Handle token retrieval while querying an API

Using a DelegatingHandler

Published on 29 January 2020

In our daily job, we often have to query secure REST APIs that require our HTTP requests to have a valid access token in their Authorization header. Of course, many API come with a SDK that makes the job easier for us as it directly takes care of retrieving a token and sending the authenticated HTTP requests. However it is not always the case and knowing how to implement that using HttpClient, IMemoryCache and DelegatingHandler can become pretty useful.

Context

Let's imagine we have a very simple API which contains the following routes :

The POST /login route returns an AuthResponse which contains the necessary Bearer token to call the 2 protected routes GET /users and PUT /users/{username}.

We want to implement an IUserService that has 2 methods:

  • GetAllUsers to retrieve the list of users that will use the GET /users route
  • UpdateUser to update a user that will use the PUT /users/{username} route

Each of these methods needs to retrieve a valid token from the POST /login route and set the Authorization header with this token in the http request to each of the protected routes.

The following code shows how to retrieve the token:

where AuthResponse is a class we defined to map the response of the POST /login route

So now we have the code to retrieve the token, how do we use it to implement our IUserService?

Retrieve the token from a private method

The easiest way to do that is to create a private method in UserService that returns this token and to call it from GetAllUsers and UpdateUser. That would give us something like that :

There are two main problems with this way of doing things:

  • We have some code duplication as we are calling the RetrieveToken in each of our methods calling the API. That could be okay here as we only have 2 methods calling the API but that can quickly be problematic if we start to have more methods and repeat the call to RetrieveTokenin each method.
  • For each call to an authenticated route of the API, we are making a call to the login route even if our token from a previous call is probably still valid.

Use a dedicated service to retrieve the token and save it for future calls

Although it's not necessary at this point, it can be interesting to move the code of our private method RetrieveToken into a separate service UserApiAuthenticationService that will be injected in UserService. That way, if the authentication method changes someday, UserService implementation won't change. Moreover we won't mess with the same HttpClient for authentication and other calls.

In order to avoid requesting always the same token to the API, we added a line to store the token in the memory cache and a line to check if the token is already in the cache before querying the API. We could also have used a class as a singleton to store the token and its expiration date, but the built-in IMemoryCache of ASP.NET Core is more convenient and handle the expiration of the token for us by removing it from the cache when date is passed. You can find more about cache memory in ASP.NET Core here.

Use a Delegating handler to directly set the token in the HttpClient request

Handling the token retrieval in a separate service is nice but that does not solve the issue of duplicated code. Even if the RetrieveToken method is now part of UserApiAuthenticationService, each method of UserService will still call RetrieveToken. Moreover setting the token on each request should not be a concern of UserService.

That's where come delegating handlers. A delegating handler is quite similar to an ASP.NET Core middleware but instead of applying a processing on an incoming request and its response, it does so on an outgoing request and its response. In concrete terms you use a delegating handler to apply something (logging, authentication, caching ...) to http requests you make to an API using an HttpClient. To learn more about delegating handlers there is a nice article from Steve Gordon on the topic.

A custom delegating handler is exactly what we need : a piece of code that all our http requests from UserService will go through and where we will be able to set the token on the authentication header of each request. Here is the code of our custom delegating handler:

That's it, we don't need anymore to handle token retrieval on UserService which becomes simpler:

To finish we just have to specify in the Startup.cs on which HttpClient to apply the delegating handler we have just created.

To conclude

To summarize, we have put the code that retrieves a token in a separate dedicated service that caches the token until it expires. And we have created a custom delegating handler that calls this service and sets the retrieved token on the authentication header of each http request to the API.

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