Microservices, API Gateways, and Frontends

Question

I have recently begun exploring the concept of microservices and API gateways and am particularly confused on how frontend endpoints should be hosted.

If I have an API gateway that acts as the middleman between requests to all of my services, where exactly should the frontend be hosted? If I request /api/example, I understand that my API gateway should route that to the appropriate service and forward that services response. I do not understand however, how an API gateway should handle /home/ in a microservice context. In this case, we want to deliver html/css/javascript corresponding to /home/ to the client making the GET request. Does this mean that we should have some sort of frontend service? Won't creating a service that just returns HTML/CSS/JS be redundant and add increased latency, since all we really need to do is just immediately return the HTML/CSS/JS associated with our frontend?

An alternative I was thinking about was to have the API gateway itself provide endpoints that return the HTML/CSS/JS required for the client to render the frontend. In other words, the API gateway could just immediately respond with the HTML corresponding to /home/ when receiving a GET request to /home/ rather than calling a service. However, I read online that API gateways should not be actually serving endpoints, rather just proxying them to services.

That is my main question: Where should frontend code go when your backend is built out using a microservice architecture?

Answer 1

I assume your frontend is Single Page Application. SPA has static content like HTML, CSS, images, fonts etc. It is the perfect candidate to be deployed as static website that gets data from backend using REST APIs.

In cloud environments like AWS, GCP it is recommended to host SPA applications separately from REST APIs. e.g. in AWS SPA can be deployed in Amazon S3 and it remains directly accessible without going through API gateway.

API gateway is supposed to be used to only route REST API calls and performing cross cutting concerns such as authentication etc. However the problem with this approach is that you will get CORS error while hitting REST APIs from frontend because SPA and API gateway will be hosted on different domains. To overcome this issue CORS need to be enabled at API gateway.

If you are looking for simpler solution, frontend can be served from a service through API gateway. Since static content is served only once in SPA it should be okay to start with this approach for simpler applications.

https://aws.amazon.com/blogs/apn/how-to-integrate-rest-apis-with-single-page-apps-and-secure-them-using-auth0-part-1/

https://docs.aws.amazon.com/AmazonS3/latest/dev/WebsiteHosting.html

Answer 2

API Gateway as you have already mentioned should act as a proxy/router with minimal logic. And as the name says it all, its main purpose is to expose API and not GUI components/pages for different types of clients and address some of the non-functional aspects e.g. security. Provide high-level API [use case driven] based on requesting client[basically BFF/Backend for Frontend approach].

When it comes to having frontend/GUI fittin to microservices, you can think of having UI Gateway/Container which is backed by Micro FrontEnds. Also, refer to MF's site for details on micro frontends.

So, following microservices architecture for frontend, your frontend code should reside in micro frontends along with other microservices.

Answer 3

Before answering your question let me go through the different rendering strategies:

Traditional Server Side Rendering

Quite uncommon nowadays. You don't need any API gateway, since everything is computed and rendered in the server you can perform the necessary calls to other microservices there.

Pros: simplicity; instant interactivity for the user on page load; SEO

Cons: full reloads each time

Client Side Rendering

Most popular option in the last years. You provide a basic static HTML/CSS/JS bundle, perform requests to an API to retrieve some data and use some sort of template engine to render new pages and components.

Pros: no full reload; lazy loading; richer interactions; benefits from CDN

Cons: SEO; page not interactive on load; slower first load

Since the rendering happens in the browser you don't need to serve the static HTML/CSS/JS files from your server. Instead, you should use a CDN to deliver them faster.

The other requests to non-static resources will be performed against the API gateway, which is responsible to forward the request to other services (or make some sort of orchestration/aggregation).

Modern Server Side Rendering

This is gaining traction nowadays thanks to frameworks like Next.js. The page is initially rendered in the server using the same template engine that it's going to be used in the browser, so you can send an interactive page faster while keeping the features and benefits of client side rendering.

In this case all the static pages can be pre-rendered, cached and served through a CDN. For dynamic pages you can still send a partially cached/rendered page that will later fetch the additional necessary information from the API gateway.

TL;DR: You don't need to serve your static HTML/CSS/JS from your API gateway or your server. You should deliver them through a CDN to improve loading times.

Otherwise the static files shouldn't be served through the API gateway, IMHO. You could make your /api/{resource} requests go through it and forward all /{page} requests to static resources.

Answer 4

Using API responses for a Frontend app, does not make sense if you are returning the WHOLE body of the Frontend, as you point out.

However, you can load (initially) a frontend, that contains let's say header, with a menu, footer, and a main body section with few elements, like articles.

Upon interacting with this frontend, an action can be triggered to that API (usually via JS Ajax calls), which will request specific portions of new data from the API, and once received, the JS will update only the relevant portions on the webpage, and not reload or replace (or refresh) the WHOLE page.

This if done correctly can save a great deal of network traffic, thus making the website more flexible, without it being fully reloaded every time you need new data to visualize.

One simple example: When you click on a link in your menu, to load a contact form, the API will return either the raw HTML for only the contact form, or it will return (usually) a JSON object/array, that will be used to further generate/replace a portion of your main body, to BECOME now the contact form..

Answer 5

Where exactly should the frontend be hosted?

Where should frontend code go when your backend is built out using a microservice architecture?

Your Front-end (web) application usually sits BEFORE the API Gateway. Requests to resources like HTML/CSS/JS are served right from the Front-end application itself, hence no API-Gateway involvement here whatsoever. If the page contains an (AJAX) call to a back-end (Micro)service, it might (should) go through the API Gateway. From hosting perspective, they are hosted as a separate web application.

how an API gateway should handle /home/ in a microservice context?

It won't (i.e not intercepted by API Gateway at all). Requests like /home are page requests served directly by Front-end application itself (Of course, it will have its own scaling mechanisms like Clustering, Caching etc)