Errors and response codes of server in testing

Client-server architecture allows operations on the Internet. In this scheme of interaction, two parties are distinguished: the customer (client) and the supplier (server). Any program connected to the network that sends formed requests and processes the server's response can act as a client.

To clients we can refer: 

• a browser;
• a mobile application;
• smart household appliances;
• a server application.

Server is a machine attached to a network whose primary job is to receive a request, process it, and send a response to the client in a specific format.

To servers belong:

• local web-server;
• any HTTP servers;
• sets of server machines.

The client and the server exchange «request-response» messages with each other – they are called network protocols.

The browser (client) starts a dialogue with the server using the HTTP protocol, sending a request specifying which information and in what form it expects to receive. The server, after receiving the request and processing it, sends a certain message (HTTP response) to the browser, which contains the requested data (usually it is an HTML document). These messages contain useful information and special status codes by which the browser can determine whether the request was successfully processed.

The result of processing the request, which is in the first line of the server's response, is called the response code or status code. These messages are sent every time the browser and the server interact, even if they are not available to the site user.

Response codes are divided among themselves into classes. The state class is indicated in the first digit, which determines the state of the page, the other two digits specify it.

There are five classes of codes: 

• 1xx – informational codes. They are responsible for processing the information, and notify that the request has been accepted and the transfer continues;
• 2xx – successful operation. The request was received by the server and executed without errors;
• 3xx – redirect code. The code indicates the need to send a request to a different address when changing the location of the requested file;
• 4xx – error on the client side. It indicates that the given file is not found at the given address;
• 5xx – server side error. A server error occurred while executing the request.

In order to find out the request status code in the browser, you need to open the web developer tools. The value of the code is in the «Status» column in the «Network» tab. After opening the console, you need to refresh the page.

Common errors

Error 400 «Bad request» – incorrectly formulated request (syntax error in the request). The user's client may try to download a file that is too large, the request may be malformed, the HTTP headers of the request may be incorrect, and so on.

Reasons: 

• the browser is blocked by antivirus;
• the browser is blocked by the Windows firewall;
• incorrect browser settings;
• bad internet connection.

Error 403 «Forbidden» – the request is not processed due to access restrictions to the requested data.

Reasons: 

• error in the index file;
• the server does not have access rights to the requested data;
• files are in the wrong directory.

Error 404 «Not Found» – the most recognizable bug on the web. It indicates that the specified file at this address was not found. Users often encounter a 404 error when visiting so-called «broken links».

Reasons: 

• invalid (non-existent) URL;
• the requested files are missing from the server. 

Error 500 «Internal Server Error» is an unexpected situation that occurs on the server, which is classified under other error codes.

Reasons:

• script is not run by the server;
• script has errors;
• incorrect syntax of the .htaccess file (an incorrect instruction is written).

 Error 502 «Bad Gateway» – the server received an error or no communication response from the next server it contacted.

Reason: 

• insufficient server resources;
•  incorrect operation of the proxy server.

Error 503 «Service Temporarily Unavailable» – unavailability of the server, which is of a technical nature.

Reason: 

• server is overloaded;
• server is under maintenance.

When testing the program interface of the application – API (Application Programming Interface), HTTP requests are used. The server, in response to the client's requests, sends various codes. They can indicate an error or inform about the state of the server. APIs provide this data to users. The information exchange procedure and data transfer format are structured so that both parties know how to interact with each other.

Using the API, you can detect the following errors in the software:

• incorrect data processing;
• absence or duplication of functionality;
• API security issues;
• performance issues;
• incorrectly structured responses (JSON or XML);
• the issue of multithreading.

It is important not only for IT specialists to understand the causes of HTTP errors. The codes are related to the functional operation of the resource and inform about the state of the pages. Thanks to the information that each code carries after a user request, the server changes the processing of the document. If an error occurs, knowing the result of the request, you can also eliminate it yourself. Also, the diagnosis of error codes allows you to identify software weaknesses.