Data Replication in .NET

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Data Replication
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Latency-Based Type 3: Semi-Synchronous Replication
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Semi-synchronous (or semi-sync ) replication is a method that was created by EMC for use in their SRDF (Symmetrix Replicated Data Facility) product, to bridge the gap between synchronous replication, with its performance problems, and asynchronous replication. SRDF does not support a form of asynchronous replication that maintains write ordering, so semi-synchronous gives their users another option besides synchronous replication. Semi-sync is designed to improve the performance of SRDF in configurations where regular synchronous mode s latency is too great to maintain reasonable application performance. Semi-sync reduces synchronous replication s performance impact by having the local disk array acknowledge the completion of a write to the application before propagating the data to the remote disk array, eliminating the latency caused by data transmission. The benefit of semi-sync is dependent on the application, the workload, and the speed at which replicated data makes the round-trip between the primary and secondary nodes. EMC says that there is a benefit only if it takes longer for writes to reach the local Symmetrix than it does for them to cross the SRDF link. On the downside, though, EMC acknowledges in their documentation that the use of semi-sync may result in inconsistent data at the remote site if there is a crash at the primary site, which, of course, is the primary purpose of using the software. EMC also says that under some circumstances the use of semi-sync mode with applications that do not defer their writes is not recommended.
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Latency-Based Type 4: Periodic, or Batch-Style, Replication
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The final type of replication goes by two different names. Under periodic, or batch-style, replication, data is saved up, and from time to time is replicated in a batch, all at once. Write ordering is not maintained, so during a replication cycle, the data that has reached the destination is not consistent. This means that if, during the replication cycle, the master system fails, the data on the destination system is not usable. The advantage to this style of replication is that it gives system and network administrators some control over when the network will be impacted by the additional load that replication introduces. Another important disadvantage to periodic replication is that the data on the destination system is very much out-of-date. If the data is updated via replication every 4 hours, for example, then on average, the data on the destination is 2 hours out-of-date. If an emergency causes a switchover from master to destination, as much as 4 hours of committed work can be lost. For most critical applications, this significant loss of data is considered unacceptable, and as a result, periodic replication is not considered acceptable for disaster recovery situations. Periodic replication is generally much more useful as a way of getting data to other systems for backups, data mining, and other noncritical uses.
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If periodic replication is to be used for disaster recovery, then it is critical that additional copies of the data be maintained. The remote side will require a multisided data mirror, and while the replication is active, one side of the mirror must be broken off to receive the updates, while the other sides of the mirror hold their static (and consistent) copy of the data. Once the replication cycle has completed, the sides that were not updated will be reattached to the updated copy and will be brought up-to-date. Some implementations will require a similar process on the sending side, as a copy of that mirror is split off from the active copy, so that it does not change during the replication. Disk vendors love this method, as it allows them to sell multiple sets of disk spindles to keep up with all of the copies of the data set. Note that if it becomes necessary to recover from an older copy of the data (because of a failure during the copy), the data you ll get dates back to the previous round of replication.
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