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Writing Open Metadata Topic Connectors

The open metadata topic connector provides a topic interface to a generic string event.

It is the interface implemented by specific event buses.

Topic connectors for specific types of events (such as the OMRS Topic Connector) are configured with an instance of an open metadata topic connector embedded inside it.

The open metadata topic connector means that only one connector need be implemented for each type of event bus - rather than one for each type of topic that Egeria supports.

Implementations of this type of connector are located in the adapters/open-connectors/event-bus-connectors/open-metadata-topic-connectors module. There are two implementations: one for Apache Kafka (KafkaOpenMetadataTopicConnector) and an in-memory implementation (InMemoryOpenMetadataTopicConnector) that is useful for testing.

Open metadata topic connector interface

The definition of the connector interface for these connectors is defined in the open metadata repository services (OMRS) repository-services-apis module in the org.odpi.openmetadata.repositoryservices.connectors.openmetadatatopic Java package.

The OpenMetadataTopic interface has two methods:

  • sendEvent(String event) for sending events to other systems/applications.
  • registerListener(OpenMetadataTopicListener newListener) for registering a listener that is called each time an event is received from the event broker. A listener implements the OpenMetadataTopicListener interface, which has a single method: processEvent(String event).

Implementing an open metadata topic connector

An open metadata topic connector extends the abstract OpenMetadataTopicConnector class, which extends the OCF's ConnectorBase and implements OpenMetadataTopic. The base class manages the registered listeners:

  • registerListener() stores the listener and returns the name of the topic, which is taken from the network address of the connection's endpoint.
  • start() reads the topic name from the endpoint and an optional sleepTime configuration property, and starts a listener thread.
  • The listener thread repeatedly calls checkForIncomingEvents() and passes each event returned to every registered listener, sleeping for sleepTime between checks.
  • disconnect() stops the listener thread.

The implementation must therefore:

  • implement sendEvent() to publish an event to the event bus.
  • override checkForIncomingEvents() to return the list of IncomingEvent objects received from the event bus since the last call. (The older checkForEvents() method, which returns a list of strings, is deprecated.)
  • override start() and disconnect() to connect to and disconnect from the event bus, calling the superclass's methods.

The connector provider extends OpenMetadataTopicProvider, which is a subclass of OpenConnectorProviderBase.

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