For every HTTP request, the server returns a status code indicating the processing status of the request. In this section, we will see some of the frequently used HTTP status codes. A basic understanding of status codes will definitely help us later while designing RESTful web services:

• 1xx Informational: This series of status codes indicates informational content. This means that the request is received and processing is going on. Here are the frequently used informational status codes:
  • 100 Continue: This code indicates that the server has received the request header and the client can now send the body content. In this case, the client first makes a request (with the Expect: 100-continue header) to check whether it can start with a partial request. The server can then respond either with 100 Continue (OK) or 417 Expectation Failed (No) along with an appropriate reason.
  • 101 Switching Protocols: This code indicates that the server is OK for a protocol switch request from the client.
  • 102 Processing: This code is an informational status code used for long running processing to prevent the client from timing out. This tells the client to wait for the future response, which will have the actual response body.
• 2xx Success: This series of status codes indicates the successful processing of requests. Some of the frequently used status codes in this class are as follows:
  • 200 OK: This code indicates that the request is successful and the response content is returned to the client as appropriate.
  • 201 Created: This code indicates that the request is successful and a new resource is created.
  • 204 No Content: This code indicates that the request is processed successfully, but there's no return value for this request. For instance, you may find such status codes in response to the deletion of a resource.
• 3xx Redirection: This series of status codes indicates that the client needs to perform further actions to logically end the request. A frequently used status code in this class is as follows:
  • 304 Not Modified: This status indicates that the resource has not been modified since it was last accessed. This code is returned only when allowed by the client via setting the request headers as If-Modified-Since or If-None-Match. The client can take appropriate action on the basis of this status code.
• 4xx Client Error: This series of status codes indicates an error in processing the request. Some of the frequently used status codes in this class are as follows:
  • 400 Bad Request: This code indicates that the server failed to process the request because of malformed syntax in the request. The client can try again after correcting the request.
  • 401 Unauthorized: This code indicates that authentication is required for the resource. The client can try again with appropriate authentication.
  • 403 Forbidden: This code indicates that the server is refusing to respond to the request even if the request is valid. The reason will be listed in the body content if the request is not a HEAD method.
  • 404 Not Found: This code indicates that the requested resource is not found at the location specified in the request.
  • 405 Method Not Allowed: This code indicates that the HTTP method specified in the request is not allowed on the resource identified by the URI.
  • 408 Request Timeout: This code indicates that the client failed to respond within the time window set on the server.
  • 409 Conflict: This code indicates that the request cannot be completed because it conflicts with some rules established on resources, such as validation failure.
• 5xx Server Error: This series of status codes indicates server failures while processing a valid request. Here is one of the frequently used status codes in this class:
  • 500 Internal Server Error: This code indicates a generic error message, and it tells that an unexpected error occurred on the server and that the request cannot be fulfilled.

With this topic, we have finished the crash course on HTTP basics. We will be resuming our discussion on RESTful web services in the next section. Take a deep breath and be ready for an exciting journey.