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My SQL : The Binary Log - Logging Transactions (part 1) - Transaction Cache

6/25/2013 8:59:17 PM
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You have now seen how individual statements are written to the binary log, along with context information. Transactions require additional treatment.

A transaction can start under several different circumstances:

  • When the user issues START TRANSACTION or BEGIN.

  • When AUTOCOMMIT=1 and a statement accessing a transactional table starts to execute. Note that a statement that writes only to nontransactional tables—for example, only to MyISAM tables—does not start a transaction.

  • When AUTOCOMMIT=0 and the previous transaction was committed or aborted either implicitly (by executing a statement that does an implicit commit) or explicitly by using COMMIT or ROLLBACK.

Not every statement that is executed after the transaction has started is part of that transaction. The exceptions require special care from the binary log.

Nontransactional statements are by their very definition not part of the transaction. When they are executed, they take effect immediately and do not wait for the transaction to commit. This also means that it is not possible to roll them back. They don’t affect an open transaction: any transactional statement executed after the nontransactional statement is still added to the currently open transaction.

In addition, several statements do an implicit commit. These can be separated into three groups based on the reason they do an implicit commit.


Statements that write files

Most DDL statements (CREATE, ALTER, etc.), with some exceptions, do an implicit commit of any outstanding transaction before starting to execute and an implicit commit after they have finished. These statements modify files in the filesystem and are for that reason not transactional.


Statements that modify tables in the mysql database

All statements that create, drop, or modify user accounts or privileges for users do an implicit commit and cannot be part of a transaction. Internally, these statements modify tables in the mysql database, which are all nontransactional.

In MySQL versions earlier than 5.1.3, these statements did not cause an implicit commit, but since they were writing to nontransactional tables, they were treated as nontransactional statements. As you will soon see, this caused some inconsistencies, so implicit commits were added for these statements over the course of several versions.


Statements that require implicit commits for pragmatic reasons

Statements that lock tables, statements that are used for administrative purposes, and LOAD DATA INFILE cause implicit commits in various situations because the implementation requires this to make them work correctly.

Statements that cause an implicit commit are clearly not part of any transaction, because any open transaction is committed before execution starts. 

1. Transaction Cache

The binary log can have statements in a different order from their actual execution, because it combines all the statements in each transaction to keep them together. Multiple sessions can execute simultaneous transactions on a server, and the transactional storage engines maintain their own transactional logs to make sure each transaction executes correctly. These logs are not visible to the user. In contrast, the binary log shows all transactions from all sessions in the order in which they were committed as if each executed sequentially.

To ensure each transaction is written as a unit to the binary log, the server has to separate statements that are executing in different threads. When committing a transaction, the server writes all the statements that are part of the transaction to the binary log as a single unit. For this purpose, the server keeps a transaction cache for each thread, as illustrated in Figure 1. Each statement executed for a transaction is placed in the transaction cache, and the contents of the transaction cache are then copied to the binary log and emptied when the transaction commits.

Figure 1. Threads with transaction caches and a binary log


Statements that contain nontransactional changes require special attention. Recall from our previous discussion that nontransactional statements do not cause the current transaction to terminate, so the changes introduced by the execution of a nontransactional statement have to be recorded somewhere without closing the currently open transaction. The situation is further complicated by statements that simultaneously affect transactional and nontransactional tables. These statements are considered transactional but include changes that are not part of the transaction.

Statement-based replication cannot handle this correctly in all situations and therefore a best-effort approach has been taken. We’ll describe the measures taken by the server, followed by the issues you have to be aware of in order to avoid the replication problems that are left over.

1.1. How nontransactional statements are logged

When no transaction is open, nontransactional statements are written directly to the binary log and do not “transit” in the transaction cache before ending up in the binary log. If, however, a transaction is open, the rules for how to handle the statement are as follows:

  1. If the statement is marked as transactional, it is written to the transaction cache.

  2. If the statement is not marked as transactional and there are no statements in the transaction cache, the statement is written directly to the binary log.

  3. If the statement is not marked as transactional, but there are statements in the transaction cache, the statement is written to the transaction cache.

The third rule might seem strange, but you can understand the reasoning if you look at Example 3-14. Returning to our employee and log tables, consider the statements in Example 1, where a modification of a transactional table comes before modification of a nontransactional table in the transaction.

Example 1. Transaction with nontransactional statement
1     START TRANSACTION;
2     SET @pass = PASSWORD('xyzzy');
3     INSERT INTO employee(name,email,password) 
      VALUES ('mats','mats@example.com', @pass);
4     INSERT INTO log(email, message)
      VALUES ('root@example.com', 'This employee was bad');
5     COMMIT;

Following rule 3, the statement on line 4 is written to the transaction cache even though the table is nontransactional. If the statement were written directly to the binary log, it would end up before the statement in line 3 because the statement in line 3 would not end up in the binary log until a successful commit in line 5. In short, the slave’s log would end up containing the comment added by the DBA in line 4 before the actual change to the employee in line 3, which is clearly inconsistent with the master. Rule 3 avoids such situations. The left side of Figure 2 shows the undesired effects if rule 3 did not apply, whereas the right side shows what actually happens thanks to rule 3.

Figure 2. Alternative binary logs depending on rule 3


Rule 3 involves a trade-off. Since the nontransactional statement is cached while the transaction executes, there is a risk that two transactions will update a nontransactional table on the master in a different order than that in which they are written to the binary log.

This situation can arise when there is a dependency between the first transactional and the second nontransactional statement of the transaction, but this cannot generally be handled by the server because it would require parsing each statement completely, including code in all triggers invoked, and performing a dependency analysis. Although technically possible, this would add extra processing to all statements during an open transaction and would therefore affect performance, perhaps significantly. Since the problem can almost always be avoided by designing transactions properly and ensuring that there are no dependencies of this kind in the transaction, the overhead was not added to MySQL.

1.2. How to avoid replication problems with nontransactional statements

A strategy for avoiding the dependencies discussed in the previous section is to ensure that statements affecting nontransactional tables are written first in the transaction. In this case, the statements will be written directly to the binary log, because the transaction cache is empty (refer to rule 2 in the preceding section). The statements are known to have no dependencies.

If you need any values from these statements later in the transaction, you can assign them to temporary tables or variables. After that, the real contents of the transaction can be executed, referencing the temporary tables or variables.

 
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