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Database / SQLite Interview questions

How does WAL mode improve concurrency compared to the default rollback journal?

In rollback journal mode, a writer modifies the actual database file directly (after saving the original data to a journal for potential rollback), which means readers have to wait during the brief window a writer is committing, since the file they'd be reading from is actively being changed. WAL mode instead has writers append new versions of changed pages to a separate write-ahead log file, leaving the main database file untouched during normal operation.

sequenceDiagram
    participant Writer
    participant WALfile as WAL file
    participant DBfile as Main DB file
    participant Reader
    Writer->>WALfile: append new page versions
    Reader->>DBfile: reads original pages (unaffected by writer)
    Reader->>WALfile: also checks WAL for newer versions
    Note over WALfile,DBfile: Periodic checkpoint merges WAL into main DB file

Because readers can see a consistent snapshot by combining the main database file with whatever's in the WAL at the moment they started reading, they never have to wait for a writer to finish — the writer and readers operate on effectively separate views that get reconciled later. This is the core mechanism behind WAL mode's readers-never-block-on-writers concurrency improvement.

Where do writes go in WAL mode, instead of directly into the main database file?
Why don't readers block on a writer in WAL mode?

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