How does the GraphQL query planning and execution pipeline work?

A GraphQL request goes through five phases: (1) Parsing: the query string is lexed and parsed into an AST (Abstract Syntax Tree) by graphql-js. Syntax errors are caught here. (2) Validation: the AST is validated against the schema — unknown fields, type mismatches, missing required arguments, and circular fragments are caught. Custom validation rules (depth limits, complexity) run here. (3) Execution planning (Federation only): the Router's query planner analyzes the query, identifies which subgraphs own which fields, and creates a parallel/sequential execution plan. (4) Execution: resolvers are called. Query field resolvers run concurrently (Promises collected, then awaited). Mutation top-level fields run serially. (5) Response formatting: results are merged, null propagation applied for non-null errors, and the response is serialized to JSON. The errors array is populated from any resolver errors. Each phase is a potential plugin hook in Apollo Server.