Use Case: Sequential Processing of Dependent Steps

Goal

Compose a batch operation from a sequence of dependent steps. Define and implement the operation only once, and allow restart after failure without having to change configuration, and without having to repeat steps that were successful.

A sub-goal is to allow the progress of a batch through the steps to be traced accurately for reporting and auditing purposes. This requires the steps to be uniquely identified.

Scope

  • Simple linear sequence of steps. Slightly more complicated requirements can be handled by putting independent steps in a sequence (no need for splits and joins).

Preconditions

  • A non-trivial sequence is defined:
    • more than one step:
    • the effects of each step can be measured.
  • The sequence can be interrupted or artificially terminated in the second or subsequent step.

Success

  • A non-trivial sequence executes successfully. The progress and success of each step can be verified by the tester.
  • The same sequence is forced to fail on second step in such a way that the first step result is not suspected of being in error, e.g. by interrupting it. When it is restarted the first step is not repeated, and the sequence is successful.
  • The same sequence is forced to fail on second step in such a way that the first step result is obviously in error, even though it completed normally. When the batch is restarted the first step is repeated, and the sequence is successful.

Description

The vanilla successful case proceeds as follows:

  1. Framework logs the start of a step, uniquely indentifying the initial conditions.
  2. Framework stores internal state so that initial conditions can be re-created in the event of a restart.
  3. Step execution proceeds as per one of the other use cases (e.g. Copy File to Database), including transactional behaviour.
  4. Client instructs Framework to store internal state needed by further steps (e.g. cached reference data).
  5. Framework logs successful completion of step, and stores
  6. Repeat for next and subsequent steps. Internal state is passed from one state to the next.

Variations

Internal Failure of Step

If a step fails internally, e.g. because of resource becoming temporarily unavailable, the sequence can be restarted without repeating the previous steps.

  1. Operator fixes resource problem (e.g. starts web service).
  2. Operator restarts batch with no configuration or input data changes.
  3. Framework resumes batch from the last commit point of the failed step.
  4. Sequence completes normally.

The process above could be carried out by the framework entirely (no need for operator intervention) if a retry policy is in effect.

Failure of Step Owing to Bad Initial State

If a step fails because it receives bad data from an earlier step, the Framework cannot recover without intervention.

  1. Operator attempts to restart without doing anything to fix the problem.
  2. Framework detects bad initial state immediately and fails fast.

If the original problem can be located and fixed (e.g. input data for earlier step is revised):

  1. Operator restarts batch signalling to framework which step to begin with.
  2. Framework locates initial state for the first step to be executed.
  3. Framework starts execution from the beginning of the desired state. This time the input data are different, so the sequence can complete normally.

Implementation

  • The need to save state for subsequent steps leads to the introduction of a batch context concept. And the need for initialising restarts leads to the context being serializable, either natively or by some pluggable strategy (this is covered in the Restart after Failure use case).

    Unfortunately, the need for parallel processing and automatic restart also makes it practically impossible for steps to handle the context at the level of a single thread of execution, where the client needs to implement business logic. If a step is executing in parallel, then each node needs to be able to restart independently, but the context needs to be a single object that can be passed on to the next step (unless all the steps are parallelised with the same multiplicity, which might not be efficient in general).

    Thus batch context must be defined and managed by the template or execution handler.

  • The requirement for steps might have implications for the implementer of the batch operation (the client). Obviously a client defines the sequence of steps according to the business requirement, but ideally we would like him to be unaware of the reporting and restart infrastructure. Maybe an array of callbacks works (the callback interface is irrelevant, except that it accepts a context object as an argument):
    batchTemplate.iterate(new RepeatCallback[] { 

    new RepeatCallback() {
        public boolean doInIteration(RepeatContext context) {
            // do stuff for step one
        };
    },

    new RepeatCallback() {
        public boolean doInIteration(RepeatContext context) {
            // do stuff for step two - the context
            // is the same...
        };
    }

});

    Notice that there is no need for the context to be set explicitly before executing the callback. The context is handled internally to the batch template using an analogue of the TransactionSynchronizationManager.

  • If we prefer that clients never need to know about batch templates, then the code above needs to be automated. This would be where an additional domain layer might come into play (c.f. Step).