Sunday, October 14, 2012

A tale of two agents

With ODI 11g, Oracle has provided integration of ODI agents with J2EE infrastructure to provide High Availability and Load Balancing in a RAC environment. However, there are several small to mid size businesses which do not want to invest in this infrastructure and hence cannot use High Availability and Load balancing feature of ODI agents in the J2EE environment.

The good news is that Oracle is continuing to maintain the Standalone agent for ODI 11g and with its basic features of load balancing, this may be enough in some situations.

We cannot achieve the high availability as promised by the Weblogic implementation but we can certainly configure the standalone agents in a way where we can load balance, have minimum downtime and yet maintenance can be done on the agents.

I have seen many implementations where it is chosen to use only One instance of an agent and everything might seem to work well initially. And then there is a long way to go.

And that does beg a question - In what cases, would a "single agent" implementation be not enough.

Will I need to perform maintenance operations on the agent, reboot the agent and if at that time a second agent is available, surely, it can come to the rescue of my long running processes and also share/transfer the load.

Here are a few cases to think:

1.) You make extensive use of Jython and external Java libraries inside the ODI JVM. This may cause the JVM overflow and make an agent unresponsive.

2.) New code needs to be deployed to the agent which warrants an agent bounce. In case of a single agent, the processes that are running will be impacted. However, if you have a multi-agent setup, then the currently running processes can be shifted over to the other running agent.

3.) The agent JVM parameters need to be changed - adding more memory to JVM or changing the path where the JVM creates its log files, etc

ODI supports the notion of a Master Agent and several slave agents and ideally each ODI implementation should have atleast 3 agents - 1 Master and 2 slave. This is how a minimal ODI configuration should look like for a Master - Slave setup:

The master agent accepts the job and passes it to the available slave agent. This configuration can be specified in the ODI topology:

The master agent is the one that is tied to a logical agent and a context. You can chose to have master agent to be a "worker agent" - it can also perform processing and share load (by selecting it as "linked" agent). Or it can be configured to be merely a delegator. Its advisable to configure the master agent in a "delegator" mode only. So, this agent can have basic ODI configuration and doesnt do anything fancy. This way there would less chances of this agent going down (either buffer overflow or other maintenance operations).

The other "n" slave agents can be tied to the Master agent as "linked" agents. Now when a job is submitted to the Master agent, it finds which slave agent that has less load and delegates the job to that agent. In case, a slave agent is unavailable, it will re-assign the job to other available agent(s).

So, in case you want to bring an agent down, the load can be diverted to the other available agent:

However, its not simple that you simply bring an agent down and the Master would divert the load other agents. The sessions that are running need to be completed. The available sessions parameter of the agent to be brought down should be set to 0. So, that no new connections are accepted. Once the existing job is complete, the "Use new Load Balancing" parameter - if set to Yes will rebalance the load and start using the other agents for subsequent steps.

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