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HTTP idempotence means that multiple identical requests have the same intended server-side effect as one request. It concerns the requested resource operation, rather than requiring every part of server state or every returned message to remain identical. The HTTP request methods defined as idempotent include PUT, DELETE and the HTTP safe methods.^[ref]

A small state model makes the distinction precise. Let $e\in \{0,1\}$ indicate whether a resource exists, and model deletion by $f(e)=0$. ThenDeleting again cannot make the resource more absent. Contrast an application operation that appends an item to a list: applying it twice can add two items.

For an invented endpoint, suppose a first DELETE removes an item and returns 204, while a second returns 404 because it is already absent. The intended final existence state agrees after one or two requests, although the HTTP status codes differ. These particular responses are stipulated for the example, not required for every deletion endpoint.

Idempotence also differs from read-only behavior. The first deletion can change a resource, whereas an HTTP safe method requests essentially read-only semantics. Separate logging or revision history can change on repeated requests without contradicting idempotence of the intended operation.^[ref]

This property matters when a connection fails before a client receives the response: repeating an idempotent request can recover the intended result without needing to know whether the earlier attempt completed. It does not establish that the operation was executed only once. A method without a general idempotence guarantee can still implement a particular application operation that is idempotent, but that requires additional knowledge of its semantics.^[ref]

HTTP pipelining sends several HTTP requests on one HTTP persistent connection without waiting for each corresponding HTTP response before sending the next request. In HTTP/1.1, the server must send the responses in request order, even when it can process requests in parallel. Parallel processing of the sequence is permitted when its methods are all HTTP safe methods.^[ref]

For example, a client can send independent GET requests for /a, /b and /c before receiving the response for /a. The response for /a must come first, followed by those for /b and /c. Each response still needs correct HTTP message framing; persistence and pipelining do not merge three responses into one representation.

A simple timing model shows the motivation. Suppose an established connection has a round-trip propagation time of $100$ milliseconds, every request takes $10$ milliseconds of server processing, the server processes them sequentially, and transmission times are negligible. Waiting for each complete response before submitting the next gives arrival times $110$, $220$ and $330$ milliseconds at the client. Sending all three requests at time zero lets them reach the server together after $50$ milliseconds; their processing finishes at $60$, $70$ and $80$ milliseconds, so responses arrive at $110$, $120$ and $130$ milliseconds. These are stipulated model calculations, not measured browser performance.

Ordered responses also impose head-of-line blocking. If the first response cannot be completed promptly, a later response cannot pass it on this connection even if its computation is ready. The processing order and the permitted transmission order are therefore different constraints.

Pipelining also requires care after a failed connection: retrying outstanding requests is governed by method semantics and the protocol's recovery rules. HTTP idempotence helps determine when automatic repetition is justified. A client must not interpret the disappearance of a connection as proof that none of its outstanding operations took effect.^[ref]

An HTTP request method identifies the operation requested for an HTTP request's target. The target and method have separate roles: the same resource can be read, replaced or processed according to the chosen method and its defined semantics. Method tokens are case-sensitive.^[ref]

For example, an invented /color resource uses GET to retrieve its current color and PUT to replace that color with the supplied value. One PUT storing blue and two identical such operations leave the same intended color. A different /log resource is stipulated to append each POST value: two appends of x then produce two entries. The append behavior belongs to this example application; it is not a universal definition of every POST.

HTTP idempotence concerns repeated intended effects, not identical response messages. A repeated replacement can leave the same state while returning a different HTTP status code because the resource already exists. An HTTP safe method requests essentially read-only semantics; incidental activities such as logging do not make every safe request a request to change the resource.^[ref]

Cacheability is another property, governed by method and response rules rather than by idempotence alone. Interpret the HTTP request method, HTTP header fields and HTTP response together before deciding what a repeated or cached exchange means.^[ref]

An HTTP safe method has essentially read-only requested semantics: the client does not ask the server to change resource state through the operation. GET and HEAD are examples. The definition concerns what the operation requests, rather than asserting that no incidental activity occurs anywhere on the server.^[ref]

For example, reading a stored color can return blue while leaving that color unchanged. The server can still append an access-log entry. A later read can return red if some separate operation changed the color in between; safe semantics do not require an unchanging response or an immutable resource.

At the intended-state level, a read-only operation can be modeled as $f(s)=s$. It follows immediately thatThis illustrates why safe methods are also idempotent. The converse fails: deletion can map both present and absent states to absent, satisfying HTTP idempotence while changing a present resource on its first application.

The chosen HTTP request method remains meaningful even when the target URL contains a query string. Naming a destructive action in a query does not redefine GET as a method for requesting that action; the standard requires such actions to be disallowed when accessed through safe methods.^[ref]

Use this term for protocol semantics. Successful transfer, access permission and the truth of returned application data are separate questions. Interpret the method with the target and application contract, rather than deciding safety from whether a response happens to contain a body.