Interface for database access objects. More...

Inheritance diagram for IDBAccessObject:

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Public Attributes
const	READ_LOCKING = self::READ_LATEST \| 2
	Constants for object loading bitfield flags (higher => higher QoS) More...

const	READ_NONE = -1

Detailed Description

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 replica DB 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.

A special constant READ_LATEST_IMMUTABLE can be used for fetching append-only data. Such data is either (a) on a replica DB and up-to-date or (b) not yet there, but on the master/quorum. Because the data is append-only, it can never be stale on a replica DB if present.

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: