Storage & Caching Architecture

Write-Ahead Logging (WAL)

An append-only disk logging technique where database state changes are recorded sequentially to durable storage before being applied to in-memory pages or data files.

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Deep Dive

How Write-Ahead Logging (WAL) Works in Production

Under-the-hood mechanics and technical implementation details.

Technical Deep Dive

Detailed Architecture

Write-Ahead Logging provides Atomicity and Durability (ACID) in database engines like PostgreSQL, MySQL (InnoDB redo log), SQLite, and LSM-tree stores (Cassandra, RocksDB). By writing changes sequentially to disk first, the database avoids random disk writes during user transactions. If the server crashes or loses power, the database can reconstruct the exact state during startup by replaying the committed log entries from the last checkpoint.

Key Architectural Rule / Formula:

Rule: Log record MUST be flushed to non-volatile disk storage before dirty buffer page is written to data files.

Engineering Trade-Offs

Trade-Off Dimensions & Analysis

Evaluating advantages and drawbacks during architecture interviews.

Dimension	Advantages (Pros)	Drawbacks & Costs (Cons)
Write Throughput	Converts slow random disk writes into high-speed sequential disk appends.	Requires regular checkpointing and log compaction to prevent disk saturation and slow recovery times.
Crash Recovery	Guarantees zero data loss for acknowledged transactions after power failure.	Increases storage footprint and requires synchronous fsync calls unless buffered asynchronously.

Interview Application

How to Frame Write-Ahead Logging (WAL) in System Design Rounds

Senior-level talking points and related interview problems.

Interview Strategy

Evaluating in Loops

Critical concept in database internals, distributed ledger designs, and high-throughput storage engines.

Applied System Design Scenarios

Double-Entry Accounting Ledger
Distributed Key-Value Store
Real-Time Chat Storage

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