> ## Documentation Index > Fetch the complete documentation index at: https://developer.upsun.com/llms.txt > Use this file to discover all available pages before exploring further. # Workers > Defines the list of worker names, which are alternate copies of the application to run as background processes. Optional in [single-runtime](https://developer.upsun.com/docs/configure-apps/app-reference/single-runtime-image.md#primary-application-properties) and [composable](https://developer.upsun.com/docs/configure-apps/app-reference/composable-image.md#primary-application-properties) images. Workers require resource definition using `upsun resources:set`, same as application containers. For more information, see how to [manage resources](https://developer.upsun.com/docs/manage-resources.md). Workers are exact copies of the code and compilation output as a `web` instance after a [`build` hook](https://developer.upsun.com/docs/configure-apps/image-properties/hooks). They use the same container image. Workers can't accept public requests and so are suitable only for background tasks. If they exit, they're automatically restarted. > [tab] Single-runtime image > > The keys of the `workers` definition are the names of the workers. > You can then define how each worker differs from the `web` instance using > the [primary application properties](https://developer.upsun.com/docs/configure-apps/app-reference/single-runtime-image.md#primary-application-properties). > > Each worker can differ from the `web` instance in all properties *except* for: > > * `build` and `dependencies` properties, which must be the same > * `crons` as cron jobs don't run on workers > * `hooks` as the `build` hook must be the same > and the `deploy` and `post_deploy` hooks don't run on workers. > > A worker named `queue` that was small and had a different start command could look like this: > > applications: > \: > type: 'python:\{\{version:python:latest}}' > source: > root: "/" > workers: > queue: > commands: > start: | > ./worker.sh > [tab] Composable image > > The keys of the `workers` definition are the names of the workers. > You can then define how each worker differs from the `web` instance using > the [primary application properties](https://developer.upsun.com/docs/configure-apps/app-reference/composable-image.md#primary-application-properties). > > Each worker can differ from the `web` instance in all properties *except* for: > > * `crons` as cron jobs don't run on workers > * `hooks` as the `build` hook must be the same > and the `deploy` and `post_deploy` hooks don't run on workers. > > A worker named `queue` that was small and had a different start command could look like this: > > applications: > \: > type: "composable:\{\{version:composable:latest}}" > source: > root: "/" > stack: > runtimes: \[ "python@\{\{version:python:latest}}" ] > workers: > queue: > commands: > start: | > ./worker.sh This documentation is built and hosted on [Mintlify](https://mintlify.com), a developer documentation platform.