八：The YARN Timeline Server
一、Overview 介绍
yarn timeline server用于存储和检查应用程序过去和现在的信息(比如job history server)。有两个功能：
1.Persisting Application Specific Information 保存应用程序特定的信息
收集和检索应用程序或者框架的特定信息。如装mr任务的map\reduce数量通过TimeClient发布到Timeline Server中。然后用户可以通过wetrest 来查询这些参数。
2.Persisting Generic Information about Completed Applications 存储已经执行完成的应用程序的通用信息
mr任务有jobhistoryServer来保存历史任务信息，但是jobhistoryServer只是yarn TimeLine server其中的一个应用。通用信息包括：
- queue-name, 队列名
- user information and the like set in the ApplicationSubmissionContext, 用户信息和applicationSubmissonContext的设置
- a list of application-attempts that ran for an application 一系列应用程序而非一个应用
- information about each application-attempt 每个应用程序的信息
- the list of containers run under each application-attempt 每个应用程序使用的container列表
- information about each container. 每个container的信息
二、Current Status and Future Plans 当前状况和远景规划
Current status 当前状况
- The core functionality of the timeline server has been completed. 核心功能已经完成
- It works in both secure and non secure clusters. 可以工作在安全和非安全模式下
- The generic history service is built on the timeline store. 测探历史服务已经在timeline store中完成
- The history can be stored in memory or in a leveldb database store; the latter ensures the history is preserved over Timeline Server restarts. 历史信息可以保存在内存或者数据库中，即使timeline server重启选样可用
- The ability to install framework specific UIs in YARN is not supported. 框架中的特殊UIs不被支持(语法有问题)
- Application specific information is only available via RESTful APIs using JSON type content. 应用程序特殊的信息只支持RESTful APIs,格式是JSON
- The “Timeline Server v1” REST API has been declared one of the REST APIs whose compatibility will be maintained in future releases. timeline server V1 的REST APIS是后向兼容的
- The single-server implementation of the Timeline Server places a limit on the scalability of the service; it also prevents the service being High-Availability component of the YARN infrastructure. 单节点的timeline server的扩展性问题，而且无法实现HA。
Future Plans 未来规划
- Future releases will introduce a next generation timeline service which is scalable and reliable, “Timeline Service v2”. 下一代的timeline server装是可扩展和稳定的
- The expanded features of this service may not be available to applications using the Timeline Server v1 REST API. That includes extended data structures as well as the ability of the client to failover between Timeline Server instances. 使用timeline server v1 的rest api可能无法使用扩展特性，包括可扩展的数据结构和clinet failover
三、Timeline Structure
Timeline domain
域，同namespace一个概念，将不同的应用和用户隔开。域中定义和用户信息、读写相关的ACL信息、时间信息等。每个域在yarn中都必须有独立的ID。
entity
实体可以是appliction\application attempt的元数据， 及用户定义的信息等。可以定义实体主过滤器( Primary filters)
event
实体中应用程序的事件。
四、配置
默认是不开启timeline server的。
具体配置请参考：
