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How Do Transaction Logs Work
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The main function of transaction logs is to ensure the integrity of changes to the database Four tests are applied to each change in the database to ensure this integrity The following list includes the four tests, which are sometimes referred to by the acronym ACID: Atomic Either all of the operations performed are completed or none of them are completed Consistent The database must be in a consistent state before and after the changes Isolated Changes to the database are not visible to the user until all of the operations have been performed and completed Durable Changes to the databases must be preserved in the event of a system or database malfunction
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Figure 15-1 Viewing a transaction log header
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We ve used a couple of terms that need to be defined An operation is the smallest unit of change that can be made to a database A series of operations, when completed, comprise a transaction; and a transaction, once written to the transaction log, is said to have been committed Let s walk though an example of how these concepts work together, and, hopefully, you ll understand the role of transaction logs Let s assume that John wants to move a piece of e-mail from his inbox to a folder he created called Important From John s perspective, this is little more than a drag-and-drop activity; however, from an ESE perspective, this represents a number of important changes to the database: The message needs to be deleted from the inbox The message needs to be inserted into the Important folder The item number for each folder needs to be updated
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Each change represents an operation All three operations represent a single transaction Once these operations have been completed and recorded in the transaction log, the transaction is considered committed Because these operations are performed in a single transaction, all or none of the operations will be performed If, during the performance of these operations, the server were to lose power, ESE will remember that this series of operations was not completed When the storeexe process is started on reboot, ESE will roll back these operations so that the ATOMIC test is passed, since none of the operations were performed Notice that in this example we were not concerned about writing this information to the database itself Instead, our focus was on writing information to the transaction log This illustrates the write-ahead logging architecture of ESE before we write new information or changes to the ESE database, we first write it to a transaction log Because we are not constantly writing to the database on disk in real time, but instead use a type of lazy write to flush changes from memory to the ESE database at a later time, Exchange Server 2007 operates much faster and more efficiently
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As we mentioned earlier, data is stored in 8-KB pages inside the ESE database When a change to a page in the database needs to be performed, and before the page is read into memory, the page number and checksum are verified to ensure that the data is the same as that written to the disk Once the page is read into memory, it is considered clean When operations are performed on the data in the page, it is marked as dirty One transaction may require changes to many pages Dirty pages are not written back to the ESE database in any particular order, so if your server loses power, for example, while the transaction is being written to disk, and some of the pages have been written and some have not, you need not worry When the storeexe process is started again, those pages that were not yet written to disk from memory will be written from the transaction log to your database and your database will be updated When you start the information store service in the Services utility, the majority of the activity is comparing the transactions in the logs to the database, making sure that all such transactions have been flushed to the database, and that the database is in a consistent state Should there be a failure in this process, you will receive an error message and the database will not be able to start There is no single way to know when committed transactions in the log files are written to the database; however, there are some guidelines Committed transactions in the log file are written to disk when one of the following occurs: The checkpoint falls too far behind in a previous log file If the number of committed transactions in the log file reaches a certain threshold, ESE will flush these changes to disk If the number of free pages in memory becomes too low and affects overall system performance, ESE will flush committed transactions in memory to the disk in an effort to free up pages for system use
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