【REVIEW】 The Hot Standby will be a pure software approach to in-depth analysis.
Database Hot with two typical methods for a more standard, two servers via a shared storage device (usually a shared disk array or storage area network SAN), and the installation of two-machine software, to achieve double hot standby, known as the shared basis. Another way is through pure software approach, commonly referred to as pure software or in mirror mode (Mirror).
Means for sharing the database on shared storage devices. When a server to provide services directly in the storage device to read and write. When the system switch, the another server also Tong Yang 读取 the storage device of Shu Ju.
For pure software approach, by mirroring software, real-time data can be copied to another server, so that the same data on two servers on the existence of a, if a server failure, you can switch to another time servers.
Pure software approach to a certain extent, reduce costs, but it also has very significant disadvantages:
1. Reliability is relatively poor, the two real-time data replication between servers is a relatively weak link.
2. Once a server interruption, even after the resumption for more sophisticated data synchronization recovery. And this time the system is unprotected.
3. No transaction mechanism, because of its replication is in the document and the disk layer, and copy the database transaction will not affect the success of operations, changes occur, the situation of incomplete data, there is considerable risk in this.
Therefore, the proposal unless a last resort, do not choose pure software solutions.
On the other hand, there is a parallel database solution, it is not in the file or copy the disk layer, but the database operations directly in front of the routing distribution, in parallel to complete the database update transaction mechanism, but also provide a database parallel service. In this way the current application for SQL SERVER very successful, much better than shared memory + Dual mode software