RocketMQ Broker's features and source line of code are already very large. In the future, if some capabilities can be split into other components, or new components, so that they can be developed like microservices, otherwise Broker will become more and more like a monolithic application, which will eventually lead to the complexity is high enough to evolve quickly.
Xiaorui Wang Apache RocketMQ PMC Chair On Thu, Sep 23, 2021 at 10:11 AM Amber Liu <ltambe...@gmail.com> wrote: > # RIP-22 Support KV semantic storage > > ## Status > - Current Status: Draft > - Authors: [ltamber](https://github.com/ltamber) > - Shepherds: [duhengforever](mailto:duhengfore...@apache.org) > - Mailing List discussion: <dev@rocketmq.apache.org> > - Pull Request: #PR_NUMBER > - Released: <released_version> > ## Background & Motivation > ### what 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 developer want to persist meta info like checkpoint. > - What can we benefit proposed changes? > rocketmq would not rely on external componet 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, in 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 availablity: if one broker in the broker group is down, > application can put/get key-value through other broker, the availablity is > same with the message of rocketmq. > 2. High capacity: the amount of key-value may very large, so the > key-value can not store in memory, we must store the key-value in disk > device. > ### Non-Goals > - What problem is this proposal NOT designed to solve? > Nothing specific. > - Are there any limits of this proposal? > Nothing specific. > ## Changes > ### Architecture >  > 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 response 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 meta data 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 flag, `kv_opType` indicate > the request type is put key-value or get key-value, and `key` indicate 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) use [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 request with key-value > flag, so broker will not receive key-value request, which keep all things > as before. > 2. new client+old broker: new clients will send key-value request, but > the broker don't recognize the request code, and will return error msg. so > we should upgrade 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 by two phases. > #### Phase 1 > 1. Implement reput logic from commitlog to rocksdb. > 2. Implement broker support key-value request and response. > 3. Implement client support key-value request and response. > 4. Implement key-value store use memory map. > 5. Implement key-value store use rocksdb. > #### Phase 2 > 1. Implement prefix get semantics. > 2. Implement compareAndSet semantics. > 3. Implement rocksdb snapshot export/import. >
