In the previous chapter, we learned about the client-server model that underpins how the internet works: when you enter a website URL in your browser, the server serves HTML, CSS, and JavaScript to your browser, which then renders it in the appropriate manner.

What does this all mean for you? Arguably the second question that you should be asking yourself when building a web application is whether you need to do any server-side processing (right after "is this really going to be worth the effort?"). Could you just create an HTML web-page with some JavaScript?

You can get away without a backend if the data needed to build the whole application is small enough: typically a few megabytes. If your application is larger, you will need a backend to transfer just the data the client currently needs. Surprisingly, you can often build visualizations without a backend: while data science is accustomed to dealing with terabytes of data, the goal of the data science process is often condensing these huge data sets to a few meaningful numbers.

Having a backend also lets you include logic invisible to the client. If you need to validate a password, you clearly cannot send the code to do that to the client computer: it needs to happen out of sight, on the server.

If your application is small enough and you do not need to do any server-side processing, stop reading this chapter, brush up on your JavaScript if you have to, and forget about Scala for now. Not having to worry about building a backend will make your life easier.

Clearly, however, we do not have that freedom for the application that we want to build: the user could enter the name of anyone on GitHub. Finding information about that user requires a backend with access to tremendous storage and querying capacity (which we simulate by just forwarding the request to the GitHub API and re-interpreting the response).