Schema Definition Language

One of the fundamental parts of any API structure is the method in which we define the schema. As outlined before, this is required to be set before either front-end or back-end teams start with their use-cases.

GraphQL APIs use a special syntax, named the Schema Definition Language (SDL) in order to create the Schema. This involves defining the types in the database, their properties and any relations which could come from the data.

Here are some examples of type definitions within the SDL:

type Person {
  name: String!
  age: Int!
  posts: [Post!]!
}

type Post {
  title: String!
  author: Person!
  rating: Int
}

This would be how we would define the data retrieved by our API. Every person item has the name, age, and posts field attached, and they must be populated, since they have an exclamation mark (!). This is how we denote fields as mandatory. In this case, if the Person has not made a post yet, this requires that Person.posts at least returns an empty array, or [].

Other than that, the types are fairly standard. We must define how our data looks (via string, or int) so that our database doesn't throw any errors,

As you can see though, we can also define custom types. These are pretty much required if you are persisting data, since you want to be able to store properties on each data type. If we want to relate these types, simply replace the regular Boolean, String, or Int with the custom type (ex. Person or Post).