XA Transactions

XA Transactions

XA is a two-phase commit protocol that is natively supported by many databases and transaction monitors. It ensures data integrity by coordinating single transactions accessing multiple relational databases. XA guarantees that transactional updates are committed in all of the participating databases, or are fully rolled back out of all of the databases, reverting to the state prior to the start of the transaction.

The X/Open XA specification defines the interactions between the Transaction Manager (TM) and the Resource Manager. The Transaction Manager, also known as the XA Coordinator, manages the XA or global transactions. The Resource Manager manages a particular resource such as a database or a JMS system. In addition, an XA Resource exposes a set of methods or functions for managing the resource.

In order to be involved in an XA transaction, the XA Resource must make itself known to the Transaction Manager. This process is called enlistment. Once an XA Resource is enlisted, the Transaction Manager ensures that the XA Resource takes part in a transaction and makes the appropriate method calls on the XA Resource during the lifetime of the transaction. For an XA transaction to complete, all the Resource Managers participate in a two-phase commit (2pc). A commit in an XA transaction is called a two-phase commit because there are two passes made in the committing process. In the first pass, the Transaction Manager asks each of the Resource Managers (through the enlisted XA Resource) whether they will encounter any problems committing the transaction. If any Resource Manager objects to committing the transaction, then all work done by any party on any resource involved in the XA transaction must all be rolled back. The Transaction Manager calls the rollback() method on each of the enlisted XA Resources. However, if no resource Managers object to committing, then the second pass involves the Transaction Manager actually calling commit() on each of the enlisted XA Resources. This process guarantees the ACID (atomicity, consistency, isolation, and durability) properties of a transaction that can span multiple resources.

Both Sun Microsystems JMS and BEA WebLogic Server implement the X/Open XA interface specifications. Because both systems support XA, the EJBs running inside the WebLogic container can subscribe or publish messages to Sun Microsystems JMS in XA mode. When running in XA mode, the EJBs subscribing or publishing to Sun Microsystems JMS can also participate in a global transaction involving other EJBs. For the example, EJBs running in XA mode, Container Managed Transactions (CMTs) are used. In other words, we define the transactional attributes of the EJBs through their deployment descriptors and allow the container to transparently handle the XA transactions on behalf of the EJBs. The WebLogic Transaction Manager coordinates the XA transactions. The Sun Microsystems JMS XA Resource is enlisted to a transaction so that the WebLogic Transaction Manager is aware of the Sun Microsystems JMS XA Resource involved in the XA transaction. The WebLogic container interacts closely with the Transaction Manager in CMT such that transactions are almost transparent to an EJB developer.

XA Transactions (WebLogic Server Components) https://docs.oracle.com/cd/E19509-01/820-5892/ref_xatrans/index.html

两阶段提交 XA事务

https://dev.mysql.com/doc/refman/8.0/en/xa.html

Support for XA transactions is available for the InnoDB storage engine. The MySQL XA implementation is based on the X/Open CAE document Distributed Transaction Processing: The XA Specification. This document is published by The Open Group and available at http://www.opengroup.org/public/pubs/catalog/c193.htm. Limitations of the current XA implementation are described in Section 13.3.8.3, “Restrictions on XA Transactions”.

mysql XA实现

XA supports distributed transactions, that is, the ability to permit multiple separate transactional resources to participate in a global transaction. Transactional resources often are RDBMSs but may be other kinds of resources.

A global transaction involves several actions that are transactional in themselves, but that all must either complete successfully as a group, or all be rolled back as a group. In essence, this extends ACID properties “up a level” so that multiple ACID transactions can be executed in concert as components of a global operation that also has ACID properties. (As with nondistributed transactions, SERIALIZABLE may be preferred if your applications are sensitive to read phenomena. REPEATABLE READ may not be sufficient for distributed transactions.)

Some examples of distributed transactions:

• An application may act as an integration tool that combines a messaging service with an RDBMS. The application makes sure that transactions dealing with message sending, retrieval, and processing that also involve a transactional database all happen in a global transaction. You can think of this as “transactional email.”

• An application performs actions that involve different database servers, such as a MySQL server and an Oracle server (or multiple MySQL servers), where actions that involve multiple servers must happen as part of a global transaction, rather than as separate transactions local to each server.

• A bank keeps account information in an RDBMS and distributes and receives money through automated teller machines (ATMs). It is necessary to ensure that ATM actions are correctly reflected in the accounts, but this cannot be done with the RDBMS alone. A global transaction manager integrates the ATM and database resources to ensure overall consistency of financial transactions.

Applications that use global transactions involve one or more Resource Managers and a Transaction Manager:

• A Resource Manager (RM) provides access to transactional resources. A database server is one kind of resource manager. It must be possible to either commit or roll back transactions managed by the RM.

• A Transaction Manager (TM) coordinates the transactions that are part of a global transaction. It communicates with the RMs that handle each of these transactions. The individual transactions within a global transaction are “branches” of the global transaction. Global transactions and their branches are identified by a naming scheme described later.

The MySQL implementation of XA enables a MySQL server to act as a Resource Manager that handles XA transactions within a global transaction. A client program that connects to the MySQL server acts as the Transaction Manager.

To carry out a global transaction, it is necessary to know which components are involved, and bring each component to a point when it can be committed or rolled back. Depending on what each component reports about its ability to succeed, they must all commit or roll back as an atomic group. That is, either all components must commit, or all components must roll back. To manage a global transaction, it is necessary to take into account that any component or the connecting network might fail.

The process for executing a global transaction uses two-phase commit (2PC). This takes place after the actions performed by the branches of the global transaction have been executed.

1. In the first phase, all branches are prepared. That is, they are told by the TM to get ready to commit. Typically, this means each RM that manages a branch records the actions for the branch in stable storage. The branches indicate whether they are able to do this, and these results are used for the second phase.

2. In the second phase, the TM tells the RMs whether to commit or roll back. If all branches indicated when they were prepared that they will be able to commit, all branches are told to commit. If any branch indicated when it was prepared that it will not be able to commit, all branches are told to roll back.

In some cases, a global transaction might use one-phase commit (1PC). For example, when a Transaction Manager finds that a global transaction consists of only one transactional resource (that is, a single branch), that resource can be told to prepare and commit at the same time.