*/
/**
- * Interface for database access objects
+ * Interface for database access objects.
+ *
+ * Classes using this support a set of constants in a bitfield argument to their data loading
+ * functions. In general, objects should assume READ_NORMAL if no flags are explicitly given,
+ * though certain objects may assume READ_LATEST for common use case or legacy reasons.
+ *
+ * There are four types of reads:
+ * - READ_NORMAL : Potentially cached read of data (e.g. from a slave or stale replica)
+ * - READ_LATEST : Up-to-date read as of transaction start (e.g. from master or a quorum read)
+ * - READ_LOCKING : Up-to-date read as of now, that locks (shared) the records
+ * - READ_EXCLUSIVE : Up-to-date read as of now, that locks (exclusive) the records
+ * All record locks persist for the duration of the transaction.
+ *
+ * Callers should use READ_NORMAL (or pass in no flags) unless the read determines a write.
+ * In theory, such cases may require READ_LOCKING, though to avoid contention, READ_LATEST is
+ * often good enough. If UPDATE race condition checks are required on a row and expensive code
+ * must run after the row is fetched to determine the UPDATE, it may help to do something like:
+ * - a) Read the current row
+ * - b) Determine the new row (expensive, so we don't want to hold locks now)
+ * - c) Re-read the current row with READ_LOCKING; if it changed then bail out
+ * - d) otherwise, do the updates
+ *
+ * @since 1.20
*/
interface IDBAccessObject {
- const LATEST_READ = 1; // read from the master
- const FOR_UPDATE = 2; // lock the rows read
- const LOCKING_READ = 3; // LATEST_READ | FOR_UPDATE
- const AVOID_MASTER = 4; // avoiding checking the master
+ // Constants for object loading bitfield flags (higher => higher QoS)
+ const READ_LATEST = 1; // read from the master
+ const READ_LOCKING = 3; // READ_LATEST (1) and "LOCK IN SHARE MODE" (2)
+ const READ_EXCLUSIVE = 7; // READ_LOCKING (3) and "FOR UPDATE" (4)
+
+ // Convenience constant for callers to explicitly request slave data
+ const READ_NORMAL = 0; // read from the slave
+
+ // Convenience constant for tracking how data was loaded (higher => higher QoS)
+ const READ_NONE = -1; // not loaded yet (or the object was cleared)
}