shows the high-level components of the ZooKeeper service. With the exception of the request processor, each of the servers that make up the ZooKeeper service replicates its own copy of each of components.
ZooKeeper Components
The replicated database is an in-memory database containing the entire data tree. Updates are logged to disk for recoverability, and writes are serialized to disk before they are applied to the in-memory database.
Every ZooKeeper server services clients. Clients connect to exactly one server to submit irequests. Read requests are serviced from the local replica of each server database. Requests that change the state of the service, write requests, are processed by an agreement protocol.
As part of the agreement protocol all write requests from clients are forwarded to a single server, called the leader. The rest of the ZooKeeper servers, called followers, receive message proposals from the leader and agree upon message delivery. The messaging layer takes care of replacing leaders on failures and syncing followers with leaders.
ZooKeeper uses a custom atomic messaging protocol. Since the messaging layer is atomic, ZooKeeper can guarantee that the local replicas never diverge. When the leader receives a write request, it calculates what the state of the system is when the write is to be applied and transforms this into a transaction that captures this new state.
展示了ZooKeeper服务的高级别的组件。除了请求处理器，组成ZooKeeper服务的每个服务器复制它本身每个组件的副本。
副本数据库是一个包含整个数据树的内存数据库。更新信息将被记录在磁盘中保证可恢复性，在它们被写到内存数据库之前序列化写到磁盘中。
每个ZooKeeper服务器服务客户端。客户端连接到一个正确的服务器来提交请求。读取请求是由每个服务器的数据库的本地副本提供服务的。改变服务的请求和写请求都是由一致性协议来处理的。
所有来自客户端的写请求作为协议的一部分都将转发给一个单独的服务器，称之为leader。剩下的ZooKeeper服务器称之为followers，接收来自leader的信息提案和达成信息传输的一致性。消息传递层负责leaders的失效替换和同步leaders和followers。