Xids

Xids is a term used in distributed transaction processing to describe transaction identifiers that uniquely identify a global transaction across multiple resource managers. The concept is central to two-phase commit protocols and to transaction managers that coordinate work across databases, message queues, and other resources.

An Xid is typically structured as three components: a formatId, a global transaction identifier (GTRID), and a branch qualifier (BQUAL). In many implementations, the formatId is an integer, while the GTRID and BQUAL are byte arrays. In the Java ecosystem, the standard interface javax.transaction.xa.Xid exposes three methods: getFormatId(), getGlobalTransactionId(), and getBranchQualifier(). Practical limits in the XA model place maximum lengths on the GTRID and BQUAL, often around 64 bytes each, though exact limits can vary by specification and implementation.

The purpose of Xids is to provide a globally unique handle that a transaction manager can use to coordinate prepare, commit, and rollback actions across all participating resource managers. Each participant tracks the Xid to associate its local work with the correct global transaction and to distinguish between different branches of the same global transaction.

Xids appear in various environments that implement distributed transactions, including Java EE/JTA environments, database transaction managers, and message-oriented middleware. They can be serialized or represented in different formats (hex, base64, or raw bytes) depending on the system, but the underlying idea remains the same: a globally unique identifier plus a branch qualifier to identify subtransactions.

See also XA, two-phase commit, Java Transaction API (JTA), and X/Open.