graphql-engine/server/documentation/deep-dives/remote-joins-execution.md

Executing remote joins

When a request has been parsed, and is ready to be executed, we start by building a JoinTree: a structure close to a prefix tree, containing all the paths in the response that will require remote joins. We call this phase the collection phase: it constructs the build tree, and transforms the request as needed.

After executing the core step of the request, if there is indeed a join tree, then we start the join phase: we fold that tree, expending the response with the result of each subsequent request.

Table of contents

• The join tree
• Collect
• Join
• Ambiguous schemas

The join tree

As mentioned, the join tree is almost like a prefix tree; the key difference is that we don't store values at arbitrary points of the tree, only at the leaves. Furthermore, while most prefix trees are indexed by character, in our case we index joins by the path through the response.

For instance, imagine that we send the following request:

query {
  authors {
    name
    articles { # remote join
      title
    }
  }
}

the join tree we would emit would have the following shape:

(Nothing, authors):
  (Nothing, articles): <join information>

Recursively, all the way down, each join information might contain its own join tree if there are any nested remote relationship.

Each key in this join tree is a pair: it contains the name of the field, but also contains an optional type information: this is used to deal with ambiguous schemas.

Collect

Implemented in Hasura.GraphQL.Execute.RemoteJoin.Collect, this phase identifies the remote joins in a request, and transforms the request accordingly. If a selection set contains a field that is a remote join, we alter the selection set:

• the field that maps to a remote join is replaced by a placeholder value, so that we can keep track of the order in the selection set (since that order must not be altered)
• we add "phantom fields": fields that were not requested by the user, but that we need to include, as they are the keys on which the join is performed

In the case where the request goes to a remote schema, we might need additional transformations (see the section on ambiguous schemas).

From a practical perspective, the collection is a glorified traverse, operating in the Collector monad, which itself is a Writer monad: whenever we encounter a remote join, we tell it to the collector, and continue our traversal. Every time we traverse a field, we use censor to wrap the resulting joins in a sub-tree. Remote joins are aggregated using the Semigroup instance of JoinTree.

Join

Implemented in Hasura.GraphQL.Execute.RemoteJoin.Join, we post-process the root request by "folding" the tree of joins: we traverse the join tree alongside the response: for each field in the response that maps to a leaf of the join tree, we recursively do the same thing: issue a query, traverse its own join tree... and on the way back, we replace the value of field by the result of the join.

Depending on whether the target is a remote schema or a local source, we call either makeRemoteSchemaJoinCall or makeSourceJoinCall, defined in Hasura.GraphQL.Execute.RemoteJoin.RemoteServer and Hasura.GraphQL.Execute.RemoteJoin.Source respectively.

Ambiguous schemas

This process is made more complicated by the fact that remote schemas, via unions and interfaces, can be ambiguous. Consider the following request:

query {
  node(id: $some_id) {
    ... on Article {
      # foo is a field, returns data of type `t`
      foo {
         # r1 is a REMOTE relationship, returns data of type `u`
         bar: r1 {
         }
      }
    }
    ... on Author {
      id
      # foo is a field, returns data of type `t`
      foo {
         # r2 is a REMOTE relationship, returns data of type `u`
         bar: r2 {
         }
      }
    }
  }
}

There are several complications with this request:

• there are two remote joins that would need to be at the same point in the join tree, node.foo.bar;
• we need to identify which of the two relationships it is when processing the joins; but we can't do so using information about foo, since its __typename will be t in both cases.

To fix this, we have altered the join tree: instead of using the field name as key at each level, we use instead a combination of optional type name and field name. We identify as "ambiguous" all selection sets of a union or an interface that either directly contain remote joins, or whose subselections contain remote joins. Whenever we encounter such a selection set, we use its type name in the corresponding keys in the join tree, and we add one more phantom field to the selection set: __hasura_internal_typename, which extracts the __typename.

When processing the joins, we look for the presence of this field: if it is there, we remove it from the response, and we do the join tree lookup using its value, instead of using Nothing.

In practice, the join tree for the aforementioned query would therefore be:

(Nothing, node):
  (Article, foo):
    (Nothing, bar): <join info>
  (Author, foo):
    (Nothing, bar): <join info>