graphql-engine/dc-agents

Data Connectors

This document describes the current specification of the new _data connectors_s feature of graphql-engine, which is under active development.

The data connectors feature allows graphql-engine to delegate the execution of operations to external web services called agents. Such agents provide access to a data set, allowing graphql-engine to query that data set over a web API.

This document specifies (1) the web API that must be presented by agents, and (2) the precise behaviour of agents for specific reference data sets.

For further reference, the directory in which this document resides contains some implementations of different agents:

• Reference Implementation - A reference implementation in TypeScript that serves some static in-memory data

Stability

This specification is complete with regards to the current implementation, but should be considered unstable until the Data Connectors feature is officially released and explicitly marked as a non-experimental feature.

Setting up Data Connector agents with graphql-engine

In order to run one of the example agents, follow the steps in its respective README document.

Once an agent is running, import the following metadata into graphql-engine:

POST /v1/metadata

{
  "type": "replace_metadata",
  "args": {
    "metadata": {
      "version": 3,
      "backend_configs": {
        "dataconnector": {
          "reference": {
            "uri": "http://localhost:8100/"
          }
        }
      },
      "sources": [
        {
          "name": "chinook",
          "kind": "reference",
          "tables": [
            {
              "table": "Album",
              "object_relationships": [
                {
                  "name": "Artist",
                  "using": {
                    "manual_configuration": {
                      "remote_table": "Artist",
                      "column_mapping": {
                        "ArtistId": "ArtistId"
                      }
                    }
                  }
                }
              ]
            },
            {
              "table": "Artist",
              "array_relationships": [
                {
                  "name": "Album",
                  "using": {
                    "manual_configuration": {
                      "remote_table": "Album",
                      "column_mapping": {
                        "ArtistId": "ArtistId"
                      }
                    }
                  }
                }
              ]
            }
          ],
          "configuration": {
            "tables": [ "Artist", "Album" ]
          }
        }
      ]
    }
  }
}

The backend_configs.dataconnector section lets you set the URIs for as many agents as you'd like. In this case, we've defined one called "reference". When you create a source, the kind of the source should be set to the name you gave the agent in the backend_configs.dataconnector section (in this case, "reference").

The configuration property under the source can contain an 'arbitrary' JSON object, and this JSON will be sent to the agent on every request via the X-Hasura-DataConnector-Config header. The example here is configuration that the reference agent uses. The JSON object must conform to the schema specified by the agent from its /config-schema endpoint.

The name property under the source will be sent to the agent on every request via the X-Hasura-DataConnector-SourceName header. This name uniquely identifies a source within an instance of HGE.

The albums and artists tables should now be available in the GraphiQL console. You should be able to issue queries via the web service. For example:

query {
  artists {
    name
    albums {
      title
    }
  }
}

Implementing Data Connector agents

This section is a guide to implementing Data Connector agents for graphql-engine. You may find it useful to consult the code examples for reference.

The entry point to the reference agent application is a Fastify HTTP server. Raw data is loaded from JSON files on disk, and the server provides the following endpoints:

• GET /capabilities, which returns the capabilities of the agent and a schema that describes the type of the configuration expected to be sent on the X-Hasura-DataConnector-Config header
• GET /schema, which returns information about the provided data schema, its tables and their columns
• POST /query, which receives a query structure to be executed, encoded as the JSON request body, and returns JSON conforming to the schema described by the /schema endpoint, and contining the requested fields.
• GET /health, which can be used to either check if the agent is running, or if a particular data source is healthy

The /schema and /query endpoints require the request to have the X-Hasura-DataConnector-Config header set. That header contains configuration information that agent can use to configure itself. For example, the header could contain a connection string to the database, if the agent requires a connection string to know how to connect to a specific database. The header must be a JSON object, but the specific properties that are required are up to the agent to define.

The /schema and /query endpoints also require the request to have the X-Hasura-DataConnector-SourceName header set. This header contains the name of the data source configured in HGE that will be querying the agent. This can be used by the agent to maintain things like connection pools and configuration maps on a per-source basis.

We'll look at the implementation of each of the endpoints in turn.

Capabilities and configuration schema

The GET /capabilities endpoint is used by graphql-engine to discover the capabilities supported by the agent, and so that it can know the correct shape of configuration data that needs to be collected from the user and sent to the agent in the X-Hasura-DataConnector-Config header. It should return a JSON object similar to the following:

{
  "capabilities": {
    "relationships": {}
  },
  "configSchemas": {
    "configSchema": {
      "type": "object",
      "nullable": false,
      "properties": {
        "tables": { "$ref": "#/otherSchemas/Tables" }
      }
    },
    "otherSchemas": {
      "Tables": {
        "description": "List of tables to make available in the schema and for querying",
        "type": "array",
        "items": { "$ref": "#/otherSchemas/TableName" },
        "nullable": true
      },
      "TableName": {
        "nullable": false,
        "type": "string"
      }
    }
  }
}

The capabilities section describes the capabilities of the service. Specifically, the service is capable of serving queries which involve relationships.

The configSchema property contains an OpenAPI 3 Schema object that represents the schema of the configuration object. It can use references ($ref) to refer to other schemas defined in the otherSchemas object by name.

graphql-engine will use the configSchema OpenAPI 3 Schema to validate the user's configuration JSON before putting it into the X-Hasura-DataConnector-Config header.

Schema and capabilities

The GET /schema endpoint is called whenever the metadata is (re)loaded by graphql-engine. It returns the following JSON object:

{
  "tables": [
    {
      "name": "Artist",
      "primary_key": ["ArtistId"],
      "description": "Collection of artists of music",
      "columns": [
        {
          "name": "ArtistId",
          "type": "number",
          "nullable": false,
          "description": "Artist primary key identifier"
        },
        {
          "name": "Name",
          "type": "string",
          "nullable": true,
          "description": "The name of the artist"
        }
      ]
    },
    {
      "name": "Album",
      "primary_key": ["AlbumId"],
      "description": "Collection of music albums created by artists",
      "columns": [
        {
          "name": "AlbumId",
          "type": "number",
          "nullable": false,
          "description": "Album primary key identifier"
        },
        {
          "name": "Title",
          "type": "string",
          "nullable": false,
          "description": "The title of the album"
        },
        {
          "name": "ArtistId",
          "type": "number",
          "nullable": false,
          "description": "The ID of the artist that created this album"
        }
      ]
    }
  ]
}

The tables section describes the two available tables, as well as their columns, including types and nullability information.

Notice that the names of tables and columns are used in the metadata document to describe tracked tables and relationships.

Type definitions

The SchemaResponse TypeScript type from the reference implementation describes the valid response body for the GET /schema endpoint.

Responding to queries

The POST /query endpoint is invoked when the user requests data from graphql-engine which is resolved by the service.

The service logs queries from the request body in the console. Here is a simple example based on a GraphQL query which fetches all artist data:

query {
  Artist {
    ArtistId
    Name
  }
}

and here is the resulting query request payload:

{
  "table": "Artist",
  "table_relationships": [],
  "query": {
    "where": {
      "expressions": [],
      "type": "and"
    },
    "order_by": [],
    "limit": null,
    "offset": null,
    "fields": {
      "ArtistId": {
        "type": "column",
        "column": "ArtistId"
      },
      "Name": {
        "type": "column",
        "column": "Name"
      }
    }
  }
}

The implementation of the service is responsible for intepreting this data structure and producing a JSON response body which is compatible with both the query and the schema.

Let's break down the request:

• The table field tells us which table to fetch the data from, namely the Artist table.
• The table_relationships field that lists any relationships used to join between tables in the query. This query does not use any relationships, so this is just an empty list here.
• The query field contains further information about how to query the specified table:
  • The where field tells us that there is currently no (interesting) predicate being applied to the rows of the data set (just an empty conjunction, which ought to return every row).
  • The order_by field tells us that there is no particular ordering to use, and that we can return data in its natural order.
  • The limit and offset fields tell us that there is no pagination required.
  • The fields field tells us that we ought to return two fields per row (ArtistId and Name), and that these fields should be fetched from the columns with the same names.

Response Body Structure

The response body for a call to POST /query should always consist of a JSON array of JSON objects. Each of those JSON objects should contain all of the same keys as the fields property of the request body, even if the values of the corresponding fields may be null.

In the case of fields appearing with type relationship, this property should apply recursively: such fields should appear in the response body as a nested array/object of record(s) (depending on whether the relationship is an object or array relationship), each with the appropriate fields described by the relationship field's query property.

Pagination

If the GraphQL query contains pagination information, then the limit and offset fields may be set to integer values, indicating the number of rows to return, and the index of the first row to return, respectively.

Filters

The where field contains a recursive expression data structure which should be interpreted as a predicate in the context of each record.

Each node of this recursive expression structure is tagged with a type property, which indicates the type of that node, and the node will contain one or more additional fields depending on that type. The valid expression types are enumerated below, along with these additional fields:

type	Additional fields	Description
and	expressions	A conjunction of several subexpressions
or	expressions	A disjunction of several subexpressions
not	expression	The negation of a single subexpression
binary_op	operator, column, value	Test the specified column against a single value using a particular binary comparison operator
binary_arr_op	operator, column, values	Test the specified column against an array of values using a particular binary comparison operator
unary_op	operator, column	Test the specified column against a particular unary comparison operator

The available binary comparison operators that can be used against a single value in binary_op are:

Binary comparison operator	Description
less_than	The < operator
less_than_or_equal	The <= operator
greater_than	The > operator
greater_than_or_equal	The >= operator
equal	The = operator

The available binary comparison operators that can be used against an array of values in binary_arr_op are:

Binary array comparison operator	Description
in	The SQL IN operator (ie. the column must be any of the array of specified values)

The available unary comparison operators that can be used against a column:

Unary comparison operator	Description
is_null	Tests if a column is null

Values (as used in value in binary_op and the values array in binary_arr_op) are specified as either a literal value, or a reference to another column, which could potentially be in another related table in the same query. The value object is tagged with a type property and has different fields based on the type.

type	Additional fields	Description
scalar	value	A scalar value to compare against
column	column	A column in the current table being queried to compare against

Columns (as used in column fields in binary_op, binary_arr_op, unary_op and in column-typed Values) are specified as a column name, as well as a path to the table that contains the column. This path is an array of relationship names that starts from the table being queried (ie the table being queried by the query that this where expression is being specified in). An empty array means the column would be on the table being queried itself.

Here is a simple example, which correponds to the predicate "first_name is John and last_name is Smith":

{
  "type": "and",
  "expressions": [
    {
      "type": "binary_op",
      "operator": "equal",
      "column": {
        "path": [],
        "name": "first_name"
      },
      "value": {
        "type": "scalar",
        "value": "John"
      }
    },
    {
      "type": "binary_op",
      "operator": "equal",
      "column": {
        "path": [],
        "name": "last_name"
      },
      "value": {
        "type": "scalar",
        "value": "John"
      }
    }
  ]
}

Here's another example, which corresponds to the predicate "first_name is the same as last_name":

{
  "type": "and",
  "expressions": [
    {
      "type": "binary_op",
      "operator": "equal",
      "column": {
        "path": [],
        "name": "first_name"
      },
      "value": {
        "type": "column",
        "column": {
          "path": [],
          "name": "last_name"
        }
      }
    }
  ]
}

Ordering

The order_by field specifies an array of zero-or-more orderings, each of which consists of a field to order records by, and an order which is either asc (ascending) or desc (descending).

If there are multiple orderings specified then records should be ordered lexicographically, with earlier orderings taking precedence.

For example, to order records principally by last_name, delegating to first_name in the case where two last names are equal, we would use the following order_by structure:

[
  {
    "field": "last_name",
    "order_type": "asc"
  },
  {
    "field": "first_name",
    "order_type": "asc"
  }
]

Relationships

If the call to GET /schema returns a capabilities record with the relationships field set to true, then the query structure may include fields corresponding to relationships.

Note :if the relationships field is set to false then graphql-engine will attempt to split queries into separate queries whenever relationships are involved. This is not always possible, depending on the structure of the query, but the fastest path to adding a data source as a Data Connector is to set relationships to false and to ignore this section. (This is currently not supported, but is intended to be in the future.)

Relationship fields are indicated by a type field containing the string relationship. Such fields will also include the name of the relationship in a field called relationship. This name refers to a relationship that is specified on the top-level query request object in the table_relationships field.

This table_relationships is a list of tables, and for each table, a map of relationship name to relationship information. The information is an object that has a field target_table that specifies the name of the related table. It has a field called relationship_type that specified either an object (many to one) or an array (one to many) relationship. There is also a column_mapping field that indicates the mapping from columns in the source table to columns in the related table.

It is intended that the backend should execute the query contained in the relationship field and return the resulting record set as the value of this field, with the additional record-level predicate that any mapped columns should be equal in the context of the current record of the current table.

An example will illustrate this. Consider the following GraphQL query:

query {
  Artist {
    Name
    Albums {
      Title
    }
  }
}

This will generate the following JSON query if the agent supports relationships:

{
  "table": "Artist",
  "table_relationships": [
    {
      "source_table": "Artist",
      "relationships": {
        "ArtistAlbums": {
          "target_table": "Album",
          "relationship_type": "array",
          "column_mapping": {
            "ArtistId": "ArtistId"
          }
        }
      }
    }
  ],
  "query": {
    "where": {
      "expressions": [],
      "type": "and"
    },
    "offset": null,
    "order_by": [],
    "limit": null,
    "fields": {
      "Albums": {
        "type": "relationship",
        "relationship": "ArtistAlbums",
        "query": {
          "where": {
            "expressions": [],
            "type": "and"
          },
          "offset": null,
          "from": "albums",
          "order_by": [],
          "limit": null,
          "fields": {
            "Title": {
              "type": "column",
              "column": "Title"
            }
          }
        }
      },
      "Name": {
        "type": "column",
        "column": "Name"
      }
    }
  }
}

Note the Albums field in particular, which traverses the Artists -> Albums relationship, via the ArtistAlbums relationship:

{
  "type": "relationship",
  "relationship": "ArtistAlbums",
  "query": {
    "where": {
      "expressions": [],
      "type": "and"
    },
    "offset": null,
    "from": "albums",
    "order_by": [],
    "limit": null,
    "fields": {
      "Title": {
        "type": "column",
        "column": "Title"
      }
    }
  }
}

The top-level table_relationships can be looked up by starting from the source table (in this case Artist), locating the ArtistAlbums relationship under that table, then extracting the relationship information. This information includes the target_table field which indicates the table to be queried when following this relationship is the Album table. The relationship_type field indicates that this relationship is an array relationship (ie. that it will return zero to many Album rows per Artist row). The column_mapping field indicates the column mapping for this relationship, namely that the Artist's ArtistId must equal the Album's ArtistId.

Back on the relationship field inside the query, there is another query field. This indicates the query that should be executed against the Album table, but we must remember to enforce the additional constraint between Artist's ArtistId and Album's ArtistId. That is, in the context of any single outer Artist record, we should populate the Albums field with the array of Album records for which the ArtistId field is equal to the outer record's ArtistId field.

Cross-Table Filtering

It is possible to form queries that filter their results by comparing columns across tables via relationships. One way this can happen in Hasura GraphQL Engine is when configuring permissions on a table. It is possible to configure a filter on a table such that it joins to another table in order to compare some data in the filter expression.

The following metadata when used with HGE configures a Customer and Employee table, and sets up a select permission rule on Customer such that only customers that live in the same country as their SupportRep Employee would be visible to users in the user role:

POST /v1/metadata

{
  "type": "replace_metadata",
  "args": {
    "metadata": {
      "version": 3,
      "backend_configs": {
        "dataconnector": {
          "reference": {
            "uri": "http://localhost:8100/"
          }
        }
      },
      "sources": [
        {
          "name": "chinook",
          "kind": "reference",
          "tables": [
            {
              "table": "Customer",
              "object_relationships": [
                {
                  "name": "SupportRep",
                  "using": {
                    "manual_configuration": {
                      "remote_table": "Employee",
                      "column_mapping": {
                        "SupportRepId": "EmployeeId"
                      }
                    }
                  }
                }
              ],
              "select_permissions": [
                {
                  "role": "user",
                  "permission": {
                    "columns": [
                      "CustomerId",
                      "FirstName",
                      "LastName",
                      "Country",
                      "SupportRepId"
                    ],
                    "filter": {
                      "SupportRep": {
                        "Country": {
                          "_ceq": ["$","Country"]
                        }
                      }
                    }
                  }
                }
              ]
            },
            {
              "table": "Employee"
            }
          ],
          "configuration": {}
        }
      ]
    }
  }
}

Given this GraphQL query (where the X-Hasura-Role header is set to user):

query getCustomer {
  Customer {
    CustomerId
    FirstName
    LastName
    Country
    SupportRepId
  }
}

We would get the following query request JSON:

{
  "table": "Customer",
  "table_relationships": [
    {
      "source_table": "Customer",
      "relationships": {
        "SupportRep": {
          "target_table": "Employee",
          "relationship_type": "object",
          "column_mapping": {
            "SupportRepId": "EmployeeId"
          }
        }
      }
    }
  ],
  "query": {
    "fields": {
      "Country": {
        "type": "column",
        "column": "Country"
      },
      "CustomerId": {
        "type": "column",
        "column": "CustomerId"
      },
      "FirstName": {
        "type": "column",
        "column": "FirstName"
      },
      "LastName": {
        "type": "column",
        "column": "LastName"
      },
      "SupportRepId": {
        "type": "column",
        "column": "SupportRepId"
      }
    },
    "where": {
      "type": "and",
      "expressions": [
        {
          "type": "binary_op",
          "operator": "equal",
          "column": {
            "path": ["SupportRep"],
            "name": "Country"
          },
          "value": {
            "type": "column",
            "column": {
              "path": [],
              "name": "Country"
            }
          }
        }
      ]
    }
  }
}

The key point of interest here is in the where field where we are comparing between columns. The first column's path is ["SupportRep"] indicating that the Country column specified there is on the other side of the Customer table's SupportRep relationship (ie. to the Employee table). The related Employee's Country column is being compared with equal to Customer's Country column (as indicated by the [] path). So, in order to evaluate this condition, we'd need to join the Employee table using the column_mapping specified in the SupportRep relationship and if any of the related rows (in this case, only one because it is an object relation) contain a Country that is equal to Employee row's Country, then the binary_op evaluates to True and we don't filter out the row.

Type Definitions

The QueryRequest TypeScript type in the reference implementation describes the valid request body payloads which may be passed to the POST /query endpoint. The response body structure is captured by the QueryResponse type.

Health endpoint

Agents must expose a /health endpoint which must return a 204 No Content HTTP response code if the agent is up and running. This does not mean that the agent is able to connect to any data source it performs queries against, only that the agent is running and can accept requests, even if some of those requests might fail because a dependant service is unavailable.

However, this endpoint can also be used to check whether the ability of the agent to talk to a particular data source is healthy. If the endpoint is sent the X-Hasura-DataConnector-Config and X-Hasura-DataConnector-SourceName headers, then the agent is expected to check that it can successfully talk to whatever data source is being specified by those headers. If it can do so, then it must return a 204 No Content response code.