Added the psycopg event broker

docs/api.rst

@@ -39,6 +39,7 @@ Event brokers
 .. autoclass:: apscheduler.abc.Subscription
 .. autoclass:: apscheduler.eventbrokers.local.LocalEventBroker
 .. autoclass:: apscheduler.eventbrokers.asyncpg.AsyncpgEventBroker
+.. autoclass:: apscheduler.eventbrokers.psycopg.PsycopgEventBroker
 .. autoclass:: apscheduler.eventbrokers.mqtt.MQTTEventBroker
 .. autoclass:: apscheduler.eventbrokers.redis.RedisEventBroker
 

docs/versionhistory.rst

@@ -4,6 +4,10 @@ Version history
 To find out how to migrate your application from a previous version of
 APScheduler, see the :doc:`migration section <migration>`.
 
+**UNRELEASED**
+
+- Added the ``psycopg`` event broker
+
 **4.0.0a5**
 
 - **BREAKING** Added the ``cleanup()`` scheduler method and a configuration option

src/apscheduler/eventbrokers/psycopg.py

@@ -0,0 +1,174 @@
+from __future__ import annotations
+
+from collections.abc import AsyncGenerator, Mapping
+from contextlib import AsyncExitStack, asynccontextmanager
+from logging import Logger
+from typing import TYPE_CHECKING, Any, NoReturn
+from urllib.parse import urlunparse
+
+import attrs
+from anyio import (
+    EndOfStream,
+    create_memory_object_stream,
+    move_on_after,
+)
+from anyio.abc import TaskStatus
+from anyio.streams.memory import MemoryObjectSendStream
+from attr.validators import instance_of
+from psycopg import AsyncConnection, InterfaceError
+
+from .._events import Event
+from .._exceptions import SerializationError
+from .._validators import positive_number
+from .base import BaseExternalEventBroker
+
+if TYPE_CHECKING:
+    from sqlalchemy.ext.asyncio import AsyncEngine
+
+
+def convert_options(value: Mapping[str, Any]) -> dict[str, Any]:
+    return dict(value, autocommit=True)
+
+
+@attrs.define(eq=False)
+class PsycopgEventBroker(BaseExternalEventBroker):
+    """
+    An asynchronous, psycopg_ based event broker that uses a PostgreSQL server to
+    broadcast events using its ``NOTIFY`` mechanism.
+
+    .. psycopg: https://pypi.org/project/psycopg/
+
+    :param conninfo: a libpq connection string (e.g.
+        ``postgres://user:pass@host:port/dbname``)
+    :param channel: the ``NOTIFY`` channel to use
+    :param max_idle_time: maximum time (in seconds) to let the connection go idle,
+        before sending a ``SELECT 1`` query to prevent a connection timeout
+    """
+
+    conninfo: str = attrs.field(validator=instance_of(str))
+    options: Mapping[str, Any] = attrs.field(
+        factory=dict, converter=convert_options, validator=instance_of(Mapping)
+    )
+    channel: str = attrs.field(
+        kw_only=True, default="apscheduler", validator=instance_of(str)
+    )
+    max_idle_time: float = attrs.field(
+        kw_only=True, default=10, validator=[instance_of((int, float)), positive_number]
+    )
+
+    _send: MemoryObjectSendStream[str] = attrs.field(init=False)
+
+    @classmethod
+    def from_async_sqla_engine(
+        cls,
+        engine: AsyncEngine,
+        options: Mapping[str, Any] | None = None,
+        **kwargs: Any,
+    ) -> PsycopgEventBroker:
+        """
+        Create a new psycopg event broker from a SQLAlchemy engine.
+
+        The engine will only be used to create the appropriate options for
+        :meth:`psycopg.AsyncConnection.connect`.
+
+        :param engine: an asynchronous SQLAlchemy engine using asyncpg as the driver
+        :type engine: ~sqlalchemy.ext.asyncio.AsyncEngine
+        :param options: extra keyword arguments passed to :func:`asyncpg.connect` (will
+            override any automatically generated arguments based on the engine)
+        :param kwargs: keyword arguments to pass to the initializer of this class
+        :return: the newly created event broker
+
+        """
+        if engine.dialect.driver != "psycopg":
+            raise ValueError(
+                f'The driver in the engine must be "psycopg" (current: '
+                f"{engine.dialect.driver})"
+            )
+
+        conninfo = urlunparse(
+            [
+                "postgres",
+                engine.url.username,
+                engine.url.password,
+                engine.url.host,
+                engine.url.database,
+            ]
+        )
+        opts = dict(options, autocommit=True)
+        return cls(conninfo, opts, **kwargs)
+
+    @property
+    def _temporary_failure_exceptions(self) -> tuple[type[Exception], ...]:
+        return OSError, InterfaceError
+
+    @asynccontextmanager
+    async def _connect(self) -> AsyncGenerator[AsyncConnection, None]:
+        async for attempt in self._retry():
+            with attempt:
+                conn = await AsyncConnection.connect(self.conninfo, **self.options)
+                try:
+                    yield conn
+                finally:
+                    await conn.close()
+
+    async def start(self, exit_stack: AsyncExitStack, logger: Logger) -> None:
+        await super().start(exit_stack, logger)
+        await self._task_group.start(self._listen_notifications)
+        exit_stack.callback(self._task_group.cancel_scope.cancel)
+        self._send = await self._task_group.start(self._publish_notifications)
+        await exit_stack.enter_async_context(self._send)
+
+    async def _listen_notifications(self, *, task_status: TaskStatus[None]) -> None:
+        task_started_sent = False
+        while True:
+            async with self._connect() as conn:
+                try:
+                    await conn.execute(f"LISTEN {self.channel}")
+
+                    if not task_started_sent:
+                        task_status.started()
+                        task_started_sent = True
+
+                    self._logger.debug("Listen connection established")
+                    async for notify in conn.notifies():
+                        event = self.reconstitute_event_str(notify.payload)
+                        await self.publish_local(event)
+                except InterfaceError as exc:
+                    self._logger.error("Connection error: %s", exc)
+
+    async def _publish_notifications(
+        self, *, task_status: TaskStatus[MemoryObjectSendStream[str]]
+    ) -> NoReturn:
+        send, receive = create_memory_object_stream[str](100)
+        task_started_sent = False
+        with receive:
+            while True:
+                async with self._connect() as conn:
+                    if not task_started_sent:
+                        task_status.started(send)
+                        task_started_sent = True
+
+                    self._logger.debug("Publish connection established")
+                    notification: str | None = None
+                    while True:
+                        with move_on_after(self.max_idle_time):
+                            try:
+                                notification = await receive.receive()
+                            except EndOfStream:
+                                return
+
+                        if notification:
+                            await conn.execute(
+                                "SELECT pg_notify(%t, %t)", [self.channel, notification]
+                            )
+                        else:
+                            await conn.execute("SELECT 1")
+
+    async def publish(self, event: Event) -> None:
+        notification = self.generate_notification_str(event)
+        if len(notification) > 7999:
+            raise SerializationError(
+                "Serialized event object exceeds 7999 bytes in size"
+            )
+
+        await self._send.send(notification)

tests/conftest.py

@@ -86,6 +86,17 @@ async def asyncpg_broker(serializer: Serializer) -> EventBroker:
     return broker
 
 
+@pytest.fixture
+async def psycopg_broker(serializer: Serializer) -> EventBroker:
+    pytest.importorskip("psycopg", reason="psycopg is not installed")
+    from apscheduler.eventbrokers.psycopg import PsycopgEventBroker
+
+    broker = PsycopgEventBroker(
+        "postgres://postgres:secret@localhost:5432/testdb", serializer=serializer
+    )
+    return broker
+
+
 @pytest.fixture(
     params=[
         pytest.param(lf("local_broker"), id="local"),
@@ -94,6 +105,11 @@ async def asyncpg_broker(serializer: Serializer) -> EventBroker:
             id="asyncpg",
             marks=[pytest.mark.external_service],
         ),
+        pytest.param(
+            lf("psycopg_broker"),
+            id="psycopg",
+            marks=[pytest.mark.external_service],
+        ),
         pytest.param(
             lf("redis_broker"),
             id="redis",