Message Facade Pattern

Consider wanting to make an Airline Reservation. The client would need to have the system check the airline for availability and seating. This can involve a number of steps as outlined below. If the client had to request each and wait for a response this could be too long for most users to sit around....you may loose a customer.

 

Problems:

  • Client does not need immediate response....so why make client wait for responses.

A Solution.....the Message Facade Pattern

 

  • Now have 1 network request to a Message Facade code (ReserveSeatMDB message-driven EJB)
  • Bundle all of the business logic in a seperated Message Facade code (ReserveSeatMDB message-driven EJB)
  • Do not wait on response:
    1) Client creates a Java Message Service (JMS) message and passes the necessary parameter.
    2) Client sends this message to a JMS destination created for the Reserve Seat use case
    3) Upon receiving the message at the destination, the client is free.
    4)The server has a "message-driven bean container" that will attempt to pass the message to the next avaialbe ReserveSeat message-driven bean. If all of these are busy the JMS server should wait until the next one becomes availab.e
    5) When a RerserveSeat MDB becomes available, the container will execute the onMessage() method.




 

QUESTION: How do we get a response from a Message-Driven class like our ReserveSeat MDB?

  • Send email to user/client.
  • Have client application poll using some unique ID to check the staus of their request.

 

 

Synopsis: Message Facade

Name: Message Facade

Context

A client wants to invoke the methods of multiple programs within the context of one use case and does not require an immediate response from the server.

Problem (forces)

How can a client invoke methods of multiple classes within one transaction, without the need to block and wait for responses from each?

Solution

Use message-driven classes (beans) to create a fault-tolerant, asynchronous facade. Clients should have access to message-driven classes (beans) only, not the underlying data layer, business logic (entity and session beans).

Related Patterns

Session Facade.

 

 

PROBLEM:

No direct response. (some possbilities with JMS as transport mechanism...see advanced book on EJB).

Message-Drive code does not throw exceptions back to clients.