photon-pump 0.5.0a6

Photon-pump is a fast, user-friendly client for Eventstore_.

It emphasises a modular design, hidden behind an interface that's written for humans.

Installation
------------

Photon pump is available on the `cheese shop`_. ::

pip install photonpump

You will need to install lib-protobuf 3.2.0 or above.

Basic Usage
-----------

Working with connections
~~~~~~~~~~~~~~~~~~~~~~~~

Usually you will want to interact with photon pump via the :class:`~photonpump.Connection` class. The :class:`~photonpump.Connection` is a full-duplex client that can handle many requests and responses in parallel. It is recommended that you create a single connection per application.

First you will need to create a connection:

>>> import asyncio
>>> from photonpump import connect
>>>
>>> loop = asyncio.get_event_loop()
>>>
>>> async with connect(loop=loop) as c:
>>> await c.ping()


The :func:`photonpump.connect` function returns an async context manager so that the connection will be automatically closed when you are finished. Alternatively, you can create a connection directly:

>>> import photonpump
>>> conn = photonpump.Connection(loop=loop)
>>> await conn.connect()

Reading and Writing single events
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

A connection can be used for both reading and writing events. You can publish a single event with the :meth:`~photonpump.Connection.publish_event` method:

>>> # When publishing events, you must provide the stream name.
>>> stream = 'ponies'
>>> event_type = 'PonyJumped'
>>>
>>> result = await conn.publish_event(stream, event_type, body={
>>> 'Pony': 'Derpy Hooves',
>>> 'Height': 10,
>>> 'Distance': 13
>>> })

We can fetch a single event with the complementary :meth:`~photonpump.Connection.get_event` method if we know its `event number` and the stream where it was published:

>>> event_number = result.last_event_number
>>> event = await conn.get_event(stream, event_number)

Assuming that your event was published as json, you can load the body with the :meth:`~photonpump.messages.Event.json` method:

.. code-block:: python

async def write_an_event():
async with photonpump.connect() as conn:
await conn.publish_event('pony_stream', 'pony.jumped', body={
'name': 'Applejack',
'height_m': 0.6
})


async def read_an_event(conn):
event = await conn.get_event('pony_stream', 1)
print(event)


async def write_two_events(conn):
await conn.publish('pony_stream', [
NewEvent('pony.jumped', body={
'name': 'Rainbow Colossus',
'height_m': 0.6
},
NewEvent('pony.jumped', body={
'name': 'Sunshine Carnivore',
'height_m': 1.12
})
])


async def read_two_events(conn):
events = await conn.get('pony_stream', max_count=2, from_event=0)
print(events[0])


async def stneve_owt_daer(conn):
events = await conn.get('pony_stream', direction=StreamDirection.backward, max_count=2)
print(events[0])


async def ticker(delay):
while True:
yield NewEvent('tick', body{ 'tick': i})
i += 1
await asyncio.sleep(delay)


async def write_an_infinite_number_of_events(conn):
await conn.publish('ticker_stream', ticker(1000))


async def read_an_infinite_number_of_events(conn):
async for event in conn.stream('ticker_stream'):
print(event)


>>> data = event.json()
>>> assert data['Pony'] == 'Derpy Hooves'

Reading and Writing in Batches
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We can read and write several events in a request using the :meth:`~photonpump.Connection.get` and :meth:`~photonpump.Connection.publish` methods of our :class:`~photonpump.Connection`. the :func:`photonpump.message.NewEvent` function is a helper for constructing events.

>>> stream = 'more_ponies'
>>> events = [
>>> NewEvent('PonyJumped',
>>> data={
>>> 'Pony': 'Peculiar Hooves',
>>> 'Height': 9,
>>> 'Distance': 13
>>> }),
>>> NewEvent('PonyJumped',
>>> data={
>>> 'Pony': 'Sparkly Hooves',
>>> 'Height': 12,
>>> 'Distance': 12
>>> }),
>>> NewEvent('PonyJumped',
>>> data={
>>> 'Pony': 'Sparkly Hooves',
>>> 'Height': 11,
>>> 'Distance': 14
>>> })]
>>>
>>> await conn.publish(stream, events)

We can get events from a stream in slices by setting the `from_event_number` and `max_count` arguments. We can read events from either the front or back of the stream.

>>> import StreamDirection from photonpump.messages
>>>
>>> all_events = await conn.get(stream)
>>> assert len(all_events) == 3
>>>
>>> first_event = await conn.get(stream, max_count=1)[0].json()
>>> assert first_event['Pony'] == 'Peculiar Hooves'
>>>
>>> second_event = await conn.get(stream, max_count=1, from_event_number=1)[0].json()
>>> assert second_event['Pony'] == 'Sparkly Hooves'
>>>
>>> reversed_events = await conn.get(stream, direction=StreamDirection.backward)
>>> assert len(reversed_events) == 3
>>> assert reversed_events[2] == first_event

Reading with Asynchronous Generators
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We can page through a stream manually by using the `from_event_number` argument of :meth:`~photonpump.Connection.get`, but it's simpler to use the :meth:`~photonpump.Connection.iter` method, which returns an asynchronous generator. By default, `iter` will read from the beginning to the end of a stream, and then stop. As with `get`, you can set the :class:`~photon.messages.StreamDirection`, or use `from_event` to control the result:

>>> async for event in conn.iter(stream):
>>> print (event)

This extends to asynchronous comprehensions:

>>> async def feet_to_metres(jumps):
>>> async for jump in jumps:
>>> data = jump.json()
>>> data['Height'] = data * 0.3048
>>> data['Distance'] = data * 0.3048
>>> yield data
>>>
>>> jumps = (event async for event in conn.iter('ponies')
>>> if event.type == 'PonyJumped')
>>> async for jump in feet_to_metres(jumps):
>>> print (event)


Persistent Subscriptions
~~~~~~~~~~~~~~~~~~~~~~~~

Sometimes we want to watch a stream continuously and be notified when a new event occurs. Eventstore supports persistent subscriptions for this use case. Multiple clients can connect to the same subscription to support competing consumer scenarios.

>>> async def create_subscription(subscription_name, stream_name, conn):
>>> await conn.create_subscription(subscription_name, stream_name)

Once we have a subscription, we can connect to it to begin receiving events. A persistent subscription exposes an `events` property, which acts like an asynchronous iterator.

>>> async def read_events_from_subscription(subscription_name, stream_name, conn):
>>> subscription = await conn.connect_subscription(subscription_name, stream_name)
>>> async for event in subscription.events:
>>> print(event)
>>> await subscription.ack(event)

Eventstore will send each event to one consumer at a time. When you have handled the event, you must acknowledge receipt. Eventstore will resend messages that are unacknowledged.


High-Availability Scenarios
~~~~~~~~~~~~~~~~~~~~~~~~~~~

Eventstore supports an HA-cluster deployment topology. In this scenario, Eventstore runs a master node and multiple slaves. Some operations, particularly subscriptions and projections, are handled only by the master node. To connect to an HA-cluster and automatically find the master node, photonpump supports cluster discovery.

The cluster discovery interrogates eventstore gossip to find the active master. You can provide the IP of a maching in the cluster, or a DNS name that resolves to some members of the cluster, and photonpump will discover the others.

>>> async def connect_to_cluster(hostname_or_ip, port=2113):
>>> with connect(discovery_host=hostname_or_ip, discovery_port=2113) as c:
>>> await c.ping()

If you provide both a `host` and `discovery_host`, photonpump will prefer discovery.

.. _Eventstore: http://geteventstore.com
.. _cheese shop: https://pypi-hypernode.com/pypi/photon-pump


## [0.5a] - 2018-04-27
### Breaking changes
- Dropped the ConnectionContextManager class.
- "Connection" class is now "Client" and acts as a context manager in its own right
- Rewrote the connection module completely.

## [0.4] - 2018-04-27
### Fixes
- Added cluster discovery for HA scenarios.

## [0.3] - 2018-04-11
### Fixes
- `iter` properly supports iterating a stream in reverse.
### Breaking change
- `published_event` reversed order of type and stream


[0.4]: https://github.com/madecom/photon-pump/compare/v0.3.0...v0.4.0
[0.3]: https://github.com/madecom/photon-pump/compare/v0.2.5...v0.3
[0.2.5]: https://github.com/madecom/photon-pump/compare/v0.2.4...v0.2.5