barcode generator project source code in vb.net Figure 8-3 compares the physical structures of some common replication scenarios. in Software

Maker QR in Software Figure 8-3 compares the physical structures of some common replication scenarios.

Figure 8-3 compares the physical structures of some common replication scenarios.
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Figure 8-3.
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Common replication scenarios
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NOTE: These roles are only logical ones. A single SQL Server can have any combination of roles. For example, one server might be configured as a Publisher only, or it may serve as a Publisher for some articles, and as a Distributor and a Subscriber to other articles.
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There are two main types of replication operations that can be performed: w Pull subscription The Subscriber initiates a request for information from a Publisher. This is useful in situations involving many Subscribers, because it eases administration. It is also useful for public information. For example, a company might make a list of its products and services available to all of its third-party business partners by allowing them to create pull subscriptions. You might also want to implement pull subscriptions when you will be replicating between servers that might not always have persistent network connections. In this case, whenever a connection and sufficient bandwidth is available, administrators at remote Subscribers can initiate a transfer of the information. Push subscription The Publisher automatically sends data to one or more SQL Servers configured as Subscribers. This is useful if you want to automatically send information when a change occurs or if you want to perform security and administration at a central server. In all types of replication operations, information is transferred between the Publisher and Subscriber, using the Distributor as an intermediate.
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After you determine what to replicate and which machines will perform these functions, it s time to define how the data should be kept consistent. A fundamental tradeoff in designing replication scenarios is between data consistency and performance of replication servers. On one end of the spectrum, information might rarely be synchronized. On the other end, data is never out of synch between the various servers. It is important to realize that greater data consistency can require more server and network resources and can thus reduce performance. Therefore, your business must accurately define data-consistency requirements. SQL Server supports several different replication methods: w Snapshot replication Used to take an image of the database at a certain point in time. This image is then made available to one or more Subscribers, either based on a schedule or on-demand. This is a good choice if you are distributing information from a central repository to many read-only servers. A common example is a retail merchant (the Publisher) making product availability and pricing information available to resellers (the Subscribers). An important
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point to remember with snapshot replication is that all information from a publication is sent to the client with each refresh. Although this reduces the processing overhead required, it can place much greater burdens on bandwidth and networking infrastructure. s Transactional replication Used to monitor and record all information about INSERT, UPDATE, and DELETE statements, which is useful if strong data consistency between servers is required. Changes to the data contained within databases can be sent to the necessary Subscribers either immediately or at predefined intervals, which ensures that information is kept up-to-date at all Subscribers, but places greater demands on network availability. An important feature of transactional replication is that a given piece of information should have only one Publisher. Therefore, this method is best for databases in which the information is already logically partitioned. For example, for a company with multiple store locations, each of which modifies its own sales data only, information can easily be collected at a corporate office. Immediate-Updating Subscribers Allows Subscribers to update information and transfer any changes back to the Publisher immediately. In the simplest configuration of snapshot or transactional replication, the data transfer will be one-way. That is, information will be modified at a Publisher, and these changes will then be transferred to Subscribers. However, various business requirements might require Subscribers to be able to update their copies of information. In this case, the Immediate-Updating Subscribers option can be used. You can enable this option when articles are created for replication. Immediate-Updating Subscribers can allow modifications of their local information as long as the Publisher is updated at the same time. All other Subscribers to the publication are then updated with the next scheduled update. This allows for distributed data changes while simultaneously ensuring that information is kept synchronized. However, since Subscribers can update information only when they have access to the Publisher, you must ensure that you are working over a reliable network connection. Merge replication Designed for situations in which you must allow multiple databases to be updated independently but still be kept synchronized. This is ideal, for example, if you have unreliable or slow network connections. In this scenario, you want users to be able to update their local copies of databases and then reconcile these updates with other servers. That s exactly the purpose of merge replication. A major challenge in merge replication is determining who wins if a conflict arises. For example, consider the situation in which the same database row is updated on two different servers. When this type of conflict occurs, SQL Server automatically resolves it based on a predefined set of rules. Either time-based replication can be used or certain servers can be given higher priorities than others. In all cases, however, the conflicts are managed and the data is kept consistent on all servers.
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