Python gRPC Client for EventStoreDB

This package provides a Python gRPC client for EventStoreDB. It has been developed and tested to work with EventStoreDB LTS version 21.10, and with Python versions 3.7, 3.8, 3.9, and 3.10.

Methods have typing annotations, the static typing is checked with mypy, and the test coverage is 100%.

Not all the features of the EventStoreDB API are presented by this client in its current form, however many of the most useful aspects are presented in an easy-to-use interface (see below). For an example of usage, see the eventsourcing-eventstoredb package.

Installation

Use pip to install this package from the Python Package Index.

$ pip install esdbclient

It is recommended to install Python packages into a Python virtual environment.

Getting started

Start EventStoreDB

Use Docker to run EventStoreDB from the official container image on DockerHub.

$ docker run -d --name my-eventstoredb -it -p 2113:2113 -p 1113:1113 eventstore/eventstore:21.10.2-buster-slim --insecure

Please note, this will start the server without SSL/TLS enabled, allowing only "insecure" connections. This version of this Python client does not support SSL/TLS connections. A future version of this library will support "secure" connections.

Construct client

The class EsdbClient can be constructed with a uri that indicates the hostname and port number of the EventStoreDB server.

from esdbclient import EsdbClient

client = EsdbClient(uri='localhost:2113')

Append events

The method append_events() can be used to append events to a stream.

Three arguments are required, stream_name, expected_position and events.

The stream_name argument is a string that uniquely identifies the stream in the database.

The expected_position argument is an optional integer that specifies the expected position of the end of the stream in the database: either a positive integer representing the expected current position of the stream, or None if the stream is expected not to exist. If there is a mismatch with the actual position of the end of the stream, an exception ExpectedPositionError will be raised by the client. This accomplishes optimistic concurrency control when appending new events. If you need to get the current position of the end of a steam, use the get_stream_position() method (see below). If you wish to disable optimistic concurrency, set the expected_position to a negative integer.

The events argument is a sequence of new event objects to be appended to the named stream. The class NewEvent can be used to construct new event objects.

In the example below, a stream is created by appending a new event with expected_position=None.

from uuid import uuid4

from esdbclient import NewEvent

# Construct new event object.
event1 = NewEvent(
    type='OrderCreated',
    data=b'{}',
    metadata=b'{}'
)

# Define stream name.
stream_name1 = str(uuid4())

# Append list of events to new stream.
commit_position1 = client.append_events(
    stream_name=stream_name1,
    expected_position=None,
    events=[event1],
)

If the append operation is successful, this method will return the database "commit position" as it was when the operation was completed. Otherwise, an exception will be raised.

The commit position value can be used to wait for downstream processing to have processed the appended events. For example, after making a command, a user interface can wait before making a query for an eventually consistent materialised view that would be stale if those events have not yet been processed.

A "commit position" is a monotonically increasing integer representing the position of the recorded event in a "total order" of all recorded events in the database. The sequence of commit positions is not gapless. It represents the position of the event record on disk, and there are usually large differences between successive commits.

In the example below, two subsequent events are appended to an existing stream. The sequences of stream positions are zero-based, and so when a stream only has one recorded event, the expected position of the end of the stream is 0.

event2 = NewEvent(
    type='OrderUpdated',
    data=b'{}',
    metadata=b'{}',
)
event3 = NewEvent(
    type='OrderDeleted',
    data=b'{}',
    metadata=b'{}',
)

commit_position2 = client.append_events(
    stream_name=stream_name1,
    expected_position=0,
    events=[event2, event3],
)

Please note, whilst the append operation is atomic, so that either all or none of a given list of events will be recorded, by design it is only possible with EventStoreDB to atomically record events in one stream.

Get current stream position

The method get_stream_position() can be used to get the position of the end of a stream (the position of the last recorded event in the stream).

stream_position = client.get_stream_position(
    stream_name=stream_name1
)

assert stream_position == 2

The sequence of stream positions is gapless. It is zero-based, so that the position of the end of the stream when one event has been appended is 0. The position is 1 after two events have been appended, 2 after three events have been appended, and so on.

If a stream does not exist, the returned stream position is None, which corresponds to the required expected position when appending events to a stream that does not exist (see above).

stream_position = client.get_stream_position(
    stream_name='stream-unknown'
)

assert stream_position == None

This method takes an optional argument timeout which is a float that sets a deadline for the completion of the gRPC operation.

Read stream events

The method read_stream_events() can be used to read the recorded events in a stream. An iterable object of recorded events is returned.

One argument is required, stream_name, which is the name of the stream to be read. By default, the recorded events in the stream are returned in the order they were recorded.

The example below shows how to read the recorded events of a stream forwards from the start of the stream to the end of the stream.

response = client.read_stream_events(
    stream_name=stream_name1
)

The iterable object is actually a Python generator, and we need to iterate over it to actually get the recorded events from gRPC. So let's convert it into a list, so that we can call len(), and so that we can index it to check we have the individual events recorded above.

events = list(response)

Now that we have an actual list of events, we can check we have the three events that we recorded in the stream above.

assert len(events) == 3

assert events[0].stream_name == stream_name1
assert events[0].stream_position == 0
assert events[0].type == event1.type
assert events[0].data == event1.data

assert events[1].stream_name == stream_name1
assert events[1].stream_position == 1
assert events[1].type == event2.type
assert events[1].data == event2.data

assert events[2].stream_name == stream_name1
assert events[2].stream_position == 2
assert events[2].type == event3.type
assert events[2].data == event3.data

The method read_stream_events() also supports four optional arguments, position, backwards, limit, and timeout.

The argument position is an optional integer that can be used to indicate the position in the stream from which to start reading. This argument is None by default, which means the stream will be read either from the start of the stream (the default behaviour), or from the end of the stream if backwards is True. When reading a stream from a specific position in the stream, the recorded event at that position WILL be included, both when reading forwards from that position, and when reading backwards from that position.

The argument backwards is a boolean, by default False, which means the stream will be read forwards by default, so that events are returned in the order they were appended, If backwards is True, the stream will be read backwards, so that events are returned in reverse order.

The argument limit is an integer which limits the number of events that will be returned.

The argument timeout is a float which sets a deadline for the completion of the gRPC operation.

The example below shows how to read recorded events in a stream forwards from a specific stream position to the end of the stream.

events = list(
    client.read_stream_events(
        stream_name=stream_name1,
        position=1,
    )
)

assert len(events) == 2

assert events[0].stream_name == stream_name1
assert events[0].stream_position == 1
assert events[0].type == event2.type
assert events[0].data == event2.data

assert events[1].stream_name == stream_name1
assert events[1].stream_position == 2
assert events[1].type == event3.type
assert events[1].data == event3.data

The example below shows how to read the recorded events in a stream backwards from the end of the stream to the start of the stream.

events = list(
    client.read_stream_events(
        stream_name=stream_name1,
        backwards=True,
    )
)

assert len(events) == 3

assert events[0].stream_name == stream_name1
assert events[0].stream_position == 2
assert events[0].type == event3.type
assert events[0].data == event3.data

assert events[1].stream_name == stream_name1
assert events[1].stream_position == 1
assert events[1].type == event2.type
assert events[1].data == event2.data

The example below shows how to read a limited number (two) of the recorded events in stream forwards from the start of the stream.

events = list(
    client.read_stream_events(
        stream_name=stream_name1,
        limit=2,
    )
)

assert len(events) == 2

assert events[0].stream_name == stream_name1
assert events[0].stream_position == 0
assert events[0].type == event1.type
assert events[0].data == event1.data

assert events[1].stream_name == stream_name1
assert events[1].stream_position == 1
assert events[1].type == event2.type
assert events[1].data == event2.data

The example below shows how to read a limited number of the recorded events in a stream backwards from a given stream position.

events = list(
    client.read_stream_events(
        stream_name=stream_name1,
        position=2,
        backwards=True,
        limit=1,
    )
)

assert len(events) == 1

assert events[0].stream_name == stream_name1
assert events[0].stream_position == 2
assert events[0].type == event3.type
assert events[0].data == event3.data

Read all recorded events

The method read_all_events() can be used to read all recorded events in the database in the order they were committed. An iterable object of recorded events is returned.

The example below shows how to read all events in the database in the order they were recorded.

events = list(client.read_all_events())

assert len(events) >= 3

The method read_stream_events() supports six optional arguments, position, backwards, filter_exclude, filter_include, limit, and timeout.

The argument position is an optional integer that can be used to specify the commit position from which to start reading. This argument is None by default, meaning that all the events will be read either from the start, or from the end if backwards is True (see below). Please note, if specified, the specified position must be an actually existing commit position, because any other number will result in a server error (at least in EventStoreDB v21.10). Please also note, when reading forwards the event at the given position WILL be included. However when reading backwards, the event at the given position will NOT be included.

The argument backwards is a boolean which is by default False meaning all the events will be read forwards by default, so that events are returned in the order they were committed, If backwards is True, all the events will be read backwards, so that events are returned in reverse order.

The argument filter_exclude is a sequence of regular expressions that match the type strings of recorded events that should not be included. This argument is ignored if filter_include is set.

The argument filter_include is a sequence of regular expressions that match the type strings of recorded events that should be included. By default, this argument is an empty tuple. If this argument is set to a non-empty sequence, the filter_include argument is ignored.

Please note, the filtering happens on the EventStoreDB server, and the limit argument is applied after filtering. See below for more information about filter regular expressions.

The argument limit is an integer which limits the number of events that will be returned.

The argument timeout is a float which sets a deadline for the completion of the gRPC operation.

The example below shows how to read all recorded events from a particular commit position.

events = list(
    client.read_all_events(
        position=commit_position1
    )
)

assert len(events) == 3

assert events[0].stream_name == stream_name1
assert events[0].stream_position == 0
assert events[0].type == event1.type
assert events[0].data == event1.data

assert events[1].stream_name == stream_name1
assert events[1].stream_position == 1
assert events[1].type == event2.type
assert events[1].data == event2.data

assert events[2].stream_name == stream_name1
assert events[2].stream_position == 2
assert events[2].type == event3.type
assert events[2].data == event3.data

The example below shows how to read all recorded events in reverse order.

events = list(
    client.read_all_events(
        backwards=True
    )
)

assert len(events) >= 3

assert events[0].stream_name == stream_name1
assert events[0].stream_position == 2
assert events[0].type == event3.type
assert events[0].data == event3.data

assert events[1].stream_name == stream_name1
assert events[1].stream_position == 1
assert events[1].type == event2.type
assert events[1].data == event2.data

assert events[2].stream_name == stream_name1
assert events[2].stream_position == 0
assert events[2].type == event1.type
assert events[2].data == event1.data

The example below shows how to read a limited number (one) of the recorded events in the database forwards from a specific commit position.

events = list(
    client.read_all_events(
        position=commit_position1,
        limit=1,
    )
)

assert len(events) == 1

assert events[0].stream_name == stream_name1
assert events[0].stream_position == 0
assert events[0].type == event1.type
assert events[0].data == event1.data

The example below shows how to read a limited number (one) of the recorded events in the database backwards from the end. This gives the last recorded event.

events = list(
    client.read_all_events(
        backwards=True,
        limit=1,
    )
)

assert len(events) == 1

assert events[0].stream_name == stream_name1
assert events[0].stream_position == 2
assert events[0].type == event3.type
assert events[0].data == event3.data

Get current commit position

The method get_commit_position() can be used to get the current commit position of the database.

commit_position = client.get_commit_position()

This method is provided as a convenience when testing, and otherwise isn't very useful. In particular, when reading all events (see above) or subscribing to all events with a catch-up subscription (see below), the commit position would normally be read from the downstream database, so that you are reading from the last position that was successfully processed.

This method takes an optional argument timeout which is a float that sets a deadline for the completion of the gRPC operation.

Catch-up subscriptions

The method subscribe_all_events() can be used to create a "catch-up subscription" to EventStoreDB.

This method takes an optional position argument, which can be used to specify a commit position from which to subscribe for recorded events. The default value is None, which means the subscription will operate from the first recorded event in the database.

This method returns an iterable object, from which recorded events can be obtained by iteration, including events that are recorded after the subscription was created.

Many catch-up subscriptions can be created, concurrently or successively, and all will receive all the events they are subscribed to receive.

The value of the commit_position attribute of recorded events can be recorded along with the results of processing recorded events, to track progress and to allow event processing to be resumed at the correct position.

The example below shows how to subscribe to receive all recorded events from a specific commit position. Three already-existing events are received, and then three new events are recorded, which are then received via the subscription.

# Get the commit position (usually from database of materialised views).
commit_position = client.get_commit_position()

# Append three events.
stream_name1 = str(uuid4())
event1 = NewEvent(
    type='OrderCreated',
    data=b'{}',
    metadata=b'{}',
)
event2 = NewEvent(
    type='OrderUpdated',
    data=b'{}',
    metadata=b'{}',
)
event3 = NewEvent(
    type='OrderDeleted',
    data=b'{}',
    metadata=b'{}',
)
client.append_events(
    stream_name=stream_name1,
    expected_position=None,
    events=[event1, event2, event3],
)

# Subscribe from the commit position.
subscription = client.subscribe_all_events(
    position=commit_position
)

# Catch up by receiving the three events from the subscription.
events = []
for event in subscription:
    # Check the stream name is 'stream_name1'.
    assert event.stream_name == stream_name1
    events.append(event)
    if len(events) == 3:
        break

# Append three more events.
stream_name = str(uuid4())
event4 = NewEvent(
    type='OrderCreated',
    data=b'{}',
    metadata=b'{}',
)
event5 = NewEvent(
    type='OrderUpdated',
    data=b'{}',
    metadata=b'{}',
)
event6 = NewEvent(
    type='OrderDeleted',
    data=b'{}',
    metadata=b'{}',
)
client.append_events(
    stream_name=stream_name,
    expected_position=None,
    events=[event4, event5, event6],
)

# Receive the three new events from the same subscription.
events = []
for event in subscription:
    # Check the stream name is 'stream_name2'.
    assert event.stream_name == stream_name
    events.append(event)
    if len(events) == 3:
        break

This method also support three other optional arguments, filter_exclude, filter_include, and timeout.

The argument filter_exclude is a sequence of regular expressions that match the type strings of recorded events that should not be included. This argument is ignored if filter_include is set.

The argument filter_include is a sequence of regular expressions that match the type strings of recorded events that should be included. By default, this argument is an empty tuple. If this argument is set to a non-empty sequence, the filter_include argument is ignored.

Please note, in this version of this Python client, the filtering happens within the client (rather than on the server as when reading all events) because passing these filter options in the read request for subscriptions seems to cause an error in EventStoreDB v21.10. See below for more information about filter regular expressions.

The argument timeout is a float which sets a deadline for the completion of the gRPC operation. This probably isn't very useful, but is included for completeness and consistency with the other methods.

Catch-up subscriptions are not registered in EventStoreDB (they are not "persistent subscriptions). It is simply a streaming gRPC call which is kept open by the server, with newly recorded events sent to the client as the client iterates over the subscription. This kind of subscription is closed as soon as the subscription object goes out of memory.

# End the subscription.
del subscription

Received events do not need to be (and indeed cannot be) acknowledged back to the EventStoreDB server. Acknowledging events is an aspect of "persistent subscriptions", which is a feature of EventStoreDB that is not (currently) supported by this client. Whilst there are some advantages of persistent subscriptions, by tracking in the upstream server the position in the commit sequence of events that have been processed, there is a danger of "dual writing" in the consumption of events. The danger is that if the event is successfully processed but then the acknowledgment fails, the event may be processed more than once. On the other hand, if the acknowledgment is successful but then the processing fails, the event may not be been processed. By either processing an events more than once, or failing to process and event, the resulting state of the processing of the recorded events might be inaccurate, or possibly inconsistent, and perhaps catastrophically so. Of course, such inaccuracies may or may not matter in your situation. But catastrophic inconsistencies may halt processing until the issue is resolved. The only protection against this danger is to avoid "dual writing" by atomically recording the commit position of an event that has been processed along with the results of process the event, that is with both things being recorded in the same transaction.

To accomplish "exactly once" processing of the events, the commit position of a recorded event should be recorded atomically and uniquely along with the result of processing recorded events, for example in the same database as materialised views when implementing eventually-consistent CQRS, or in the same database as a downstream analytics or reporting or archiving application. This avoids "dual writing" in the processing of events.

The subscription object might be used directly when processing events. It might also be used within a thread dedicated to receiving events, with recorded events put on a queue for processing in a different thread. This package doesn't provide such thread or queue objects, you would need to do that yourself. Just make sure to reconstruct the subscription (and the queue) using your last recorded commit position when resuming the subscription after an error, to be sure all events are processed once.

The NewEvent class

The NewEvent class can be used to define new events.

The attribute type is a unicode string, used to specify the type of the event to be recorded.

The attribute data is a byte string, used to specify the data of the event to be recorded. Please note, in this version of this Python client, writing JSON event data to EventStoreDB isn't supported, but it might be in a future version.

The attribute metadata is a byte string, used to specify metadata for the event to be recorded.

new_event = NewEvent(
    type='OrderCreated',
    data=b'{}',
    metadata=b'{}',
)

The RecordedEvent class

The RecordedEvent class is used when reading recorded events.

The attribute type is a unicode string, used to indicate the type of the event that was recorded.

The attribute data is a byte string, used to indicate the data of the event that was recorded.

The attribute metadata is a byte string, used to indicate metadata for the event that was recorded.

The attribute stream_name is a unicode string, used to indicate the type of the name of the stream in which the event was recorded.

The attribute stream_position is an integer, used to indicate the position in the stream at which the event was recorded.

The attribute commit_position is an integer, used to indicate the position in total order of all recorded events at which the event was recorded.

from esdbclient.events import RecordedEvent

recorded_event = RecordedEvent(
    type='OrderCreated',
    data=b'{}',
    metadata=b'{}',
    stream_name='stream1',
    stream_position=0,
    commit_position=512,
)

Filter regular expressions

The filter_exclude and filter_include arguments in read_all_events() and subscribe_all_events() are applied to the type attribute of recorded events.

The default value of the filter_exclude arguments is designed to exclude EventStoreDB "system events", which otherwise would be included. System events, by convention in EventStoreDB, all have type strings that start with the $ sign.

Please note, characters that have a special meaning in regular expressions will need to be escaped (with double-backslash) when matching these characters in type strings.

For example, to match EventStoreDB system events, use the sequence ['\\$.*']. Please note, the constant ESDB_EVENTS_REGEX is set to '\\$.*'. You can import this value (from esdbclient import ESDB_EVENTS_REGEX) and use it when building longer sequences of regular expressions. For example, to exclude system events and snapshots, you might use the sequence [ESDB_EVENTS_REGEX, '.*Snapshot'] as the value of the filter_exclude argument.

Stop EventStoreDB

Use Docker to stop and remove the EventStoreDB container.

$ docker stop my-eventstoredb
$ docker rm my-eventstoredb

Developers

Clone the project repository, set up a virtual environment, and install dependencies.

Use your IDE (e.g. PyCharm) to open the project repository. Create a Poetry virtual environment, and then update packages.

$ make update-packages

Alternatively, use the make install command to create a dedicated Python virtual environment for this project.

$ make install

The make install command uses the build tool Poetry to create a dedicated Python virtual environment for this project, and installs popular development dependencies such as Black, isort and pytest.

Add tests in ./tests. Add code in ./esdbclient.

Start EventStoreDB.

$ make start-eventstoredb

Run tests.

$ make test

Stop EventStoreDB.

$ make stop-eventstoredb

Check the formatting of the code.

$ make lint

Reformat the code.

$ make fmt

Add dependencies in pyproject.toml and then update installed packages.

$ make update-packages