How Hasura GraphQL engine works

Given a Postgres database, the Hasura GraphQL engine can automatically generate a GraphQL schema and process GraphQL queries, subscriptions and mutations. Here’s what the Hasura GraphQL engine does under the hood.

Schema generation

The Hasura GraphQL engine automatically generates GraphQL schema components when you track a Postgres table/view in Hasura and create relationships between them.

Tables

When you track a Postgres table in the Hasura GraphQL engine, it automatically generates the following for it:

  • A GraphQL type definition for the table
  • A query field with where, order_by, limit and offset arguments
  • A subscription field with where, order_by, limit and offset arguments
  • An insert mutation field with on_conflict argument that supports upsert and bulk inserts
  • An update mutation field with where argument that supports bulk updates
  • A delete mutation field with where argument that supports bulk deletes

Views

When you track a Postgres view in Hasura GraphQL engine, it automatically generates the following for it:

  • A GraphQL type definition for the view
  • A query field with where, order_by, limit and offset arguments
  • A subscription field with where, order_by, limit and offset arguments

Essentially the Hasura GraphQL engine does the same thing it would do for a table, but without creating the insert, update and delete mutations.

Relationships

When you create a relationship between a table/view with another table/view in the Hasura GraphQL engine, it does the following:

  • Augments the type of the table/view by adding a reference to the nested type to allow fetching nested objects.
  • Augments the where and order_by clauses to allow filtering and sorting based on nested objects.

Resolvers

The Hasura GraphQL engine does not have any resolvers. The Hasura GraphQL engine is actually a compiler that compiles your GraphQL query into an efficient SQL query.

Hasura’s GraphQL syntax is also optimized to expose the power of the underlying SQL so that you can make powerful queries via GraphQL.

Metadata

All the information required for schema generation is stored by the Hasura GraphQL engine as metadata in a specific Postgres schema in the database. See metadata schema for more details.