Data Replication in VS .NET

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Data Replication
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Why Replicate
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An enterprise might choose to replicate its data for several reasons:
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Probably the most popular use of replication is for disaster recovery. Sending data over a wide area network to a remote site means that the data will be protected in the event that something happens to the facility that contains the master server. It is this use of replication that leads us to place replication so high on the Availability Index. Other uses are valuable, but are not as critical to achieving high availability. Wide area replication, in fact, enables (but does not define) the 10th and final level of the Index, disaster recovery. Data can be replicated from one system to another within the same facility. In this model, the destination system can work on and with the replicated data independently of the master. The destination system might be used to generate reports, mine data, or perform tape backups, all without affecting the original copy of the data, or impacting the CPU, and therefore the users, of the master. Some FMS packages use replication to share their data. As we said in 15, Local Clustering and Failover, even though some vendors recommend it, we maintain that data sharing via replication is inferior to data sharing via shared disks.
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Two Categories of Replication Types
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Replication types or styles can be categorized in two ways:
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I I I I
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By the amount of latency they introduce (none, a little, or a lot) By the entity that manages and initiates the replication (hardware, software, filesystem, or an application)
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Within both categories, each type has its distinct advantages and disadvantages, and we spend this section discussing them in some detail. In some cases we mention specific vendors who offer products in each area. The two replication categories are independent of each other. In other words, each initiatorbased type of replication can create any amount of latency.
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Four Latency-Based Types of Replication
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The four types of replication, relative to the latency that they introduce, are: (1) synchronous, (2) asynchronous, (3), semi-synchronous, and (4) periodic, or
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batch-style, replication. The impact on both network and system performance decreases as you move from synchronous to batch style, but in return, the data on the destination system is more and more out-of-date.
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Latency-Based Type 1: Synchronous Replication
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Synchronous replication is the copying of data from the master system, ebony, to its destination, ivory, in real time. As a block of data is written to ebony s disks, the block is sent across the network. The data must arrive at ivory, which must acknowledge receipt of the data by sending a packet back across the network to ebony. Only once it has received the acknowledgement does ebony report to the application that generated the write that it has completed. Since every single write causes a round-trip on the network (once when the data packet itself travels to the remote site and once when the acknowledgment makes the trip back), writes can be significantly delayed under synchronous replication. That delay is called latency. The latency is not only caused by the length of network cable and the time it takes for data to traverse it at roughly the speed of light, but also any additional delays caused by equipment, such as routers, switches, and repeaters, located along the network. This equipment may simply receive and then resend the data, or it may receive the data and perform some processing on it before sending it on. In either case, these devices introduce additional latency to the replication process. For systems that are located in the same data center, the same building, or even in the same neighborhood, the performance impact of synchronous replication will be acceptable on all but the busiest systems. Once the distance between the systems exceeds about 20 miles, the overhead that the latency introduces may start to have some mild impact on some busy applications, and once it exceeds about 50 miles, the delays will likely become unacceptable for many applications. These distances are, of course, just a suggestion, and your (ahem) mileage will surely vary. The reasons for this variance include the amount of data and the number of packets being replicated, the method of implementation of synchronous replication that you choose, and the amount of equipment between the two sites. In return for this delay, synchronous replication provides the ultimate in data integrity. It ensures that the data on ivory is as up-to-date as it can be. Ebony cannot generate its next write until the current write s acknowledgment has completed the round-trip and been received on ivory. As a result, every write that makes it to ebony s disks also makes it to ivory s disks. This means that if something happens to ebony, all of its data is guaranteed to be present and up-to-date on ivory. For some extremely critical applications, this guarantee is worth the impact on performance. For others, the impact will be too great, and different options must be considered.
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