What is a GraphQL schema and what is SDL?
Assesses fundamental understanding of GraphQL conventions, runtime behavior, and memory/performance considerations.
Hiring managers look for precision, avoidance of ambiguous jargon, and ability to explain trade-offs under real production conditions.
The schema is the contract between client and server: it declares the types, fields, arguments, return types and nullability that all operations are validated against. The Schema Definition Language (SDL) is the human-readable syntax for writing it.
type User {
id: ID!
name: String!
email: String
orders(first: Int = 10): [Order!]!
}
type Query {
user(id: ID!): User
}
A ! marks a non-null field. The schema also defines input types, enums, interfaces, unions and directives. There are two authoring styles: schema-first, where SDL is the source of truth and resolvers are bound to it, and code-first, where the schema is generated from typed code. Introspection exposes the schema for tooling such as GraphiQL and code generators. Because clients build against the schema, treat it as a public contract and evolve it only with backward-compatible additions.
Candidate Response Strategy & Interview Tips
- Start with a concise one-sentence summary: Deliver a direct, confident answer first before expanding into nuances.
- Demonstrate real-world trade-offs: Discuss where this approach excels and when you would avoid it in production systems.
- Discuss complexity & edge cases: Proactively explain time/space complexity or boundary conditions (null values, scale limits).
- Prepare for interviewer follow-ups: Technical hiring panels frequently probe deeper into concurrency, backward compatibility, or alternative libraries.