How does WAL (Write-Ahead Log) work during writing?

Write-Ahead Log (WAL) is a mechanism for ensuring data reliability during writing. How it works:

1. Before changing data in the main database or file, the changes are first recorded in a log (journal) — WAL.

2. Writing to WAL is sequential and fast, minimizing the risk of data loss in case of a failure.

3. After a successful write to WAL, the changes are applied to the main database.

4. In case of a failure, the system can restore the state by replaying the WAL entries that have not yet been applied to the main database.

An example of a simplified process:

// Pseudocode for writing with WAL
void writeData(Data data) {
    wal.append(data); // write to log
    database.apply(data); // apply to database
}

Thus, WAL ensures atomicity and fault tolerance, allowing data recovery after a crash.