Learn by changing the system

Systems playground.

Five small experiments. Change one thing, watch what happens, and leave with a better mental model.

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Five concepts, explained

Crash recovery & idempotency: A checkpoint records where to resume. If a write lands before its checkpoint, recovery may repeat it. Keying the write by a stable event ID prevents an extra row.
Consumer lag & bounded queues: When production exceeds consumption, a queue grows. A bounded buffer eventually fills. More processing capacity or backpressure addresses the mismatch; more buffering buys time.
TCP byte-stream reassembly: Segments can arrive out of order. Bytes behind a gap wait in a buffer until the missing bytes arrive, so the application receives an ordered byte stream.
Endianness: Bytes 0x12 and 0x34 at increasing addresses encode 0x1234 in big-endian order, or 0x3412 in little-endian order.
Write-quorum availability: Two acknowledgements form a majority of three replicas. One can be unavailable and the write quorum can still be met; losing two removes the majority. This is only one part of a full consensus protocol.