Publish/Subscribe

1. Publish/Subscribe

2. Messaging Pattern • Publish/subscribe (or pub/sub) is a messaging pattern where senders (publishers) of messages are not programmed to send their messages to specific receivers (subscribers) • Published messages are characterized into classes, without knowledge of what, if any, subscribers there may be • Subscribers express interest in one or more classes, and only receive messages that are of interest, without knowledge of what, if any, publishers there are • Many-to-many relationships • Decoupling of publishers and subscribers can allow for greater scalability and a more dynamic network topology

3. Message Filtering • Subscribers typically receive only a subset of the total messages published • Topic-based: messages are published to "topics“, “subjects”, or named logical channels - subscribers in a topic-based system will receive all messages published to the topics to which they subscribe, and all subscribers to a topic will receive the same messages • Content-based: messages are only delivered to a subscriber if the attributes or content of those messages match constraints defined by the subscriber • Filters may be strings consist ing of logical combinations of name-value pairs, comparison operators, wildcards • Or may be template objects (type-based) or executable code • Some systems support a hybrid where publishers post messages to a topic while subscribers register content-based subscriptions to one or more topics

4. Topology • Publishers post messages to an intermediary message broker (aka event notification service) and subscribers register subscriptions with that broker • The broker performs a store and forward function to route messages from publishers to subscribers • Publishers are not blocked while producing events • Subscribers can get asynchronous notification of events

5. Event Services

6. COM Events(Request-Reply)

7. Choice between two techniques Interface callback mechanism Connectable Objects Original COM Events

8. Client implements a COM interface defined by the event publisher component, and passes to the component a pointer to this interface Client then receives notifications (i.e., callbacks) when the component calls a method through the interface implemented by the client code Interface Callbacks

9. Also known as connection points Client implements a COM interface defined by COM’s standard IConnectionPoint interface (and other related interfaces), and passes a pointer to this interface Connect (Advise) and disconnect (Unadvise) Enumerate connections (EnumConnections) Client then receives notifications (i.e., callbacks) when the component calls a method through the interface implemented by the client code Connectable Objects

10. Example

11. Only a series of interfaces - developers still have to write all the code to implement these interfaces (no infrastructure) Implementing a complex application making heavy use of events may require complex coding to handle multiplexing events to multiple connected clients, circular references, deadlock situations, etc. Client and component lifetimes are tightly coupled through the exchanged interface pointer - the client must be running and connected to receive events It is difficult to get between a component instance and its clients to monitor the connection, provide trace information, etc. COM Events Limitations

12. Publish-subscribe model rather than request-reply An intermediary object manages communication between a publisher and its subscribers Publishers and subscribers are not tightly bound Asynchronous: Publishers do not block when firing an event and subscribers do not wait to receive COM+ Events

13. An event class component sits between a publisher of information and any potential subscribers COM+ Events system provides the actual implementation of this intermediate object Eliminates the need to directly pass an interface pointer Event Class

14. The event class looks like a subscriber to the publisher When a publisher wants to “fire” an event, it creates an instance of the event class, calls the appropriate method, and then releases the interface (as in queued components) The runtime then determines how and when to notify any subscribers Publisher

15. To receive events, need only implement the event interface Registers with the COM+ Events service by creating a subscription object, through the IEventSubscription interface The component will be (activated and) notified as events are published Either persistent or transient subscriptions Subscriber

16. Example

17. Example

18. Provides a “third-party” publish-subscribe environment: Once an event class is created, anyone can become a publisher or subscriber of the events Supports a rich filter mechanism: one can filter at the publisher method level – IPublisherFilter allows the event class object to decide which subscribers receive a particular event, or at the method parameter level – ISubscribeControl supports a complex criteria string per subscriber Improved COM+ Events

19. Filtering Example

20. EJB Events Message-Driven Beans

21. Messaging enables distributed communication that is loosely coupled A component sends a message to a destination, and the recipient retrieves the message from the destination The sender and the receiver do not have to be available at the same time The sender does not need to know anything about the receiver, nor vice versa Both only need to know which message format and which destination to use Differs from tightly coupled technologies, such as Remote Method Invocation (RMI), which require an application to know a remote application’s methods Messaging

22. Synchronous (pull): A subscriber explicitly fetches the message from the destination by calling the receive method - the receive method can block until a message arrives or can time out if a message does not arrive within a specified time limit Asynchronous (push): A client can register a message listener with a consumer - Whenever a message arrives at the destination, the provider delivers the message by calling the listener’s onMessage method, which acts on the contents of the message Message Consumption

23. Allows Java Enterprise Edition applications to process messages asynchronously (session beans can only receive synchronous messages) Acts as a JMS (Java Message Service) message listener Messages can be sent by an application client, another enterprise bean, a web component, or a JMS system that does not use Java EE technology Message-Driven Beans

24. Common set of interfaces and associated semantics that allow programs written in Java to communicate with other messaging implementations The JMS API can ensure that a message is delivered once and only once (PERSISTENT) Lower reliability, at most once (NON_PERSISTENT), is available for applications that can afford to miss messages JMS API

25. A JMS provider is a messaging system that implements the JMS interfaces and provides administrative and control features (included in Java EE) JMS clients are the programs or components that produce and consume messages Messages are the objects that communicate information between JMS clients Administered objects are preconfigured JMS objects (destinations and connection factories) created by an administrator for the use of clients via Java Naming and Directory Interface (JNDI) JMS API Architecture

26. JMS API Architecture

27. Either point-to-point or publish/subscribe JMS API provides common interfaces not specific to either model Messaging Domains

28. Built on the concept of message queues, senders and receivers Each message is addressed to a specific queue, and receiving clients extract messages from the queues established to hold their messages Queues retain all messages sent to them until the messages are consumed or until the messages expire Point-to-Point

29. Each message has only one consumer A sender and a receiver of a message have no timing dependencies - the receiver can fetch the message whether or not it was running when the client sent the message The receiver acknowledges the successful processing of a message Point-to-Point

30. Clients address messages to a topic Each message can have multiple consumers Publishers and subscribers are anonymous and can dynamically publish or subscribe to the content hierarchy The system distributes the messages arriving from a topic’s multiple publishers to its multiple subscribers Topics retain messages only as long as it takes to distribute them to current subscribers Publish/Subscribe

31. Publishers and subscribers have a timing dependency – a client that subscribes to a topic can consume only messages published after the client has created a subscription, and normally the subscriber must continue to be active in order for it to consume messages JMS relaxes this timing dependency by allowing durable subscriptions, which receive messages sent while the subscribers are not active Publish/Subscribe

32. Developer does not define any interfaces, only a bean class that implements the MessageListener interface Otherwise resembles a stateless session bean: Retains no data or conversational state for a specific client All instances equivalent, allowing EJB container to assign a message to any bean instance in a pool Can process messages from multiple clients (one at a time) Client-independent state can be retained across messages (e.g., JMS API connection, open database connection, object reference to an enterprise bean) How do Message-Driven Beans and Session Beans differ?

33. The container usually creates a pool of message-driven bean instances For each, the container calls the @PostConstruct method, if any Lifecycle of a Message-Driven Bean

34. A message-driven bean is never passivated, and it has only two states: nonexistent and ready to receive messages At the end of the life cycle, the container calls the @PreDestroy method, if any Lifecycle of a Message-Driven Bean