Xid

Xid is an interface in Java that represents a global transaction identifier used in distributed transaction processing, notably under the X/Open XA standard and the Java Transaction API (JTA). It provides a way for a transaction manager to identify and coordinate a global transaction and its constituent branches across multiple resource managers such as databases and messaging systems.

A Xid consists of three components: a format identifier, a global transaction identifier (GTRID), and a branch qualifier (BQUAL). In the Java Xid interface, these correspond to getFormatId(), getGlobalTransactionId(), and getBranchQualifier(). The formatId signals the encoding scheme used by the transaction manager for the GTRID and BQUAL; the GTRID uniquely identifies the global transaction across all participating resources; the BQUAL distinguishes a particular branch or subtransaction within that global transaction. Both the GTRID and BQUAL are byte arrays, with the standard allowing up to a certain length (commonly up to 64 bytes each, though exact limits are defined by the implementation and the XA specification).

Usage of Xid occurs when resource managers interact with the transaction manager via the XAResource interface. Methods such as start, end, prepare, commit, and rollback use a Xid to identify the transaction or its branches. The lifecycle and correctness of a Xid are critical for ensuring the integrity of a distributed transaction across heterogeneous resources.

Implementation considerations: many transaction managers provide concrete Xid implementations. Applications typically obtain Xids from the transaction manager rather than implementing Xid themselves. If an application does implement Xid, it should be immutable, provide proper equals and hashCode semantics, and return copies of byte arrays to preserve immutability.

See also: JTA, X/Open XA, XAResource, javax.transaction.xa.Xid.