Writing stages for the storage pipelines


A pipeline is a set of stages needed to process a request. When a new request comes to Zaqar, first the message goes through the transport layer pipeline and then through one of the storage layer pipelines depending on the type of operation of each particular request. For example, if Zaqar receives a request to make a queue-related operation, the storage layer pipeline will be queue pipeline. Zaqar always has the actual storage controller as the final storage layer pipeline stage.

By setting the options in the [storage] section of zaqar.conf you can add additional stages to these storage layer pipelines:

  • Claim pipeline

  • Message pipeline with built-in stage available to use:

    • zaqar.notification.notifier - sends notifications to the queue subscribers on each incoming message to the queue, i.e. enables notifications functionality.

  • Queue pipeline

  • Subscription pipeline

The storage layer pipelines options are empty by default, because additional stages can affect the performance of Zaqar. Depending on the stages, the sequence in which the option values are listed does matter or not.

You can add your own external stages to the storage layer pipelines.

Things to know before writing the stage

Stages in the pipeline must implement storage controller methods they need to hook. You can find all available to hook methods in the abstract classes in zaqar/storage/base.py. For example, if you’re looking for all methods available to hook for the queue storage layer pipeline, see Queue class in zaqar/storage/base.py. As you can see, Zaqar’s built-in stage zaqar.notification.notifier implements post method of zaqar.storage.base.Message abstract class.

A stage can halt the pipeline immediate by returning a value that is not None; otherwise, processing will continue to the next stage, ending with the actual storage controller.


For the most of the cases it does not matter what non-None value the storage pipeline returns, but sometimes the returned value is used by the transport layer and you have to be careful. For example, during queue creation request, if the storage driver returns True, the transport layer responds to the client with the 201 http response code, if False, it responds with 204 http response code. See: zaqar.transport.wsgi.v2_0.queues.ItemResource#on_put.

Zaqar finds stages with their source codes through the Python entry points mechanism. All Python packages containing stages for Zaqar must register their stages under zaqar.storage.stages entry point group during their install either by setup.py or by setup.cfg. If the stage is registered, and the name of the stage’s entry point is specified by the user in the one of zaqar.conf storage layer pipeline options, the stage will be loaded to the particular storage layer pipeline. Zaqar imports stages as plugins. See zaqar.storage.pipeline#_get_storage_pipeline.

For additional information about plugins see: Stevedore - Creating Plugins and Stevedore - Loading the Plugins.

Example of external stage (written outside Zaqar package)

This is an example of small package with a stage that can process queue-related requests in Zaqar. The stage does not do anything useful, but is good as example.

File tree structure of the package:

├── setup.py
└── ubershystages
    ├── __init__.py
    └── queues
        ├── __init__.py
        └── lovely.py

2 directories, 4 files


from setuptools import setup, find_packages


    description='Demonstration package for Zaqar with plugin pipeline stage',



    classifiers=['Development Status :: 3 - Alpha',
                 'License :: OSI Approved :: Apache Software License',
                 'Programming Language :: Python',
                 'Programming Language :: Python :: 2',
                 'Programming Language :: Python :: 2.7',
                 'Programming Language :: Python :: 3',
                 'Programming Language :: Python :: 3.5',
                 'Intended Audience :: Developers',
                 'Environment :: Console',




        'zaqar.storage.stages': [
            'ubershy.lovelyplugin = ubershystages.queues.lovely:LovelyStage',



class LovelyStage(object):
    """This stage:
    1. Prints 'Lovely stage is processing request...' on each queue creation or
       deletion request.
    2. Prints 'Oh, what a lovely day!' on each creation request of a queue
       named 'lovely'.
    3. Prevents deletion of a queue named 'lovely' and prints 'Secretly keeping
       lovely queue' on such attempt.

    def __init__(self, *args, **kwargs):
        print("Lovely stage is loaded!")

    def create(self, name, metadata=None, project=None):
        """Stage's method which processes queue creation request.

        :param name: The queue name
        :param project: Project id

        if name == 'lovely':
            print("Oh, what a lovely day!")

    def delete(self, name, project=None):
        """Stage's method which processes queue deletion request.

        :param name: The queue name
        :param project: Project id
        :returns: Something non-None, if the queue has a name 'lovely'. It will
        stop further processing through the other stages of the pipeline, and
        the request will not reach the storage controller driver, preventing
        queue deletion from the database.

        if name == 'lovely':
            print('Secretly keeping lovely queue')
            something = "shhh... it's a bad practice"
            return something

    def printprocessing(self):
        print('Lovely stage is processing request...')

To install the package to the system in the root directory of the package run:

# pip install -e .

In zaqar.conf add ubershy.lovelyplugin to the queue_pipeline option:

queue_pipeline = ubershy.lovelyplugin

Start Zaqar:

$ zaqar-server

If the stage has successfully loaded to Zaqar you will see amongst terminal output lines the Lovely stage is loaded! line. Then you can try to perform queue create and queue delete operations with the queue ‘lovely’ and see what will happen in Zaqar’s database.


You can hold multiple stages in one package, just be sure that all stages will be registered as entry points. For example, in the setup.py you can register additional ubershy.nastyplugin stage:

    'zaqar.storage.stages': [
        'ubershy.lovelyplugin = ubershystages.queues.lovely:LovelyStage',
        'ubershy.nastyplugin = ubershystages.messages.nasty:NastyStage',