Quorum
一、"quorum"的意思是"法定人数",也就是说多少人同意了才作数;一般我们会将它的值设置为majority,那么此时的含义就是: "大多数人同意才算数"、"少数服从多数"。
二、它在distributed computing中有着非常广泛的使用
Multiple model
一、通过**投票**来达成共识。
二、既然有multiple entity,那么由它们来进行vote从而进行决策是自然而然的想法了。
少数服从多数
wikipedia Quorum (distributed computing)
A quorum is the minimum number of votes that a distributed transaction has to obtain in order to be allowed to perform an operation in a distributed system. A quorum-based technique is implemented to enforce consistent operation in a distributed system.
Quorum-based techniques in distributed database systems
Quorum-based voting can be used as a replica control method,[1] as well as a commit method to ensure transaction atomicity in the presence of network partitioning.[1]
Quorum-based voting in commit protocols
In a distributed database system, a transaction could be executing its operations at multiple sites. Since atomicity requires every distributed transaction to be atomic, the transaction must have the same fate (commit or abort) at every site. In case of network partitioning, sites are partitioned and the partitions may not be able to communicate with each other. This is where a quorum-based technique comes in. The fundamental idea is that a transaction is executed if the majority of sites vote to execute it.
Every site in the system is assigned a vote V_i. Let us assume that the total number of votes in the system is V and the abort and commit quorums are V_a and V_c, respectively. Then the following rules must be obeyed in the implementation of the commit protocol:
- V_a + V_c > V, where 0 < V_c, V_a $ \leq $ V.
- Before a transaction commits, it must obtain a commit quorum V_c. The total of at least one site that is prepared to commit and zero or more sites waiting $ \geq $ V_c.[2]
- Before a transaction aborts, it must obtain an abort quorum V_a The total of zero or more sites that are prepared to abort or any sites waiting $ \geq $ Va.
The first rule ensures that a transaction cannot be committed and aborted at the same time. The next two rules indicate the votes that a transaction has to obtain before it can terminate one way or the other.
Quorum-based voting for replica control
In replicated databases, a data object has copies present at several sites. To ensure serializability, no two transactions should be allowed to read or write a data item concurrently. In case of replicated databases, a quorum-based replica control protocol can be used to ensure that no two copies of a data item are read or written by two transactions concurrently.
The quorum-based voting for replica control is due to [Gifford, 1979].[3] Each copy of a replicated data item is assigned a vote. Each operation then has to obtain a read quorum (Vr) or a write quorum (Vw) to read or write a data item, respectively. If a given data item has a total of V votes, the quorums have to obey the following rules:
- Vr + Vw > V
- Vw > V/2
The first rule ensures that a data item is not read and written by two transactions concurrently. Additionally, it ensures that a read quorum contains at least one site with the newest version of the data item. The second rule ensures that two write operations from two transactions cannot occur concurrently on the same data item. The two rules ensure that one-copy serializability is maintained.
See also
Case study
Case: ZooKeeper
在 ZooKeeper Getting Started Guide 中的"Running Replicated ZooKeeper"章节进行了介绍:
But in production, you should run ZooKeeper in replicated mode. A replicated group of servers in the same application is called a quorum, and in replicated mode, all servers in the quorum have copies of the same configuration file.