iPOPO is a Python-based Service-Oriented Component Model (SOCM) based on Pelix, a dynamic service platform. They are inspired on two popular Java technologies for the development of long-lived applications: the iPOJO component model and the OSGi Service Platform. iPOPO enables to conceive long-running and modular IT services.

See https://ipopo.readthedocs.io/ for documentation and more information.

This is the 3.x branch of iPOPO, which is intended to work with Python 3.10+. The iPOPO API didn't change much between v1 and v3: all code working with iPOPO v1 should be compatible with iPOPO v3 (as long as the Python syntax and packages are compatible with newer Python versions). If that's not the case, please fill in a bug report on GitHub issues.

If you are working with Python between 2.7 and 3.7, you must instead use the [1.x branch](https://github.com/tcalmant/ipopo/tree/v1) of iPOPO. Note that iPOPO has not been tested with versions 3.8 and 3.9.

[Version 2.x](https://github.com/tcalmant/ipopo/tree/v2) was a trial to implement iPOPO with asyncio for Python 3.7, but has been stalled due to various issues and lack of contributions.

In order to gain insight from the iPOPO community, I've put a really short survey on Google Forms (no login required).

Please, feel free to answer it, the more answers, the better. All feedback is really appreciated.

iPOPO is available on PyPI and can be installed using pip:

# Install system-wide
$ sudo pip install iPOPO

# ... or user-wide installation
$ pip install --user iPOPO

$ git clone https://github.com/tcalmant/ipopo.git
$ cd ipopo
$ python setup.py install

To check if Pelix is installed correctly, run the following command:

$ python -m pelix.shell --version
Pelix 3.0.0 from /home/tcalmant/git/ipopo/pelix/__init__.py

Pelix brings the concept of bundle in Python. A bundle is a module with a life cycle: it can be installed, started, stopped, updated and uninstalled.

A bundle can declare a class acting as bundle activator, using the @BundleActivator decorator. This class will be instantiated by the framework and its start() and stop() method will be called to notify the bundle about its activation and deactivation.

When it is active, a bundle can register services. A service is an object implementing a specification and associated to a set of properties. A component will then be able to select and consume a service according to the specification(s) it provides and to its properties.

The components are a concept brought by iPOPO. A component, or component instance, is an object managed by a container. The container handles the interactions between the component and the Pelix framework. That way, the component contains only the code required for its task, not for its bindings with the framework. A component is an instance of a component factory, a class manipulated by iPOPO decorators.

For more information, see the concepts page.

This sample gives a quick overview of the usage of iPOPO. For more information, take a look at iPOPO in 10 minutes.

In iPOPO v3, you can use a Python procotol to define the specification of a service. A specification class/protocol should be decorated with @Specification to be given a unique name. The components providing that specification should then inherit that specification class/protocol in order for the development tools you use to be able to warn if a method is missing or uses invalid types.

Note that it is possible to skip that step and use a string constant as specification like in iPOPO v1.

Here is a sample description of an Hello World service specification:

from typing import Protocol
from pelix.constants import Specification

@Specification("sample.hello")
class HelloWorld(Protocol):
    """
    Hello world specification: definition of the methods a component providing
    that service must implement
    """

    def hello(self, name: str) -> None:
        """
        Prints hello
        """
        ...

    def bye(self, name: str) -> None:
        """
        Prints bye
        """
        ...

The following code defines a component factory (a class) which instances will provide a sample.hello service.

 # iPOPO decorators
 from pelix.ipopo.decorators import ComponentFactory, Provides, Instantiate

# Import the specification, if we want to use its type
from specification import HelloWorld

 # Manipulates the class and sets its (unique) factory name
 @ComponentFactory("hello-provider-factory")
 # Indicate that the components will provide a service
 @Provides(HelloWorld)
 # Like in iPOPOv1, We could also use the specification name directly:
 # @Provides("sample.hello")
 # Tell iPOPO to instantiate a component instance as soon as the file is loaded
 @Instantiate("hello-provider-auto")
 # When using Python protocols, it is recommended to inherit from it to
 # benefit from types handling of IDEs.
 class HelloProvider(HelloWorld):
     """
     A sample service provider
     """
     def hello(self, name="world"):
         """
         Says hello
         """
         print("Hello,", name, "!")

     def bye(self, name="cruel world"):
         """
         Says bye
         """
         print("Bye,", name, "!")

Due to the @Instantiate decorator, iPOPO will automatically instantiate a component when the bundle providing this component factory will be started. It is also possible to instantiate a component using shell commands or via the iPOPO runtime service (see use_ipopo).

Each component instance will provide a sample.hello service, which can be consumed by any bundle or any other component.

The following code defines a component factory (a class) which instances will consume a sample.hello service. If multiple services are available, iPOPO will select the one with the highest rank and the lowest service ID (i.e. the oldest service).

In iPOPO v3, it is again recommended to use typing as much as possible. For injected fields and properties, the fields injected with @Requires should be defined at class level with the right type hint: type, optional, list, ... based on the requirement configuration.

from pelix.ipopo.decorators import ComponentFactory, Instantiate, Invalidate, Requires, Validate
from specification import HelloWorld

# Manipulates the class and sets its (unique) factory name
@ComponentFactory("hello-consumer-factory")
# Indicate that the components require a sample.hello service to work
# and to inject the found service in the _svc field
# We could also use the specification name instead of the type
@Requires("_svc", HelloWorld)
# Tell iPOPO to instantiate a component instance as soon as the file is loaded
@Instantiate("hello-consumer-auto")
class HelloConsumer:
    """
    A sample service consumer
    """

    # Define the injected field type for static typing (optional)
    _svc: HelloWorld

    @Validate
    def validate(self, context):
        """
        Component validated: all its requirements have been injected
        """
        self._svc.hello("Consumer")

    @Invalidate
    def invalidate(self, context):
        """
        Component invalidated: one of its requirements is going away
        """
        self._svc.bye("Consumer")

When the bundle providing this component factory will be started, iPOPO will automatically instantiate a component, due to the @Instantiate decorator.

Each component instance will require a sample.hello service. Once iPOPO has injected all the required services (here, a single sample.hello service) in a component instance, this instance will be considered valid and iPOPO will call its method decorated by @Validate. There, the component can consume its dependencies, start threads, etc. It is recommended for this method to start threads and to return quickly, as it blocks iPOPO and the Pelix framework.

When a required service is unregistered by its provider, the component instances consuming it are invalidated. When the method decorated by @Invalidate is called, the service is still injected and should be usable (except for special cases, like remote services).

To run this sample, you'll need to copy the snippets above in different files:

• copy the Service specification snipper in a file named specification.py
• copy the Service provider snippet in a file named provider.py
• copy the Service consumer snippet in a file named consumer.py

You can also find those files in the project repository in the samples/hello_world folder.

Then, run a Pelix shell in the same folder as those files, and execute the commands listed in this trace:

$ python -m pelix.shell
** Pelix Shell prompt **
$ # Install the bundles
$ install provider
Bundle ID: 15
$ install consumer
Bundle ID: 16
$ # Start the bundles (the order isn't important here)
$ start 15 16
Starting bundle 15 (provider)...
Starting bundle 16 (consumer)...
Hello, Consumer !
$ # View iPOPO instances
$ instances
+----------------------+------------------------------+-------+
|         Name         |           Factory            | State |
+======================+==============================+=======+
| hello-consumer-auto  | hello-consumer-factory       | VALID |
+----------------------+------------------------------+-------+
| hello-provider-auto  | hello-provider-factory       | VALID |
+----------------------+------------------------------+-------+
| ipopo-shell-commands | ipopo-shell-commands-factory | VALID |
+----------------------+------------------------------+-------+
3 components running
$ # View details about the consumer
$ instance hello-consumer-auto
Name.....: hello-consumer-auto
Factory..: hello-consumer-factory
Bundle ID: 16
State....: VALID
Services.:
Dependencies:
        Field: _svc
                Specification: sample.hello
                Filter......: None
                Optional.....: False
                Aggregate....: False
                Handler......: SimpleDependency
                Bindings:
                        ServiceReference(ID=18, Bundle=15, Specs=['sample.hello'])
Properties:
        +---------------+---------------------+
        |      Key      |        Value        |
        +===============+=====================+
        | instance.name | hello-consumer-auto |
        +---------------+---------------------+

$ # Modify the provider file (e.g. change the 'Hello' string by 'Hi')
$ # Update the provider bundle (ID: 15)
$ update 15
Updating bundle 15 (provider)...
Bye, Consumer !
Hi, Consumer !
$ # Play with other commands (see help)

First, the install commands are used to install the bundle: they will be imported but their activator won't be called. If this command fails, the bundle is not installed and is not referenced by the framework.

If the installation succeeded, the bundle can be started: it's activator is called (if any). Then, iPOPO detects the component factories provided by the bundle and instantiates the components declared using the @Instantiate decorator.

The instances and instance commands can be use to print the state and bindings of the components. Some other commands are very useful, like sl and sd to list the registered services and print their details. Use the help command to see which ones can be used.

The last part of the trace shows what happens when updating a bundle. First, update the source code of the provider bundle, e.g. by changing the string it prints in the hello() method. Then, tell the framework to update the bundle using the update command. This command requires a bundle ID, which has been given as a result of the install command and can be found using bl.

When updating a bundle, the framework stops it and reloads it (using importlib.reload). If the update fails, the old version is kept. If the bundle was active before the update, it is restarted by the framework.

Stopping a bundle causes iPOPO to kill the component instance(s) of the factories it provided. Therefore, no one provides the sample.hello service, which causes the consumer component to be invalidated. When the provider bundle is restarted, a new provider component is instantiated and its service is injected in the consumer, which becomes valid again.

Pelix/iPOPO comes with some useful services:

• Pelix Shell: a simple shell to control the framework (manage bundles, show the state of components, ...). The shell is split in 5 parts:

  • the parser: a shell interpreter class, which can be reused to create other shells (with a basic support of variables);
  • the shell core service: callable from any bundle, it executes the given command lines;
  • the UIs: text UI (console) and remote shell (TCP/TLS, XMPP)
  • the commands providers: iPOPO commands, report, EventAdmin, ...
  • the completion providers: Pelix, iPOPO

  See the shell tutorial for more information.

• An HTTP service, based on the HTTP server from the standard library. It provides the concept of servlet, borrowed from Java.

  See the HTTP service reference for more information.

  There is also a routing utility class, based on decorators, which eases the development of REST-like servlets.

• Remote Services: export and import services to/from other Pelix framework or event Java OSGi frameworks!

  See the remote services reference and the Remote Service Admin reference for more information. The former should be used to link iPOPO instances while the latter targets both iPOPO and Java OSGi frameworks.

Pelix also provides an implementation of the EventAdmin service, inspired from the OSGi specification.

Feel free to send feedback on your experience of Pelix/iPOPO, via the mailing lists:

• User list: https://groups.google.com/g/ipopo-users
• Development list: https://groups.google.com/g/ipopo-dev
• GitHub Discussions: https://github.com/tcalmant/ipopo/discussions

Bugs and features requests can be submitted using the Issue Tracker on GitHub.

All contributions are welcome!

1. Create an issue to discuss about your idea or the problem you encounter
2. Fork the project
3. Develop your changes
4. Check your code with pylint and pep8
5. If necessary, write some unit tests
6. Commit your changes, indicating in each commit a reference to the issue you're working on
7. Push the commits on your repository
8. Create a Pull Request
9. Enjoy!

Please note that your contributions will be released under the project's license, which is the Apache Software License 2.0.

Pelix and iPOPO are tested using GitHub actions targetting Python 3.10, 3.11 and 3.12.

iPOPO v3 doesn't support Python 2 neither versions earlier than 3.10. If you need to work with those versions of Python, please use iPOPO v1. You can then use Remote Services to allow interactions between iPOPO v1 and v3.

iPOPO is released under the Apache Software License 2.0.