HTTP requests, representations, and caching

Updated 06 Oct 2026

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Definition
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  HTTP → application-layer protocol defining how clients request resources and servers respond.
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  a resource can have multiple representations, such as different image formats or languages.
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Clients
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  browsers, command-line tools, mobile applications, backend services, and other programs can act as HTTP clients.
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Typical navigation
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  interpret the URL; reuse a suitable cahce response or connection where possible; resolve the hostname when needed; exchange a request and response. the application processes it, and a browser may request furtehr resources
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Protocol and browser behavior
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  HTTP/1.1 message examples show a textual wire format; HTTP/2 and HTTP/3 use different framing.
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  Browser rendering is applictaion behavior build on top of HTTP

Messages

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a request identifies a method, target, and protocol version; it may carry headers and a body.
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a response carries a status code, headers, and sometimes a body.
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headers carry metadata such as content type, accepted formats, cache policy, authentication, and connection details.
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Body → contains representation data and is not necessarily HTML.

sequenceDiagram
    autonumber
    participant Client as Client Browser/App
    participant Server as Server Backend

    Note over Client,Server: 1. First request - no cache
    Client->>Server: GET /api/v1/users/101
    Note over Client,Server: Headers: Host, Accept, User-Agent
    Note over Client,Server: Body: empty
    Note over Server: Fetch data, generate ETag "a8f3-v1"
    Server-->>Client: 200 OK
    Note over Client,Server: Content-Type: application/json
    Note over Client,Server: ETag: "a8f3-v1"
    Note over Client,Server: Body: JSON id 101, name Alice

    Note over Client,Server: 2. Second request - ETag revalidation
    Client->>Server: GET /api/v1/users/101
    Note over Client,Server: If-None-Match: "a8f3-v1"
    Note over Server: ETag matches, data unchanged
    Server-->>Client: 304 Not Modified
    Note over Client,Server: ETag: "a8f3-v1"
    Note over Client,Server: Body: empty, use local cache

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Methods
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  GET retrieves a representation and is intended to be safe.
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  HEAD asks for response metadata without the representation body.
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  POST commonly submits data or requests processing.
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  PUT commonly replaces a resource representation; PATCH applies a partial change; DELETE requests removal.
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  safety and idempotence describe intended semantics, not a guarantee that every server implementation is correct.
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Status codes
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  Status classes → 1xx informational; 2xx success; 3xx redirection or revalidation; 4xx request/client problem; 5xx server problem. Examples: 200 OK, 304 Not Modified, 404 Not Found, and 500 Internal Server Error.
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  inspect headers and body as well as the status.
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Testing
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  curl makes requests and inspects responses; the practicum varies methods and header. curl -i includes response headers; curl -I makes a HEAD request
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Negotiation
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  clients express preferences using headers such as Accept, Accept-Language, and Accept-Encoding
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  servers describe the selected representation with headers such as Content-Type ad Content-Encoding
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  a server may return 406 Not Acceptable when it cannot meet acceptable constraints
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Caching
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  vary tells caches which request headers influenced the selected representation
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  withotu correct variation metadata, a cahce could serve the wrong language or format
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Cache policy
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  caching stores responses so later requests can avoid transferring or recomputing unchanged content
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  Cache-Control -> defines policy; max-age sets a freshness lifetime
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  no-store asks chaces not to store a response
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  no-cache permits storage but requires validation before reuse
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Validation
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  a stale response can be checked with If-None-Match and an ETag, or If-Modified-Since and a modification date
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  if unchanged, the server can return 304 Not Modified and the client can reuse the stored body
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  expires, age, and shared-cache directives also affect behavior
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Asset versioning
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  change an asset URL when its content changes, often by adding a content hash to the filename
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  Versioned assets -> allow long-lived caching; the HTML that points to them can use a shroter policy

Linked references 2

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HTTP requests, representations, and caching

Definition

Clients

Typical navigation

Protocol and browser behavior

Messages

Methods

Status codes

Testing

Negotiation

Caching

Cache policy

Validation

Asset versioning

Linked references 2