What is an idempotent operation

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It is any operation that every nth result will result in an output matching the value of the 1st result. For instance the absolute value of -1 is 1. The absolute value of the absolute value of -1 is 1. The absolute value of the absolute value of absolute value of -1 is 1. And so on.

See also: When would be a really silly time to use recursion?

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In short  Idempotent operations means that the operation will not result in different results no matter how many times you operate the idempotent operations   For example  according to the definition of the spec of HTTP  GET  HEAD  PUT  and DELETE are idempotent operations  however POST and PATCH are not  That s why sometimes POST is replaced by PUT

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In computing  an idempotent operation is one that has no additional effect if it is called more than once with the same input parameters  For example  removing an item from a set can be considered an idempotent operation on the set   In mathematics  an idempotent operation is one where f f x     f x   For example  the abs   function is idempotent because abs abs x     abs x  for all x    These slightly different definitions can be reconciled by considering that x in the mathematical definition represents the state of an object  and f is an operation that may mutate that object  For example  consider the Python set and its discard method  The discard method removes an element from a set  and does nothing if the element does not exist  So   my set discard x    has exactly the same effect as doing the same operation twice   my set discard x  my set discard x    Idempotent operations are often used in the design of network protocols  where a request to perform an operation is guaranteed to happen at least once  but might also happen more than once  If the operation is idempotent  then there is no harm in performing the operation two or more times   See the Wikipedia article on idempotence for more information     The above answer previously had some incorrect and misleading examples  Comments below written before April 2014 refer to an older revision

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Just wanted to throw out a real use case that demonstrates idempotence  In JavaScript  say you are defining a bunch of model classes  as in MVC model   The way this is often implemented is functionally equivalent to something like this  basic example    function model name      function Model         this name   name         return Model      You could then define new classes like this   var User   model  user    var Article   model  article      But if you were to try to get the User class via model  user    from somewhere else in the code  it would fail   var User   model  user           then somewhere else in the code  in a different scope  var User   model  user      Those two User constructors would be different  That is   model  user       model  user      To make it idempotent  you would just add some sort of caching mechanism  like this   var collection        function model name      if  collection name       return collection name      function Model         this name   name         collection name    Model    return Model      By adding caching  every time you did model  user   it will be the same object  and so it s idempotent  So   model  user       model  user

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retry-safe   Is usually the easiest way to understand its meaning in computer science

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Idempotent Operations  Operations that have no side-effects if executed multiple times   Example  An operation that retrieves values from a data resource and say  prints it  Non-Idempotent Operations  Operations that would cause some harm if executed multiple times   As they change some values or states   Example  An operation that withdraws from a bank account

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No matter how many times you call the operation  the result will be the same

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An idempotent operation over a set leaves its members unchanged when applied one or more times   It can be a unary operation like absolute x  where x belongs to a set of positive integers  Here absolute absolute x     x   It can be a binary operation like union of a set with itself would always return the same set   cheers

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An idempotent operation produces the result in the same state even if you call it more than once  provided you pass in the same parameters

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Idempotence means that applying an operation once or applying it multiple times has the same effect   Examples    Multiplication by zero  No matter how many times you do it  the result is still zero  Setting a boolean flag  No matter how many times you do it  the flag stays set  Deleting a row from a database with a given ID  If you try it again  the row is still gone    For pure functions  functions with no side effects  then idempotency implies that f x    f f x     f f f x      f f f f x              for all values of x  For functions with side effects  idempotency furthermore implies that no additional side effects will be caused after the first application  You can consider the state of the world to be an additional  hidden  parameter to the function if you like   Note that in a world where you have concurrent actions going on  you may find that operations you thought were idempotent cease to be so  for example  another thread could unset the value of the boolean flag in the example above   Basically whenever you have concurrency and mutable state  you need to think much more carefully about idempotency   Idempotency is often a useful property in building robust systems  For example  if there is a risk that you may receive a duplicate message from a third party  it is helpful to have the message handler act as an idempotent operation so that the message effect only happens once

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my 5c  In integration and networking the idempotency is very important  Several examples from real-life  Imagine  we deliver data to the target system  Data delivered by a sequence of messages   1  What would happen if the sequence is mixed in channel   As network packages always do       If the target system is idempotent  the result will not be different  If the target system depends of the right order in the sequence  we have to implement resequencer on the target site  which would restore the right order  2  What would happen if there are the message duplicates  If the channel of target system does not acknowledge timely  the source system  or channel itself  usually sends another copy of the message  As a result we can have duplicate message on the target system side   If the target system is idempotent  it takes care of it and result will not be different  If the target system is not idempotent  we have to implement deduplicator on the target system side of the channel

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A good example of understanding an idempotent operation might be locking a car with remote key   log Car state     unlocked  Remote lock    log Car state     locked  Remote lock    Remote lock    Remote lock    log Car state     locked   lock is an idempotent operation  Even if there are some side effect each time you run lock  like blinking  the car is still in the same locked state  no matter how many times you run lock operation

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Quite a detailed and technical answers  Just adding a simple definition      Idempotent   Re-runnable   For example   Create operation in itself is not guaranteed to run without error if executed more than once  But if there is an operation CreateOrUpdate then it states re-runnability  Idempotency

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An idempotent operation is an operation  action  or request that can be applied multiple times without changing the result  i e  the state of the system  beyond the initial application    EXAMPLES  WEB APP CONTEXT    IDEMPOTENT  Making multiple identical requests has the same effect as making a single request  A message in an email messaging system is opened and marked as  opened  in the database  One can open the message many times but this repeated action will only ever result in that message being in the  opened  state  This is an idempotent operation  The first time one PUTs an update to a resource using information that does not match the resource  the state of the system   the state of the system will change as the resource is updated  If one PUTs the same update to a resource repeatedly then the information in the update will match the information already in the system upon every PUT  and no change to the state of the system will occur  Repeated PUTs with the same information are idempotent  the first PUT may change the state of the system  subsequent PUTs should not    NON-IDEMPOTENT  If an operation always causes a change in state  like POSTing the same message to a user over and over  resulting in a new message sent and stored in the database every time  we say that the operation is NON-IDEMPOTENT   NULLIPOTENT  If an operation has no side effects  like purely displaying information on a web page without any change in a database  in other words you are only reading the database   we say the operation is NULLIPOTENT  All GETs should be nullipotent    When talking about the state of the system we are obviously ignoring hopefully harmless and inevitable effects like logging and diagnostics

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An idempotent operation can be repeated an arbitrary number of times and the result will be the same as if it had been done only once   In arithmetic  adding zero to a number is idempotent   Idempotence is talked about a lot in the context of  RESTful  web services   REST seeks to maximally leverage HTTP to give programs access to web content  and is usually set in contrast to SOAP-based web services  which just tunnel remote procedure call style services inside HTTP requests and responses   REST organizes a web application into  resources   like a Twitter user  or a Flickr image  and then uses the HTTP verbs of POST  PUT  GET  and DELETE to create  update  read  and delete those resources   Idempotence plays an important role in REST   If you GET a representation of a REST resource  eg  GET a jpeg image from Flickr   and the operation fails  you can just repeat the GET again and again until the operation succeeds   To the web service  it doesn t matter how many times the image is gotten   Likewise  if you use a RESTful web service to update your Twitter account information  you can PUT the new information as many times as it takes in order to get confirmation from the web service   PUT-ing it a thousand times is the same as PUT-ing it once   Similarly DELETE-ing a REST resource a thousand times is the same as deleting it once   Idempotence thus makes it a lot easier to construct a web service that s resilient to communication errors   Further reading  RESTful Web Services  by Richardson and Ruby  idempotence is discussed on page 103-104   and Roy Fielding s PhD dissertation on REST   Fielding was one of the authors of HTTP 1 1  RFC-2616  which talks about idempotence in section 9 1 2

[language-agnostic] What is an idempotent operation?

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