c# gtin Lesson 2: Transactional Replication in C#

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Lesson 2: Transactional Replication
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If your database is in the Simple recovery model, the inactive portion of the log is removed at each checkpoint. Normally, a transaction log backup removes the inactive portion of the log at the end of the backup process. The backup engine removes the inactive portion of the log by beginning at the head of the log and reading forward until it reaches the oldest open transaction. After it reaches the oldest open transaction, the backup process termi nates. To determine which transactions are open and which are committed, the backup process reads a bit flag in each transaction log record that indicates whether it was committed or not. The replication engine needs to guarantee that all transactions written to the pub lisher reach the subscriber. However, the transaction log backup process, or the checkpoint process in the Simple recovery model, can interfere with this process. If SQL Server allowed the inactive portion of the log to be removed before the Log Reader Agent could write the transactions to the distribution database, the transac tions could be lost. For this reason, when a database is participating in transactional replication, a second flag is enabled within a transaction log record. A transaction log backup still processes the inactive portion of the log, but it is not allowed to remove a record from the log until both the committed flag and the replicated flag are set. Therefore, if the Log Reader Agent is not writing transactions to the distribution data base, the transaction log on the publisher continues to grow even if transaction log backups are being executed or the database checkpoints with the Simple recovery model. This effect also means that transactional replication runs with any recovery model. The same basic process occurs with the distribution database. A transaction is not removed from the distribution database until it has been successfully written to each subscriber. Therefore, having a subscriber offline and inaccessible means that the dis tribution database continues to grow.
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Transactional Options
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You can configure transactional replication with two options, the Queued Updat ing Subscriber option and the Immediate Updating Subscriber option, as shown in Figure 11-5.
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Publisher
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Agent
Distributor Network
Queue
Immediate Updating Subscriber
Read-Only Subscriber
Queued Updating Subscriber
Figure 11-5 Transactional replication
Immediate Updating Subscriber Option
The Immediate Updating Subscriber option enables changes to be made at the sub scriber that are propagated back to the publisher. The change is then picked up by the transactional engine and sent to all other subscribers but does not get reapplied to the subscriber that originated the change. To prevent the change from being sent back to the originating subscriber, a time-stamp column is required on each table that is par ticipating in replication. The process of applying changes at a subscriber configured with the Immediate Updating Subscriber option is the following: 1. The application issues the transaction at the subscriber. 2. The trigger is fired. 3. The trigger enlists Microsoft Distributed Transaction Coordinator (MS DTC) to connect to the publisher and reissue the transaction. 4. The transaction commits on the publisher. 5. The trigger commits on the subscriber. 6. The transaction commits on the subscriber.
Lesson 2: Transactional Replication
The main issue concerning high-availability architectures with immediate updating subscribers is that changes must be applied to the publisher. If the publisher is not available, the distributed transaction fails. Because the distributed transaction is being executed from a trigger, the initiating transaction also fails and rolls back. Therefore, the Immediate Updating Subscriber option is an incompatible replication option for high availability.
Queued Updating Subscriber Option
The Queued Updating Subscriber option also allows changes to be made at the sub scriber and then propagated back to the publisher, but it does so via a much different mechanism than the Immediate Updating Subscriber option. The process by which a change is applied to a subscriber is as follows: 1. The application issues a transaction. 2. The trigger is fired. 3. The trigger logs the transaction into a queue (a table within the database). 4. The trigger commits. 5. The transaction commits. The Queue Reader Agent periodically transfers the queue back to the publisher and issues all the transactions. A change transferred to the publisher is not reapplied to the subscriber that initiated the transaction. This requires a time stamp column on the table. Because of the asynchronous processing of a queued update, data conflicts can occur. To mitigate data conflicts, when you use the Queued Updating Subscriber option in high-availability architectures, changes are applied to the subscriber only during a fail ure scenario. This ensures that changes are always made to a single copy of the data, thereby enabling normal application processing to handle conflicting changes. The advantage of the Queued Updating Subscriber option is that when the publisher is brought back online, all the changes that occurred at the subscriber during the out age can be automatically flushed to the publisher. All the planning and processes required to fail back with other architectures are eliminated because the Queued Updating Subscriber option has the built-in capability to bring the publisher back up to date following a failover. The only step required to fail back to the publisher is to repoint the applications.
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