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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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What is SQLite? What are the key features of SQLite? How is SQLite different from client-server databases like MySQL or PostgreSQL? What is the file format of a SQLite database? What are the supported data types in SQLite? What is dynamic typing (type affinity) in SQLite? How do you create a table in SQLite? How do you insert data into a SQLite table? How do you query data from a SQLite table? How do you update a record in SQLite? How do you delete a record in SQLite? What is a PRIMARY KEY in SQLite? What is AUTOINCREMENT in SQLite? What is a UNIQUE constraint in SQLite? What is a FOREIGN KEY in SQLite and how do you enable it? What is the sqlite3 command-line shell used for? How do you create an index in SQLite? What is a NULL value in SQLite? What is the difference between CHAR, VARCHAR, and TEXT in SQLite? What is the ROWID in a SQLite table? What is an in-memory SQLite database? How do you back up a SQLite database? What is the PRAGMA statement used for? What is a VIEW in SQLite? What is a TRIGGER in SQLite? What is the difference between DELETE, TRUNCATE, and DROP in SQLite? What is a transaction in SQLite and how do you use BEGIN/COMMIT/ROLLBACK? What is the difference between WAL mode and rollback journal mode? Why does SQLite recommend enabling foreign keys explicitly, given they're off by default? What is the difference between INTEGER PRIMARY KEY and a regular PRIMARY KEY in SQLite? How does SQLite handle concurrent writes? What is the difference between a SQLite VIEW and a TABLE? How do you perform a JOIN in SQLite? What is an UPSERT in SQLite (INSERT... ON CONFLICT)? How do you use the EXPLAIN QUERY PLAN statement? What is a composite primary key in SQLite? How does SQLite handle database locking? What is the difference between VACUUM and ANALYZE in SQLite? Explain the internal working of SQLite's B-tree storage structure? How does WAL mode improve concurrency compared to the default rollback journal? Explain the lifecycle of a SQLite transaction from BEGIN to COMMIT? Why might full-text search (FTS5) be needed instead of a LIKE query? How do you optimize a slow SQLite query? What are the limitations of SQLite for large-scale, high-concurrency applications? How does SQLite ensure ACID compliance without a separate server process? Explain how SQLite's write-ahead logging (WAL) checkpoint process works? What isolation level does SQLite provide, compared to other RDBMS? What is the STRICT keyword used for in SQLite table definitions?
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