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Publish and Subscribe ​

This guide shows the practical, Python side of Pub/Sub programming against Bondy — assuming you already know what a subscription, a publication, and a topic are. For the protocol-level mechanics of each advanced feature, see Advanced Publish/Subscribe; this guide only shows how to reach for it in code.

How Subscriptions Work ​

A subscription binds a topic URI to your session on the router; Bondy delivers any matching publication to it, rather than a publisher addressing subscribers directly — publishers and subscribers never need to know about each other. See Publish/Subscribe for why WAMP works this way.

Basic Subscription ​

Typically you subscribe on the session's on_join callback, the same way you'd register a procedure:

Python
async def _on_join(self, session, details):
    self._session = session
    await self._session.subscribe(self.on_order_created, "com.example.order.created")

def on_order_created(order_id, **kwargs):
    print(f"Order created: {order_id}")

And publishing:

Python
await self._session.publish("com.example.order.created", order_id)

By default a publication doesn't wait for acknowledgement — pass options=PublishOptions(acknowledge=True) if you need to confirm the router accepted it.

Subscriber Black- and Whitelisting ​

Restrict which sessions receive a specific publication, regardless of who else is subscribed:

Python
from autobahn.wamp.types import PublishOptions

# Only these sessions receive it
await self._session.publish(
    "com.example.admin.command", command,
    options=PublishOptions(eligible=[session_id_1, session_id_2])
)

# Everyone except these sessions receives it
await self._session.publish(
    "com.example.broadcast", announcement,
    options=PublishOptions(exclude=[session_id_3])
)

# Only sessions holding the operator role receive it
await self._session.publish(
    "com.example.ops.alert", alert,
    options=PublishOptions(eligible_authrole=["operator"])
)

An empty eligible list delivers the event to no subscriber.

See Subscriber Black- and White-listing for combining this with excluding the publisher itself.

Publisher Exclusion ​

By default Bondy does not deliver a publication to its own publisher, even when the publisher is subscribed to the topic. exclude_me defaults to true. To receive your own publication, set exclude_me to False:

Python
from autobahn.wamp.types import PublishOptions

await self._session.publish(
    "com.example.state.updated", state,
    options=PublishOptions(exclude_me=False)
)

Keep the default for chat and collaborative-editing topics, where a publisher does not need its own message echoed back.

Publisher Identification ​

Bondy discloses the publisher to subscribers by default. To read the publisher's identity, subscribe with SubscribeOptions(details_arg=...) and read details.publisher, details.publisher_authid and details.publisher_authrole:

Python
from autobahn.wamp.types import SubscribeOptions

# Subscriber: read the publisher's identity
def on_event(data, details=None):
    print("Published by session:", details.publisher)
    print("Authenticated as:", details.publisher_authid)

await self._session.subscribe(
    on_event, "com.example.events",
    options=SubscribeOptions(details_arg="details")
)

# Publisher: no option is needed
await self._session.publish("com.example.events", data)

Bondy omits the publisher only when the PUBLISH sets disclose_me to false. See Publisher Identification.

Pattern-Based Subscriptions ​

Subscribe to a URI prefix or wildcard instead of an exact topic to receive events from a whole family of topics on one subscription:

Python
from autobahn.wamp.types import SubscribeOptions

def on_user_event(*args, details=None, **kwargs):
    print("Event on topic:", details.topic)

await self._session.subscribe(
    on_user_event, "com.example.users.",
    options=SubscribeOptions(match="prefix", details_arg="details")
)

# Receives com.example.users.created, com.example.users.updated, etc.

details.topic carries the exact topic a publication was sent to, so one subscription can dispatch by suffix. See Pattern-based Subscriptions for prefix vs. wildcard matching.

Event Retention ​

Bondy can retain the last event on a topic and deliver it to a subscriber that was not connected when it was published — configured node-wide via wamp.message_retention.*. A publisher opts a specific event into retention, and a subscriber opts into receiving it on subscribe:

Python
from autobahn.wamp.types import PublishOptions, SubscribeOptions

# Publisher retains this event
await self._session.publish(
    "com.example.service.status", None,
    status="operational",
    options=PublishOptions(retain=True)
)

# A subscriber joining later still gets the last retained event immediately
await self._session.subscribe(
    on_status, "com.example.service.status",
    options=SubscribeOptions(get_retained=True)
)

This is retention of the last event per topic, not a queryable history of many past events — see Event Retention for the exact semantics, and Message Retention for the storage limits (max_messages, max_memory, max_message_size, default_ttl) that bound it.

Subscription Meta Events and Procedures ​

Bondy publishes a subscription's lifecycle as WAMP meta-events, and lets a session introspect the registry directly, both under the reserved wamp.subscription.* namespace:

Python
# Watch subscribe/unsubscribe on this realm
await self._session.subscribe(
    on_subscribed, "wamp.subscription.on_subscribe"
)
await self._session.subscribe(
    on_unsubscribed, "wamp.subscription.on_unsubscribe"
)

# List the subscriptions an event on a topic would reach
sub_ids = await self._session.call("wamp.subscription.match", "com.example.users.created")

Use this for operational visibility — knowing when a topic gains or loses subscribers — rather than as part of your application's own event path. Bondy does not publish wamp.subscription.on_create or wamp.subscription.on_delete. To learn whether a topic still has subscribers, call wamp.subscription.lookup or wamp.subscription.match.

See also ​