Introduction

ConstDB is an in-memory cache store which aims at master-master replications. A group of ConstDB instances can be deployed across many regions, and each of them is available for reading and writing. Every instance keeps in touch with all other ones. When a write command has been executed in one instance, it is asynchronously replicated to all the other instances. The data structures that are used for storing the client's input messages implement CRDT(concurrent replicated data structures), thus we are sure data in different instances are eventually consistent.
ConstDB is compatible with redis protocol, and I have implemented a portion of redis's data types and commands. I'll keep developing it and take as much features in redis as possible into ConstDB. Contribution is also welcomed.

Features:

• CRDT: Each write command is associated with an increasing 64-bit uuid and an unique node id, therefore the synchronization is idempotent and loopback is avoided. The data stored in cache are also kept with a timestamp at which time they are generated(actually we use the uuid as a timestamp, because it's first 41-bits form a timestamp in milliseconds).So in this way, if two commands modifying the same key are executed at different nodes, we solve the conflicts through Last-Write-Win and Add-Win strategies when they are merged at one node. We also support other crdt types including PNCounter, LWWRegister, MultiValueRegister and ORSet and so on. We also implemented the gc mechanism.
• Online scaling: Nodes are free to join or leave a group. When we want to add a node(B) into an already running group, we simply let it to MEET a node(A) that is contained in that group. The new node(B) firstly connects to that node. Secondly they exchange their whole data set through their own snapshot file(this is safe because their data both implement CRDT)——in which all the necessary information of their replicas are also listed. When one node has merged the other's snapshot, it also records it's replicas. Then, each of them connects to the newly found replicas and exchange their data(probably a partial replication instead of a full snapshot dumping is used in this step, because the node(A) has told the new node(B) the replicate positions itself has received from it's replicas). Finnaly the new node(B) keeps pace with all other nodes in this group.
• Multiple io threads: Input requests and output responses are read or written on io threads, and each command is executed sequentially in the main thread. This way the performance is much efficient.

Test

You can experience the master-master replications by the following test.

1. Download the project and build.

git clone https://github.com/tancehao/ConstDB && cd ConstDB
cargo build

2. Run 2 processes listening to different local ports. Make 2 copies of the file constdb.toml.

mkdir server9001 server9002

cp constdb.toml server9001/constdb.toml && cp constdb.toml server9002/constdb.toml

And then edit the constdb.toml config files in each directory(change the id, node_alias and port options). Finally start 2 process with these different configs.

cd server9001 && ../target/debug/constdb-server constdb.toml cd .. cd server9002 && ../target/debug/constdb-server constdb.toml

3. Now a happy moment has come. We modify different keys in each node and make them keep in touch.

redis-cli -p 9001 incr k1 // 1
redis-cli -p 9002 incr k2 // 1
redis-cli -p 9002 meet 127.0.0.1:9001 // 1

wait for some seconds.

redis-cli -p 9001 get k2 // 1
redis-cli -p 9002 get k1 // 1
redis-cli -p 9002 incr k2 // 2
redis-cli -p 9001 get k2 // 2
redis-cli -p 9001 hset h1 k1 v1
redis-cli -p 9002 hget h1 k1 // v1

we can see that the each node stays abreast of the other.

Commands available

• get
• set
• del
• incr
• decr
• sadd
• srem
• spop
• smembers
• hset
• hget
• hgetall
• hdel

Contact

Email: tancehao93@163.com
Wechat id: xiaotanzi-bien