Initiator-Based Type 5: Transaction Processing Monitors in .NET framework

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As with all commercial tools, you should get better results using the commercial tool rather than building your own log transfer mechanism. Be sure to look for third-party tools that handle log transfers before you delve into log copying.
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Initiator-Based Type 5: Transaction Processing Monitors
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Transaction processing monitors (TPMs), now sometimes referred to as transactional middleware, are your best bet for hard, reliable database replication. They re also the slowest and hardest to implement, a trade-off you d expect for a solid guarantee. You can use some distributed transaction features of some databases to accomplish the same results described in this section, with the same upsides and drawbacks. Two-phase commit (see Figure 18.3) ensures that two databases are updated in a consistent manner transaction by transaction. The TPM, or resource manager, makes sure that both systems can proceed with the transaction, then it applies the transaction to each, verifies completion, and completes the higherlevel transaction it is managing for the application. If either side fails to complete the transaction, the TPM can roll back the side that succeeded, aborting the transaction. TPMs do not require that the transactions be the same; they are commonly used to ensure that updates to multiple, different databases occur as a single transactional unit. The problem with two-phase commit is that it s slow. The TPM has to coordinate with each database, and every intermediate transaction has to complete before the application is notified of a successful transaction. So what is the advantage Recovery is no different than reconnecting to a single database. After all, the TPM was managing all updates in lockstep, so if the application sees the last transaction rolled back, it s been rolled back in all copies of the databases. You ll need to make sure your application uses the appropriate transaction interfaces supported by the TPM and databases, and this may require some application coding changes. You ll also need to make sure you can find the redundant copies of the database and reconnect the client application quickly.
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1. You ready 3. I m ready 5. Commit 7. Done
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Resource Manager
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Figure 18.3 Two-phase commit.
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Data Replication
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So how does having a redundant, live copy of the database allow for predictable failover times The trick is moving the point of control from the system administration domain, where systems monitor each other for failure or signs of failure, to the application domain, where the application itself or the transaction processing monitor drives recovery via a timeout or dead man timer.3 Let s say you want less than 10 seconds of downtime for any transaction. After submitting the transaction, set a timer for 8 seconds; if the timer expires, then you can forcibly abort the transaction, apply it to the secondary system only, and proactively declare the primary system dead. Of course, you may want your application to begin queuing updates for the primary, or to note that the primary fell behind at a certain time so that it can be recovered later. Many TPM systems allow dead man timers to be set by the resource manager coordinating the transactions, proactively alerting applications if the entire set of updates cannot be performed within the desired time window. What do you do if you can t trade off insert and update performance for fast failover times Look for an intermediate solution that guarantees transactional integrity but allows for asynchronous updates to the replica systems. So-called asynchronous, or reliable, queuing mixes the best of the TPM and log replication models.
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Queuing Systems
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Most transaction processing monitors operate in a bulletin board, or memorybased mode. Transactions that are in-flight are posted to the bulletin board until they are committed to all systems. Recovering a bulletin board system is similar to recovering a database; the log of applied transactions results in rollforwards or rollbacks to bring the transaction monitor into a state of consistency with the databases it is front-ending. A reliable queue, or transactional queue, uses disk-based transaction waiting areas to ensure that no transaction accepted by the queue is ever lost. Reliable queues that allow transactions to be inserted and processed later are known as store-and-forward queues or asynchronous queues. Examples of reliable queuing systems include the IBM/Transarc Encina RQS, IBM s MQ Series, Microsoft s MSMQ, and BEA s Tuxedo/Q and MessageQ products. There are other commercial products that perform some of the store-and-forward distribution of messages but lack strong recovery or replication features. Use the design rules and requirements in this chapter to help guide you through product selection; you may not need all of the transactional recovery if you handle corner cases in your application.
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A dead man timer is a mechanism where any sign of activity from a system resets the timer. When no activity occurs, the timer counts, and when it reaches its expiration point, it acts as though the nonresponding system has failed.
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