What is AST in GraphQL?

AST stands for “Abstract Syntax Tree,” and it is an essential concept in GraphQL. AST is a data structure that represents the syntactic structure of a GraphQL query or schema in a tree-like format. The AST serves as an intermediate representation of the GraphQL query or schema, enabling efficient parsing, validation, and execution of GraphQL operations.

When a GraphQL query is received by the server, the first step is to parse the query string into an AST. The AST provides a structured representation of the query that the server can easily traverse and analyze.

Similarly, when defining a GraphQL schema, the schema definition is parsed into an AST, allowing the server to understand and work with the defined types, fields, and operations.

The AST for a GraphQL query or schema is composed of nodes, where each node represents a specific part of the query or schema. The nodes form a tree-like structure with parent-child relationships, reflecting the hierarchical nature of GraphQL queries and schema definitions.

For example, consider the following GraphQL query:

query {
  user(id: "123") {
    name
    email
  }
}

After parsing, the query is represented as an AST, which might look like:

{
  "kind": "Document",
  "definitions": [
    {
      "kind": "OperationDefinition",
      "operation": "query",
      "selectionSet": {
        "kind": "SelectionSet",
        "selections": [
          {
            "kind": "Field",
            "name": {
              "kind": "Name",
              "value": "user"
            },
            "arguments": [
              {
                "kind": "Argument",
                "name": {
                  "kind": "Name",
                  "value": "id"
                },
                "value": {
                  "kind": "StringValue",
                  "value": "123"
                }
              }
            ],
            "selectionSet": {
              "kind": "SelectionSet",
              "selections": [
                {
                  "kind": "Field",
                  "name": {
                    "kind": "Name",
                    "value": "name"
                  }
                },
                {
                  "kind": "Field",
                  "name": {
                    "kind": "Name",
                    "value": "email"
                  }
                }
              ]
            }
          }
        ]
      }
    }
  ]
}

The above JSON represents the AST for the GraphQL query. Each object in the AST corresponds to a different part of the query (e.g., “Document,” “OperationDefinition,” “Field,” “SelectionSet,” etc.), forming a tree structure that captures the query’s syntactic structure.

ASTs are fundamental in enabling various features of GraphQL, such as query validation, introspection, and query execution. They provide an efficient way for GraphQL servers to process and work with GraphQL queries and schemas.

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