Python Client

The Python package umadb is available on PyPI and provides a synchronous client for reading and appending events to UmaDB over its gRPC API using Rust-powered bindings via PyO3.

It is adapted into the Python eventsourcing library via the eventsourcing-umadb package.

Installation

Add umadb to your Python project:

bash

uv add "umadb"

Connecting to UmaDB

Use the Client class as the main entry point for connecting to an UmaDB server.

python

class Client(
    url: str,
    ca_path: str | None = None,
    api_key: str | None = None,
    batch_size: int | None = None,
):
    ...

Parameters

Name	Type	Description
`url`	`str`	Required connection string. If the argument starts with `https` or `grpcs`, a secure TLS channel is created; otherwise an insecure channel is used.
`ca_path`	`str\|None`	Optional path to a PEM-encoded root/CA certificate for TLS connections (useful for self-signed servers).
`api_key`	`str\|None`	Optional API key used for authenticating with the server.
`batch_size`	`int\|None`	Optional hint for how many events to buffer per batch when reading. The server may cap this; a sensible default is used if unset..

Connection Examples

python

from umadb import Client

# Insecure (no TLS)
client = Client("http://localhost:50051")

# Secure with TLS (system CAs)
client = Client("https://example.com:50051")

# Secure with TLS using self-signed CA
client = Client(
    url="https://localhost:50051",
    ca_path="server.pem",
)

# TLS + API key
client = Client(
    url="https://example.com:50051",
    ca_path="server.pem",
    api_key="umadb:example-api-key-4f7c2b1d9e5f4a038c1a",
)

# TLS + API key + batch size hint
client = Client(
    url="https://example.com:50051",
    ca_path="server.pem",
    api_key="umadb:example-api-key-4f7c2b1d9e5f4a038c1a",
    batch_size=1000,
)

Appending Events

The Client.append() method writes new events to an UmaDB server.

python

def append(
    events: list[Event],
    condition: AppendCondition | None = None,
    tracking_info: TrackingInfo | None = None,
) -> int:
    ...

The Client.append() method can be used to append new Event instances to UmaDB atomically, with an optional append condition, and optional tracking information. Events are written in order.

Conditional appends with event UUIDs are idempotent. The server does not enforce uniqueness of events IDs.

Parameters

Name	Type	Description
`events`	`list[Event]`	The list of events to append. Each includes an event type, tags, and data payload.
`condition`	`AppendCondition\|None`	Optional append condition to ensure no conflicting writes occur.
`tracking_info`	`TrackingInfo\|None`	Optional tracking information – for event-processing components only.

Return Value

Returns the sequence number (int) of the last successfully appended event from this operation. This value can be used to wait for downstream event-processing components in a CQRS system to become up-to-date.

Example

python

import uuid

from umadb import AppendCondition, Client, Event, IntegrityError, Query, QueryItem

client = Client("http://localhost:50051")

# Define a consistency boundary (same query you use while reading)
cb = Query(items=[QueryItem(types=["example"], tags=["tag1", "tag2"])])

# Read to build decision model
read_resp = client.read(query=cb)
for r in read_resp:
    print(f"Existing event at {r.position}: {r.event}")

last_known = read_resp.head()
print("Last known position:", last_known)

# Produce a new event with a UUID (for idempotent retries)
ev = Event(
    event_type="example",
    tags=["tag1", "tag2"],
    data=b"Hello, world!",
    uuid=str(uuid.uuid4()),
)

# Append with an optimistic condition: fail if conflicting events exist after last_known
cond = AppendCondition(fail_if_events_match=cb, after=last_known)
position1 = client.append([ev], condition=cond)
print("Appended at:", position1)

# Conflicting append should raise an error (e.g. ValueError)
try:
    client.append(
        [
            Event(
                event_type="example",
                tags=["tag1", "tag2"],
                data=b"Hello, world!",
                uuid=str(uuid.uuid4()),
            )
        ],
        condition=cond,
    )
except IntegrityError as e:
    print("Conflicting event was rejected:", e)

# Idempotent retry with same event UUID and condition should return same commit position
position2 = client.append([ev], condition=cond)
assert position1 == position2
print("Idempotent retry returned position:", position2)

Reading Events

The Client.read() method returns recorded events from an UmaDB server.

python

def read(
    query: Query | None = None,
    start: int | None = None,
    backwards: bool = False,
    limit: int | None = None,
    subscribe: bool = False,
) -> ReadResponse:
    ...

The Client.read() method can be used both for constructing decision models in a domain layer, and for projecting events into materialized views in CQRS. An optional Query can be provided to select by tags and types.

Parameters

Name	Type	Description
`query`	`Query\|None`	Optional structured query to filter events (by tags, event types, etc).
`start`	`int\|None`	Read events from this sequence number. Only events with positions greater than or equal will be returned (or less than or equal if `backwards` is `True`.
`backwards`	`bool`	If `True` events will be read backwards, either from the given position or from the last recorded event.
`limit`	`int\|None`	Optional cap on the number of events to retrieve.
`subscribe`	`bool`	If `True`, keeps the stream open to deliver future events as they arrive.

Return Value

Returns an iterable "read response" instance from which SequencedEvent instances, and the most relevant "last known" sequence number, can be obtained.

Example

python

from umadb import Client, Query, QueryItem

client = Client("http://localhost:50051")

# Filter by type(s) and tag(s)
q = Query(items=[QueryItem(types=["example"], tags=["tag1", "tag2"])])

resp = client.read(
    query=q, start=None, backwards=False, limit=None, subscribe=False
)
for item in resp:
    print(f"Got event at position {item.position}: {item.event}")

last_known = resp.head()
print("Last known position:", last_known)

# Subscribe to new events
subscription = client.read(subscribe=True)
for se in subscription:
    print("New event:", se.position, se.event)
    # Break for demo purposes
    break

Getting Head Position

The Client.head() method returns the position of the last event recorded in an UmaDB server.

python

def head(self) -> int | None: ...

The Client.head() method can be used for counting the number of recorded events in the database, or for determining the position of the last recorded event when subscribing only to new events.

Return Value

Returns the position (u64) of the last recorded event in the event store, or None if no events have been recorded yet.

Getting Tracking Info

The Client.get_tracking_info() method returns the last recorded position in an upstream sequence of events.

python

def get_tracking_info(self, source: str) -> int | None: ...

The Client.get_tracking_info() method can be used when starting or resuming an event-processing component. The event-processing component will start by requesting new events from the upstream sequence after this position. The position of an upstream event that has been processed successfully can be recorded atomically when appending new events generated by processing that event.

Parameters

Name	Type	Description
`source`	`str`	Upstream sequence name.

Returns the last recorded upstream position (int), or None if the sequence name is not found.

Event

An Event represents a single event either to be appended or already stored in the event log.

Field	Type	Description
`event_type`	`str`	The event’s logical type (e.g. `"UserRegistered"`).
`tags`	`list<str>`	Tags assigned to the event (used for filtering and indexing).
`data`	`bytes`	Binary payload associated with the event.
`uuid`	`str\|None`	Unique event ID.

Idempotent support for append operations is activated by setting a UUID on appended events.

Include in:

Append requests when writing new events to the store.

Included in:

SequencedEvent objects when the server responds to read requests.

Matched by:

QueryItem during read() and append() operations.

Append Condition

An AppendCondition causes an append request to fail if events match its Query, optionally after a sequence number.

Field	Type	Description
`fail_if_events_match`	`Query`	Query for conflicting events.
`after`	`int\|None`	Sequence number.

Include in:

Append requests to define optimistic concurrent control.

To implement a consistency boundary, command handlers can use the same Query used when reading events as the value of fail_if_events_match, and the "head" sequence number received from the read response as the value of after.

Tracking Info

A TrackingInfo instance indicates the source and position of an upstream event.

Field	Type	Description
`source`	`str`	Upstream sequence name.
`position`	`int`	Upstream sequence number.

Include in:

Append requests when recording the results of processing an upstream event.

To implement exactly-once semantics in event-processing components, pull events from an upstream source after the last recorded position, then record the upstream positions of upstream events along with new state that results from processing those events. By processing event sequentially in this way, each event will be processed at least once. And by recording tracking information along with new state, the new state will be recorded at most once. The combination of "at least once" processing and "at most once" recording gives "exactly once" semantics from the point of view of consumers of the recorded state.

Query

A Query defines criteria for selecting events in the event store.

Field	Type	Description
`items`	`list[QueryItem]`	A list of selection criteria (logical OR).

An Event is selected if any QueryItem matches or the items field is empty.

Include in:

Read requests to select events returned by the server.
An AppendCondition to select conflicting events.

Query Item

A QueryItem defines a criterion for matching events.

Field	Type	Description
`types`	`list[str]`	List of event types (logical OR).
`tags`	`list[str]`	List of tags (logical AND).

A QueryItem will match an `Event if:

one of its types matches the Event.event_type or its types field is empty; AND
all of its tags match one of the Event.tags or its tags field is empty.

Sequenced Event

A SequencedEvent represents a recorded Event along with its assigned sequence number.

Field	Type	Description
`position`	`int`	The sequence number.
`event`	`Event`	The recorded event.

Included in:

Read responses when the server responds to read requests.

Error Handling

The Python client raises Python exceptions on error:

Integrity/condition failure: IntegrityError
Transport/connection errors: TransportError
Authentication failures: AuthenticationError
Invalid argument errors: ValueError
Other internal errors: RuntimeError or OSError

Your application should catch these as appropriate.

Complete Example

python

import uuid

from umadb import AppendCondition, Client, Event, IntegrityError, Query, QueryItem

# Connect to the gRPC server (TLS + API key)
client = Client(
    url="http://localhost:50051",
)

# Define a consistency boundary
cb = Query(items=[QueryItem(types=["example"], tags=["tag1", "tag2"])])

# Read events for a decision model
read_response = client.read(query=cb)
for result in read_response:
    print(f"Got event at position {result.position}: {result.event}")

# Remember the last-known position
last_known_position = read_response.head()
print("Last known position is:", last_known_position)

# Create an event with a UUID to enable idempotent append
event = Event(
    event_type="example",
    tags=["tag1", "tag2"],
    data=b"Hello, world!",
    uuid=str(uuid.uuid4()),
)

# Append event within the consistency boundary
condition = AppendCondition(fail_if_events_match=cb, after=last_known_position)
commit_position1 = client.append([event], condition=condition)
print("Appended event at position:", commit_position1)

# Append conflicting event — expect an error
try:
    conflicting_event = Event(
        event_type="example",
        tags=["tag1", "tag2"],
        data=b"Hello, world!",
        uuid=str(uuid.uuid4()),  # different UUID
    )
    client.append([conflicting_event], condition=condition)
except IntegrityError as e:
    print("Conflicting event was rejected:", e)

# Idempotent retry — same event ID and condition
print("Retrying to append event at position:", last_known_position)
commit_position2 = client.append([event], condition=condition)
assert commit_position1 == commit_position2
print("Append returned same commit position:", commit_position2)

# Subscribe to all events for a projection
subscription = client.read(subscribe=True)
for ev in subscription:
    print(f"Processing event at {ev.position}: {ev.event}")
    if ev.position == commit_position2:
        print("Projection has processed new event!")
        break

Example with Tracking

python

import uuid

from umadb import (
    AppendCondition,
    Client,
    Event,
    IntegrityError,
    Query,
    QueryItem,
    TrackingInfo,
)

# Connect to the gRPC server (TLS + API key)
client = Client(
    url="http://localhost:50051",
)

# Get last processed upstream event position
last_processed_position = client.get_tracking_info("upstream")

# Pull next unprocessed upstream event...
next_upstream_event_position = 1 + (last_processed_position or 0)

# Construct tracking information from next unprocessed event
tracking_info = TrackingInfo("upstream", next_upstream_event_position)

# Define a consistency boundary
cb = Query(items=[QueryItem(types=["example"], tags=["tag1", "tag2"])])

# Read events for a decision model
read_response = client.read(query=cb)
for result in read_response:
    print(f"Got event at position {result.position}: {result.event}")

# Remember the last-known position
last_known_position = read_response.head()
print("Last known position is:", last_known_position)

# Create an event with a UUID to enable idempotent append
event = Event(
    event_type="example",
    tags=["tag1", "tag2"],
    data=b"Hello, world!",
    uuid=str(uuid.uuid4()),
)

# Append event within the consistency boundary
condition = AppendCondition(fail_if_events_match=cb, after=last_known_position)
commit_position1 = client.append(
    [event], condition=condition, tracking_info=tracking_info
)
print("Appended event at position:", commit_position1)

# Idempotent retry — same event ID and condition
print("Retrying to append event at position:", last_known_position)
commit_position2 = client.append(
    [event], condition=condition, tracking_info=tracking_info
)
assert commit_position1 == commit_position2
print("Append returned same commit position:", commit_position2)

# Check tracking information
assert tracking_info.position == client.get_tracking_info("upstream")

# Unconditional append with conflicting tracking information — expect an error
try:
    conflicting_event = Event(
        event_type="example",
        tags=["tag1", "tag2"],
        data=b"Hello, world!",
        uuid=str(uuid.uuid4()),  # different UUID
    )
    client.append([conflicting_event], condition=None, tracking_info=tracking_info)
except IntegrityError as e:
    print("Conflicting event was rejected:", e)

Notes

Python client is synchronous and blocking; if you need async integration, run client calls in a thread pool or use an async worker that offloads to threads.
Event data is binary (bytes). Use a consistent serialization (e.g., JSON serialized to UTF-8 bytes, protobuf, msgpack) for your domain.
API keys must match the server configuration.
For TLS with self-signed certs, pass ca_path with your root/CA certificate.

Python Client ​

Installation ​

Connecting to UmaDB ​

Parameters ​

Connection Examples ​

Appending Events ​

Parameters ​

Return Value ​

Example ​

Reading Events ​

Parameters ​

Return Value ​

Example ​

Getting Head Position ​

Return Value ​

Getting Tracking Info ​

Parameters ​

Event ​

Append Condition ​

Tracking Info ​

Query ​

Query Item ​

Sequenced Event ​

Error Handling ​

Complete Example ​

Example with Tracking ​

Notes ​

Python Client

Installation

Connecting to UmaDB

Parameters

Connection Examples

Appending Events

Parameters

Return Value

Example

Reading Events

Parameters

Return Value

Example

Getting Head Position

Return Value

Getting Tracking Info

Parameters

Event

Append Condition

Tracking Info

Query

Query Item

Sequenced Event

Error Handling

Complete Example

Example with Tracking

Notes