RIP-22 Support KV semantic storageStatus - Current Status: Draft - Authors: ltamber <https://github.com/ltamber>
- Shepherds: duhengforever <[email protected]> - Mailing List discussion: [email protected] - Pull Request: #PR_NUMBER - Released: <released_version> Background & Motivationwhat do we need to do - Will we add a new module? *no*. - Will we add new APIs? *yes*. - Will we add new feature? *yes*. Why should we do that - Are there any problems of our current project? Currently, we can't get/put key-value from/into rocketmq, so if we use connector <https://github.com/apache/rocketmq-externals>, like FileSource, BinlogSource, we can't persist current read position/dump position to rocketmq rather than an external meta store like zookeeper/mysql, this will bring more operator risk by introduce another component. This issue was also in streaming <https://github.com/apache/rocketmq-streams> scenarios when developers want to persist meta info like checkpoint. - What can we benefit proposed changes? rocketmq would not rely on external component such as zookeeper/etcd to support meta data storage. Goals - What problem is this proposal designed to solve? Design a distribution persistent key-value store, application can put key-value into broker, and then get the value after a while, at the same time, it can also have the ability like compareAndSet, prefix get and so on. - To what degree should we solve the problem? This RIP must guarantee below point: 1. High availability: if one broker in the broker group is down, application can put/get key-value through another broker, the availability is same with the message of rocketmq. 2. High capacity: the amount of key-value may be very large, so the key-value can not be stored in memory, we must store the key-value in a disk device. Non-Goals - What problem is this proposal NOT designed to solve? Nothing specific. - Are there any limits of this proposal? Nothing specific. ChangesArchitecture We will introduce rocksdb <https://github.com/facebook/rocksdb> to persist key-value data, to say it more accurately, we use rocksdb to compact the value with the same key, we will not enable WAL in rocksdb to decrease write amplification (most case), instead we can recover the rocksdb state and consistency by redo rocketmq commitlog. so the put/get flow showed on the above figure is: put: the key-value message will put into commitlog first, and then through the reputService redo commitlog, the key-value will put to rocksdb asynchronous, until this reput finished broker will not respond to client. get: application will get key-value from rocksdb thought broker directly. In addition, if we don't want introduce rocksdb <https://github.com/facebook/rocksdb> and the metadata content will not occupy too many memory, we can also use a key-value store base on memory map, there will a periodic serialization and persistence thread to guarantee data won't loss if broker restart or system abnormal shutdown, and the memory state consistency will also guaranteed by redo rocketmq commitlog. Interface Design/Change - Method signature changes. *No* - Method behavior changes. *No* - CLI command changes. *No* - Log format or content changes. the properties of the message will add two flags, kv_opType indicates the request type is put key-value or get key-value, and key indicates the request key both in put or get operation. In order to pass the key through the network in the request header, we will encode/decode the key(byte array format) using the base64 <https://docs.oracle.com/javase/8/docs/api/java/util/Base64.html> encoding method. Compatibility, Deprecation, and Migration Plan - Are backward and forward compatibility taken into consideration? New RequestCode between client and broker are added, so there are 2 compatibility situations: 1. old client+new broker: old clients won't make requests with key-value flag, so broker will not receive key-value requests, which keep all things as before. 2. new client+old broker: new clients will send key-value requests, but the broker doesn't recognize the request code, and will return error msg. so we should upgrade the broker first to support this feature. - Are there deprecated APIs? Nothing specific. - How do we do migration? Nothing specific. Implementation Outline We will implement the proposed changes in two phases. Phase 1 1. Implement reput logic from commitlog to rocksdb. 2. Implement broker support key-value request and response. 1. Implement client support key-value request and response. 2. Implement key-value store using memory map. 1. Implement key-value store using rocksdb. Phase 2 1. Implement prefix get semantics. 2. Implement compareAndSet semantics. 1. Implement rocksdb snapshot export/import.
