Run background processes
- Prerequisites
- Run a process in the background

Run background processes

Last updated Oct 23, 2026 Start a process that keeps running after the request ends, such as a build, a test suite, or an agent task. The process object that exec() returns belongs to the request that started it, so later requests manage the process through files in the sandbox. The process writes its process ID, output, and exit code to those files.

For a process that runs until the container stops, such as a development server or a tunnel, refer to Run a server in the background.

Prerequisites

A Worker with a Durable Object that starts a container with the Durable Object scheduling policy. To create one, refer to Run a Linux command.

Run a process in the background

Add shell scripts that run a process and report its state:

src/index.jsjs

```js const ROOT = "/var/lib/processes"; const INACTIVITY_TIMEOUT_MS = 10 * 60 * 1000;

// Runs the command in its own process group. Records its process ID when // it starts and its exit code when it ends. const RUN = dir=$1; shift setsid sh -c 'echo "$$ $(cat /proc/sys/kernel/random/boot_id)" >"$0/pid"; exec "$@"' \\ "$dir" "$@" >"$dir/stdout.log" 2>"$dir/stderr.log" echo "$?" >"$dir/exit-code.tmp" && mv "$dir/exit-code.tmp" "$dir/exit-code";

// Defines current(), which reads the process ID in a directory into $pid // when the process started in this instance. const CURRENT = current() { read -r pid boot 2>/dev/null <"$1/pid" && [ "$boot" = "$(cat /proc/sys/kernel/random/boot_id)" ] };

// Prints the state of the process that owns the directory. const STATUS = ${CURRENT} dir=$1 if [ ! -d "$dir" ]; then echo missing elif [ -e "$dir/exit-code" ]; then echo "exited $(cat "$dir/exit-code")" elif [ ! -e "$dir/pid" ]; then echo starting elif current "$dir" && kill -0 "$pid" 2>/dev/null; then echo "running $pid" elif [ -e "$dir/exit-code" ]; then echo "exited $(cat "$dir/exit-code")" else echo lost fi;

async function run(container, argv) { const process = await container.exec(argv); const output = await process.output();

return { exitCode: output.exitCode, stdout: new TextDecoder().decode(output.stdout), }; } ```

src/index.tsts

```ts const ROOT = "/var/lib/processes"; const INACTIVITY_TIMEOUT_MS = 10 * 60 * 1000;

type Status = | { state: "starting" | "lost" } | { state: "running"; pid: number } | { state: "exited"; exitCode: number };

async function run(container: Container, argv: string[]) { const process = await container.exec(argv); const output = await process.output();

return { exitCode: output.exitCode, stdout: new TextDecoder().decode(output.stdout), }; } ```

setsid starts the command in a new process group. Stopping the group also stops the processes the command starts, such as the node process behind npm run dev, unless a process moves to its own group. The script writes the exit code to a temporary file and then renames it, so a reader never sees a partial file. Do not start the command with & instead: a shell starts background jobs with SIGINT ignored.

Each process keeps its files in its own directory under /var/lib/processes on the instance disk. The logs grow until the process ends. When the disk fills, writes from the process fail, so delete old process directories, or send output that you want to keep to a mounted bucket. 2. Add a constructor to your Durable Object. It sets the inactivity timeout again when a restarted Durable Object finds the container running:

src/index.tsts

ts export class MyContainer extends DurableObject<Env> { constructor(ctx: DurableObjectState, env: Env) { super(ctx, env); const container = ctx.container; if (container?.running) { void ctx.blockConcurrencyWhile(() => container.setInactivityTimeout(INACTIVITY_TIMEOUT_MS), ); } } }

Then add a method that starts a process:

src/index.tsts

```ts export class MyContainer extends DurableObject { // ...

async startProcess(id: string, argv: string[]): Promise { const container = this.ctx.container;

if (!container) { throw new Error("No container is configured"); }

if (!container.running) { container.start({ image: "cloudflare/debian-trixie", entrypoint: ["sleep", "infinity"], enableInternet: false, }); await container.setInactivityTimeout(INACTIVITY_TIMEOUT_MS); }

// mkdir fails when another process already uses the ID. const dir = ${ROOT}/${id}; await run(container, ["mkdir", "-p", ROOT]); const created = await run(container, ["mkdir", dir]);

if (created.exitCode !== 0) { return false; }

// Ignoring the output lets the process outlive this request. await container.exec(["sh", "-c", RUN, "sh", dir, ...argv], { stdout: "ignore", stderr: "ignore", });

// exec() returns before the process records its ID. Wait for it, so // a request that follows the output right away finds it running. for (let i = 0; i < 50; i++) { if ((await this.status(id))?.state !== "starting") break; await scheduler.wait(100); }

await this.ctx.storage.put(process:${id}, argv); await this.ctx.storage.setAlarm(Date.now() + 60_000); return true; } } ```

Add a method that reports the state of a process:

src/index.tsts

```ts export class MyContainer extends DurableObject { // ...

async status(id: string): Promise { const container = this.ctx.container;

if (!container?.running) { return undefined; }

const { stdout } = await run(container, [ "sh", "-c", STATUS, "sh", ${ROOT}/${id}, ]); const [state, value] = stdout.trim().split(" ");

if (state === "running") { return { state, pid: Number(value) }; }

if (state === "exited") { return { state, exitCode: Number(value) }; }

if (state === "starting" || state === "lost") { return { state }; }

return undefined; } } ```

kill -0 checks that the process exists without sending it a signal. A process is lost when it ended without recording an exit code, for example because something sent SIGKILL to the script that records it.

The process ID file also records the boot ID, which changes in every instance. A snapshot restores the file, and a new instance reuses the same process IDs, so a restored process ID can belong to an unrelated process. current() ignores a process ID from another instance, so a process that was running when the snapshot was taken is lost. 4. Add a method that returns the output of a process so far:

src/index.tsts

```ts export class MyContainer extends DurableObject { // ...

async logs(id: string, stream: "stdout" | "stderr"): Promise { if (!(await this.status(id))) { return undefined; }

const path = ${ROOT}/${id}/${stream}.log; return (await run(this.ctx.container!, ["cat", path])).stdout; } } ```

To stream the output while the process runs, or to wait until it prints a line, refer to Follow a background process. 5. Add a method that stops a process:

src/index.tsts

```ts export class MyContainer extends DurableObject { // ...

async stop(id: string): Promise { const status = await this.status(id);

if (status?.state !== "running") { return false; }

// kill is a shell built-in. A negative ID signals the process group. const { exitCode } = await run(this.ctx.container!, [ "sh", "-c", 'kill -s TERM -- "$1"', "sh", -${status.pid}, ]); return exitCode === 0; } } ```

A process stopped by a signal exits with 128 plus the signal number, such as 143 for SIGTERM. 6. Add an alarm handler that keeps the sandbox running while any process runs: