Backend practice
Every Backend exercise, built from a topic you can open beside it.
139 exercises runs offline
All tracks Python 139 JavaScript 184 TypeScript 76 Go 106 Rust 75 Java 81 Kotlin 66 C++ 92 C# 88 Swift 68 PHP 69 Frontend 82 Backend 139 Architecture and system design 25 DevOps and cloud 15 Data and databases 5 Data science 31 AI and LLM engineering 75 Security 26 Game development 26 Coding in the AI era 101 CS foundations 164
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api Quiz new Which response best describes a successful POST that created order o-42? Design HTTP APIs as evolvable contracts, including resources, methods, errors, concurrency control, pagination, and safe retries. api Predict the output new What does this program print for a successful deletion? Design HTTP APIs as evolvable contracts, including resources, methods, errors, concurrency control, pagination, and safe retries. api Review AI code new Review a generated transfer endpoint Design HTTP APIs as evolvable contracts, including resources, methods, errors, concurrency control, pagination, and safe retries. api Spot the bug new Spot the bug in the product update Design HTTP APIs as evolvable contracts, including resources, methods, errors, concurrency control, pagination, and safe retries. api Quiz new Orders are sorted by createdAt, which is not unique. What must a cursor design add first? Design HTTP APIs as evolvable contracts, including resources, methods, errors, concurrency control, pagination, and safe retries. api Quiz new Which practice makes an OpenAPI description part of an API-first process? Make the API contract a reviewable, executable deliverable before implementations, clients, mocks, and tests begin to depend on it. api Quiz new What evidence does a consumer-driven contract add beside an OpenAPI description? Make the API contract a reviewable, executable deliverable before implementations, clients, mocks, and tests begin to depend on it. api Predict the output new What does this contract check print? Make the API contract a reviewable, executable deliverable before implementations, clients, mocks, and tests begin to depend on it. api Quiz new A provider adds an optional field to a response. When is that change compatible with an e… Make the API contract a reviewable, executable deliverable before implementations, clients, mocks, and tests begin to depend on it. api Review AI code new Review a generated create-order endpoint Make the API contract a reviewable, executable deliverable before implementations, clients, mocks, and tests begin to depend on it. api Spot the bug new Spot the bug in the contract test Make the API contract a reviewable, executable deliverable before implementations, clients, mocks, and tests begin to depend on it. api Quiz new When is adding a response field backward-compatible? Evolve an HTTP API without silently changing the contract that existing clients depend on. api Quiz new Which pair uses the current standard syntax for a deprecation date and a later sunset? Evolve an HTTP API without silently changing the contract that existing clients depend on. api Predict the output new What does this version selector print? Evolve an HTTP API without silently changing the contract that existing clients depend on. api Quiz new Which artifact should normally determine whether an HTTP API needs a new public version? Evolve an HTTP API without silently changing the contract that existing clients depend on. api Review AI code new Review a generated versioned order handler Evolve an HTTP API without silently changing the contract that existing clients depend on. api Spot the bug new Spot the bug in the version cache Evolve an HTTP API without silently changing the contract that existing clients depend on. api Quiz new A Django 6.0 async view must perform several ORM writes in one transaction. What is the s… A batteries-included Python web framework for routing requests, validating input, persisting data, rendering HTML, and operating secure applications. api Quiz new A RunPython data migration needs the Order model. Which lookup is safe for the recorded m… A batteries-included Python web framework for routing requests, validating input, persisting data, rendering HTML, and operating secure applications. api Predict the output new What does this URL resolution program print? A batteries-included Python web framework for routing requests, validating input, persisting data, rendering HTML, and operating secure applications. api Quiz new Which operation normally builds a new QuerySet without executing its SELECT? A batteries-included Python web framework for routing requests, validating input, persisting data, rendering HTML, and operating secure applications. api Review AI code new Review a generated profile endpoint A batteries-included Python web framework for routing requests, validating input, persisting data, rendering HTML, and operating secure applications. api Spot the bug new Spot the bug in the order view A batteries-included Python web framework for routing requests, validating input, persisting data, rendering HTML, and operating secure applications. api Quiz new A plugin has a default local `onBeforeHandle` hook and an `/account` route. The parent us… A Bun-first TypeScript framework that combines route schemas, lifecycle hooks, and an inferred Eden client contract. api Predict the output new What statuses does this Elysia program print? A Bun-first TypeScript framework that combines route schemas, lifecycle hooks, and an inferred Eden client contract. api Review AI code new Review a generated Elysia order route A Bun-first TypeScript framework that combines route schemas, lifecycle hooks, and an inferred Eden client contract. api Quiz new What does a declared Elysia body schema provide that a TypeScript interface alone cannot? A Bun-first TypeScript framework that combines route schemas, lifecycle hooks, and an inferred Eden client contract. api Spot the bug new Spot the bug in Elysia route typing A Bun-first TypeScript framework that combines route schemas, lifecycle hooks, and an inferred Eden client contract. api Quiz new Why prefer `return status(404, payload)` when a route declares a `404` response schema? A Bun-first TypeScript framework that combines route schemas, lifecycle hooks, and an inferred Eden client contract. api Quiz new Which declaration lets a client omit the query parameter q? Build Python APIs with typed request, response, and dependency declarations that produce a testable OpenAPI contract. api Predict the output new What does this program print? Build Python APIs with typed request, response, and dependency declarations that produce a testable OpenAPI contract. api Quiz new A route has a broad return annotation and response_model=PublicUser. Which shape does Fas… Build Python APIs with typed request, response, and dependency declarations that produce a testable OpenAPI contract. api Review AI code new Review generated document updater Build Python APIs with typed request, response, and dependency declarations that produce a testable OpenAPI contract. api Spot the bug new Spot the bug in the async order reader Build Python APIs with typed request, response, and dependency declarations that produce a testable OpenAPI contract. api Quiz new Which implementation preserves backpressure while writing an upload? Receive, validate, quarantine, and publish untrusted bytes without exhausting the service or exposing unsafe content. api Quiz new Which statement correctly distinguishes multipart/form-data from an object-store multipar… Receive, validate, quarantine, and publish untrusted bytes without exhausting the service or exposing unsafe content. api Predict the output new What does this publication gate print? Receive, validate, quarantine, and publish untrusted bytes without exhausting the service or exposing unsafe content. api Review AI code new Review generated profile image uploader Receive, validate, quarantine, and publish untrusted bytes without exhausting the service or exposing unsafe content. api Spot the bug new Spot the bug in direct-upload completion Receive, validate, quarantine, and publish untrusted bytes without exhausting the service or exposing unsafe content. api Quiz new What does a Flask blueprint do before it is registered? Flask is a lightweight Python WSGI web framework for building testable HTTP services with explicit routes, contexts, and application factories. api Quiz new What should a view pass to a durable background job that needs the current request ID? Flask is a lightweight Python WSGI web framework for building testable HTTP services with explicit routes, contexts, and application factories. api Predict the output new What status-code list does this program print? Flask is a lightweight Python WSGI web framework for building testable HTTP services with explicit routes, contexts, and application factories. api Review AI code new Review generated ticket endpoint Flask is a lightweight Python WSGI web framework for building testable HTTP services with explicit routes, contexts, and application factories. api Spot the bug new Spot the bug in the export endpoint Flask is a lightweight Python WSGI web framework for building testable HTTP services with explicit routes, contexts, and application factories. api Quiz new Which response best represents a task that was successfully created at /tasks/t-9? Backend development starts at the HTTP boundary: route requests, validate input, persist state, and return API contracts clients can safely depend on. api Quiz new Why is a module-level array unsuitable as the only task store in a deployed backend? Backend development starts at the HTTP boundary: route requests, validate input, persist state, and return API contracts clients can safely depend on. api Predict the output new What does this router print for the request? Backend development starts at the HTTP boundary: route requests, validate input, persist state, and return API contracts clients can safely depend on. api Review AI code new Review generated task endpoint Backend development starts at the HTTP boundary: route requests, validate input, persist state, and return API contracts clients can safely depend on. api Spot the bug new Spot the bug in shared request state Backend development starts at the HTTP boundary: route requests, validate input, persist state, and return API contracts clients can safely depend on. api Quiz new What does GraphQL guarantee for two top-level fields in one mutation operation? GraphQL constrains client-selected data with a typed schema; understand execution, null bubbling, resolvers, and batching to build evolvable, bounded APIs. api Predict the output new The nullable order field contains a Non-Null total that resolves to null. What does resul… GraphQL constrains client-selected data with a typed schema; understand execution, null bubbling, resolvers, and batching to build evolvable, bounded APIs. api Review AI code new Review a generated order graph GraphQL constrains client-selected data with a typed schema; understand execution, null bubbling, resolvers, and batching to build evolvable, bounded APIs. api Spot the bug new Spot the bug in the relationship resolver GraphQL constrains client-selected data with a typed schema; understand execution, null bubbling, resolvers, and batching to build evolvable, bounded APIs. api Quiz new A query selects Product.internalCost, which is absent from the schema. What should happen? GraphQL constrains client-selected data with a typed schema; understand execution, null bubbling, resolvers, and batching to build evolvable, bounded APIs. api Predict the output new What does this program print? Define typed RPC contracts with Protocol Buffers, then handle streaming, deadlines, status codes, retries, and compatible evolution correctly. api Quiz new What should a schema author do after removing field 4 named warehouse? Define typed RPC contracts with Protocol Buffers, then handle streaming, deadlines, status codes, retries, and compatible evolution correctly. api Quiz new A reservation RPC returns UNAVAILABLE after the server may have committed. What makes an … Define typed RPC contracts with Protocol Buffers, then handle streaming, deadlines, status codes, retries, and compatible evolution correctly. api Review AI code new Review a generated reservation client Define typed RPC contracts with Protocol Buffers, then handle streaming, deadlines, status codes, retries, and compatible evolution correctly. api Spot the bug new Spot the bug in the gRPC handler Define typed RPC contracts with Protocol Buffers, then handle streaming, deadlines, status codes, retries, and compatible evolution correctly. api Quiz new What must still be audited before moving a Web-standard Hono application to another runti… A Web-standards TypeScript HTTP framework that unifies routing, middleware, runtime validation, testing, and typed clients across runtimes. api Predict the output new What does this Hono program print? A Web-standards TypeScript HTTP framework that unifies routing, middleware, runtime validation, testing, and typed clients across runtimes. api Review AI code new Review a generated Hono order route A Web-standards TypeScript HTTP framework that unifies routing, middleware, runtime validation, testing, and typed clients across runtimes. api Quiz new Which type should an `hc` client receive for reliable route inference? A Web-standards TypeScript HTTP framework that unifies routing, middleware, runtime validation, testing, and typed clients across runtimes. api Quiz new What does `await c.req.json<OrderInput>()` guarantee about an incoming body? A Web-standards TypeScript HTTP framework that unifies routing, middleware, runtime validation, testing, and typed clients across runtimes. api Spot the bug new Spot the bug in Hono admin middleware A Web-standards TypeScript HTTP framework that unifies routing, middleware, runtime validation, testing, and typed clients across runtimes. api Predict the output new What does this program print? Build controlled synchronous and asynchronous Python HTTP clients with correct pooling, phase timeouts, streaming cleanup, errors, and retries. api Review AI code new Review a generated order fetcher Build controlled synchronous and asynchronous Python HTTP clients with correct pooling, phase timeouts, streaming cleanup, errors, and retries. api Spot the bug new Spot the bug in the async HTTPX client Build controlled synchronous and asynchronous Python HTTP clients with correct pooling, phase timeouts, streaming cleanup, errors, and retries. api Quiz new Why should a streaming response normally be used as a context manager? Build controlled synchronous and asynchronous Python HTTP clients with correct pooling, phase timeouts, streaming cleanup, errors, and retries. api Quiz new What does timeout=5.0 guarantee for a normal HTTPX request? Build controlled synchronous and asynchronous Python HTTP clients with correct pooling, phase timeouts, streaming cleanup, errors, and retries. api Quiz new Where should a JWT verifier get its allowed algorithms? JWT authentication carries signed identity claims; safe implementations validate the algorithm, key, issuer, audience, time, and token purpose. api Predict the output new What does this program print? JWT authentication carries signed identity claims; safe implementations validate the algorithm, key, issuer, audience, time, and token purpose. api Review AI code new Review generated JWT middleware JWT authentication carries signed identity claims; safe implementations validate the algorithm, key, issuer, audience, time, and token purpose. api Quiz new A token has a valid signature and has not expired. Which check still prevents use at the … JWT authentication carries signed identity claims; safe implementations validate the algorithm, key, issuer, audience, time, and token purpose. api Quiz new Which shape actually starts two independent calls before waiting for either result? Build testable HTTP services in Kotlin with clear request boundaries, structured concurrency, isolated blocking calls, and reliable error handling. api Predict the output new What does this boundary parser print? Build testable HTTP services in Kotlin with clear request boundaries, structured concurrency, isolated blocking calls, and reliable error handling. api Review AI code new Review a generated Ktor order route Build testable HTTP services in Kotlin with clear request boundaries, structured concurrency, isolated blocking calls, and reliable error handling. api Spot the bug new Spot the bug in notification launch Build testable HTTP services in Kotlin with clear request boundaries, structured concurrency, isolated blocking calls, and reliable error handling. api Spot the bug new Spot the bug in the coroutine error mapper Build testable HTTP services in Kotlin with clear request boundaries, structured concurrency, isolated blocking calls, and reliable error handling. api Quiz new What does adding `suspend` to a function that calls blocking JDBC guarantee? Build testable HTTP services in Kotlin with clear request boundaries, structured concurrency, isolated blocking calls, and reliable error handling. api Quiz new Which test best proves an Eloquent N+1 regression is fixed? Build testable PHP HTTP applications with Laravel 13 by understanding its request lifecycle, container, validation boundary, and Eloquent data access. api Quiz new A service stores per-job correlation data and a queue worker handles many jobs in one pro… Build testable PHP HTTP applications with Laravel 13 by understanding its request lifecycle, container, validation boundary, and Eloquent data access. api Predict the output new What does this Laravel container program print? Build testable PHP HTTP applications with Laravel 13 by understanding its request lifecycle, container, validation boundary, and Eloquent data access. api Review AI code new Review a generated order endpoint Build testable PHP HTTP applications with Laravel 13 by understanding its request lifecycle, container, validation boundary, and Eloquent data access. api Spot the bug new Spot the bug in the Laravel controller Build testable PHP HTTP applications with Laravel 13 by understanding its request lifecycle, container, validation boundary, and Eloquent data access. api Quiz new A Form Request validates an order ID as an integer. What must still happen before updatin… Build testable PHP HTTP applications with Laravel 13 by understanding its request lifecycle, container, validation boundary, and Eloquent data access. api Predict the output new What does this program print? Use Python Requests to build HTTP calls, handle responses, reuse sessions, and set explicit boundaries for timeouts, retries, streaming, and TLS. api Quiz new A POST may have committed before its response was lost. What makes retrying the create op… Use Python Requests to build HTTP calls, handle responses, reuse sessions, and set explicit boundaries for timeouts, retries, streaming, and TLS. api Review AI code new Review a generated report downloader Use Python Requests to build HTTP calls, handle responses, reuse sessions, and set explicit boundaries for timeouts, retries, streaming, and TLS. api Spot the bug new Spot the bug in the HTTP client function Use Python Requests to build HTTP calls, handle responses, reuse sessions, and set explicit boundaries for timeouts, retries, streaming, and TLS. api Quiz new What does timeout=(2, 5) guarantee for a Requests call? Use Python Requests to build HTTP calls, handle responses, reuse sessions, and set explicit boundaries for timeouts, retries, streaming, and TLS. api Quiz new Which statement describes HTTP idempotency correctly? Design understandable, retryable RESTful APIs with resources, HTTP methods, status codes, conditional requests, and stable error contracts. api Predict the output new What does this resource handler print? Design understandable, retryable RESTful APIs with resources, HTTP methods, status codes, conditional requests, and stable error contracts. api Review AI code new Review generated order endpoint Design understandable, retryable RESTful APIs with resources, HTTP methods, status codes, conditional requests, and stable error contracts. api Spot the bug new Spot the bug in the order replacement Design understandable, retryable RESTful APIs with resources, HTTP methods, status codes, conditional requests, and stable error contracts. api Quiz new Orders are sorted by a non-unique createdAt field. What must a cursor add? Design understandable, retryable RESTful APIs with resources, HTTP methods, status codes, conditional requests, and stable error contracts. api Predict the output new What does this Rails strong-parameters script print? Build Rails 8.1 HTTP applications by understanding the request lifecycle, conventions, Active Record boundaries, and a testable write path. api Quiz new Which test best proves that a Rails N+1 regression is fixed? Build Rails 8.1 HTTP applications by understanding the request lifecycle, conventions, Active Record boundaries, and a testable write path. api Review AI code new Review a generated order update Build Rails 8.1 HTTP applications by understanding the request lifecycle, conventions, Active Record boundaries, and a testable write path. api Spot the bug new Spot the bug in the Rails controller Build Rails 8.1 HTTP applications by understanding the request lifecycle, conventions, Active Record boundaries, and a testable write path. api Quiz new An order ID exists and its submitted status passes validation. What must happen before th… Build Rails 8.1 HTTP applications by understanding the request lifecycle, conventions, Active Record boundaries, and a testable write path. api Quiz new A timeout expires while awaiting a spawn_blocking task that has started. What can the cal… Build typed HTTP services with Rust 1.98, Axum, and Tokio while controlling request extraction, shared state, timeouts, and task lifetimes. api Quiz new Why must Json<T> be the last Axum handler argument when other arguments read request part… Build typed HTTP services with Rust 1.98, Axum, and Tokio while controlling request extraction, shared state, timeouts, and task lifetimes. api Predict the output new What does this program print? Build typed HTTP services with Rust 1.98, Axum, and Tokio while controlling request extraction, shared state, timeouts, and task lifetimes. api Review AI code new Review a generated Axum write handler Build typed HTTP services with Rust 1.98, Axum, and Tokio while controlling request extraction, shared state, timeouts, and task lifetimes. api Spot the bug new Spot the bug in the async cache refresh Build typed HTTP services with Rust 1.98, Axum, and Tokio while controlling request extraction, shared state, timeouts, and task lifetimes. api Quiz new What does `IO.blocking(jdbcCall())` provide? Build clear HTTP service boundaries with Scala 3, Cats Effect, and http4s while controlling effects, concurrency, resources, and errors. api Quiz new When does the body of `IO.println("saved")` normally run? Build clear HTTP service boundaries with Scala 3, Cats Effect, and http4s while controlling effects, concurrency, resources, and errors. api Predict the output new What does this boundary parser print? Build clear HTTP service boundaries with Scala 3, Cats Effect, and http4s while controlling effects, concurrency, resources, and errors. api Spot the bug new Spot the bug in the Resource client helper Build clear HTTP service boundaries with Scala 3, Cats Effect, and http4s while controlling effects, concurrency, resources, and errors. api Review AI code new Review a generated http4s order route Build clear HTTP service boundaries with Scala 3, Cats Effect, and http4s while controlling effects, concurrency, resources, and errors. api Spot the bug new Spot the bug in the eager Future wrapper Build clear HTTP service boundaries with Scala 3, Cats Effect, and http4s while controlling effects, concurrency, resources, and errors. api Quiz new Which Actuator endpoint is exposed over HTTP by default in Spring Boot 4.1.1? Spring Boot assembles production-ready Spring applications from explicit dependencies, conditional defaults, and external configuration. api Quiz new Where should the primary application class normally live? Spring Boot assembles production-ready Spring applications from explicit dependencies, conditional defaults, and external configuration. api Quiz new What does `@ConditionalOnMissingBean(PaymentClient.class)` mean on a default bean? Spring Boot assembles production-ready Spring applications from explicit dependencies, conditional defaults, and external configuration. api Predict the output new What is the final line printed by this application? Spring Boot assembles production-ready Spring applications from explicit dependencies, conditional defaults, and external configuration. api Review AI code new Review a generated order controller Spring Boot assembles production-ready Spring applications from explicit dependencies, conditional defaults, and external configuration. api Spot the bug new Spot the bug in the invoice controller Spring Boot assembles production-ready Spring applications from explicit dependencies, conditional defaults, and external configuration. api Quiz new Two mutations are sent together through httpBatchLink. What transaction guarantee does th… tRPC turns a TypeScript router into a typed client contract; learn its runtime boundaries, validation, context, middleware, and deployment traps. api Quiz new Why does a tRPC procedure still need a runtime input parser when its TypeScript client ha… tRPC turns a TypeScript router into a typed client contract; learn its runtime boundaries, validation, context, middleware, and deployment traps. api Predict the output new What does this program print? tRPC turns a TypeScript router into a typed client contract; learn its runtime boundaries, validation, context, middleware, and deployment traps. api Review AI code new Review a generated tRPC admin router tRPC turns a TypeScript router into a typed client contract; learn its runtime boundaries, validation, context, middleware, and deployment traps. api Spot the bug new Spot the bug in the tRPC rename mutation tRPC turns a TypeScript router into a typed client contract; learn its runtime boundaries, validation, context, middleware, and deployment traps. api Quiz new A POST is cancelled after its bytes may have reached the server. What may the client safe… Build, send, and test HTTP requests with URLSession while handling status codes, decoding, caching, cancellation, and bounded retries. api Quiz new A session has already been initialized from a configuration. How do you apply a different… Build, send, and test HTTP requests with URLSession while handling status codes, decoding, caching, cancellation, and bounded retries. api Predict the output new What does this program print? Build, send, and test HTTP requests with URLSession while handling status codes, decoding, caching, cancellation, and bounded retries. api Review AI code new Review a generated URLSession client Build, send, and test HTTP requests with URLSession while handling status codes, decoding, caching, cancellation, and bounded retries. api Spot the bug new Spot the bug in the URLSession user loader Build, send, and test HTTP requests with URLSession while handling status codes, decoding, caching, cancellation, and bounded retries. api Quiz new `data(for:)` returns without throwing and the response status is 500. Which interpretatio… Build, send, and test HTTP requests with URLSession while handling status codes, decoding, caching, cancellation, and bounded retries. api Quiz new Which statement about WSGI concurrency is correct? WSGI defines the callable protocol between synchronous Python web servers and applications, including environ, middleware, and byte responses. api Quiz new What is the safest portable way to read a normal WSGI request body? WSGI defines the callable protocol between synchronous Python web servers and applications, including environ, middleware, and byte responses. api Predict the output new What does this program print? WSGI defines the callable protocol between synchronous Python web servers and applications, including environ, middleware, and byte responses. api Review AI code new Review generated WSGI logging middleware WSGI defines the callable protocol between synchronous Python web servers and applications, including environ, middleware, and byte responses. api Spot the bug new Spot the bug in the WSGI text response WSGI defines the callable protocol between synchronous Python web servers and applications, including environ, middleware, and byte responses. api Quiz new Which sequence completes a two-chunk ASGI HTTP response correctly? The two interfaces between Python web servers and applications, with their call shapes, event flows, compatibility limits, and async review traps. api Predict the output new What does this WSGI driver print? The two interfaces between Python web servers and applications, with their call shapes, event flows, compatibility limits, and async review traps. api Review AI code new Review generated ASGI middleware The two interfaces between Python web servers and applications, with their call shapes, event flows, compatibility limits, and async review traps. api Spot the bug new Spot the bug in the ASGI echo application The two interfaces between Python web servers and applications, with their call shapes, event flows, compatibility limits, and async review traps. api Quiz new What does mounting a WSGI application through an ASGI adapter actually provide? The two interfaces between Python web servers and applications, with their call shapes, event flows, compatibility limits, and async review traps.
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