Example analysis of redis cluster principle

Principle of redis cluster

If you rely on sentinels to achieve high availability of redis, and if you want to support high concurrency while accommodating massive amounts of data, you need a redis cluster. Redis cluster is a distributed data storage solution provided by redis. The cluster shares data through data sharding and provides replication and failover functions.

Node

A redis cluster consists of multiple nodes, and multiple nodes are connected through the cluster meet command. The handshake process of the nodes:

1. Node A receives the cluster meet command from the client

2. A sends a meet message to B based on the received IP address and port number

3. Node B receives the meet message and returns pong

4. A knows that B has received the meet message, returns a ping message, and the handshake is successful

5. Finally, node A will spread the information of node B to other nodes in the cluster through the gossip protocol, and other nodes will also shake hands with B

slot

redis saves data in the form of cluster sharding. The entire cluster database is divided into 16384 slots. Each node in the cluster can handle 0-16384 slots. When all 16384 slots in the database are processed by nodes, the cluster is in Online status, otherwise as long as one slot is not processed, the offline status will be processed. Slots can be assigned to corresponding nodes for processing through the cluster addslots command.

slot is a bit array, the length of the array is 16384/8=2048, and each bit of the array is represented by 1 to be processed by the node, and 0 represents not processed. As shown in the figure, it means that node A processes 0 -7 slot.

When the client sends a command to the node, if it happens to find that the slot belongs to the current node, the node will execute the command. Otherwise, a MOVED command will be returned to the client to guide the client to the correct node. (The MOVED process is automatic)

If you add or remove nodes, it is also very convenient to reallocate slots. Redis provides tools to help realize slot migration. The entire process is completely online and does not need to stop the service. .

Failover

If node A sends a ping message to node B, and node B does not respond to pong within the specified time, then node A will mark node B as pfail suspected offline state, and at the same time Send the status of B to other nodes in the form of messages. If more than half of the nodes mark B as pfail status, B will be marked as fail offline. At this time, failover will occur, and priority will be given to replicating data from Multiple slave nodes choose one to become the master node and take over the slot of the offline node. The whole process is very similar to that of the sentinel, both of which are based on the Raft protocol for election.

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