Skip to main content

Parallelizing work

This guide shows how to use the Restate to execute a list of tasks in parallel and then gather their result, also known as fan-out, fan-in.

Fan out: You can fan out tasks with Restate by creating a handler that processes a single subtask, and then scheduling it repeatedly from another handler. Restate guarantees and manages the execution of all the tasks across failures.

Fan in: You can fan in the results of the subtasks by using Restate's Promise Combinators to wait for all promises to resolve.

Example

The example implements a worker service:

  1. It splits a task into subtasks.
  2. It schedules all the subtasks. Each subtask results in a promise that gets added to a list.
  3. The result is gathered by waiting for all promises to resolve.

You can run this on FaaS infrastructure, like AWS Lambda, and it will scale automatically. The run handler will then suspend while it waits for all subtasks to finish. Restate will then resume the handler when all subtasks are done.

const fanOutWorker = restate.service({

    name: "worker",

    handlers: {

        run: async (ctx: Context, task: Task) => {

            // Split the task in subtasks

            const subtasks: SubTask[] = await ctx.run("split task", () =>

                split(task)

            );



            // Fan out the subtasks - run them in parallel

            const resultPromises = [];

            for (const subtask of subtasks) {

                const subResultPromise = ctx

                    .serviceClient(fanOutWorker)

                    .runSubtask(subtask);

                resultPromises.push(subResultPromise);

            }



            // Fan in - Aggregate the results

            const results = await CombineablePromise.all(resultPromises);

            return aggregate(results);

        },



        // Can also run on FaaS

        runSubtask: async (ctx: Context, subtask: SubTask) => {

            // Processing logic goes here ...

            // Can be moved to a separate service to scale independently

        },

    },

});



export const handler = restate.endpoint().bind(fanOutWorker).handler();

Running the example

1
Download the example

restate example typescript-patterns-use-cases && cd typescript-patterns-use-cases

2
Start the Restate Server

restate-server

3
Start the Service

npx tsx watch ./src/parallelizework/fan_out_worker.ts

4
Register the services

restate deployments register localhost:9080

5
Send a request

curl localhost:8080/worker/run \

    --json '{"description": "get out of bed,shower,make coffee,have breakfast"}'

4
Check the service logs

See how all tasks get spawned in parallel, finish at different times, and then get aggregated.

[restate] [worker/runSubtask][inv_17jBqoqRG0TN3msVqHEpZn2aQMOX5kSKrf][2025-01-17T08:51:44.993Z] INFO:  Started executing subtask: get out of bed

[restate] [worker/runSubtask][inv_1f8R1NuF0LF27EdQ0R6s7PR8hld245OM8h][2025-01-17T08:51:44.995Z] INFO:  Started executing subtask: shower

[restate] [worker/runSubtask][inv_101oPhGwxQqZ0sQebkQnpGyV9Rp3oj9CSJ][2025-01-17T08:51:44.997Z] INFO:  Started executing subtask: make coffee

[restate] [worker/runSubtask][inv_1eKDShaxMCEB6DXasrR5OtRXJEvA2je33X][2025-01-17T08:51:44.998Z] INFO:  Started executing subtask: have breakfast

[restate] [worker/runSubtask][inv_17jBqoqRG0TN3msVqHEpZn2aQMOX5kSKrf][2025-01-17T08:51:47.003Z] INFO:  Execution subtask finished: get out of bed

[restate] [worker/runSubtask][inv_101oPhGwxQqZ0sQebkQnpGyV9Rp3oj9CSJ][2025-01-17T08:51:48.007Z] INFO:  Execution subtask finished: make coffee

[restate] [worker/runSubtask][inv_1f8R1NuF0LF27EdQ0R6s7PR8hld245OM8h][2025-01-17T08:51:48.999Z] INFO:  Execution subtask finished: shower

[restate] [worker/runSubtask][inv_1eKDShaxMCEB6DXasrR5OtRXJEvA2je33X][2025-01-17T08:51:49.001Z] INFO:  Execution subtask finished: have breakfast

[restate] [worker/run][inv_18QHSeAYfvim1oNXRl9I5105veQcTW3BEl][2025-01-17T08:51:49.007Z] INFO:  Aggregated result: get out of bed: DONE,shower: DONE,make coffee: DONE,have breakfast: DONE