computer science Computer science is the study of computation, information, and automation. Computer science spans Theoretical computer science, theoretical disciplines (such as algorithms, theory of computation, and information theory) to Applied science, ...

, message queues and mailboxes are software-engineering

components Component may refer to: In engineering, science, and technology Generic systems *System components, an entity with discrete structure, such as an assembly or software module, within a system considered at a particular level of analysis * Lumped e ...

typically used for

inter-process communication In computer science, interprocess communication (IPC) is the sharing of data between running Process (computing), processes in a computer system. Mechanisms for IPC may be provided by an operating system. Applications which use IPC are often cat ...

(IPC), or for inter- thread communication within the same process. They use a queue for

messaging A message is a unit of communication that conveys information from a sender to a receiver. It can be transmitted through various forms, such as spoken or written words, signals, or electronic data, and can range from simple instructions to co ...

– the passing of control or of content. Group communication systems provide similar kinds of functionality. The message queue paradigm is a sibling of the publisher/subscriber pattern, and is typically one part of a larger

message-oriented middleware Message-oriented middleware (MOM) is software or hardware infrastructure supporting sending and receiving messages between distributed systems. Message-oriented middleware is in contrast to streaming-oriented middleware where data is communicate ...

system. Most messaging systems support both the publisher/subscriber and message queue models in their

API An application programming interface (API) is a connection between computers or between computer programs. It is a type of software interface, offering a service to other pieces of software. A document or standard that describes how to build ...

, e.g.

Java Message Service The Jakarta Messaging API (formerly Java Message Service or JMS API) is a Java application programming interface (API) for message-oriented middleware. It provides generic messaging models, able to handle the producer–consumer problem, that can ...

(JMS). Competing Consumers pattern enables multiple concurrent consumers to process messages on the same message queue.

Remit and ownership

Message queues implement an asynchronous communication pattern between two or more processes/threads whereby the sending and receiving party 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.

Remit

Many implementations of message queues function internally within an

operating system An operating system (OS) is system software that manages computer hardware and software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ...

or within an application. Such queues exist for the purposes of that

system A system is a group of interacting or interrelated elements that act according to a set of rules to form a unified whole. A system, surrounded and influenced by its open system (systems theory), environment, is described by its boundaries, str ...

only. Other implementations allow the passing of messages between different computer systems, potentially connecting multiple applications and multiple operating systems. These message queuing systems typically provide 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 queuing

software Software consists of computer programs that instruct the Execution (computing), execution of a computer. Software also includes design documents and specifications. The history of software is closely tied to the development of digital comput ...

(also known as

) include IBM MQ (formerly MQ Series) and Oracle Advanced Queuing (AQ). There is a

Java Java is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea (a part of Pacific Ocean) to the north. With a population of 156.9 million people (including Madura) in mid 2024, proje ...

standard called

, which has several

proprietary {{Short pages monitor Many of the more widely known

communications protocol A communication protocol is a system of rules that allows two or more entities of a communications system to transmit information via any variation of a physical quantity. The protocol defines the rules, syntax, semantics (computer science), sem ...

s in use operate synchronously. The HTTP protocol – used in the

World Wide Web The World Wide Web (WWW or simply the Web) is an information system that enables Content (media), content sharing over the Internet through user-friendly ways meant to appeal to users beyond Information technology, IT specialists and hobbyis ...

and in

web service A web service (WS) is either: * a service offered by an electronic device to another electronic device, communicating with each other via the Internet, or * a server running on a computer device, listening for requests at a particular port over a n ...

s – 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 Ajax may refer to: Greek mythology and tragedy * Ajax the Great, a Greek mythological hero, son of King Telamon and Periboea * Ajax the Lesser, a Greek mythological hero, son of Oileus, the king of Locris * Ajax (play), ''Ajax'' (play), by the an ...

(

Asynchronous Asynchrony is any dynamic far from synchronization. If and as parts of an asynchronous system become more synchronized, those parts or even the whole system can be said to be in sync. Asynchrony or asynchronous may refer to: Electronics and com ...

JavaScript JavaScript (), often abbreviated as JS, is a programming language and core technology of the World Wide Web, alongside HTML and CSS. Ninety-nine percent of websites use JavaScript on the client side for webpage behavior. Web browsers have ...

and

XML Extensible Markup Language (XML) is a markup language and file format for storing, transmitting, and reconstructing data. It defines a set of rules for encoding electronic document, documents in a format that is both human-readable and Machine-r ...

) can be used to asynchronously send text, JSON or XML messages to update part of a web page with more relevant information.

Google Google LLC (, ) is an American multinational corporation and technology company focusing on online advertising, search engine technology, cloud computing, computer software, quantum computing, e-commerce, consumer electronics, and artificial ...

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 interested 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. * An application may need to notify another that an event has occurred, but does not need to wait for a response. * In publish/subscribe systems, an application "publishes" information for any number of clients to read. 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 behavior of the overall system.

Implementation in UNIX

There are two common message queue implementations in

UNIX Unix (, ; trademarked as UNIX) is a family of multitasking, multi-user computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, a ...

. One is part of the SYS V API, the other one is part of

POSIX The Portable Operating System Interface (POSIX; ) is a family of standards specified by the IEEE Computer Society for maintaining compatibility between operating systems. POSIX defines application programming interfaces (APIs), along with comm ...

SYS V

UNIX SYS V implements message passing by keeping an array of linked lists as message queues. Each message queue is identified by its index in the array, and has a unique descriptor. A given index can have multiple possible descriptors. UNIX gives standard functions to access the message passing feature. ;msgget(): This system call takes a key as an argument and returns a descriptor of the queue with the matching key if it exists. If it does not exist, and the IPC_CREAT flag is set, it makes a new message queue with the given key and returns its descriptor. ;msgrcv(): Used to receive a message from a given queue descriptor. The caller process must have read permissions for the queue. It is of two types. :*Blocking receive puts the child to sleep if it cannot find a requested message type. It sleeps until another message is posted in the queue, and then wakes up to check again. :*Non-blocking receive returns immediately to the caller, mentioning that it failed. ;msgctl(): Used to change message queue parameters like the owner. Most importantly, it is used to delete the message queue by passing the IPC_RMID flag. A message queue can be deleted only by its creator, owner, or the superuser.

POSIX

The POSIX.1-2001 message queue API is the later of the two UNIX message queue APIs. It is distinct from the SYS V API, but provides similar function. The unix man page mq_overview(7) provides an overview of POSIX message queues.

Graphical user interfaces

Graphical user interface A graphical user interface, or GUI, is a form of user interface that allows user (computing), users to human–computer interaction, interact with electronic devices through Graphics, graphical icon (computing), icons and visual indicators such ...

s (GUIs) employ a message queue, also called an ''event queue'' or ''input queue'', to pass graphical input actions, such as mouse clicks, keyboard events, or other user inputs, to the

application program Application software is any computer program that is intended for end-user use not operating, administering or programming the computer. An application (app, application program, software application) is any program that can be categorized as ...

. The windowing system places messages indicating user or other events, such as timer ticks or messages sent by other threads, into the message queue. The GUI application removes these events one at a time by calling a routine called getNextEvent() or similar in an

event loop In computer science, the event loop (also known as message dispatcher, message loop, message pump, or run loop) is a programming construct or design pattern that waits for and dispatches events or messages in a program. The event loop works by m ...

, and then calling the appropriate application routine to process that event.

References

{{DEFAULTSORT:Message queue Inter-process communication Events (computing) Queue management