Ultimate Guide to HTTP Status Codes

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If asked by an interviewer, “What are the common HTTP status codes? What do they mean?“, how should we respond?

Brief Answer

The ten most frequently encountered HTTP status codes are as follows:

200: OK, indicates that the request has been successfully processed (e.g., webpage loaded successfully, API successfully returned data).
301: Moved Permanently, indicates that the resource has been permanently redirected to a new URL (e.g., when a website changes its domain, the old link redirects).
302: Found, indicates that the resource has been temporarily redirected to a new URL (e.g., redirecting back to the original page after login).
304: Not Modified, indicates that the resource has not been modified, and the client can use the local cache directly (this is the core status code of caching mechanism).
400: Bad Request, indicates that the client’s request format is incorrect (e.g., missing parameters, JSON syntax errors).
401: Unauthorized, indicates not authenticated (e.g., accessing a privileged interface without logging in).
403: Forbidden, indicates no permission to access the resource (e.g., a regular user accessing an admin interface).
404: Not Found, indicates that the resource does not exist (e.g., URL path error, file deleted).
500: Internal Server Error, indicates server internal error (e.g., code exception, database crash).
503: Service Unavailable, indicates that the service is temporarily unavailable (e.g., server maintenance, overload circuit breaker).

Detailed Answer

1. 2xx (Success)

200:

Definition: OK, indicates that the request has been successfully processed by the server; a GET request generally returns the resource content, while a POST request may return the operation result or a new resource link.
Typical Scenarios: Successfully loading a webpage (HTML content); API returning JSON data.

2. 3xx (Redirection)

301:

Definition: Moved Permanently, indicates that the requested resource has been permanently redirected to a new URL, and the client should update bookmarks or links; the browser will cache this redirection, and subsequent requests will directly access the new URL.
Typical Scenarios: Website changing domain (e.g., programmercarl.com → notes.kamacoder.com); global redirection from HTTP to HTTPS.

302:

Definition: Found, indicates that the resource has been temporarily redirected to a new URL, and subsequent requests may still access the original URL; the browser defaults to using the GET method for redirection, even if the original request was POST.
Typical Scenarios: Temporarily redirecting to the user homepage after login; callback redirection in OAuth authorization flow.

304:

Definition: Not Modified, indicates that the resource has not changed, and the client can use the cached copy directly; the server does not return a response body, only the 304 status and cache-related headers.
Typical Scenarios: Browser cache validation (through If-Modified-Since request header); CDN node cache validity checks.

3. 4xx (Client Error)

400:

Definition: Bad Request, indicates that there is a syntax or parameter error in the client’s request, and the server cannot process it; issues should be investigated in conjunction with the error description in the response body.
Typical Scenarios: JSON format error in request body; missing required parameters leading to request format errors.

401:

Definition: Unauthorized, indicates that the request did not provide valid credentials, and authentication is required to retry.
Typical Scenarios: An unauthenticated user accessing a privileged interface; token expired or invalid.

403:

Definition: Forbidden, indicates that the client’s identity has been authenticated, but they have no permission to access the target resource; a 404 may be returned instead when hiding inaccessible resources.
Typical Scenarios: A regular user attempting to access an admin interface; IP address blacklisted.

404:

Definition: Not Found, indicates that the server could not find the requested resource; it is possible that the server deliberately hides the existence of the resource (e.g., to avoid sensitive information leakage).
Typical Scenarios: URL path error; resource has been deleted (e.g., a user accessing a discontinued product).

4. 5xx (Server Error)

500:

Definition: Internal Server Error, indicates a server internal error, unable to complete the request; in development environments, detailed error stacks should be returned, while in production environments, sensitive information should be hidden.
Typical Scenarios: Unhandled exceptions in code (e.g., null pointer, database connection failure); server configuration errors.

503:

Definition: Service Unavailable, indicates that the server is temporarily unable to process the request (usually due to overload or maintenance).
Typical Scenarios: Server maintenance window; high concurrent traffic causing service circuit breaker (e.g., flash sales).

Knowledge Expansion

Detailed explanation of frequently used HTTP status codes is shown in the image below:

More HTTP status codes are shown in the table below:

Status Code	Status Code English Name	Chinese Description
`100`	Continue	Continue. The client should continue its request.
`101`	Switching Protocols	Switching protocols. The server switches protocols based on the client’s request (e.g., upgrading to WebSocket).
`200`	OK	Request successful. Generally used for GET and POST requests.
`201`	Created	Created. The request was successful and a new resource was created.
`202`	Accepted	Accepted. The request has been accepted but not yet processed (commonly used for asynchronous tasks).
`203`	Non-Authoritative Information	Non-authoritative information. The returned metadata comes from a copy rather than the original server.
`204`	No Content	No content. The server successfully processed the request but did not return any content (e.g., DELETE request).
`205`	Reset Content	Reset content. The client should reset the document view (e.g., clear the form).
`206`	Partial Content	Partial content. The server successfully processed part of the GET request (e.g., resume download).
`300`	Multiple Choices	Multiple choices. The resource has multiple addresses available, and the client needs to choose.
`301`	Moved Permanently	Moved permanently. The resource has been permanently redirected to a new URI (the browser will cache the new address).
`302`	Found	Found. The resource is temporarily redirected to a new URI (subsequent requests still use the original address).
`303`	See Other	See other address. Forces the client to access the new URI using the GET method (commonly used for redirection after POST).
`304`	Not Modified	Not modified. The client’s cached resource is still valid, no need to retransmit.
`305`	Use Proxy	Use proxy. Request must be accessed through a proxy (modern browsers have deprecated this).
`306`	Unused	Deprecated (originally designed for subsequent requests through a proxy, now replaced by 307/308).
`307`	Temporary Redirect	Temporary redirect. Keeps the original request method for redirection (e.g., temporary redirect for a POST request).
`400`	Bad Request	Client request syntax error, server cannot understand.
`401`	Unauthorized	Unauthorized. Valid credentials are required (e.g., login token).
`402`	Payment Required	Reserved status (actually used in payment scenarios, such as Stripe API calls).
`403`	Forbidden	Forbidden. Authenticated but no permission to access the resource.
`404`	Not Found	Not found. The server cannot find the resource based on the request.
`405`	Method Not Allowed	Method not allowed. The HTTP method requested is not allowed (e.g., using GET to access an interface that only supports POST).
`406`	Not Acceptable	Not acceptable. The server cannot provide the media type requested by the client (e.g., only supports JSON but requests XML).
`407`	Proxy Authentication Required	Proxy authentication required. Must authenticate through a proxy server.
`408`	Request Time-out	Request timeout. The server waited too long for the client to send the request.
`409`	Conflict	Conflict. The request conflicts with the current state of the server (e.g., concurrent editing resource version conflict).
`410`	Gone	Gone. The resource no longer exists (distinct from 404, clearly indicating that the resource has been actively deleted).
`411`	Length Required	Length required. The request must include a `Content-Length` header.
`412`	Precondition Failed	Precondition failed. The conditions in the request header are not met (e.g., `If-Match` validation failed).
`413`	Request Entity Too Large	Request entity too large. The server refuses to process (e.g., uploading a file exceeds the limit).
`414`	Request-URI Too Large	Request-URI too large. The requested URL exceeds the server limit (e.g., GET parameters too long).
`415`	Unsupported Media Type	Unsupported media type. The server cannot process the format attached to the request (e.g., uploaded file type not supported).
`416`	Requested range not satisfiable	Requested range invalid. The client requests data beyond the available range of the resource (e.g., requesting data beyond the end of a file).
`417`	Expectation Failed	Expectation failed. The server cannot meet the requirements of the `Expect` request header (e.g., `Expect: 100-continue` not supported).
`500`	Internal Server Error	Internal server error. Code exception or configuration error.
`501`	Not Implemented	Not implemented. The server does not support the functionality requested (e.g., using an unsupported HTTP method).
`502`	Bad Gateway	Bad gateway. The proxy server received an invalid response from upstream (e.g., backend service crashed).
`503`	Service Unavailable	Service unavailable. The server is overloaded or undergoing maintenance (can specify retry time using `Retry-After` header).
`504`	Gateway Time-out	Gateway timeout. The proxy server did not receive a timely response from upstream (e.g., backend service response timeout).
`505`	HTTP Version not supported	Unsupported HTTP version. The server does not support the protocol version in the request (e.g., requesting HTTP/3 but the server only supports HTTP/1.1).

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