Process a list in parallel

Transform a batch of tickets with a typed target, bounded concurrency and ordered results. This first walkthrough uses neither a model nor a handler: verify composition before adding external effects.

1. Write the program

Create batch.tb:

batch.tb

module examples.parallel_batch

strategy Label
    takes ticket as Text
    gives Text
    do
        give "Ticket: " ++ ticket

strategy Run
    takes tickets as List of Text
    gives List of Text
    uses Label
    do
        parallel Label
            each ticket in tickets
            with ticket as ticket
            max_concurrency 2
            max_items 8
            into labels
        give labels

Run orchestrates; Label produces a value for each input. The group admits at most eight tickets and executes at most two branches simultaneously.

2. Execute

touba compile batch.tb --out batch.air.json
touba run batch.air.json \
  --entry strategy:examples.parallel_batch/Run \
  --inputs-json '{"tickets":["T-3","T-1","T-2"]}' \
  --output result

Expected result: ["Ticket: T-3","Ticket: T-1","Ticket: T-2"]. It follows input order even when branches finish in a different order. This small pure transformation does not measure a performance improvement.

3. Test bounds

• With {"tickets":[]}, the result is [].
• With nine tickets, the run refuses with ParallelItemLimitExceeded before the group's first call.
• A non-text value in the list causes an input refusal.

The business testing guide provides an executable test file for this program, with success and refusal assertions.

4. Keep a recovery point

touba run batch.air.json \
  --entry strategy:examples.parallel_batch/Run \
  --inputs-json '{"tickets":["T-3","T-1"]}' \
  --parallel-checkpoint-out progress.json \
  --output result
touba run batch.air.json \
  --entry strategy:examples.parallel_batch/Run \
  --parallel-resume-checkpoint progress.json \
  --output result

The second command restores completed results without calling the targets again. Treat this checkpoint as private data: it may contain business arguments and results. Do not alter its identity to force resumption of an incompatible program.

Move to agents or tools

The target can be a skill, agent or tool with an explicit output signature. Add its actual authorizations and dependencies; parallelism does not supply them. The multi-agent guide demonstrates composition, and the parallel guide explains budgets, errors, shared memory and human pauses.