Tools: Pager Interface Functions in SQLite: The Narrow Doorway to Persistence

sqlite3PagerOpen: Creating a Storage Universe ## sqlite3PagerClose: Tearing Everything Down Safely ## sqlite3PagerGet: Turning a Page Number into Memory ## First Access Responsibilities ## Cache Validation ## Deferred Recovery ## Page Loading Rules ## sqlite3PagerWrite: Declaring Intent to Modify ## Transaction Setup ## Journaling Discipline ## Statement Journaling ## sqlite3PagerLookup: Cache, If You Have It ## A Pattern Worth Noticing ## What Comes Next ## References: Hello, I'm Maneshwar. I'm working on FreeDevTools online currently building "one place for all dev tools, cheat codes, and TLDRs" — a free, open-source hub where developers can quickly find and use tools without any hassle of searching all over the internet. So far, we’ve talked around the pager what it represents, how higher layers interact with it, and why it exists at all. Today we finally look at the actual doorway. The pager doesn’t expose its internals freely. Instead, it offers a tight, carefully designed set of interface functions that higher layers (mainly the tree module) are allowed to use. These functions live in pager.c, are all prefixed with sqlite3Pager*, and are strictly internal. Application developers never touch them, and that’s very much intentional. Each function enforces a rule. Break the rule, and SQLite’s correctness guarantees collapse. Let’s walk through the most important ones. This is where everything begins. At this stage, no locks are taken, no journal is created and no recovery* is performed That last part is subtle and important. SQLite uses deferred recovery. It doesn’t eagerly inspect or replay journals when the database is opened. Recovery only happens when a page is actually accessed. This keeps startup cheap and avoids unnecessary work. At this moment, the pager exists, but it’s still passive. Closing a pager is not just “close the file”. But there’s a sharp edge here. If a transaction is still in progress when this function is called SQLite forces an abort and all changes are rolled back immediately There is no “best effort” here. Consistency wins over convenience. Once this function returns: This strict cleanup is what allows SQLite to stay memory-safe without reference counting everywhere. This is the most frequently used pager function. Behind the scenes, a lot happens: Before returning the page: This prevents one connection from using stale pages after another connection commits changes. If a hot journal is detected: By the time sqlite3PagerGet returns, the caller is holding a valid, safe, in-memory page. This function is the gatekeeper for correctness. Before the tree module modifies any byte of a page, it must call: This call tells the pager: “I’m about to change this page. Prepare accordingly.” What the pager does next depends on state: If the lock can’t be obtained → SQLITE_BUSY Crucially, this function does not write to the database file. It only prepares the ground so that writing later is safe. This function is intentionally limited. It does not do any disk I/O or locking This is used as a fast-path optimization when the caller knows the page might already be resident. Across all these functions, a consistent philosophy emerges: The tree module never “accidentally” modifies persistent state. The pager forces discipline. Today we focused on the front door of the pager: In the next post, we’ll cover the functions that control lifecycle and boundaries: That’s where we’ll finally trace an entire transaction, from first page touch to durable commit or clean rollback. My experiments and hands-on executions related to SQLite will live here: lovestaco/sqlite SQLite Database System: Design and Implementation. N.p.: Sibsankar Haldar, (n.d.). 👉 Check out: FreeDevTools Any feedback or contributors are welcome! 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Hide child comments as well For further actions, you may consider blocking this person and/or reporting abuse - Allocates a new Pager object - Opens the database file - Creates an empty page cache - Initializes pager state - Destroys the Pager object - Closes the database file - Frees the entire page cache - Releases all OS resources - All cached pages are invalid - All pointers to page memory are dangling - Touching them is undefined behavior (often a crash) - Takes a page number - Returns a pointer to the in-memory page image - Pins the page in the cache so it won’t be recycled - The pager acquires a shared lock on the database file - If the lock cannot be obtained → SQLITE_BUSY - The pager checks the database file change counter - If it has changed, the entire cache is purged - Recovery is performed right here - Not at open time, but exactly when it becomes necessary - If the page exists → read it from disk into cache - If the file is smaller than the requested page → return a zero-filled page - For in-memory databases → no disk I/O at all - Acquire a reserved lock - Create the rollback journal - Enter a write transaction - If the page has not yet been logged: Its original contents are written to the rollback journal - Its original contents are written to the rollback journal - If it has already been logged: No additional journal write occurs - No additional journal write occurs - Its original contents are written to the rollback journal - No additional journal write occurs - The page is marked dirty - The update is happening inside a user transaction - And the page was already journaled earlier - A statement journal record may be written - This allows SQLite to roll back just one statement without aborting the full transaction - Returns a pointer to a cached page only if it already exists - Pins the page if found - Returns NULL if the page is not cached - Page access is explicit - Write intent must be declared - Journaling happens before mutation - Locks are acquired before danger - Disk I/O is deferred until unavoidable - Opening and closing - Reading pages - Declaring write intent - Cache lookups - sqlite3PagerRef - sqlite3PagerUnref - sqlite3PagerBegin - sqlite3PagerCommitPhaseOne - sqlite3PagerCommitPhaseTwo - sqlite3PagerRollback - Savepoint-related pager calls