REST APIs vs. Backend for Frontend

APIs and BFFs both serve JSON. What's the difference?

API Everything?

As of 2021, JSON is dominating as the format used for exchanging structured data. XML is still a thing of course, but JSON is the primary go-to solution, thus I'll only focus on it.

What is JSON over HTTP? API, you might say.

API is an acronym for Application Programming Interface, that is a very broad term. My very first encounter with APIs was the WinApi which was basically a C library exposed via DLLs and it was made available for other languages as well. No HTTP, no JSON of course, we're in 1998.

Let's jump to REST APIs. REST is basically a style how functionality of systems is being made available for other systems over HTTP. The most important paradigms are HTTP, JSON and resource oriented CRUD via HTTP verbs.

Another popular "style" is GraphQL which shares HTTP and JSON with REST but has a different control language.

Despite the common characteristics, there are quite several differences between various "JSON over HTTP" backends. This article will discuss the most important ones.

Consumer Difference

It's important to distinct the actors of the transactions. Interactions can be made between two machines or between a human and a machine.

Machine to Machine

Both REST and GraphQL are very popular for integrating systems, written at a different time by different people, likely in a different programming language.

These APIs provide a straightforward contract between these systems and tell what and how can be done. An important factor is that in this scenario that two automated systems are talking to each other.

Human (Browser) to Machine

Long before SPAs and the Frontend's takeover of rendering HTML, we already had AJAX. Back in the days, small JSON snippets were travelling over HTTP asynchronously, mostly with the help of jQuery. This has made applications faster and more usable.

Classic examples were fetching items of a dropdown depending on the selection in another dropdown; or adding items to a shopping cart without reloading the entire page.

Back in the days, to fetch and store these JSON snippets, web backends used to access specific endpoints (controller actions in MVC terms) that were part of the application but "talked" JSON instead of HTML.

Did jQuery talk to an API? I wouldn't say, unless we call every web backend (ie. a collection of HTTP endpoints) an API.

An important differentiator is that in this case a human, with the help of a browser is interacting with the backend.

Backend for Frontend

Earlier, we used to have mostly HTML but partially JSON speaking backends targeted for browsers. Machines talk a structured language (JSON, XML) with each other.

Today with SPAs, browser based interaction has also become mostly JSON but partially HTML.

Have they become the same? I think not, and software architecture has a invented a term for that: Backend for Frontend.

Backend for Frontend - or short BFF - is simply a backend dedicated to User Interfaces. As usual, BFF is more than just that, but in general the term can help mind-mapping the conceptual difference.

Terminology

I prefer to call backends that serve JSON for User Interfaces = BFFs; and backends that serve JSON for other backends = APIs.

This is not a 100% precise terminology, but in the daily practice helps to separate apples from oranges.

Authentication Difference

Regardless of whether both actors are machines or one of them is a human, we can differentiate the actors as Provider and Consumer (aka client/server).

The Provider is the one that exposes functionality, thus defines the rules.

The Consumer must comply and in most of the cases must be identified.

The Provider can either identify concrete users (eg. via user+pass, token) or identify anonymous users across requests, with the help of cookies/sessions.

And here comes another important difference between machine-to-machine and browser-to-machine interactions*:

  • machine-to-machine is rarely anonymous and has no browser involved;
  • browser-to-machine can be anonymous and there is a browser

*: One may note that a phone app is a UI, therefore it's human-to-machine but (usually) there's no browser. That is true, and albeit the topic is closely related, it's not being further discussed in this article.

Token or Not Token

Token based authentication is where the consumer is passing a unique string (a token) to the provider to identify itself.

We tend to think that when there's no session and cookies involved, then we're in the brave new world of tokens.

In fact, JWT Tokens - a widely used format - brings a fundamentally different concept, it is a so-called Self-contained Token.

The most important thing to understand is that a JWT token works like a passport, ie it contains who you are, and the reader will read your data from the passport (token) and not from the database. This article explains the concept with a brilliant analogy I wish I would've read when I first learned about JWT.

API Tokens in turn are static, and basically replace username and password. Whenever the Consumer passes the token to the Provider, mostly in an HTTP header it needs to do extra work to find out which user it belongs to, and then what the user can do. This extra work often requires a database lookup.

Cookie Based Authentication is when you store a string (session id) in your browser and the browser keeps passing that string to the server as an HTTP header. Basically the same thing as the API Token does, except that API tokens never expire.

As you see, Cookie based authentication and API tokens are fundamentally the same. Cookies are being managed by your browser, but API tokens are the application's responsibility to store, fetch and pass with every request. Cookies and API tokens have much more in common than API tokens and JWT tokens, despite the name.

Security Considerations

API tokens are the least secure since basically it is the same as passing the username and password in each request. These tokens rarely expire, therefore should be handled with care.

Sessions stored as cookies are more secure because they have an expiry, which is not only known by the server, but the browser as well.

Both API tokens and sessions can be revoked any time at the server.

JWT tokens are cryptographically signed, but they give permissions to their holders until they expire. The JWT token is like a VIP card, it contains who you are and what you can do. Compromised tokens can't be revoked at the server, unless you maintain a blacklist of tokens at every edge that accepts tokens (a feature JWT was not designed for).

Anonymous vs.Logged-in Users

When you're browsing the web, you're mostly anonymous. You still want to be distinguished from other anonymous users, eg. when placing a product in the cart of a webshop or when watching a youtube video.

This is called identification of anonymous users.

API tokens were not designed to identify anonymous users.

JWT tokens technically can be used for that, but its sub claim typically stores the user id (client_id in OAuth terms).

Cookie based sessions were designed to identify both anonymous users and users with an identity (ie. logged in users).

API vs. BFF Authentication

Returning to the machine vs. browser and BFF vs. API division, the followings are the key differences regarding Authentication:

Machine to machine involves a user with an identity and there's no browser. As a verdict, APIs should use either a JWT or a static API token.

Browser to machine can be both a user with an identity or an anonymous user and there's a browser. As a verdict, BFFs are best to use cookie based sessions for that purpose as it's safe enough and works out of the box.

Laravel's Sanctum (subsystem for SPA authentication) just does the very same thing: https://laravel.com/docs/8.x/sanctum#how-it-works-spa-authentication

Sometimes we try to fix things that aren't broken. Cookies are not bad. Since you've been reading this article, millions of cookies have been created, used and expired.

It doesn't mean they're perfect or that you can't have problems with your app's authentication, but the problem you have, may be completely unrelated to cookies and sessions.

Structural Difference

A third difference between APIs and BFFs is their structure.

REST APIs are built around resources which can be imagined as tables and records in a relational database. It's a bit vague, but what actually happens is the mapping of tables and records to HTTP endpoints and HTTP verbs as CRUD operations.

REST APIs are very versatile but are not perfect for complex querying and telling the backend to omit or include parts of the response. GraphQL can solve the latter two problems which makes it a good candidate to act as backend for frontends.

There can be however still some endpoints that are particularly necessary for the UI to work and make no sense for an API. Such endpoints could be auth endpoints like login, logout, password reminder, or the typical "me" endpoint which returns the user data of the current user.

A machine user will typically not have "a cart" which is a singleton in the browser. The currently identified user in the browser will want to have a single cart, whereas a "machine user" can create as many carts or orders as many it wants and operate on them.

An API user can have access to all the carts in the system whereas the plain user coming from a browser only to his/her own ones.

All these differences require you to structure the two backend endpoints differently and end up having an API and a BFF.

APIs (both RESTful and GraphQL style ones) are resource oriented and BFFs are goal oriented. It's perfectly fine to add utility endpoints to BFFs which would be weird to see in APIs.

Couldn't We Just Have One API?

Yes we could. Such as we could have a backend rendering the HTML and no fancy react 💩. Similarly, we can keep CSS next to HTML.

I'm not arguing for atomic fragmentation, or for a micro<XYZ> approach.

What I'm arguing for is:

  • Segment your application along well defined use cases.
  • Don't mix machine targeted APIs and JSON speaking backends targeted for UI rendering.
  • Don't ditch sessions and cookies when in a browser context.

It will keep things simple and straightforward.

duplication is far cheaper than the wrong abstraction
--Sandy Metz