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# SDK Harness Configuration

Beam allows configuration of the [SDK harness]({{ site.baseurl }}/roadmap/portability/) to
accommodate varying cluster setups.
(The options below are for Python, but much of this information should apply to the Java and Go SDKs
as well.)

- `environment_type` determines where user code will be executed.
  `environment_config` configures the environment depending on the value of `environment_type`.
  - `DOCKER` (default): User code is executed within a container started on each worker node.
    This requires docker to be installed on worker nodes.
    - `environment_config`: URL for the Docker container image. Official Docker images
    are available [here](https://hub.docker.com/u/apachebeam) and are used by default.
    Alternatively, you can build your own image by following the instructions
    [here]({{ site.baseurl }}/documentation/runtime/environments/).
  - `PROCESS`: User code is executed by processes that are automatically started by the runner on
    each worker node.
    - `environment_config`: JSON of the form `{"os": "<OS>", "arch": "<ARCHITECTURE>",
    "command": "<process to execute>", "env":{"<Environment variables 1>": "<ENV_VAL>"} }`. All
    fields in the JSON are optional except `command`.
      - For `command`, it is recommended to use the bootloader executable, which can be built from
        source with `./gradlew :sdks:python:container:build` and copied from
        `sdks/python/container/build/target/launcher/linux_amd64/boot` to worker machines.
        Note that the Python bootloader assumes Python and the `apache_beam` module are installed
        on each worker machine.
  - `EXTERNAL`: User code will be dispatched to an external service. For example, one can start
    an external service for Python workers by running
    `docker run -p=50000:50000 apachebeam/python3.6_sdk --worker_pool`.
    - `environment_config`: Address for the external service, e.g. `localhost:50000`.
    - To access a Dockerized worker pool service from a Mac or Windows client, set the
      `BEAM_WORKER_POOL_IN_DOCKER_VM` environment variable on the client:
      `export BEAM_WORKER_POOL_IN_DOCKER_VM=1`.
  - `LOOPBACK`: User code is executed within the same process that submitted the pipeline. This
    option is useful for local testing. However, it is not suitable for a production environment,
    as it performs work on the machine the job originated from.
    - `environment_config` is not used for the `LOOPBACK` environment.
- `sdk_worker_parallelism` sets the number of SDK workers that will run on each worker node.