parent of the fields under it.null and adds an entry to errors. The rest of the data still arrives, usually with HTTP 200.POST. That is why HTTP caching takes more work than in REST.POST /graphqlA JSON body with query, variables and operationName.GET, with the query in the URL. Mutations must use POST.200, with data and errors side by side.TrapMonitoring that reads only status codes misses most GraphQL errors. Read the body too.
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AskedHow does GraphQL work over HTTP?Asked in interviews
AskedWhat are the limitations of GraphQL?TipMany systems use both, with gRPC between services and GraphQL in front of the apps.
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AskedWhat is the difference between GraphQL and gRPC?class UserAPI extends RESTDataSourceResolvers call the existing REST endpoints.TrapResolvers that call REST once per item bring the N+1 problem back. Batch them with DataLoader where REST has a bulk endpoint.
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AskedHow do you migrate from REST to GraphQL?type User { id: ID! }name: String!Fields are nullable unless their type ends in !.posts: [Post!]!type Query { me: User }Int, Float, String, Boolean and ID. ID is sent as a string.scalar DateServer code parses and validates its values.enum Role { ADMIN USER }input NewUser { name: String! }TrapYou can't use an object type like User as an argument. Inputs hold only scalars, enums and other inputs, or lists of them.
type User implements Nodeunion Result = User | PostTrapA union has no shared fields. Every field except __typename needs an inline fragment.
TipIn code-first, the schema exists only once the code runs. Generate the SDL to review changes.
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AskedWhat is the difference between schema-first and code-first development?@deprecated(reason: "Use name")Add the new field and mark the old one. Remove it once usage tracking shows no client needs it.TrapOld mobile app versions stay installed, so they keep asking for old fields.
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AskedHow do you version a GraphQL API?query reads data, mutation changes it, and subscription gets updates the server pushes.users(role: ADMIN)Any field can take arguments, nested ones too.query GetUser($id: ID!)user(id: $id)admins: users(role: ADMIN)TrapNever paste user input into the query text. Send it in variables.
fragment UserInfo on User...UserInfo... on Post { title }TipColocate each fragment with the component that renders its fields. Relay combines them into one query.
email @include(if: $full)posts @skip(if: $lite)@include, @skip, @deprecated and @specifiedBy, plus @oneOf for inputs that take exactly one field.Asked in interviews
AskedWhat are directives in GraphQL?node and its cursor, beside a pageInfo object.users(first: 10, after: $cursor)Pass the last endCursor as after.hasNextPageFetch one row more than first. If it comes back, there is a next page.Asked in interviews
AskedHow do you implement cursor-based pagination?(parent, args, context, info)context. The server builds it once per request.parent.async. Start independent calls together with Promise.all.parent of the fields nested under it.null, and its error and path go into errors.applyMiddleware(schema, logger)From the graphql-middleware library.mapSchema transformer.TrapSchema no longer works. GraphQL Tools 8 removed it, and Apollo Server 3 dropped schema.
startStandaloneServer(server) expressMiddleware(server) args and context.server.executeOperation({ query })TrapApollo Server 3 dropped apollo-server-testing and its create, which old guides still use.
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AskedWhat are some GraphQL testing strategies?TrapSmall development data hides it, so N+1 often shows up only in production.
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AskedWhat is the N+1 query problem in GraphQL?new DataLoader(batchFn) It gets every key from one tick of the event loop. Return one value per key, in the same order.loaders.posts.load(user.id) context function.TrapA loader shared across requests keeps serving values cached for earlier requests.
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AskedHow do you solve the N+1 problem with DataLoader?@cacheControl(maxAge: 60)A response lives as long as its lowest maxAge.GET, often as a persisted query hash.TrapKeep user-specific data out of shared caches.
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AskedWhat are some GraphQL caching strategies?TrapAPQ blocks nothing, since any client can register any query. Only an allowlist stops attackers.
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AskedWhat are persisted queries and query allowlisting?createPost(input: CreatePostInput!) CreatePostPayload It holds the changed objects, so the client can update its view.publishPost, not one generic update for the whole record.Asked in interviews
AskedWhat is the best practice for mutation input design?errors: [UserError!], each with a field and a code.union Result = User | EmailTakenextensions: { code: 'USER_NOT_FOUND' }Pass it to GraphQLError.formatErrorSwap database messages for a generic one, and log the real error.TrapKeep the top-level errors array for failures the client can't fix, like an outage.
SELECT ... FOR UPDATE.TrapEach top-level mutation field commits on its own. If a later one fails, the earlier one stays.
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AskedHow do you handle database transactions in GraphQL?scalar UploadFollow the multipart request spec, with graphql-upload.TrapMultipart requests skip the browser's preflight, so they need CSRF protection. Apollo Server blocks them without an Apollo-Require-Preflight header.
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AskedHow do you handle file uploads in GraphQL?context function, from the Authorization header.code: 'UNAUTHENTICATED'Throw a GraphQLError with this code. Apollo Server 4 removed AuthenticationError .TrapAuthentication only says who the user is. Each field still needs an authorization check.
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AskedHow do you implement authentication in GraphQL?@auth(requires: ADMIN)It needs server code to enforce it.null unless the viewer owns the record.TrapA check only at the top misses other paths to the same field. Check access on each field.
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AskedHow do you implement authorization in GraphQL?depthLimit(10)maximumCost: 1000A list multiplies its children's cost by first. So 50 users with 50 posts each count as 2,500 posts.Trapgraphql-rate-limit counts in memory by default. Use its RedisStore to share counts across servers.
__schema query, which returns every type and field. Tools build docs from it.introspection: falseApollo Server 4 and later turn it off by default when NODE_ENV is production.hideSchemaDetailsFromClientErrors: trueTrapHiding the schema is only obscurity. Authorization and cost limits are the real defense.
new ApolloClient({ link, cache })new HttpLink({ uri: '/graphql' })useQuery(GET_USER) useMutation(ADD_TODO) TrapApollo Client 4 removed the constructor's uri option and moved the hooks to @apollo/.
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AskedWhat is Apollo Client?User:1Its __typename plus its id.fetchPolicy: 'cache-first'A query fully in the cache skips the network.update function, or refetch the list.TrapHave each mutation return the id and changed fields. Otherwise the cache can't update the object.
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AskedHow does a normalized client cache like Apollo Client's work?optimisticResponse The object you expect back, with __typename and id.TrapSkip it where a false success is costly, like a payment.
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AskedHow would you handle optimistic UI after a mutation?npx graphql-codegenpreset: clientIt generates a typed graphql() function.TrapThe old apollo CLI no longer generates code. Apollo recommends GraphQL Code Generator.
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AskedWhat are some popular GraphQL code generators?subscription { messageAdded { id } }A subscription takes exactly one root field.Asked in interviews
AskedWhat are GraphQL subscriptions?graphql-ws. Server-Sent Events are the main alternative.expressMiddleware beside a graphql-ws server. startStandaloneServer has no support for them.pubsub.publish('MSG', payload)withFilter(subscribe, filterFn)PubSub for one backed by Redis or Kafka.Trapsubscriptions-transport-ws is deprecated and doesn't work with graphql-ws.
type User @key(fields: "id")That shared type is an entity.__resolveReference TrapIn Federation 2, skip extend type and @external on the key. Just declare the type with @key again.
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AskedWhat is GraphQL Federation?stitchSchemas({ subschemas })Nothing matches. Try a shorter word, or clear the search.
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