# Execution backends

Compute-intensive applications can be executed in parallel, either locally or on backends
like the HTCondor system.
The backend is selected with `--backend` and configured with `--backend_config`.

Available backends:

- `local` (default) uses `--max_workers` processes on the controller host.
- `htcondor` submits one scheduler process per job; `max_workers` is not a queue limit.


## HTCondor installation and runtime

Install the optional Python bindings on the submission host:

```bash
pip install "gammasimtools[htcondor]"
```

HTCondor uses a shared filesystem: payloads, outputs, logs, containers, and environment files
must be visible at the same absolute paths on the submit and execute hosts. Execute hosts need a
compatible Python environment and the dependencies required by the job or configured container.

## Backend configuration

Pass a YAML mapping with `--backend_config`:

```yaml
request_cpus: 1
priority: 0
request_memory: 4GB
request_disk: 10GB
container_image: /shared/containers/simtools.sif
# Keep the HTCondor scratch mount away from the image's /workdir environment.
container_target_dir: /simtools-run
environment_file: /shared/config/simtools.env
log_path: scheduler.log
poll_interval: 60
timeout: null
cancel_on_interrupt: false
keep_successful_artifacts: false
extra_submit_attributes: {}
```

For container jobs, the backend runs `python` inside the image by default. Set
`python_executable` only when the image uses a different command, such as `python3` or
`/opt/conda/bin/python`.

`poll_interval` is used only when an application waits for submitted jobs, for example with
`simtools-simulate-prod --wait`.