Initiator-Based Type 4: Application-Based Replication in .NET

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Database replication buys you protection against several disasters: corruption of your primary database, failure of an operational system when the MTTR is measured in seconds not minutes, and in some cases protection from a fullfledged physical disaster. Database replication can rely on variations of file distribution tricks, using log replay, built-in database features such as distributed transaction management, or third-party transaction processing and queuing systems that guarantee delivery of transactions to multiple database instances on multiple servers.
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Log Replay
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Database log replay is the simplest approach to database replication. Taking a page from the filesystem replication playbook, a database logs all changes to permanent structures, typically in an undo or redo log that is used for
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rolling the transaction forward in the event of a system failure. Log replay involves manually (or via a scheduled script) copying these log files to another machine, and then reapplying the transactions from the logs to the replica databases using the recovery manager, creating a similar copy of the primary database with some minor changes, including, but not limited to, timestamps and transaction IDs. One concern with log-replay-style replication is that it is possible for some transactions to fall into the gutter between the primary and secondary database machines, if the primary crashes and takes its log files with it before the final transactions are copied from the log. If you replay the log file, then accept five new transactions in the primary database, then suffer a crash, those five transactions won t make it to the secondary server. Another concern with log replay is that the database on the remote side is not, in fact, identical to the one on the primary side. As a result, it would be more difficult, and, in fact, probably impossible, to take an extract from it and apply that extract back to the primary side, in the event of data corruption. As we mentioned previously, timestamps and record identifiers are going to be different on the remote side. If it were necessary to copy the replicated database back to the primary database server, serious confusion and corruption will result. However, database log replay is a relatively big win because it requires no application changes to implement replication. The database administrators can configure the log copy and recovery steps, setting the time interval between copies and the verification of updates on the replicated machines. Some applications are better suited to this technique than others. The following is a brief checklist of features that allow an application to fit the log replay model:
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Applications that can easily detect missing transactions when connecting to a database for the first time and that know how to find the right starting point. This is a critical consideration for stating that no changes are necessary to the application. If the application can look for transactions that fell into the gutter, then it will tolerate log replication well. Low volumes of inserts and updates so that the log files do not grow rapidly. Applications with high rates of change generate large log files and can induce a backlog on the replica database servers as they struggle to keep up. Applications that can tolerate a small time skew between the live and redundant sites. If the primary site goes down and the application has to reconnect to the redundant site, it may have to wait for several transactions to be played through the recovery manager before it can reconnect to the database. This delay is likely to be similar to the delay imposed by a failover.
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In some cases, asking the user for verification of a missing transaction (resulting in a user query of the secondary database) is warranted or desired; this puts recovery with precision back into the hands of the end user and helps ensure the integrity of the data. In some cases, if a few minutes have gone by between the time of the original transaction and the time the redundant database is available, the user may no longer wish to resume the old transaction. One example is that of a trading floor, in which a user may choose to hold a trade that was not accepted by the primary database if the market has moved adversely during the recovery window. It s better to let the user make this call than to blindly apply the transaction under a different set of environmental conditions.
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