TDIing out loud

Ramblings on the paradigm-shift that is TDI.

Monday, October 29, 2007

Why throw multiple ALs at a single problem?

I get this one occasionally and it's a very valid question from a simplicity standpoint (a TDI mantra). However, there are good reasons for dividing a workload up among multiple AssemblyLines, and better performance is one of them -- as long as the task at hand allows for multi-threading; For example, data movement/migration/sync where the ordering of updates is not significant as long as all are processed. Here is a whitepaper that outlines a number of patterns that lend themselves to multi-AL solutions:

Performance Best Practices Paper for TDI 6.1 and 6.1.1

Another reason is to improve availability. Having several simultaneous worker ALs as mentioned above also means no single point of failure. Imagine a scenario where one or more AssemblyLines (on one or more TDI Servers) read source data and write to MQ. At the other end of this secure, persistent data pipe is another set of ALs picking up these messages on a first-come-first-serve basis. If an AssemblyLine fails then the integration service is degraded; not dead.

One oft-used multi-AL technique is to have a secondary "launcher" AL with a "while(true)" loop that starts the primary long-running AssemblyLine, waiting for it to complete - which it should never do (in theory). If for some reason the mission-critical AL stops then postmortem failure handling, including logs/alerts can be made by the calling AL; Finally, the AL can be re-started again in the next cycle of the loop.

I have also seen complex logic divided up into smaller, simpler ALs for reasons of simplicity and maintenance. The developer specs out each step in their integration flows - along with constraints, schema and invariants - and them implements these as individual ALs. This allows for incremental upgrades to a solution as well as unit testing.

Finally, entire AssemblyLines can be exposed as components (called "Adapters") facilitating sharing and reuse. You can find more on this topic here:

Introducing Adapters with Tivoli Directory Integrator 6.1

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