Skip to main content
Workers are lightweight polling services that retrieve scheduled runs from a work pool and execute them. Workers each have a type corresponding to the execution environment to submit flow runs to. Workers can only poll work pools that match their type. As a result, when deployments are assigned to a work pool, you know in which execution environment scheduled flow runs for that deployment will run. The following diagram summarizes the architecture of a worker-based work pool deployment: The worker is in charge of provisioning the flow run infrastructure.

​
Worker types

Below is a list of available worker types. Most worker types require installation of an additional package. If you don’t see a worker type that meets your needs, consider developing a new worker type.

​
Worker options

Workers poll for work from one or more queues within a work pool. If the worker references a work queue that doesn’t exist, it is created automatically. The worker CLI infers the worker type from the work pool. Alternatively, you can specify the worker type explicitly. If you supply the worker type to the worker CLI, a work pool is created automatically if it doesn’t exist (using default job settings). Configuration parameters you can specify when starting a worker include: You must start a worker within an environment to access or create the required infrastructure to execute flow runs. The worker will deploy flow runs to the infrastructure corresponding to the worker type. For example, if you start a worker with type kubernetes, the worker deploys flow runs to a Kubernetes cluster. Prefect must be installed in any environment (for example, virtual environment, Docker container) where you intend to run the worker or execute a flow run.
PREFECT_API_URL and PREFECT_API_KEYsettings for workersPREFECT_API_URL must be set for the environment where your worker is running. When using Prefect Cloud, you must also have a user or service account with the Worker role, which you can configure by setting the PREFECT_API_KEY.

​
Worker status

Workers have two statuses: ONLINE and OFFLINE. A worker is online if it sends regular heartbeat messages to the Prefect API. If a worker misses three heartbeats, it is considered offline. By default, a worker is considered offline a maximum of 90 seconds after it stopped sending heartbeats, but you can configure the threshold with the PREFECT_WORKER_HEARTBEAT_SECONDS setting.

​
Worker logs

Workers send logs to the Prefect Cloud API if you’re connected to Prefect Cloud.
  • All worker logs are automatically sent to the Prefect Cloud API
  • Logs are accessible through both the Prefect Cloud UI and API
  • Each flow run will include a link to its associated worker’s logs

​
Worker details

The Worker Details page shows you three key areas of information:
  • Worker status
  • Installed Prefect version
  • Installed Prefect integrations (e.g., prefect-aws, prefect-gcp)
  • Live worker logs (if worker logging is enabled)
Access a worker’s details by clicking on the worker’s name in the Work Pool list.

​
Start a worker

Use the prefect worker start CLI command to start a worker. You must pass at least the work pool name. If the work pool does not exist, it will be created if the --type flag is used.
For example:
Results in output like this:
In this case, Prefect automatically discovered the worker type from the work pool. To create a work pool and start a worker in one command, use the --type flag:
In addition, workers can limit the number of flow runs to start simultaneously with the --limit flag. For example, to limit a worker to five concurrent flow runs:

​
Configure prefetch

By default, the worker submits flow runs 10 seconds before they are scheduled to run. This allows time for the infrastructure to be created so the flow run can start on time. In some cases, infrastructure takes longer than 10 seconds to start the flow run. You can increase the prefetch time with the --prefetch-seconds option or the PREFECT_WORKER_PREFETCH_SECONDS setting. If this value is more than the amount of time it takes for the infrastructure to start, the flow run will wait until its scheduled start time.

​
Polling for work

Workers poll for work every 15 seconds by default. You can configure this interval in your profile settings with the PREFECT_WORKER_QUERY_SECONDS setting.

​
Install policy

The Prefect CLI can install the required package for Prefect-maintained worker types automatically. Configure this behavior with the --install-policy option. The following are valid install policies:
Installing packages at runtime with always or if-not-present policies can lead to nondeterministic behavior in production environments. When integration libraries are installed without pinned versions, minor version updates or changes to upstream dependencies may introduce breaking changes that cause unexpected failures.For production deployments, consider using prebuilt Docker images with pre-installed integration libraries instead of installing packages at runtime. You can pin specific versions by using image tags (e.g., prefecthq/prefect-aws:0.7.5-python3.12-prefect3.6.20) for a more stable and reproducible experience.

​
Further reading