Parallel Execution

Use .parallel() to run multiple handlers concurrently and merge their results into state.

flow.parallel('fetchAll', 'deep',
  async (ctx) => ({ users: await fetchUsers() }),
  async (ctx) => ({ posts: await fetchPosts() }),
);
// ctx.state now has both `users` and `posts`

Merge strategies

The second argument to .parallel() determines how results from concurrent handlers are merged.

Strategy	Behavior
'shallow'	Object.assign() — later results override earlier ones
'error-on-conflict'	Throws if any keys overlap between results
'deep'	Recursive merge; arrays are concatenated

'shallow'

Simple key-level merge. If two handlers return the same key, the later one wins.

'error-on-conflict'

Throws an error if two handlers return overlapping keys. Use this when you expect all handlers to produce distinct state.

'deep'

Recursively merges nested objects and concatenates arrays. Useful when handlers return data with shared structure.

Type safety

TypeScript infers the combined return type from all parallel handlers. The state after a .parallel() step includes all keys from all handlers.

Error handling

If any parallel handler throws, all results are discarded and the error propagates. There is no partial-success mode — either all handlers succeed or the step fails.

Parallel steps support .withRetry() — the entire parallel group is retried as a unit.