Engineering guides

Learn backend engineering by understanding how real systems fail.

Practical guides for backend interviews: concurrency, databases, distributed systems, production errors and more. Most guides link to a real repository with failing tests, so you can debug the thing you just read about.

402 guides9 topics203 with real code

real repositories. failing tests.

inventory-service

▾ src

▾ main/java

▾ service

InventoryService.java

▾ test

InventoryServiceTest.java

✗ 3/16 tests failed

AssertionFailedError:

expected: 10 but was: 12

✗ testConcurrentReservations()

✗ testStockNotNegative()

✗ testIdempotentRequests()

✗ testHighLoad()

▶ Run tests

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Distributed Systems

Idempotency

An operation is idempotent when performing it more than once has the same effect as performing it once — the property that makes retrying safe.

8 min readWith code example

Databases

Transaction isolation levels

An isolation level defines which effects concurrent transactions may observe, trading anomalies against blocking, aborts, and database overhead.

3 min read

Concurrency & Performance

Race conditions

A race condition exists when the correctness of a result depends on the relative timing of operations that the system is free to interleave however it likes.

9 min readWith code example

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Distributed Systems23 guidesReplication, consensus, messaging and coordination Databases33 guidesTransactions, isolation, indexing and consistency Concurrency & Performance29 guidesLocks, race conditions, deadlocks and latency APIs & Platforms30 guidesAPI design, idempotency and platform choices Production Errors120 guidesOOM, latency, CPU, GC and real incidents Coding Problems58 guidesMachine coding, LLD and debugging problems Interview Rounds67 guidesCompany-specific rounds and what to expect Prep by Role & Level37 guidesSDE1, SDE2, backend, data, ML and DSA Linux & Shell5 guidesLog parsing, permissions and scripting

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