Guys,
since this topic has come up again, I thought this would a useful point to braindump my current ideas for comment and as a common point of reference...
Here goes :
Each session has one 'primary' node and n 'replicant' nodes associated with it.
Sticky load-balancing is a hard requirement.
Changes to the session may only occur on the primary node.
Such changes are then replicated (possibly asynchronously, depending on data integrity requirements) to the replicant nodes.
If, for any reason, a session is required to 'migrate' to another node (fail-over or clusterwide-state-balancing), this 'target' node makes a request to the cluster for this session, the current 'source' node handshakes and the migration ensues, after which the target node is promoted to primary status.
Any inbound request landing on a node that is not primary for the required session results in a forward/redirect of the request to it's current primary, or a migration of the session to the receiving node and it's promotion to primary.
A shared store is used to passivate sessions that have been inactive for a given period, or are surplus to constraints on a node's session cache size.
Once in the shared store, a session is disassociated from it's primary and replicant nodes. Any node in the cluster, receiving a relevant request, may load the session, become it's primary and choose replicant nodes for it.
Correct tuning of this feature, in a situation where frequent migration is taking place, might cut this dramatically.
The reason for the hard node-level session affinity requirement is to ensure maximum cache hits in e.g. the business tier. If a web session is interacting with cached resources that are not explicitly tied to it (and so could be associated with the same replicant nodes), the only way to ensure that subsequent uses of this session hit resources in these caches is to ensure that these occur on the same node as the cache - i.e. the session's primary node.
By only having one node that can write to a session, we remove the possibility of concurrent writes occurring on different nodes and the subsequent complexity of deciding how to merge them.
The above strategy will work for a 'implicit-affinity' lb (e.g. BigIP), which remembers the last node that a session was successfully accessed on and rolls this value forward as and when it has to fail-over to a new node. We should be able to migrate sessions forward to the next node picked by the lb, underneath it, keeping the two in sync.
With an 'explicit-affinity' lb (e.g. mod_jk), where routing info is
actually encoded into the jsessionid/JSESSIONID value (or maybe an
auxiliary path param or cookie), it should be possible, in the case of
fail-over, to choose a (probably) replicant node to promote to primary and to stick
requests falling elsewhere to this new primary by resetting this
routing info on their jsessionid/JSESSIONID and redirecting/forwarding
them to it.
If, in the future, we write/enhance an lb to be Geronimo-aware, we can
be even smarter in the case of fail-over and just ask the cluster to
choose a (probably) replicant node to promote to primary and then direct requests
directly to this node.
The cluster should dynamically inform the lb about joining/leaving nodes, and sessions should likewise maintain their primary/replicant lists accordingly.
LBs also need to be kept up to date with the locations and access points of the various webapps deployed around the cluster, relevant node and webapp stats (on which to base balancing decisions), etc...
All of this information should be available to any member of the cluster and a Geronimo-aware lb should be a full cluster member.
On shutting down every node in the cluster all session state should end up in the shared store.
These are fairly broad brushstrokes, but they have been placed after some thought and outline the sort of picture that I would like to see.
Your thoughts ?
Jules
Bhagwat, Hrishikesh wrote:
I am also not convinced it reduces the amount of net traffic. After eachI agree. However, this is based on the assumption that only one unicast write is required. In other words, this is a primary/secondary topology. I think that hd did not intended such a topology and hence his statement.
request the MS must write to the shared store, which is the same traffic as
a unicast write to another node or a multicast write to the partition
(discounting the processing power needed to receive the message).
[hb] Yes i was not assuming a Pri/Sec design but a layout where any active server can be request to pick up a client request which is destined to server that has just failed
-----Original Message----- From: gianny DAMOUR [mailto:[EMAIL PROTECTED] Sent: Sunday, October 19, 2003 7:35 AM To: [EMAIL PROTECTED] Subject: [Re] Web Clustering : Stick Sessions with Shared Store
Jeremy Boynes wrote:
However, as Andy says, the cost of storing a serialized object in a BLOB is
significant. Other forms of shared store are available though which may
offer better performance (e.g. a hi-av NFS server).
Do we need a shared repository or a replicated repository?
The issue I have with hb's approach is the reliance on an Admin Server, ofI agree. It seems that in such a design an Admin Server is only used to route incoming requests to the relevant node.
which there would need to be at least two and they would need to co-operate
between themselves and with any load-balancers. I think this can be handled
by the regular servers themselves just as efficiently.
However, I do not believe that regular servers can do this job. I assume that they will implement a standard peer-to-peer cluster topology to provide redundancies, however I do not see how they can handle the dispatch of incoming requests.
This feature seems to be either a client or a proxy one: I mean it should be done prior to reach the nodes.
For instance, this feature is treated on the client-side via a stub aware of the available nodes in WebLogic. It seems that JBoss (correct me if I am wrong) has also followed this design.
I am also not convinced it reduces the amount of net traffic. After eachI agree. However, this is based on the assumption that only one unicast write is required. In other words, this is a primary/secondary topology. I think that hd did not intended such a topology and hence his statement.
request the MS must write to the shared store, which is the same traffic as
a unicast write to another node or a multicast write to the partition
(discounting the processing power needed to receive the message).
Gianny
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-- /************************************* * Jules Gosnell * Partner * Core Developers Network (Europe) * http://www.coredevelopers.net *************************************/
