Enterprise Integration Patterns: Designing, Building, and Deploying Messaging Solutions (Addison-Wesley Signature Series (Fowler))

by Hohpe Gregor

Thread Management. Asynchronous communication means that one application does not have to block while waiting for another application to perform a task, unless it wants to. Rather than blocking to wait for a reply, the caller can use a callback that will alert the caller when the reply arrives. (See the Request-Reply [154] pattern.) A large number of blocked threads or threads blocked for a long time can leave the application with too few available threads to perform real work. Also, if an application with a dynamic number of blocked threads crashes, reestablishing those threads will be difficult when the application restarts and recovers its former state. With callbacks, the only threads that block are a small, known number of listeners waiting for replies. This leaves most threads available for other work and defines a known number of listener threads that can easily be reestablished after a crash.