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My SQL : The Binary Log - Logging Transactions (part 2) - Distributed Transaction Processing Using XA

6/25/2013 9:00:10 PM
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2. Distributed Transaction Processing Using XA

MySQL version 5.0 lets you coordinate transactions involving different resources by using the X/Open Distributed Transaction Processing model XA. Although currently not very widely used, XA offers attractive opportunities for coordinating all kinds of resources with transactions.

In version 5.0, the server uses XA internally to coordinate the binary log and the storage engines.

A set of commands allows the client to take advantage of XA synchronization as well. XA allows different statements entered by different users to be treated as a single transaction. On the other hand, it imposes some overhead, so some administrators turn it off globally.

XA includes a transaction manager that coordinates a set of resource managers so that they commit a global transaction as an atomic unit. Each transaction is assigned a unique XID, which is used by the transaction manager and the resource managers. When used internally in the MySQL server, the transaction manager is usually the binary log and the resource managers are the storage engines. The process of committing an XA transaction is shown in Figure 3 and consists of two phases.

Figure 3. Distributed transaction commit using XA

In phase 1, each storage engine is asked to prepare for a commit. When preparing, the storage engine writes any information it needs to commit correctly to safe storage and then returns an OK message. If any storage engine replies negatively—meaning that it cannot commit the transaction—the commit is aborted and all engines are instructed to roll back the transaction.

After all storage engines have reported that they have prepared without error, and before phase 2 begins, the transaction cache is written to the binary log. In contrast to normal transactions, which are terminated with a normal Query event with a COMMIT, an XA transaction is terminated with an Xid event containing the XID.

In phase 2, all the storage engines that were prepared in phase 1 are asked to commit the transaction. When committing, each storage engine will report that it has commit⁠ted the transaction in stable storage. It is important to understand that the commit cannot fail: once phase 1 has passed, the storage engine has guaranteed that the transaction can be committed and therefore is not allowed to report failure in phase 2. A hardware failure can, of course, cause a crash, but since the storage engines have stored the information in durable storage, they will be able to recover properly when the server restarts. 

After phase 2, the transaction manager is given a chance to discard any shared resources, should it choose to. The binary log does not need to do any such cleanup actions, so it does not do anything special with regard to XA at this step.

In the event that a crash occurs while committing an XA transaction, the recovery procedure in Figure 4 will take place when the server is restarted. At startup, the server will open the last binary log and check the Format description event. If the binlog-in-use flag described earlier is set, it indicates that the server crashed and XA recovery has to be executed.

The server starts by walking through the binary log that was just opened and finding the XIDs of all transactions in the binary log by reading the Xid events. Each storage engine loaded into the server will then be asked to commit the transactions in this list. For each XID in the list, the storage engine will determine whether a transaction with that XID is prepared but not committed, and commit it if that is the case. If the storage engine has prepared a transaction with an XID that is not in this list, the XID obviously did not make it to the binary log before the server crashed, so the transaction should be rolled back.

Figure 4. Procedure for XA recovery

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