高可用性(High Availability)

High Availability, HA

Wikipedia 對 HA 的條目說明, http://en.wikipedia.org/wiki/High_availability

High availability is a system design protocol and associated implementation that ensures a certain absolute degree of operational continuity during a given measurement period.

Users want their systems, for example wrist watches, hospitals, airplanes or computers, to be ready to serve them at all times. Availability refers to the ability of the user community to access the system, whether to submit new work, update or alter existing work, or collect the results of previous work. If a user cannot access the system, it is said to be unavailable. [1] Generally, the term downtime is used to refer to periods when a system is unavailable.






Glossary


  • 管理節點(Manager Node)
  • SQL節點(SQL Node), 存儲表結構
  • 資料節點(Data Node)


主從式複寫架構(Master/Multi-Slaves)


Replication

can manage multiple PostgreSQL servers. Using the replication function enables creating a realtime backup on 2 or more physical disks, so that the service can continue without stopping servers in case of a disk failure.



負載平衝(Loading Balance, LB)


Load Balance

If a database is replicated, executing a SELECT query on any server will return the same result. pgpool-II takes an advantage of the replication feature to reduce the load on each PostgreSQL server by distributing SELECT queries among multiple servers, improving system's overall throughput. At best, performance improves proportionally to the number of PostgreSQL servers. Load balance works best in a situation where there are a lot of users executing many queries at the same time.



連接池/動態過度連接限制


Connection Pooling

pgpool-II saves connections to the PostgreSQL servers, and reuse them whenever a new connection with the same properties (i.e. username, database, protocol version) comes in. It reduces connection overhead, and improves system's overall throughput.

Limiting Exceeding Connections

There is a limit on the maximum number of concurrent connections with PostgreSQL, and connections are rejected after this many connections. Setting the maximum number of connections, however, increases resource consumption and affect system performance. pgpool-II also has a limit on the maximum number of connections, but extra connections will be queued instead of returning an error immediately.


平行查詢(Parallel Query)


Parallel Query

Using the parallel query function, data can be divided among the multiple servers, so that a query can be executed on all the servers concurrently to reduce the overall execution time. Parallel query works the best when searching large-scale data.



叢集(Clusters)/分散式運算架構


強調高可用性, 高可靠性, 可伸縮性





References






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