RPC for Service Workers -- move that heavy computation off of your UI thread!

Features

• Fully typesafe
• Cancelable requests
• Parallelization with multiple worker instances
• Automatic transfer of transferable values from- and to- worker code
• A way to polyfill a pre-filled localStorage to be accessed within the worker code
• Supports Service workers, Shared workers and Dedicated workers

Installation

npm add swarpc arktype

If you want to use the latest commit instead of a published version, you can, either by using the Git URL:

npm add git+https://github.com/gwennlbh/swarpc.git

Or by straight up cloning the repository and pointing to the local directory (very useful to hack on sw&rpc while testing out your changes on a more substantial project):

mkdir -p vendored
git clone https://github.com/gwennlbh/swarpc.git vendored/swarpc
npm add file:vendored/swarpc

This works thanks to the fact that dist/ is published on the repository (and kept up to date with a CI workflow).

Usage

1. Declare your procedures in a shared file

import type { ProceduresMap } from "swarpc";
import { type } from "arktype";

export const procedures = {
  searchIMDb: {
    // Input for the procedure
    input: type({ query: "string", "pageSize?": "number" }),
    // Function to be called whenever you can update progress while the procedure is running -- long computations are a first-class concern here. Examples include using the fetch-progress NPM package.
    progress: type({ transferred: "number", total: "number" }),
    // Output of a successful procedure call
    success: type({
      id: "string",
      primary_title: "string",
      genres: "string[]",
    }).array(),
  },
} as const satisfies ProceduresMap;

2. Register your procedures in the service worker

In your service worker file:

import fetchProgress from "fetch-progress"
import { Server } from "swarpc"
import { procedures } from "./procedures.js"

// 1. Give yourself a server instance
const swarpc = Server(procedures)

// 2. Implement your procedures
swarpc.searchIMDb(async ({ query, pageSize = 10 }, onProgress) => {
  const queryParams = new URLSearchParams({
    page_size: pageSize.toString(),
    query,
  })

  return fetch(`https://rest.imdbapi.dev/v2/search/titles?${queryParams}`)
    .then(fetchProgress({ onProgress }))
    .then((response) => response.json())
    .then(({ titles } => titles)
})

// ...

// 3. Start the event listener
swarpc.start(self)

3. Call your procedures from the client

Here's a Svelte example!

<script>
    import { Client } from "swarpc"
    import { procedures } from "./procedures.js"

    const swarpc = Client(procedures)

    let query = $state("")
    let results = $state([])
    let progress = $state(0)
</script>

<search>
    <input type="text" bind:value={query} placeholder="Search IMDb" />
    <button onclick={async () => {
        results = await swarpc.searchIMDb({ query }, (p) => {
            progress = p.transferred / p.total
        })
    }}>
        Search
    </button>
</search>

{#if progress > 0 && progress < 1}
    <progress value={progress} max="1" />
{/if}

<ul>
    {#each results as { id, primary_title, genres } (id)}
        <li>{primary_title} - {genres.join(", ")}</li>
    {/each}
</ul>

Configure parallelism

By default, when a worker is passed to the Client's options, the client will automatically spin up navigator.hardwareConcurrency worker instances and distribute requests among them. You can customize this behavior by setting the Client:options.nodes option to control the number of nodes (worker instances).

When Client:options.worker is not set, the client will use the Service worker (and thus only a single instance).

Send to multiple nodes

Use Client#(method name).broadcast to send the same request to all nodes at once. This method returns a Promise that resolves to an array of PromiseSettledResult (with an additional property, node, the ID of the node the request was sent to), one per node the request was sent to.

For example:

const client = Client(procedures, {
  worker: "./worker.js",
  nodes: 4,
});

for (const result of await client.initDB.broadcast("localhost:5432")) {
  if (result.status === "rejected") {
    console.error(
      `Could not initialize database on node ${result.node}`,
      result.reason,
    );
  }
}

Make cancelable requests

Implementation

To make your procedures meaningfully cancelable, you have to make use of the AbortSignal API. This is passed as a third argument when implementing your procedures:

server.searchIMDb(async ({ query }, onProgress, abort) => {
  // If you're doing heavy computation without fetch:
  let aborted = false
  abort?.addEventListener("abort", () => {
    aborted = true
  })

  // Use `aborted` to check if the request was canceled within your hot loop
  for (...) {
    /* here */ if (aborted) return
    ...
  }

  // When using fetch:
  await fetch(..., { signal: abort })
})

Call sites

Instead of calling await client.myProcedure() directly, call client.myProcedure.cancelable(). You'll get back an object with

• async cancel(reason): a function to cancel the request
• request: a Promise that resolves to the result of the procedure call. await it to wait for the request to finish.

Example:

// Normal call:
const result = await swarpc.searchIMDb({ query });

// Cancelable call:
const { request, cancel } = swarpc.searchIMDb.cancelable({ query });
setTimeout(() => cancel().then(() => console.warn("Took too long!!")), 5_000);
await request;

Polyfill a localStorage for the Server to access

You might call third-party code that accesses on localStorage from within your procedures.

Some workers don't have access to the browser's localStorage, so you'll get an error.

You can work around this by specifying to swarpc localStorage items to define on the Server, and it'll create a polyfilled localStorage with your data.

An example use case is using Paraglide, a i18n library, with the localStorage strategy:

// In the client
import { getLocale } from "./paraglide/runtime.js";

const swarpc = Client(procedures, {
  localStorage: {
    PARAGLIDE_LOCALE: getLocale(),
  },
});

await swarpc.myProcedure(1, 0);

// In the server
import { m } from "./paraglide/runtime.js";
const swarpc = Server(procedures);

swarpc.myProcedure(async (a, b) => {
  if (b === 0) throw new Error(m.cannot_divide_by_zero());
  return a / b;
});