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The Procedure v2 Implementation of WAL Splitting and ACL
该议题由来自小米的 HBase Committer 梅祎分享，她也是中国区唯一的女性Committer.
分享主要分为3个部分：
第一部分：主要介绍了 ProcedureV2 的核心原理，在 PPT 中，她对 ProcedureV2 各组件的介绍以及执行回滚流程的演示，应该是我见过的所有讲 ProcedureV2 的文档中最清晰易懂的了。非常推荐对 ProcedureV2 感兴趣的朋友去学习一下这个PPT。
第二部分：介绍了如何用 ProcedureV2 重构社区的 HBase Grant/Revoke ACL 流程。重构的目的主要有几个：
原始设计采用 Zookeeper 通知机制来实现各 RegionServer的ACL 更新，整个过程依赖 Zookeeper,而且流程相当于是异步的。一旦某些 RS 的 ACL 缓存更新失败（有可能但概率很低），则容易造成各节点之间的 ACL 权限不一致。而采用ProcedureV2 重写之后，整个流程变成同步流程，则不再存在这个问题了，此外还去掉该功能了对 Zookeeper 服务的依赖。
重构的另一个初衷在于，期望在执行 Grant 和 Revoke 时能暴露一些 Coprocessor 接口。例如有一个非常经典的场景就是，某些用户期望通过扫Snapshot 来跑离线任务，但由于扫 Snapshot 需要 HDFS 的权限，而 HBase 的权限管理跟 HDFS 的权限管理完全是两套机制。这时候，就可以实现一个Coprocessor 在 Grant 和 Revoke 时完成 HBase 权限和 HDFS 权限的同步，从而让那些有表权限的用户能立马访问表的 Snapshot.这个功能将在 HBASE-18659中支持。
第三部分：介绍了基于 ProcedureV2 重写了 WAL Splitting 的流程。考虑的点跟 ACL类似，主要是异步流程重写成更可控的同步流程，同时去掉了对 Zookeeper 的依赖。更多细节请参考演讲 PPT 和视频。
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1 .
2 .The Procedure v2 Implementation of WAL Splitting and ACL meiyi@xiaomi.com HBase Committer
3 . Abstract ❏ Introduction of Procedure v2 ❏ Overview ❏ Execution and Rollback ❏ Models ❏ ACL ❏ ACL based on ZK Notification ❏ ACL based on Procedure v2 ❏ WAL Splitting ❏ WAL Splitting based on ZK Coordination ❏ WAL Splitting based on Procedure v2
4 . Abstract ❏ Introduction of Procedure v2 ❏ Overview ❏ Execution and Rollback ❏ Models ❏ ACL ❏ ACL based on ZK Notification ❏ ACL based on Procedure v2 ❏ WAL Splitting ❏ WAL Splitting based on ZK Coordination ❏ WAL Splitting based on Procedure v2
5 .Goal of Procedure v2 Aims to provide a unified way to build: • multi-steps procedure in case of failure (e.g. Create table) • notifications across multiple machines (e.g. ACLs/Quota cache updates) • coordination of long-running/heavy procedures (e.g. splits) • procedures across multiple machines (e.g. Assignment)
6 .Build and run state machines do some checks step1 create table fs layout step1 step2 add regions to meta step2 step3-1 step3-2 step3-3 step3 assign region assign region assign region subprocedures step4 update descriptors cache (steps of create table)
7 .Overview ProcedureExecutor Push Worker Worker ... Worker Insert Update Delete ProcedureScheduler Poll meta Queue Load server Queue ProcedureStore peer Queue table Queue HDFS
8 .Overview 1. Submit ProcedureExecutor 3. Push Worker Worker ... Worker 2. Insert 5. Update Delete ProcedureScheduler 4. Poll meta Queue Load server Queue ProcedureStore peer Queue table Queue HDFS
9 .Procedure execution A B C D 1 2 3 0 A B C G state 0 D E F procedure
10 .Procedure execution Queue Back 0 Queue Front Stack Top Stack Bottom A B D C 1 2 3 state 0 procedure
11 .Procedure execution Queue Back Queue Front Stack Top Stack Bottom A B D C 1 2 3 state 0 procedure
12 .Procedure execution Queue Back Queue Front Stack Top 0 Stack Bottom A B D C 1 2 3 state 0 procedure
13 .Procedure execution Queue Back Queue Front Stack Top 0 0 Stack Bottom A B D C 1 2 3 state 0 procedure
14 .Procedure execution Queue Back 3 2 1 Queue Front Stack Top 0 0 0 Stack Bottom A B D C 1 2 3 state 0 procedure
15 .Procedure execution Queue Back 3 2 Queue Front Stack Top 0 0 0 Stack Bottom A B D C 1 2 3 state 0 procedure
16 .Procedure execution Queue Back 3 Queue Front Stack Top 1 0 0 0 Stack Bottom A B D C 1 2 3 state 0 procedure
17 .Procedure execution Queue Back Queue Front Stack Top 2 1 0 0 0 Stack Bottom A B D C 1 2 3 state 0 procedure
18 .Procedure execution Queue Back 0 Queue Front Stack Top 3 2 1 0 0 0 Stack Bottom A B D C 1 2 3 state 0 procedure
19 .Procedure execution Queue Back Queue Front Stack Top 3 2 1 0 0 0 Stack Bottom A B D C 1 2 3 state 0 procedure
20 .Procedure execution Queue Back Queue Front Stack Top 0 3 2 1 0 0 0 Stack Bottom A B D C 1 2 3 state 0 procedure
21 .Procedure Rollback Queue Back Queue Front Stack Top 1 0 0 Stack Bottom A B 1 state 0 procedure failed procedure
22 .Procedure Rollback Queue Back Queue Front Stack Top 0 0 Stack Bottom A B 1 state 0 procedure failed procedure
23 .Procedure Rollback Queue Back Queue Front Stack Top 0 Stack Bottom A B 1 state 0 procedure failed procedure
24 .Procedure Rollback Queue Back Queue Front Stack Top Stack Bottom A B 1 state 0 procedure failed procedure
25 .StateMachineProcedure ● enum of states, describing the various steps of the procedure ● transition from one state to another after calling executeFromState method do some checks CREATE_TABLE_PRE_OPERATION create table fs layout CREATE_TABLE_WRITE_FS_LAYOUT add regions to meta CREATE_TABLE_ADD_TO_META assign region assign region assign region CREATE_TABLE_ASSIGN_REGIONS update descriptors cache CREATE_TABLE_UPDATE_DESC_CACHE
26 .RemoteProcedureDispatcher • Dispatch aggregated RPCs to remote server • Remote server report procedures execute states in a heartbeat remoteDispatch RegionServer master RegionServer reportProcedureDone RegionServer
27 . Abstract ❏ Introduction of Procedure v2 ❏ Overview ❏ Execution and Rollback ❏ Models ❏ ACL ❏ ACL based on ZK Notification ❏ ACL based on Procedure v2 ❏ WAL Splitting ❏ WAL Splitting based on ZK Coordination ❏ WAL Splitting based on Procedure v2
28 .ACL Overview • Every server keeps permission cache. • Check if operation is allowed by hooks of AccessController. Zookeeper RegionServer Master PermissionCache PermissionCache AccessController AccessController Region AccessController
29 .The Process of Grant/Revoke Master RegionServer PermissionCache (hbase: acl region) Zookeeper 4 2 RegionServer put or delete in acl table table 4 PermissionCache 3 @namespace update ZK hbase:acl 4 RegionServer 4 PermissionCache PermissionCache notification 1 grant/revoke Client
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