Context

In any GraphQL framework, including Tailcall, Context is a fundamental mechanism used for data sharing amongst various parts of your application. It is an adaptable object that is made available to every resolver in GraphQL.

Context in Tailcall

In Tailcall, as in all GraphQL implementations, Context is a variable that is accessible to every Operator. It is used to store and access data that needs to be shared between operators.

The Context can be described using the following Typescript interface:

interface Context {
  args: Map<string, Json>
  value: Json
  parent: Context
  env: Map<string, string>
  headers: Map<string, string>
}

args

These are the arguments passed to the current query. They can be used to access the arguments of the query. For example,

type Query {
  user(id: ID!): User @http(path: "/users/{{args.id}}")
}

In this example, args.id is used to access the id argument passed to the user query.

value

This represents the value of the current node. For instance,

type Post {
  id: ID!
  title: String!
  body: String!
  comments: [Comment] @http(path: "/posts/{{value.id}}/comments")
}

In the example above, value.id is used to access the id field of the Post type.

parent

This denotes the context of the parent node.

type Query {
  posts: [Post] @http(path: "/posts")
}
type Post {
  id: Int!
  userId: Int!
  title: String!
  body: String!
  user: User
    @http(path: "/users", query: [{key: "id", value: "{{value.userId}}"}], matchPath: ["id"], matchKey: "userId")
}

In this case, value.userId is a way to get the userId information from the "parent" context of the Post type. Essentially, it's extracting a list or "array" of userId fields from multiple Post types. Think of value as a container that holds the results of a post query, with userId being the specific key you want to fetch from that container.

env

This represents global environment variables for the server. This is set once when the server starts.

type Query {
  users: [User]! @http(baseUrl: "{{env.API_ENDPOINT}}", path: "/users")
}

In the above example, env.API_ENDPOINT refers to an environment variable called API_ENDPOINT, which should be defined in your server settings.

headers

These are the headers of the request that was received by the Tailcall server.

type Query {
  commentsForUser: [Comment] @http(path: "/users/{{headers.userId}}/comments")
}

Here, headers.userId refers to a header called userId that should be present in the context. The server can use this userId to fetch comments for the specified user.