<Consistency><of HBase><CAP><ACID>

Overview

• 讨论一些(分布式)(存储)系统的一致性

CAP原理

• 随着分布式事务的出现，传统的单机事务模型（ACID）已经无法胜任，尤其是对于一个高访问量、高并发的互联网分布式系统来说。
• 如何构建一个兼顾可用性和一致性的分布式系统成为了无数工程师探讨的难题。
• CAP定理：一个分布式系统不可能同时满足一致性（C：Consistency）、可用性（A：Availability）和分区容错性（P：Partition tolerance）这三个基本需求，最多只能同时满足其中的两项。

Consistency

• 在分布式环境中，一致性是指数据在多个副本之间是否能够保持一致的特性。
• 通俗来说就是，All the servers in the system will have the same data so anyone using the system will get the same copy regardless of which server answers their request.
• CAP中的一致性与ACID中的一致性是完全不同的定义。

Availability

• 可用性是指系统提供的服务必须一直处于可用的状态，对于用户的每一个操作请求总是能够在有限的时间内返回结果。

  [“有限的时间内”是一个在系统设计之初就设定好的系统运行指标，通常不同的系统之间会有很大的不同。

   “返回结果”要求系统在完成对用户请求的处理后，返回一个正常的响应结果。]

• The system will always respond to a request (even if it's not the latest data or consistent across the system or just a message saying the system isn't working).

Partition Tolerance

• “网络分区”是指在分布式系统中，不同的节点分布在不同的子网络（机房或异地网络等）中，由于一些特殊的原因导致这些子网络之间出现网络不连通的状况，但各个子网的内部网络是正常的，从而导致整个系统的网络环境被切分成了若干个孤立的区域。
•  The system continues to operate as a whole even if individual servers fail or can't be reached.

取舍

• 首先，要明确的是对于一个分布式系统，不可能舍弃Partition Tolerance，否则系统便不再是一个分布式了。【放弃分区容错性的话，则放弃了分布式，放弃了系统的可扩展性】[No distributed system is safe from network failures, thus network partitioning generally has to be tolerated.]
• 放弃Availability：一旦系统遇到网络分区或其它故障时，那么受到影响的服务需要等待一定的时间，在此期间系统无法对外提供正常的服务，即不可用。
• 放弃Consistency：放弃C，指的是放弃数据的强一致性（实时一致性），而保留数据的最终一致性。这就引入了一个时间窗口的概念【在时间窗口内，数据是不一致的】，具体多久能够达到数据一致取决于系统的设计，主要包括主副本在不同节点之间的复制时间长短。

Consistency VS Availability

• Web-scale companies such as LinkedIn, Netflix, Google, Facebook, etc... have several requirements of their database systems around scalability, availability, and performance.
• For performance first: A company will deploy a few data centers in different parts of the world and partition their users using something like IP Anycast so that all of their users experience the fewest possible hops by being routed to the closest data center.
• 进而，table按user partitioned，但是为了failover between all data centers，所有data center都保留有所有的数据。但是实际场景是一个特点用户的写操作是发生在一个数据中心的，进而需要被复制到其他数据中心。
• This is where CAP comes in.

Consistency over Availability

• If the replication is synchronous, then you can achieve "consistency".
• The problem is that these protocols reduce the throughput of transactions. They take longer to run and hence less work gets done. As a result, transactions "back-up", connection pools are drained, and your scalability (number of concurrent transaction) drops.
• You are now prone to hitting a scalability bottleneck, which will cause intermittent outages of sorts (e.g. every other click on your site will timeout or fail fast). Hence, at the cost of consistency, you have compromised availability.
• eg: HBase is CP.

Availability over Consistency

• To replicate data asynchronously. In this model, every application writes to its local data base and immediately returns.
• All transactions remain fast and the transaction throughput remains high. Availability is not impacted.
• However, views are not consistent between data centers because data is delayed by definition, though this window of inconsistency can be made to be few minutes or better on average. This model is also called eventual consistency.
• eg: Cassandra is an AP system.

Summary：

• 在有分区的情况下，只能选择可用性 or 一致性。
  • When choosing consistency over availability, the system will return an error or a time-out if particular information cannot be guaranteed to be up to date due to network partitioning.
  • When choosing availability over consistency, the system will always process the query and try to return the most recent available version of the information, even if it cannot guarantee it is up to date due to network partitioning.
    
• 如果没有网络故障，也即分布式系统保持正常运行，那么可用性和一致性就都能被满足。[在实际场景中并不可用]

HBase

• HBase是拥有强一致性的CP系统。
• 这是因为对每一个region同时只有一台region server为它服务,对一个region所有的操作请求,都由这一台region server来响应,自然是强一致性的。【与HBase底层的HDFS副本存储无关。】
• 在这台region server fail的时候,它管理的region failover到其他region server时,需要根据WAL log来redo,这时候进行redo的region应该是unavailable的,所以hbase降低了可用性,提高了一致性。

HDFS

正常情况下，client会等待所有data packages(minimum number of replicas)的ack。

当副本数小于required number of replicas(不同于minimum number of replicas)时，块会被标记成unreplicated，然后NameNode会异步地复制。

dfs.namenode.replication.min (default 1)

dfs.replication (default 3)

HDFS Availability: NameNode是单点故障点。即使2.x引入了High Availability，也不是complete-availability。

【以上，有点乱，TBD...先别看】

FYI

• Quora：what is the relation between SQL, NoSQL, the CAP theorem and ACID?
• wiki: CAP theorem

ACID

• Atomicity - Everything in a transaction must happen successfully or none of the changes are committed. This avoids a transaction that changes multiple pieces of data from failing halfway and only making a few changes.
• Consistency - The data will only be committed if it passes all the rules in place in the database (ie: data types, triggers, constraints, etc).
• Isolation - Transactions won't affect other transactions by changing data that another operation is counting on; and other users won't see partial results of a transaction in progress (depending on isolation mode).
• Durability - Once data is committed, it is durably stored and safe against errors, crashes or any other (software) malfunctions within the database.

SQL/Relational DB

• ACID is commonly provided by most classic relational databases like MySQL, Microsoft SQL Server, Oracle and others. These databases are known for storing data in spreadsheet-like tables that have their columns and data types strictly defined. The tables can have relationships between each other and the data is queried with SQL (Structured Query Language), which is a standardized language for working with databases.

FYI

• HBase on CAP: With respect to CAP, HBase is decidedly CP. HBase makes strong consistency guarantees. If a client succeeds in writing a value, other clients will receive the updated value on the next request.
  
  In HBase, data is only served by one region server (even if it resides on multiple data nodes). If region server dies, clients need to wait for a long time because the fact of the region reassignment and log replay.
  
  HBase isn't designed that multiple region servers can simultaneously serve the same region, because that would be difficult or impossible to achieve otherwise features:single-row put atomicity, atomic check-and-set operations, atomic increment operations, etc. That are only possible if you know for sure exactly one machine is in control of the row.
  
  HBase does trade some availability to achieve a stronger level of consistency. 
  
  Partition tolerance in CAP, in short, is the ability of a system to survive despite  message loss (due to server failure, network problem, etc.). HBase does this of course, a server failure or message loss does not damage the database. When that happens, we give up availability or give up consistency. In HBase's case we choose consistency, so we have to give up some availability.