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Database / SQLite Interview questions

How does SQLite handle database locking?

SQLite uses a small set of lock states applied to the whole database file to coordinate access between connections: UNLOCKED, SHARED (for reading), RESERVED (a writer intends to write soon but hasn't yet), PENDING, and EXCLUSIVE (actively writing).

flowchart LR
    A[UNLOCKED] --> B[SHARED - readers]
    B --> C[RESERVED - writer intends to commit]
    C --> D[PENDING - waiting for readers to finish]
    D --> E[EXCLUSIVE - actively writing]
    E --> A

Multiple connections can hold a SHARED lock simultaneously (concurrent readers), but only one connection at a time can hold RESERVED/EXCLUSIVE, and a write can't finalize until existing readers release their SHARED locks. If a lock can't be acquired immediately, SQLite returns a SQLITE_BUSY error (or waits, if a busy timeout is configured), which application code needs to handle by retrying rather than treating it as a fatal error.

How many connections can hold a SHARED lock at the same time?
What error does SQLite return if a lock can't be acquired immediately?

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