Apex Chainable

Apex Batches can be chained by calling the successor batch from the finish() method of the previous batch. But such hardcoding makes this model inflexible. It's hard to build the chain from outside, neighter from a central class nor on runtime dependant on business logic.

The same applies when the execute() method of Schedulable or Queueable classes call other classes.

With Chainable

The Chainable wrapper class of this repository overcomes those drawbacks.

• No need to hardcode successor batch in finish() method
• Created batch chains of arbitrary length without changing existing Batch classes
• Support Batchable, Queueable and Schedulable classes as chain members
• Allows sharing and passing of variables between chain members

      new FirstBatch().setShared('result', new Money(0))
            .then(AnotherBatch())
            .then(QueueableJob())
            .then(ScheduledJob())
            ...
            .execute();

Without Chainable

class FirstBatch implements Batchable<SObject> {
    Iterator<SObject> start(BatchableContext ctx) { ... }

    void execute(BatchableContext ctx, List<Account> scope) { ... }

    void finish(BatchableContext ctx) {
        Database.enqueueBatch(new SecondBatch()); 
    }
}

class AnotherBatch implements Batchable<SObject> {
    Iterator<SObject> start(BatchableContext ctx) { ... }

    void execute(BatchableContext ctx, List<Account> scope) { ... }

    void finish(BatchableContext ctx) {
        System.schedule('name', cron, new ScheduledJob()); 
    }
}

Deferring

When all the "links" in the chain cannot be completely identified beforehand in order to assemble them in a single chain and trigger its execution, the chain execution can be deferred until the end of the transaction. All chainable processes that have been deferred will be automatically chained together in a single chain and executed sequentually.

// automation 1
new FirstBatch()
        .then(AnotherBatch())
        .setShared('result', new Money(0)) // shared variables will be available across other following deferred chainables
        .executeDeferred();

// automation 2
new QueueableJob()
        .then(ScheduledJob())
        ...
        .executeDeferred();

// the framework would internally build the chain in a separate transaction with a definition like this
new FirstBatch()
        .then(AnotherBatch())
        .setShared('result', new Money(0))
        .then(QueueableJob())
        .then(ScheduledJob())
        .execute();

Considerations

In order to leverage the deferring of the Chainable instances there are some nuances compared to its direct execution.

• Override the getDeferArgs and setDeferredArgs if the class receives external input before its execution via constructor parameters or setters to serialize and deserialize them (See the SampleDeferArgQueueable).
• Must have a no-arg constructor (if no explicit constructor exists, the default implicit one will be used)
• Must not be an inner class (due to difficulty on dynamic name inferring)