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GraphQL Resolver Context

Within Tailcall, Context is a pivotal component that allows for dynamic retrieval of values during the resolution of fields for a given type within the schema.

Schema Definition​

type Context = {
  args: Map<string, JSON>
  value: JSON
  env: Map<string, string>
  vars: Map<string, string>
  headers: Map<string, string>
}

Context operates by storing values as key-value pairs, which can be accessed through mustache template syntax.

args​

This property facilitates access to query arguments. Consider the example:

type Query {
  user(id: ID!): User
    @http(
      url: "http://jsonplaceholder.typicode.com/users/{{.args.id}}"
    )
}

Here, args.id is utilized to retrieve the id argument provided to the user query.

value​

This enables access to the fields of the specified type.

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

In this case, value.id accesses the id field of the Post type.

env​

Environment variables, set at server startup, allow directives to dynamically adapt behavior based on external configurations without altering the server configuration itself.

Example:

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

env.API_ENDPOINT references an environment variable named API_ENDPOINT, which specifies the base URL for HTTP requests.

vars​

vars offers a mechanism for defining reusable variables within the configuration. Unlike env, these are embedded and can be universally applied across configurations.

schema
  @server(
    vars: {key: "apiKey", value: "{{.env.AUTH_TOKEN}}"}
  ) {
  query: Query
}

type Query {
  user(id: ID!): [User]
    @http(
      url: "/users"
      headers: [
        {
          key: "Authorization"
          value: "Bearer {{.vars.apiKey}}"
        }
      ]
    )
}

Here, the variable apiKey is set using an environment variable and subsequently utilized in the Authorization header for HTTP requests.

headers​

Headers originate from the request made to the GraphQL server.

type Query {
  commentsForUser: [Comment]
    @http(
      url: "http://jsonplaceholder.typicode.com/users/{{.headers.x-user-id}}/comments"
    )
}

In this example, headers.x-user-id extracts the value of the x-user-id header present in the request, dynamically constructing the request path.