Redirection of redis cluster

This article mainly introduces redis Clustered Redirect problem .

One 、 The reason for redirection

about Redis In terms of clusters , Because cluster nodes can't proxy （proxy） Command request , So the client should return  -MOVED  perhaps  -ASK  to turn to （redirection） When it's wrong , Forward the command request to other nodes .

Common optimization methods in use ：  The client can save the mapping information between key and node , It can effectively reduce the number of possible turns , So as to improve the efficiency of command execution .

When a node needs to make a client long-term （permanently） When a command request for a slot is sent to another node , The node returns... To the client  MOVED  to turn to . This situation is usually , When the client requests an operation that does not exist on the current node , We can usually solve this problem by maintaining a mapping relationship between nodes and key values on the client side .

On the other hand , When the node needs to let the client only move to another node in the next command request , The node returns... To the client  ASK  to turn to . Here redirection is generally redis When data is being migrated between different nodes in the cluster , To use this ASK to turn to , for example redis When the cluster expands or shrinks .

Two 、MOVED to turn to

One Redis The client can go to any node in the cluster （ Include slave nodes ） Send command request . If the slot you are looking for is not handled by this node , The node will view the hash slot saved in itself to the node ID Mapping records for , And reply to the client with a  MOVED  error .

    （ come from ： https://www.huaweicloud.com/articles/38e2316d01880fdbdd63d62aa26b31b4.html）

Execution process ：

     
      

       
 
        
 
         
 
        
 
        
 
         

           1.Redis A single node of the socket Communications , 
         
 
        
       
 
        
 
         
 
        
 
        
 
         

            Will share Redis Cluster The relationship between the slot and the corresponding node in the cluster . 
         
 
        
       
 
        
 
         
 
        
 
        
 
         

           2. Client to Redis Cluster Send commands to any node of the , 
         
 
        
       
 
        
 
         
 
        
 
        
 
         

            The node receiving the command will respond to CRC16 Rules carry out  
          hash Operation and 16383 Remainder , 
         
 
        
       
 
        
 
         
 
        
 
        
 
         

            Calculate your own slot and corresponding node  . 
         
 
        
       
 
        
 
         
 
        
 
        
 
         

           3. If the slot that holds the data is assigned to the current node , Then execute the command in the slot , 
         
 
        
       
 
        
 
         
 
        
 
        
 
         

            And return the result of command execution to the client . 
         
 
        
       
 
        
 
         
 
        
 
        
 
         

           4. If the slot where the data is stored is not within the management scope of the current node , 
         
 
        
       
 
        
 
         
 
        
 
        
 
         

            Return... To the client moved Redirection exception  . 
         
 
        
       
 
        
 
         
 
        
 
        
 
         

           5. The client receives the result returned by the node , If it is moved abnormal , 
         
 
        
       
 
        
 
         
 
        
 
        
 
         

            From moved Exception to get the information of the target node . 
         
 
        
       
 
        
 
         
 
        
 
        
 
         

           6. The client sends commands to the target node , Get command execution result . 
         
 
        
      
     
      

3、 ... and 、ASK to turn to

When the node needs to let the client only move to another node in the next command request , The node returns... To the client  ASK  to turn to , Often occurs in data from the node A Migrate to node B When .

（ come from ： https://www.huaweicloud.com/articles/38e2316d01880fdbdd63d62aa26b31b4.html）

Execution process ：

     
      

       
 
        
 
         
 
        
 
        
 
         
 
          1. When a client sends a command to a node in the cluster , 
         
 
        
       
 
        
 
         
 
        
 
        
 
         

            The node returns... To the client moved abnormal , Tell the client the node information of the corresponding target slot . 
         
 
        
       
 
        
 
         
 
        
 
        
 
         
 
          2. The client sends the command to the target node , 
         
 
        
       
 
        
 
         
 
        
 
        
 
         

            The slot in the target node has been migrated to another node , At this point, the target node will return ask Redirect to client . 
         
 
        
       
 
        
 
         
 
        
 
        
 
         
 
          3. Client to new target The node sends Asking command , 
         
 
        
       
 
        
 
         
 
        
 
        
 
         

            Then send the request command to the new node again . 
         
 
        
       
 
        
 
         
 
        
 
        
 
         
 
          4. New node target Carry out orders , Return the result of command execution to the client . 
         
 
        
      
     
      

Example ：

The slots listed in our previous section  8  In the case of , Because of the slot  8  The bonds contained are scattered in the nodes A And nodes B in , So when the client is at the node A When a key is not found in , It should turn to the node B In search of , But this shift should only affect one command query , Instead of having the client look up the node directly every time B ： At the node A What is held belongs to the slot  8  There is no key for All moved to node B Before , The client should access the node first A , Then visit the node B .

Reference documents ：

http://redisdoc.com/topic/cluster-spec.html

https://www.huaweicloud.com/articles/38e2316d01880fdbdd63d62aa26b31b4.html

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