Motivation

Before this document, only publishers were discoverable via ZeroConf, and the announced information was weak for filtering for supported events and any other semantics. Also, subscribers always subscribed automatically to every discovered publisher which used the same schema.

To overcome already applied workarounds and missing features for filtering, application-compatible subscriptions and user-controllable subscriptions, this document describes necessary changes to ZeroEQ.

Use-cases are coupling of applications that belong to a certain user/session, not coupling all applications on the same network. User-interfaces for selection of applications that should connect to each other need more information for the user and for the system to perform filtering.

Overview of changes

Introduction of Session: only publishers of the same session and compatible events are automatically subscribed in the subscriber. The default session name is the current username. It can be set to a different value at runtime with the ZEROEQ_PUB_SESSION environment variable. If the session name is empty/non-existant, no filtering is performed and automatic subscription to all discovered publishers happens.
Schemas in URIs are superseded by the session; user-specified schemas are ignored. Filtering is now done via sessions and compatible events. As a future use-case, the schema might control the transport for zmq (tcp, inproc, ...)
The ZeroConf record contains information about published/subscribed events, application name (derived automatically), username and session.
Vocabulary stores event type information from an associated publisher or subscriber. The vocabulary information can be requested (this class implements request-reply using the REQ-REP socket from zmq). One vocabulary is associated with exactly one publisher or with one subscriber.
Broker can handle explicit (un)subscriptions instructed by a remote application.

Changes for Publisher

ZeroConf/Servus service name is _zeroeq_pub._tcp
Schema is fixed, internal and ignored if specified
URI can contain hostname and port
ZeroConf announcement:
- Instance name is hostname:port
- txt record:
  - vocabulary_request = zmq connection string
  - vocabulary = semi-colon separated list of published event UUIDs
  - session = <user-name> by default, env ZEROEQ_PUB_SESSION if specified
  - user = <user-name>
  - application = <application-name>
Published events must be registered in the publisher. Non-registered events cannot be published. (TBD: Published events are auto-registered).
Add (un)registerEvent()

Changes for Subscriber

ZeroConf/Servus service name is _zeroeq_sub._tcp
Schema is fixed, internal and ignored if specified
URI can contain hostname and port
Automatic subscription only via session (default session is username), otherwise explicit subscription via Broker.
ZeroConf announcement:
- Instance name is a UUID to uniquely identify this subscriber
- txt record:
  - vocabulary_request = zmq connection string
  - vocabulary = semi-colon separated list of published event UUIDs
  - session = <user-name> by default, env ZEROEQ_PUB_SESSION if specified
  - user = <user-name>
  - application = <application-name>
Event registration for vocabulary happens in registerHandler/subscribe

Changes for Broker/Management port

Supports subscribe/unsubscribe requests for any remote application
- Tell application to subscribe to an event of a publisher
List of current subscriptions (for NodeGraph UI for instance)

New class Vocabulary

Provides a request-reply for vocabulary information
Holds registered events from a publisher or subscriber