General.Synchronous communication.Synchronous communication represents a tight coupling between a sender and receiver: • the sender and receiver must both be available - ie real-time communication • the sender will block until the response is available • communication is one-to-one • sender / receiver have some knowledge of each other - eg location, protocol, methods, etc. • the sender is responsible for dealing with failures - eg retries, etc. "request / response" is an example of a synchronous communication paradigm.Example : a browser sends a request to a web server and "waits" for the response The "wait" can be indefinite or until a timeout occurs.Asynchronous communication.Asynchronous communication represents a loose coupling between senders / receivers:. • sender and receiver communicate Via A Middle-man / Broker / Mediator • No Requirement for sender or receiver to Both Be available - IE Deferred Communication • sender doesn't Wait for a response (if any) • Communication Can Be O ne-to-one or one-to-many • sender and receiver have no direct knowledge of each other so promotes reuse / extension - eg new senders / receivers can be added without impact; senders / receivers can be replaced as required • the broker Is Responsible for DEALING WITH FAILES - EG RETRIES, ETC. • Messages May Be delayed and / or arrive out of sequence
JMS.JMS (Java Message Service) is an API from Sun which allows generic access to MOM services - in a similar way that JDBC allows generic access to databases.Concepts • Provider -. Vendor implementation of the JMS API • Messaging domains - two types of domain, Point To Point (PTP) or publisher / subscriber • Message consumption - two types of consumption, synchronous and asynchronous (see below) • Administered objects -. JMS bootstrap objects (eg connection factories, destinations) bound in the JNDI namespace by an administrator • ConnectionFactory - Queue / TopicConnectionFactory, used to create a connection to the Provider • Destination - Queue / Topic, target of message production / consumption • Connection - Queue / TopicConnection, encapsulates connection to the Provider; start () indicates that message delivery SHOULD BEGIN • SESSION - Queue / TopicSession, Used to create producer geConsumer - QueueReceiver / TopicSubscriber; consume messages synchronously using the receive () methods or asynchronously by passing a MessageListener to setMessageListener () • Message - object produced or consumed; composed of header, properties and body Body types are: text, name / value. pairs, bytes, stream of primitives, serialized object or none.PTP messaging. • based on Queues, Senders and Receivers • Senders and Receivers address the Queue by name • one or more Senders but only one Receiver • messages stay in the Queue until consumed OR PAST EXPIRY TIME • Receiver Acknowledges Successful Processing of a Message
Publisher / subscriber. • based on Topics, Publishers and Subscribers • Publishers and Subscribers address the Topic by name • one or more Publishers publish to the Topic subscribed to by one or more Subscribers • messages only stay in the Topic for as long as it takes to send them to the active Subscribers • a subscriber can only consume "active" messages - ie those published since the subscriber became active (unless durable subscriptions are in use) List Benefits of Synchronous and Asynchronous MessagingThe benefits of synchronous messaging follow: • Because both parties must be active to participate in synchronous messaging, if either party is not active, the message transaction can not be completed. • A message must be acknowledged before proceeding to the next message. If it is not acknowledged, the message can not be considered processed.
The benefits of asynchronous messaging are as follows: • As the volume of traffic increases, asynchronous messaging is better able to handle the spike in demand by keeping a backlog of requests in its queue and then operating at maximum capacity over a period of time instead of . needing to service the requests instantaneously • Asynchronous messaging is less affected by failures at the hardware, software, and network levels • When capacities are exceeded, information is not lost;. instead, it is delayed.
Identify Scenarios That Are Appropriate to Implementation Using Messaging • Scenarios appropriate to implementation using message include asynchronous communication, one-to-many communication, guaranteed messaging, and transactional messaging.
Identify Scenarios That Are More Appropriate to Implementation Using Asynchronous Messaging, Rather Than Synchronous • You need to implement a messaging system in which a response is not required or not immediately required. • You need a high-volume transaction processing capability for sending messages. • You want a messaging system that uses your system hardware in an efficient manner.Identify Scenarios that Are More Appropriate to Implementation Using Synchronous Messaging, Rather Than Asynchronous • One scenario more appropriate to synchronous messaging includes that in which a response to the message is required before CONTINUING, For Example, For Transactions Requiring Credit Card or User Login Authentication. • A Transaction Where Both Parties Must Be Active Participants.
Identify Scenarios That Are Appropriate to Implementation Using Messaging, Enterprise JavaBeans Technology, or Both • The scenarios appropriate for messaging technology include broadcasting stock prices to traders, instant messages, and in situations when integration of incompatible systems is necessary. • The scenarios appropriate for EJB technology include those that perform business logic and those that maintain persistent data. • The scenarios appropriate for messaging and EJB technology including those that require maintenance of distributed transactions and those that send an order to another system.
