You're Not Using HTTP Status Codes Right

HTTP status codes are like short messages returned from a server whenever we request or interact with a resource on the server. They are an invaluable asset for diagnosing application errors and help to fix them quickly.

Each HTTP status code has a specific and unique meaning, however, there are cases where developers might confuse the purpose of a status code with another status code with almost similar usage. In this article, we will be discussing some of the common mistakes that developers make when using HTTP status codes.

HTTP Status Codes For Invalid Data: 400 vs 422

To understand the difference between these two status codes, lets assume a case where you pass the string value password for the key password in the request body but the developer has blacklisted this string value on the server side to prevent users from using this as their password. In this case, what should be returned as the response status code: 400 or 422?

If you were thinking 400, in that case, lets think it out loud once. You have passed a string value to the API endpoint that expected a string value, but the value of the string contained data that has been blacklisted or we can say, isnt satisfying the condition for further processing on the server. Hence, the syntax of the request isnt wrong, so, we cant return a 400 status code saying bad request. According to w3.org, 400 Bad Request means:

The server was unable to understand the request due to incorrect syntax. The client SHOULD NOT REPEAT the request UNALTERED.

Therefore, we can use the status code 422 Unprocessable Entity in this scenario. That would mean that the server understands what you are trying to do, and it understands the data that you are submitting, but it simply wont let that data be processed further.

The 422 (Unprocessable Entity) status code indicates that the server recognized the requested entity's content type but was unable to process the instructions contained within it.

The status code 422 can be returned in cases such as an error condition where the XML request body contains well-formed and syntactically correct XML but it is semantically erroneous.

Handling Forbidden RESTful Requests: 401 vs 403 vs 404
To explain this issue, say we have two users in our system: Sam with ID 4 and Rick with ID 7. Assuming Sam attempts an authorized API call to view Rick's profile resources, as shown below:

GET /users/7/profile HTTP/1.1Authorization: Basic YmVuK2F206dGVzdA==Accept: application/json

Here, Sam is using Basic Authorization to identify himself as Sam, but he's making a call to access another person's profile, and let's suppose that a user can only read his or her own profile in this application.

So the question here would be what status code should be returned? The most appropriate ones could be:

Sam is not authorized to view Ricks profile, so 401 Unauthorized?

Sam is forbidden from viewing someone elses profile, so 403 Forbidden?

Sam cant simply see resources that hes not allowed to see, so 404 Not Found?

However, if you return 401 or 403, you will be disclosing secure information since, in both circumstances, we are declaring that the resource exists but you can't view it, thereby verifying the resource's existence. So, can we use 404 in this case? Let's see what happens.

The description of 403 Forbidden says that:

The server comprehended the request but refused to perform it. Authorization is useless and the request SHOULD NOT BE REPEATED.

If the request method is not HEAD and the server intends to make public the reason why the request was not completed, the entity SHOULD specify the reason for the rejection.

If the server does not want the client to get this information, the status code 404 (Not Found) might be used instead.

Well, this should solve the problem. If you dont want to expose the information, you should return a 404 Not Found instead.

Empty resource HTTP status code: 200 vs 204 vs 404

To illustrate this, imagine there is a method GET /users that returns a list of all users and GET /users?name=sam provides a list of only users with the name sam. What HTTP status code should be issued if there is no user named Sam?

The first and most likely option is 200 Success, as the request will be successful and an empty list will be returned. It might be claimed that the /users collection/list resource exists and that the name query param is used to filter the list's content.

However, the 204 No Content status code definition says:

The 204 (No Content) status code indicates that the server has successfully fulfilled the request and that there is no additional content to be sent in the response payload body.

While 200 OK is a common and acceptable answer, 204 No Content may make sense if there is nothing to return. It is most typically used in response to a PUT (replace) or a PATCH (partial update) when servers do not wish to deliver the replaced/updated resource, or in reaction to a DELETE, because there is usually nothing to return after a deletion. It may, however, be used on a GET.

If the request is valid, has been correctly fulfilled, and there is no more information to send (which is the case because the returned list is empty), the answer 204 No Content is understandable and valid.

However, the 404 Not Found status codes definition says:

The 404 (Not Found) response code indicates that the origin server was unable to locate a current representation of the requested resource or is unwilling to reveal the existence of one.

Assuming that /users is the resource used even when doing a GET /users?name=sam, and obtaining a 404 HTTP status code makes no sense because the resource /users exists, it's only that the list it contains may be empty.

This concludes this article; please share any additional use cases in which you encountered confusion and how you resolved it. I hope you found this article useful.

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