Backend System¶

Introduction¶

The backend system in oslo.service provides a modular architecture that allows you to choose between different concurrency models without modifying your service logic.

Currently, two backends are implemented and supported:

Eventlet backend: The traditional concurrency backend based on green threads and cooperative multitasking (greenlet library).
Threading backend: A new backend using native Python threads, Cotyledon, and Futurist, designed to be compatible with modern Python versions, simplify debugging, and improve maintainability.

Why a Backend System?¶

This system was introduced to enable a gradual transition away from the Eventlet backend, which has known limitations for scaling, cooperative multitasking complexity, and debugging in some Python environments. By using the backend system, you can test, adopt, or migrate to the Threading backend while keeping the same public API and service structure.

In the future, the Eventlet backend may be deprecated once the Threading backend covers all use cases. Some related features tied to Eventlet, like the embedded WSGI server and the backdoor shell, are also planned for removal.

For more context, see:

How It Works¶

The backend system exposes the same public components (ServiceLauncher, ProcessLauncher, ThreadGroup, LoopingCall, etc.) through a dynamic selection mechanism.

At runtime, you select the backend you want to use by calling the init_backend() function and providing the desired BackendType:

from oslo_service.backend import init_backend, BackendType

# Use the Threading backend
init_backend(BackendType.THREADING)

If you do not explicitly initialize a backend, the system will use the Eventlet backend by default.

Once the backend is initialized, it cannot be changed. Trying to re-initialize it will raise a BackendAlreadySelected exception.

If you see this exception, ensure that your backend is initialized only once early during your application startup (for example in your main() entry point). Avoid calling init_backend() in shared libraries or multiple times. If you need to reset the backend in unit tests, use the _reset_backend() helper to clear the backend cache:

from oslo_service.backend import _reset_backend

_reset_backend()

How to Select a Backend¶

There are two supported ways to select which backend to use:

Explicit initialization

Call init_backend() early in your application entry point:
```
from oslo_service.backend import init_backend, BackendType

init_backend(BackendType.THREADING)
```
This will load the Threading backend and cache its components.

For practical usage, see how Nova does it: https://review.opendev.org/c/openstack/nova/+/948311

Dynamic hook

You may register a Python hook to dynamically decide the backend if init_backend() is not called:

from oslo_service.backend \
    import register_backend_default_hook, BackendType

def my_backend_decider():
    # Add custom logic here if needed
    return BackendType.THREADING

register_backend_default_hook(my_backend_decider)

If no explicit backend is set, the system will call your hook to decide which backend to load.

Note

There is no automatic oslo.config option for service_backend. If you want to make the backend configurable through your application’s config file, you need to define and parse that option yourself, then call init_backend() with the selected value.

Accessing Backend Components¶

Once the backend is initialized, you can access its components via the get_component() helper function.

Example: getting a ProcessLauncher instance:

from oslo_service.backend import get_component

ProcessLauncher = get_component("ProcessLauncher")

launcher = ProcessLauncher(conf)
launcher.launch_service(my_service)
launcher.wait()

Available components include:

ServiceLauncher
ProcessLauncher
ThreadGroup
LoopingCall variants
SignalHandler
And other related service utilities

If you try to access a component that does not exist in the current backend, a KeyError will be raised.

Example Usage¶

Here is a minimal example that shows how to initialize the Threading backend and launch a service:

from oslo_service.backend import (
    init_backend,
    get_component,
    BackendType
)

def main():
    # Select the Threading backend
    init_backend(BackendType.THREADING)

    # Get the ProcessLauncher component
    ProcessLauncher = get_component("ProcessLauncher")

    # Initialize your service and launch it
    launcher = ProcessLauncher(conf)
    launcher.launch_service(my_service)
    launcher.wait()

if __name__ == "__main__":
    main()

Understanding the Implications of Migrating from Eventlet to Threading¶

The following table is not about choosing a backend, but about understanding the implications of migrating away from Eventlet to the Threading backend:

Compared Aspect	Eventlet Backend	Threading Backend
Concurrency model	Cooperative multitasking using green threads (greenlet)	Native Python threads (preemptive)
Process management	Uses built-in eventlet processes	Uses Cotyledon for multi-process support
Looping calls	Uses eventlet green thread pools	Uses Futurist thread pools
Embedded WSGI server	Provided (with eventlet)	Planned for removal; use an external WSGI server instead
Backdoor shell	Provided (eventlet only)	Planned for removal
API compatibility	Same public API	Same public API
Debugging	More complex due to cooperative multitasking and green threads. See eventlet_backdoor.py	May produce clearer native thread stack traces and benefits from recent CPython improvements (e.g. PEP 768).
Python compatibility	Known issues with CPython native threads and RLocks. May break with new Python versions.	Fully compatible with modern CPython threading.

Test Setup in Devstack and CI Gates¶

If you want to test the Threading backend in Devstack or your CI gates, refer to:

References¶

For any questions, please refer to the OpenStack mailing list or the oslo.service maintainers.

Backend System