总结



ZooKeeper



ZooKeeper is a centralized service for maintaining configuration information, naming, providing distributed synchronization, and providing group services. All of these kinds of services are used in some form or another by distributed applications. Each time they are implemented there is a lot of work that goes into fixing the bugs and race conditions that are inevitable. Because of the difficulty of implementing these kinds of services, applications initially usually skimp on them, which make them brittle in the presence of change and difficult to manage. Even when done correctly, different implementations of these services lead to management complexity when the applications are deployed.



Server: The server sends an acknowledge when any client connects. In the case when there is no response from the connected server, the client automatically redirects the message to another server.



Client: Client is one of the nodes in the distributed application cluster. It helps you to accesses information from the server. Every client sends a message to the server at regular intervals that helps the server to know that the client is alive.



Leader: One of the servers is designated a Leader. It gives all the information to the clients as well as an acknowledgment that the server is alive. It would performs automatic recovery if any of the connected nodes failed.



Follower: Server node which follows leader instruction is called a follower.



• Client read requests are handled by the correspondingly connected Zookeeper server

• The client writes requests are handled by the Zookeeper leader.



Ensemble/Cluster: Group of Zookeeper servers which is called ensemble or a Cluster. You can use ZooKeeper infrastructure in the cluster mode to have the system at the optimal value when you are running the Apache.



ZooKeeper WebUI: If you want to work with ZooKeeper resource management, then you need to use WebUI. It allows working with ZooKeeper using the web user interface, instead of using the command line. It offers fast and effective communication with the ZooKeeper application.



Paxos

Paxos is an algorithm that is used to achieve consensus among a distributed set of computers that communicate via an asynchronous network. One or more clients proposes a value to Paxos and we have consensus when a majority of systems running Paxos agrees on one of the proposed values. Paxos is widely used and is legendary in computer science since it is the first consensus algorithm that has been rigorously proved to be correct.



CAP

• Consistency

• Availability

• Partition Tolerant



Although the CAP Theorem might seem somewhat outdated, it is valuable in providing a way to think about database architecture design. It not only forces engineers and architects to ask questions about what they want from the technologies they use, but it also forces them to think carefully about the requirements of a given project.



ACID

features are usually applied to relational DBs. If you want to apply ACID in a distributed fashion (distributed DB), ACID uses 2PC(two-phase commit) to force consistency across partitions. However since ACID provides consistency and partitioning, applying the CAP theorem for (distributed environments) this will mean that availability is compromised.