November 18, 2020

SDL Support in Apollo Android

Martin Bonnin

Martin Bonnin

If you’re using Apollo Android, the chances are that you are using a schema.json file: an introspection schema. While introspection schemas work well, GraphQL has SDL (Schema Definition Language), which is another, more concise, way to describe a schema. 

The excellent news is that Apollo Android supports SDL!

Read below to learn more about SDL and how to enable it in your project.

Why SDL?

Since Apollo Android already supports introspection schemas, why bother with a new schema format? Well, there are a ton of reasons for that:

1. SDL is more concise

Taking a look at the commit that moved the Apollo Android test schema from introspection to SDL: 

 The SDL file has almost ten times fewer lines. It’s also significantly smaller (9KB vs. 106KB), making your repository faster to download.

2. SDL is easier to read (and write!)

SDL being smaller also means it’s easier to read. Let’s take an example from our favorite schema (simplified here):

"""A character from the Star Wars universe"""
interface Character {
  """The ID of the character"""
  id: ID!

  """The name of the character"""
  name: String!

  """The date character was born."""
  birthDate: Date!

The same thing in JSON (with only the “id” field or it would take too much space!):

// Introspection
  "kind": "INTERFACE",
  "name": "Character",
  "description": "A character from the Star Wars universe",
  "fields": [
      "name": "id",
      "description": "The ID of the character",
      "args": [],
      "type": {
        "kind": "NON_NULL",
        "name": null,
        "ofType": {
          "kind": "SCALAR",
          "name": "ID",
          "ofType": null        }
      "isDeprecated": false,
      "deprecationReason": null    },

Adding a new field to an SDL type is a few lines, and you won’t have to worry about creating a valid JSON document or nesting the LIST and NON_NULL types; this makes the experience of writing unit tests for Apollo Android way more enjoyable.

3. Introspection isn’t always an option

While introspection is a useful tool during development, it exposes your schema’s structure to anyone to access your GraphQL endpoint. Unfortunately, exposing your schema can have security repercussions, and a lot of server implementations like apollo-server will disable introspection in production, making it harder to obtain an introspection schema.

4. SDL carries more information

Last but not least, SDL carries more information than plain introspection JSON. Custom directives definition, for example, are stripped out in introspection responses. While Apollo Android does not use this, having this information gives client developers more context about the schema they are using. There are other useful ways we could utilize it in the future too! 

Using SDL in Apollo Android

Using SDL in Apollo Android is as simple as replacing your schema.json file with a schema.sdl file. You might sometimes see this file named schema.graphql. We decided to go with the explicit schema.sdl name to distinguish from operation files, which also have a .graphql extension. 

We also built a few tools in the Gradle plugin to make it easier to work with SDL files:

Transforming an introspection schema to a SDL schema

The Gradle plugin in Apollo Android can automatically translate your schema.json to schema.sdl:

./gradlew convertApolloSchema --from schema.json --to schema.sdl

Note that it works both ways if for some reason you want to get a JSON file:

./gradlew convertApolloSchema --from schema.sdl --to schema.json

Downloading a SDL schema from introspection

Since the plugin can translate between both formats, it can save the results of an introspection query as SDL.

./gradlew downloadApolloSchema --endpoint --schema app/src/main/graphql/com/example/schema.sdl

Downloading an SDL schema from Studio

Finally, if you have a Studio integration, you can also download the SDL directly from Apollo Studio. Get your API key and type:

./gradlew downloadApolloSchema --key $API_KEY --graph $GRAPH --variant current  --schema app/src/main/graphql/com/example/schema.sdl


The initial SDL support was committed a couple of months ago and we’re still working on improving it. convertApolloSchema, for example, is a very recent addition.

It’s still the early days of SDL support for Apollo Android but we’ve been using it successfully to write a lot of unit tests and it really made our life better. We hope it can make yours better too!

Try it out and let us know how you liked it!

Written by

Martin Bonnin

Martin Bonnin

Read more by Martin Bonnin