The `@custom` directive - Graphql

diggy · August 28, 2020, 9:16pm

The @custom directive is used to define custom queries, mutations and fields.

In all cases, the result type (of the query, mutation or field) can be either:

a type that’s stored in Dgraph (that’s any type you’ve defined in your schema), or
a type that’s not stored in Dgraph and is marked with the @remote directive.

Because the result types can be local or remote, you can call other HTTP endpoints, call remote GraphQL, or even call back to your Dgraph instance to add extra logic on top of Dgraph’s graph search or mutations.

Here’s the GraphQL definition of the directives:

directive @custom(http: CustomHTTP) on FIELD_DEFINITION
directive @remote on OBJECT | INTERFACE
input CustomHTTP {
	url: String!
	method: HTTPMethod!
	body: String
	graphql: String
	mode: Mode
	forwardHeaders: [String!]
	secretHeaders: [String!]
	introspectionHeaders: [String!]
	skipIntrospection: Boolean
}
enum HTTPMethod { GET POST PUT PATCH DELETE }
enum Mode { SINGLE BATCH }

Each definition of custom logic must include:

the url where the custom logic is called. This can include a path and parameters that depend on query/mutation arguments or other fields.
the HTTP method to use in the call. For example, when calling a REST endpoint with GET, POST, etc.

Optionally, the custom logic definition can also include:

a body definition that can be used to construct a HTTP body from from arguments or fields.
a list of forwardHeaders to take from the incoming request and add to the outgoing HTTP call. Used, for example, if the incoming request contains an auth token that must be passed to the custom logic.
a list of secretHeaders to take from the Dgraph.Secret defined in the schema file and add to the outgoing HTTP call. Used, for example, for a server side API key and other static value that must be passed to the custom logic.
the graphql query/mutation to call if the custom logic is a GraphQL server and whether to introspect or not (skipIntrospection) the remote GraphQL endpoint.
mode which is used for resolving fields by calling an external GraphQL query/mutation. It can either be BATCH or SINGLE.
a list of introspectionHeaders to take from the Dgraph.Secret defined in the schema file and added to the introspection requests sent to the graphql query/mutation.

The result type of custom queries and mutations can be any object type in your schema, including @remote types. For custom fields the type can be object types or scalar types.

The method can be any of the HTTP methods: GET, POST, PUT, PATCH, or DELETE, and forwardHeaders is a list of headers that should be passed from the incoming request to the outgoing HTTP custom request. Let’s look at each of the other http arguments in detail.

Dgraph.Secret

Sometimes you might want to forward some static headers to your custom API which can’t be exposed to the client. This could be an API key from a payment processor or an auth token for your organization on GitHub. These secrets can be specified as comments in the schema file and then can be used in secretHeaders and introspectionHeaders while defining the custom directive for a field/query.

	type Query {
		getTopUsers(id: ID!): [User] @custom(http: {
			url: "http://api.github.com/topUsers",
			method: "POST",
			introspectionHeaders: ["Github-Api-Token"],
			secretHeaders: ["Authorization:Github-Api-Token"],
            graphql: "..."
		})
}
# Dgraph.Secret Github-Api-Token "long-token"

In the above request, Github-Api-Token would be sent as a header with value long-token for the introspection request. For the actual request, the value Authorization would be sent along with the value long-token. Note Authorization:Github-Api-Token syntax tells us to use the value for the Github-Api-Token dgraph secret but to forward it to the custom API with the header key as Authorization.

The URL and method

The URL can be as simple as a fixed URL string, or include details drawn from the arguments or fields.

A simple string might look like:

type Query {
    myCustomQuery: MyResult @custom(http: {
        url: "https://my.api.com/theQuery",
        method: GET
    })
}

While, in more complex cases, the arguments of the query/mutation can be used as a pattern for the URL:

type Query {
    myGetPerson(id: ID!): Person @custom(http: {
        url: "https://my.api.com/person/$id",
        method: GET
    })
    getPosts(authorID: ID!, numToFetch: Int!): [Post] @custom(http: {
        url: "https://my.api.com/person/$authorID/posts?limit=$numToFetch",
        method: GET
    })
}

In this case, a query like

query {
    getPosts(authorID: "auth123", numToFetch: 10) {
        title
    }
}

gets transformed to an outgoing HTTP GET request to the URL https://my.api.com/person/auth123/posts?limit=10.

When using custom logic on fields, the URL can draw from other fields in the type. For example:

type User {
    username: String! @id
    ...
    posts: [Post] @custom(http: {
        url: "https://my.api.com/person/$username/posts",
        method: GET
    })
}

Note that:

Fields or arguments used in the path of a URL, such as username or authorID in the exapmles above, must be marked as non-nullable (have ! in their type); whereas, those used in parameters, such as numToFetch, can be nullable.
Currently, only scalar fields or arguments are allowed to be used in URLs or bodies; though, see body below, this doesn’t restrict the objects you can construct and pass to custom logic functions.
Currently, the body can only contain alphanumeric characters in the key and other characters like _ are not yet supported.
Currently, constant values are not also not allowed in the body template. This would soon be supported.

The body

Many HTTP requests, such as add and update operations on REST APIs, require a JSON formatted body to supply the data. In a similar way to how url allows specifying a url pattern to use in resolving the custom request, Dgraph allows a body pattern that is used to build HTTP request bodies.

For example, this body can be structured JSON that relates a mutation’s arguments to the JSON structure required by the remote endpoint.

type Mutation {
    newMovie(title: String!, desc: String, dir: ID, imdb: ID): Movie @custom(http: {
            url: "http://myapi.com/movies",
            method: "POST",
            body: "{ title: $title, imdbID: $imdb, storyLine: $desc, director: { id: $dir }}",
    })

A request with newMovie(title: "...", desc: "...", dir: "dir123", imdb: "tt0120316") is transformed into a POST request to http://myapi.com/movies with a JSON body of:

{
    "title": "...",
    "imdbID": "tt0120316",
    "storyLine": "...",
    "director": {
        "id": "dir123"
    }
}

url and body templates can be used together in a single custom definition.

For both url and body templates, any non-null arguments or fields must be present to evaluate the custom logic. And the following rules are applied when building the request from the template for nullable arguments or fields.

If the value of a nullable argument is present, it’s used in the template.
If a nullable argument is present, but null, then in a body null is inserted, while in a url nothing is added. For example, if the desc argument above is null then { ..., storyLine: null, ...} is constructed for the body. Whereas, in a URL pattern like https://a.b.c/endpoint?arg=$gqlArg, if gqlArg is present, but null, the generated URL is https://a.b.c/endpoint?arg=.
If a nullable argument is not present, nothing is added to the URL/body. That would mean the constructed body would not contain storyLine if the desc argument is missing, and in https://a.b.c/endpoint?arg=$gqlArg the result would be https://a.b.c/endpoint if gqlArg were not present in the request arguments.

Calling GraphQL custom resolvers

Custom queries, mutations and fields can be implemented by custom GraphQL resolvers. In this case, use the graphql argument to specify which query/mutation on the remote server to call. The syntax includes if the call is a query or mutation, the arguments, and what query/mutation to use on the remote endpoint.

For example, you can pass arguments to queries onward as arguments to remote GraphQL endpoints:

type Query {
    getPosts(authorID: ID!, numToFetch: Int!): [Post] @custom(http: {
        url: "https://my.api.com/graphql",
        method: POST,
        graphql: "query($authorID: ID!, $numToFetch: Int!) { posts(auth: $authorID, first: $numToFetch) }"
    })
}

You can also define your own inputs and pass those to the remote GraphQL endpoint.

input NewMovieInput { ... }
type Mutation {
    newMovie(input: NewMovieInput!): Movie @custom(http: {
        url: "http://movies.com/graphql",
        method: "POST",
        graphql: "mutation($input: NewMovieInput!) { addMovie(data: $input) }",
    })

When a schema is uploaded, Dgraph will try to introspect the remote GraphQL endpoints on any custom logic that uses the graphql argument. From the results of introspection, it tries to match up arguments, input and object types to ensure that the calls to and expected responses from the remote GraphQL make sense.

If that introspection isn’t possible, set skipIntrospection: true in the custom definition and Dgraph won’t perform GraphQL schema introspection for this custom definition.

Remote types

Any type annotated with the @remote directive is not stored in Dgraph. This allows your Dgraph GraphQL instance to serve an API that includes both data stored locally and data stored or generated elsewhere. You can also use custom fields, for example, to join data from disparate datasets.

Remote types can only be returned by custom resolvers and Dgraph won’t generate any search or CRUD operations for remote types.

The schema definition used to define your Dgraph GraphQL API must include definitions of all the types used. If a custom logic call returns a type not stored in Dgraph, then that type must be added to the Dgraph schema with the @remote directive.

For example, you api might use custom logic to integrate with GitHub, using either https://api.github.com or the GitHub GraphQL api https://api.github.com/graphql and calling the user query. Either way, your GraphQL schema will need to include the type you expect back from that remote call. That could be linking a User as stored in your Dgraph instance with the Repository data from GitHub. With @remote types, that’s as simple as adding the type and custom call to your schema.

# GitHub's repository type
type Respository @remote { ... }
# Dgraph user type
type User {
    # local user name = GitHub id
    username: String! @id
    # ...
    # other data stored in Dgraph
    # ...
    # join local data with remote
    repositories: [Repository] @custom(http: {
        url:  "https://api.github.com/users/$username/repos",
        method: GET
    })
}

Just defining the connection is all it takes and then you can ask a single GraphQL query that performs a local query and joins with (potentialy many) remote data sources.

How Dgraph processes custom results

Given types like

type Post @remote {
    id: ID!
    title: String!
    datePublished: DateTime
    author: Author
}
type Author { ... }

and a custom query

type Query {
    getCustomPost(id: ID!): Post @custom(http: {
        url: "https://my.api.com/post/$id",
        method: GET
    })
    getPosts(authorID: ID!, numToFetch: Int!): [Post] @custom(http: {
        url: "https://my.api.com/person/$authorID/posts?limit=$numToFetch",
        method: GET
    })
}

Dgraph turns the getCustomPost query into a HTTP request to https://my.api.com/post/$id and expects a single JSON object with fields id, title, datePublished and author as result. Any additional fields are ignored, while if non-nullable fields (like id and title) are missing, GraphQL error propagation will be triggered.

For getPosts, Dgraph expects the HTTP call to https://my.api.com/person/$authorID/posts?limit=$numToFetch to return a JSON array of JSON objects, with each object matching the Post type as described above.

If the custom resolvers are GraphQL calls, like:

type Query {
    getCustomPost(id: ID!): Post @custom(http: {
        url: "https://my.api.com/graphql",
        method: POST,
        graphql: "query(id: ID) { post(postID: $id) }"
    })
    getPosts(authorID: ID!, numToFetch: Int!): [Post] @custom(http: {
        url: "https://my.api.com/graphql",
        method: POST,
        graphql: "query(id: ID) { postByAuthor(authorID: $id, first: $numToFetch) }"
    })
}

then Dgraph expects a GraphQL call to post to return a valid GraphQL result like { "data": { "post": {...} } } and will use the JSON object that is the value of post as the data resolved by the request.

Similarly, Dgraph expects postByAuthor to return data like { "data": { "postByAuthor": [ {...}, ... ] } } and will use the array value of postByAuthor to build its array of posts result.

How custom fields are resolved

When evaluating a request that includes custom fields, Dgraph might run multiple resolution stages to resolve all the fields. Dgraph must also ensure it requests enough data to forfull the custom fields. For example, given the User type defined as:

type User {
    username: String! @id
    ...
    posts: [Post] @custom(http: {
        url: "https://my.api.com/person/$username/posts",
        method: GET
    })
}

a query such as:

query {
    queryUser {
        username
        posts
    }
}

is executed by first querying in Dgraph for username and then using the result to resolve the custom field posts (which relies on username). For a request like:

query {
    queryUser {
        posts
    }
}

Dgraph works out that it must first get username so it can run the custom field posts, even though username isn’t part of the original query. So Dgraph retrieves enough data to satisfy the custom request, even if that involves data that isn’t asked for in the query.

There are currently a few limitations on custom fields:

each custom call must include either an ID or @id field
arguments are not allowed (soon custom field arguments will be allowed and will be used in the @custom directive in the same manner as for custom queries and mutations), and
a custom field can’t depend on another custom field (longer term, we intend to lift this restriction).

Restrictions / Roadmap

Our custom logic is still in beta and we are improving it quickly. Here’s a few points that we plan to work on soon:

adding arguments to custom fields
relaxing the restrictions on custom fields using id values
iterative evaluation of @custom and @remote - in the current version you can’t have @custom inside an @remote type once we add this, you’ll be able to extend remote types with custom fields, and
allowing fine tuning of the generated API, for example removing of customizing the generated CRUD mutations.

This is a companion discussion topic for the original entry at https://dgraph.io/docs/graphql/custom/directive/

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