In computer science, message queues and mailboxes are software-engineering components used for interprocess communication, or for inter-thread communication within the same process. They use a queue for messaging – the passing of control or of content. Group communication systems provide similar kinds of functionality.
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Message queues provide an asynchronous communications protocol, meaning that the sender and receiver of the message do not need to interact with the message queue at the same time. Messages placed onto the queue are stored until the recipient retrieves them. Message queues have implicit or explicit limits on the size of data that may be transmitted in a single message and the number of messages that may remain outstanding on the queue.
Many implementations of message queues function internally: within an operating system or within an application. Such queues exist for the purposes of that system only.[1][2]
Other implementations allow the passing of messages between different computer systems, potentially connecting multiple applications and multiple operating systems.[3] These message queueing systems typically provide enhanced resilience functionality to ensure that messages do not get "lost" in the event of a system failure. Examples of commercial implementations of this kind of message queueing software (also known as message-oriented middleware) include IBM's WebSphere MQ (formerly MQ Series) and Oracle Advanced Queuing (AQ). There is a Java standard called Java Message Service, which has several proprietary and free software implementations.
There are a number of open source choices of messaging middleware systems, including JBoss Messaging, JORAM, Apache ActiveMQ, Sun Open Message Queue, Apache Qpid[4], RabbitMQ, and HTTPSQS[5]
In addition to open source systems, hardware-based messaging middleware is a growing trend with vendors like Solace Systems, Sonoa / Apigee and Tervela offering queuing through silicon or silicon/software datapaths.
Most RTOSes, such as VxWorks and QNX operating systems encourage the use of message queueing as the primary IPC or inter-thread communication mechanism. The resulting tight integration between message passing and CPU scheduling is attributed as a main reason for the usability of RTOSes for real time applications. Early examples of commercial RTOSes that encouraged a message-queue basis to inter-thread communication also include VRTX and pSOS+, both of which date to the early 1980s.
In a typical message-queueing implementation, a system administrator installs and configures off-the-shelf message-queueing software (a queue manager), and defines a named message queue.
An application then registers a software routine that "listens" for messages placed onto the queue.
Second and subsequent applications may connect to the queue and transfer a message onto it.
The queue-manager software stores the messages until a receiving application connects and then calls the registered software routine. The receiving application then processes the message in an appropriate manner.
There are often numerous options as to the exact semantics of message passing, including:
These are all considerations that can have substantial effects on transaction semantics, system reliability, and system efficiency.
Many of the more widely-known communications protocols in use operate synchronously. The HTTP protocol – used in the World Wide Web and in web services – offers an obvious example where a user sends a request for a web page and then waits for a reply.
However, scenarios exist in which synchronous behaviour is not appropriate. For example, AJAX (Asynchronous Javascript and XML) can be used to asynchronously send text or XML messages to update part of a web page with more relevant information. Google uses this approach for their Google Suggest, a search feature which sends the user's partially typed queries to Google's servers and returns a list of possible full queries the user might be in the process of typing. This list is asynchronously updated as the user types.
Other asynchronous examples exist in event notification systems and publish/subscribe systems.
In both of the above examples it would not make sense for the sender of the information to have to wait if, for example, one of the recipients had crashed.
Applications need not be exclusively synchronous or asynchronous. An interactive application may need to respond to certain parts of a request immediately (such as telling a customer that a sales request has been accepted, and handling the promise to draw on inventory), but may queue other parts (such as completing calculation of billing, forwarding data to the central accounting system, and calling on all sorts of other services) to be done some time later.
In all these sorts of situations, having a subsystem which performs message-queuing (or alternatively, a broadcast messaging system) can help improve the behaviour of the overall system.
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