster hashes the session id, determines the data is local on node 1, so routes the request there.

    3) Node 1 crashes but another request comes in. mod_cluster checks the session hash, sees the secondary node for the session is node 3, so it routes the request to node 3, which handles it using its locally cached backup data.

    4) The AS side recognizes the failure of node 1, so the topology information that is an input to the hashing function changes. Following such changes, some portion of cached data (no more than 1/n, where n is the number of cluster members) will need to be moved to a new host.  In this case, the session in question needs to be moved and ends up on node 2.

    5) Another request comes in, but mod_cluster is not yet aware of the new topology having changed the hashing function input, so it routes the request to node 3. Node 3 handles the request by making remote calls to node 2 to read/write the session data.

    6) The AS side informs mod_cluster of the change to the topology.

    7) Next request comes in, mod_cluster uses the new topology in its hashing function and routes the request to node 2, where the data resides.


    The above is a deliberately complex scenario where the session data is moving around the AS-side cluster and requests are coming in while the topology info is inconsistent between the mod_cluster side and the AS side.


    Possible Simpler Approach


    A simpler approach is to continue to use the jvmRoute suffix in the session id to control routing on the httpd side, but allow JBoss AS nodes to alter an emitted session cookie to point to a node other than themselves. Example:


    1) Initial request comes in to node 1 which, as above, generates a session id that points to itself.  Session cookie is 123xxx.node1.

    2) Node 1 fails, so a subsequent request is randomly sent by mod_cluster to node 2. The 123xxx session data is not stored locally on node 2, but rather on node 3. Node 2 handles the request by making remote calls to node 3 to read/write the session data.

    3) Node 2 recognizes that making remote calls to handle requests is inefficient, so in its response to the failover request, it alters the session cookie, not to 123xxx.node2 but to 123xxx.node3.

    4) Next request that comes in is routed by mod_cluster to node 3, where the data is local.


    This approach eliminates the need for mod_cluster to understand the consistent hashing algorithm.