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Shard Proxy and Global Routing

A distributed system consists of multiple servers. A lot of complicated traffic and coordination mechanisms are implemented to make the data stored in all servers balanced and the service equally scaled. But users usually don't care and have little intention to understand the distributed detail. Therefore, we designed a Shard Proxy to get a simple user experience just like working with a standalone database.The Shard Proxy is a central module to accept all user read/write requests.

For example, when a user is quering for a certain table from a database. For a distributed database system, this request is actually looking for a Shard in a ceratin Store.

A user can send this request to any Store of the system. The Shard Proxy of the Store will take that request, it will look up for the Global Routing table to find the correct Store.

As we have explained in the Raft Group and Leader,Leader is the representative of a Raft Group, all read and write requests are handled only by the leader.

Therefore, when a request for certain rows are executed. * Firstly, we need to locate the Shard where these rows are stored. * Secondly, locate the Leader Replica of this Shard group. * Thirdly, route the request to the Store where the Leader Replica is located. * Finally, Leader Replica executes the request and returns response.

Example

We have a cluster of 3 Stores, and their status are as below:

Range Store1 Store2 Store3
Shard1 [key1-key10) Leader Follower Follower
Shard2 [key10-key20) Follower Leader Follower
Shard3 [key20-key30) Follower Follower Leader

A user sends requests on key1, key10 and key20, the following diagram illustrates how the requests gets through Shard Proxy and being routed.

User Request Routing Diagram