Merge "Improved some thumb.php error messages."

[lhc/web/wiklou.git] / includes / dao / IDBAccessObject.php

diff --git a/includes/dao/IDBAccessObject.php b/includes/dao/IDBAccessObject.php
index cd5dda9..f847b98 100644 (file)
--- a/includes/dao/IDBAccessObject.php
+++ b/includes/dao/IDBAccessObject.php
@@ -22,11 +22,36 @@
  */
 
 /**
- * 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 three 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 the records for 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 and "FOR UPDATE"
+
+       // 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)
 }