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

, message queues and mailboxes are software-engineering components typically used for

inter-process communication In computer science, inter-process communication or interprocess communication (IPC) refers specifically to the mechanisms an operating system provides to allow the processes to manage shared data. Typically, applications can use IPC, categori ...

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

queue __NOTOC__ Queue () may refer to: * Queue area, or queue, a line or area where people wait for goods or services Arts, entertainment, and media *''ACM Queue'', a computer magazine * ''The Queue'' (Sorokin novel), a 1983 novel by Russian author ...

for

messaging A message is a discrete unit of communication intended by the source for consumption by some recipient or group of recipients. A message may be delivered by various means, including courier, telegraphy, carrier pigeon and electronic bus. ...

– the passing of control or of content.

Group communication system {{Unreferenced, date=April 2021 The term Group Communication System (GCS) refers to a software platform that implements some form of group communication. Examples of group communication systems include IS-IS, Spread Toolkit The Spread Toolkit ...

s 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. MOM allows application modules to be distributed over heterogeneous platforms and reduces the comple ...

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

API An application programming interface (API) is a way for two or more computer programs to communicate with each other. It is a type of software interface, offering a service to other pieces of software. A document or standard that describes how ...

, 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).

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, software resources, and provides common services for computer programs. Time-sharing operating systems schedule tasks for efficient use of the system and may also in ...

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

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

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 is a set of computer programs and associated documentation and data. This is in contrast to hardware, from which the system is built and which actually performs the work. At the lowest programming level, executable code consists ...

(also known as

) include

IBM MQ IBM MQ is a family of message-oriented middleware products that IBM launched in December 1993. It was originally called MQSeries, and was renamed ''WebSphere MQ'' in 2002 to join the suite of IBM WebSphere, WebSphere products. In April 2014, it was ...

(formerly MQ Series) and

Oracle Advanced Queuing In computing, Oracle Advanced Queuing (AQ) is a sort of message-oriented middleware developed by Oracle Corporation and integrated into its Oracle database. AQ uses database structures as a repository for asynchronous queuing as an element in va ...

(AQ). There is a

Java Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's List ...

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 kind of variation of a physical quantity. The protocol defines the rules, syntax, semantics and synchr ...

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

World Wide Web The World Wide Web (WWW), commonly known as the Web, is an information system enabling documents and other web resources to be accessed over the Internet. Documents and downloadable media are made available to the network through web se ...

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 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), by the ancient Greek ...

( Asynchronous

JavaScript JavaScript (), often abbreviated as JS, is a programming language that is one of the core technologies of the World Wide Web, alongside HTML and CSS. As of 2022, 98% of Website, websites use JavaScript on the Client (computing), client side ...

and

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

) 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 technology company focusing on search engine technology, online advertising, cloud computing, computer software, quantum computing, e-commerce, artificial intelligence, and consumer electronics. ...

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, multiuser 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, and ot ...

. 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 both the system- and user-level application programming interf ...

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 The GUI ( "UI" by itself is still usually pronounced . or ), graphical user interface, is a form of user interface that allows users to interact with electronic devices through graphical icons and audio indicator such as primary notation, inste ...

s (GUIs) employ a message queue, also called an ''event queue'' or ''input queue'', to pass graphical input actions, such as

mouse click In programming and software design, an event is an action or occurrence recognized by software, often originating asynchronously from the external environment, that may be handled by the software. Computer events can be generated or triggered ...

s, keyboard events, or other user inputs, to the

application program Application may refer to: Mathematics and computing * Application software, computer software designed to help the user to perform specific tasks ** Application layer, an abstraction layer that specifies protocols and interface methods used in a ...

. 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 is a programming construct or design pattern that waits for and dispatches events or messages in a program. The event loop works by making a request to some internal or external "event provider" (that generally ...

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

References

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