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GraphQL Schema Design
What are our best practices for GraphQL Schema Design?

Notes

How to model our graph

  • As with most GraphQL advice, our schema should strive to always be backwards compatible. Thus careful consideration is needed about naming (be as unambiguous as possible) and the shape of the data returned by a field.

  • Whenever a ‘namespace’ would be used in a field by adding a prefix or suffix (e.g. for_sale_artwork or location_city), this should be taken as a clue that this data needs to be nested instead. E.g.

    {
      show(id: "kate-oh-gallery-metropolis") {
        # Don’t do this:
        location_city
        location_country
        # Instead do this:
        location {
          city
          country
        }
      }
    }
  • Design the schema around first class domain-models, not functional details.

    For instance, rather than mimicking a back-end endpoint that allows one to filter artworks by defining a filter_artworks field that has a nested artworks connection, expose the ability to filter artworks in a plain artworks connection field instead.

    The key aspect here is that we are trying to expose artworks, which are the same as artworks retrieved through other means, only the ‘feed’ they are retrieved from is different and that’s an implementation detail, there is no such model as filter artworks.

    # Bad
    {
      filter_artworks(aggregations: [TOTAL]) {
        counts {
          total
        }
        artworks(first: 10) {
          edges {
            node {
              title
            }
          }
        }
      }
    }
    
    # Good
    {
      artworks(aggregations: [TOTAL], first: 10) {
        counts {
          total
        }
        edges {
          node {
            title
          }
        }
      }
    }

Root fields

  • These are entry points into the graph that is Artsy’s data set.

  • Not all types necessarily make sense as root fields.

    • In general only domain models should be exposed as root fields. An ‘artwork’ is such a model, Artsy’s system time (for clock synchronization) probably is not.
  • For the cases where there is no root field, but you still need to be able to retrieve an arbitrary node of the graph (e.g. when you need to re-fetch a node without needing to re-fetch all parent nodes along the path from the root to said node), there is the special node root field.

    In short, this field is able to retrieve an arbitrary node by using a special ‘global’ ID, which has data encoded needed for our system to know the type of that entity and how to retrieve it. For example, based on just banksy-champagne-formica-flag the system wouldn’t be able to know what type of entity this refers to, a global ID would rather encode it like Artwork:banksy-champagne-formica-flag.

Connections over Lists

It is undesirable to have multiple fields that semantically refer to the same data. So rather than defining both e.g. an artworks field and an artworks_connection, simply choose one form applicable to the data and call it artworks.

  • A paginated list (such as most associated types) should use ‘connections’ (see the spec and this blog post).

    • These are more forward-compatible, as they allow for adding metadata to the association itself and the ‘edge’ (the relation between the parent entity and the associated entity).
    • Connections use cursor based pagination, which has benefits over ‘page’ based pagination in systems where data availability could change (e.g. removing/adding of artworks) and it’s an abstraction that allows the underlying system to change the way it does pagination whilst staying API compatible.
  • In rare cases an immediate list may be used, but this should only be done in cases where the list has few entries and doesn’t require pagination, i.e. fetch all entries at once.

Schema stitching

As we expand our microservices oriented architecture and take our use of GraphQL more serious, a need has arisen to model GraphQL schemas more closely to the data source (i.e. database) in an effort to colocate schema code next to the rest of the code related to that data and to make these schema resolvers more performant.

In this new scenario, metaphysics would be an orchestration layer that stitches together these various schemas into a single coherent schema that clients can consume without needing to know about the existence of these various microservices.

  • Services should only model the data they themselves are responsible for and leave it up to metaphysics to model that further. For example, consider a list of consignment submissions for a user, rather than modelling the submissions under an ‘authenticated user’ in your upstream API like this:

    {
      me {
        submissions {
          # ...
        }
      }
    }

    Make submissions a root field that takes a user_id argument:

    {
      submissions(user_id: 42) {
        # ...
      }
    }

    then Metaphysics can then stitch that together the submissions query with a User type and model the schema as per the first example.

  • Schemas that are exposed to Metaphysics for stitching, should preferably name the fields and mutations they define in such a way that doesn't leak any implementation details about the underlying service, and reflects actual real-world 'business' groupings.

    For instance, Convection has a Submission model (which represents a consignment), but addSubmission is probably not a great name to expose at the Metaphysics orchestration layer (even though in a more old-school approach where your client directly used Convection, this may have been fine).

    Additionally, addSubmissionToConvection is probably not a great name to expose at Metaphysics either. This leaks the underlying implementation (Convection), which will make it harder to update in the future.

    So, perhaps something like addConsignment or addConsignmentSubmission is the best name to give this mutation, in your Convection GraphQL schema.

Unions instead of Merging Responsibilities

When you have a response that could be multiple things, instead of inlining the data into an existing object, consider using unions to separate the responsibilities. For example, if you have an Order object which represents sending a physical object to a person, they could get it in a few ways. Instead of having a type like:

type Order {
  item: Thing

  is_pickup: Boolean!

  address_1: String!
  address_2: String
  country: String!
  phone: String!
}

Use a union to force clients to cover all potential cases:

type Pickup {
  time: String
}

type Mail {
  address_1: String!
  address_2: String
  country: String!
  phone: String!
}

union Shipping = Pickup | Mail

type Order {
  item: Thing

  shipping: Shipping
}

This ensures that:

  • You can never end up in a state where is_pickup is true, but there is address metadata available
  • You can safely extend Shipping with a new type (like a digital work with a url/email)
  • Clients need to specify and be aware of the objects they want to handle when making queries

Mutation Responses as Unions

The GraphQL community still hasn't consolidated on how to handle errors in mutations. We're currently thinking that there are two types of issues: exceptions and errors. An exception is something unexpected, and this shows up in root of a response under errors.

Errors that you expect to happen can be modeled as a union:

# A known fail-state
type CreditCardMutationFailure {
  mutationError: GravityMutationError
}

# A known success state
type CreditCardMutationSuccess {
  creditCard: CreditCard
}

# The response to a mutation being either a success or a failure
union CreditCardMutationType = CreditCardMutationSuccess | CreditCardMutationFailure

type Mutation {
  # Create a credit card
  createCreditCard(input: CreditCardInput!): CreditCardMutationType
}

This has all of the same advantages as above, but that you can also customise the Error object to fit the domain of the mutation. For example, when working with forms the API can pass back the name and reasons for failing field validations.

Partial Types over nullability

When you have data that can be partially completed, or in a draft stage, consider using the type system to your advantage. For example - take submitting a consignment as an example. During the user's drafting phase, you can use an object with all of the fields that need to be filled in as nullable:

type ConsignmentDraft {
  title: String
  location: String
  category: String

  # This is optional
  signatureExplanation: String
}

Which gives a time for the user to fill out all these different fields over time. Then, when submitting the object, and storing it for long-term, switch it's type:

type ConsignmentSubmission {
  title: String!
  location: String!
  category: String!

  # This is still optional
  signatureExplanation: String
}

This means clients can make stronger assumptions about the data they're working with. It can take your server-side validation, and allow API clients to rely on your validations when the object has migrated into a finalized state. For example:

# A changing, not fully validated version of a Consignment
type ConsignmentDraft {
  id: ID
  title: String
  location: String
  category: String

  # This is optional
  signatureExplanation: String
}

# A fully-validated Consignment, with non-null versions of the Consignment's inputs
type ConsignmentSubmission {
  id: ID
  title: String!
  location: String!
  category: String!

  # This is still optional
  signatureExplanation: String
}

# The root query, so the fields you can use in a request
type Query {
  # All your finalized consignments, with guaranteed/validated fields
  submitted: ConsignmentSubmissionConnection!

  # All your WIP consignments that could be half-finished
  drafts: ConsignmentDraftConnection!
}

type Mutation {
  # The input can be incrementally sent, as all the fields are optional
  draftConsignment(input: ConsignmentDraft!): ConsignmentDraftMutationType

  # Takes the ID of a draft consignment (effectively a mutable consignment
  # and submits it transforming it into a submission)
  submitConsignment(id: ID!): ConsignmentSubmissionMutationType
}

Note: You don't have to structure the data in your database like this. The difference in the resolvers for submitted and drafts could be a lookup for a state field on an object being "submitted". The key concept is that you can declare something as being after data validation has occurred.