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An HTTP conditional request carries one or more preconditions to be evaluated before applying its HTTP request method. The condition can ask whether the selected HTTP representation still has a previously observed entity tag. This makes the request depend on the state the client knows, rather than applying an operation unconditionally.^[ref]

For HTTP cache validation, suppose an earlier retrieval of /color stored the text blue with entity tag "v7". A later GET with If-None-Match: "v7" asks for the representation only if its tag no longer matches. If the selected representation still matches, the condition is false and the server returns status 304 instead of transferring the representation data again. If it has changed, an otherwise successful retrieval can return the new data.^[ref]

A second use prevents overwriting another client's work. Suppose two clients read version "v7". The first makes an update and the server assigns version "v8". The second submits a different PUT with If-Match: "v7". The stale tag does not match, so the server must not apply that update; in this example it returns 412. Checking and applying the condition as part of the operation prevents the second client from silently replacing the first client's changes.^[ref]

The two field names deliberately use different comparisons: If-Match requires strong entity tag comparison, while If-None-Match uses weak comparison and can match a weak entity tag. Failed If-Match need not always yield 412: the specification permits a success response when the requested change appears already applied. Authentication, redirects and other normal request checks also precede precondition evaluation. An HTTP conditional request therefore combines method semantics, a particular condition and the specified evaluation order.

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]

An HTTP request is a message asking a server to apply an HTTP request method to a target resource. It includes control information and HTTP header fields, and can include content according to the applicable method and message rules. Its meaning is distinct from the textual framing used by one protocol version.^[ref]

For an illustrative HTTP/1.1 request, the line GET /color?view=plain HTTP/1.1 selects a method and request target, while Host: example.org supplies host information. An empty line ends the header section. These pieces belong to the version's request-line and field syntax.^[ref]

Starting from https://example.org/color?view=plain#legend, the ordinary origin-form target is /color?view=plain. The query string participates in that target, while the fragment identifier does not: fragment processing is performed on the client side rather than by including the fragment in the target sent to the server.^[ref][ref]

The method cannot be inferred from a content string alone. Sending the text blue to an endpoint stipulated to replace a color and sending it to an endpoint stipulated to append a log entry request different operations. The target, method and application contract supply that distinction.

An HTTP request also differs from a returned HTTP response. A successfully transmitted request can receive an error status, and a connection can end before a final response is obtained. Reasoning about the requested operation, its eventual outcome and the transport event as separate observations makes failures easier to interpret.

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.

The Hypertext Transfer Protocol is an application-level communication protocol for exchanging requests and responses about resources. In the client-server model, an HTTP request supplies an HTTP request method and target; an HTTP response supplies an HTTP status code and associated information. HTTP header fields convey metadata, and messages can carry content. These semantics are distinct from the particular wire format used by a protocol version.^[ref]

For a concrete exchange, a client can send method GET for https://example.org/manual. A successful response can carry status 200 and an HTML representation of the resource. A response with status 404 instead says that the server did not find a current representation, or is unwilling to disclose one; successful communication does not require successful retrieval.^[ref]

The request's URL identifies the target, while the response describes the outcome of this exchange. A later exchange can return different content or a different status. The protocol's stateless request semantics also do not prevent an application from maintaining state: the distinction concerns what a request must mean, not whether a user can have an ongoing session.^[ref]