READWRITE ORG MOD
The remainder of this section outlines various failover, replication, and load balancing solutions. The algorithm generates key mod 2n ( 2n is usually the sharding amount of tables or databases) in different milliseconds and the result is always 0 or 1. For example, a fully synchronous solution over a slow network might cut performance by more than half, while an asynchronous one might have a minimal performance impact. There is usually a trade-off between functionality and performance. Performance must be considered in any choice. Some solutions can deal only with an entire database server, while others allow control at the per-table or per-database level. Solutions can also be categorized by their granularity. Asynchronous communication is used when synchronous would be too slow. In contrast, asynchronous solutions allow some delay between the time of a commit and its propagation to the other servers, opening the possibility that some transactions might be lost in the switch to a backup server, and that load balanced servers might return slightly stale results. This guarantees that a failover will not lose any data and that all load-balanced servers will return consistent results no matter which server is queried. Some solutions are synchronous, meaning that a data-modifying transaction is not considered committed until all servers have committed the transaction. A standby server that cannot be connected to until it is promoted to a primary server is called a warm standby server, and one that can accept connections and serves read-only queries is called a hot standby server. Servers that track changes in the primary are called standby or secondary servers. Servers that can modify data are called read/write, master or primary servers. Some solutions deal with synchronization by allowing only one server to modify the data. Each solution addresses this problem in a different way, and minimizes its impact for a specific workload. Because there is no single solution that eliminates the impact of the sync problem for all use cases, there are multiple solutions. This synchronization problem is the fundamental difficulty for servers working together. The World Wide Web in 2003 is beginning to fulfil the hopes that Tim Berners-Lee had for it over 10 years ago when he created it. This is because though read-only data needs to be placed on each server only once, a write to any server has to be propagated to all servers so that future read requests to those servers return consistent results. Unfortunately, most database servers have a read/write mix of requests, and read/write servers are much harder to combine. In fact, read-only database servers can be combined relatively easily too. Web servers serving static web pages can be combined quite easily by merely load-balancing web requests to multiple machines. Ideally, database servers could work together seamlessly.
Caveatsĭatabase servers can work together to allow a second server to take over quickly if the primary server fails (high availability), or to allow several computers to serve the same data (load balancing). Preparing the Primary for Standby Servers 27.2.4.
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