How to ensure the high availability of Redis HA

This article mainly introduces how Redis ensures high availability.

1.About Redis

Redis (Remote Dictionary Server) is an open source, memory-based data structure storage system that can be used as a database, cache, and message middleware.

Redis supports multiple types of data structures, such as Strings, Hashes, Lists, Sets, Sorted Sets, Bitmaps, HyperLogLogs and geospatial Index radius query, etc.

Redis has the following features:

1. Efficient performance: Since all data is stored in memory, Redis can provide high-speed read and write rates.
2. Persistence: Redis can persist data in memory to disk to prevent data loss.
3. Supports multiple data structures: Redis supports multiple data structures to meet different data storage needs.
4. Support transactions: Redis supports transactions, which can ensure the atomicity of a series of commands.
5. Support publish-subscribe mode: Redis supports publish-subscribe mode and can be used to implement message queues.
6. Support Lua scripts: Redis supports Lua scripts, which can perform complex operations on the server side.
7. Support master-slave replication: Redis supports master-slave replication, which can be used for data backup or to improve read performance.
8. Support high availability and partitioning: Redis supports high availability and data partitioning through Redis Sentinel and Redis Cluster.

2.Redis usage scenarios

Redis can be used in a variety of scenarios. Here are some common uses:

1. Cache system: Due to Redis's high performance and rich data structure, it is often used as a cache system, which can effectively reduce access to the back-end database and improve the response speed of the application.
2. Message queue: Redis's publish and subscribe function can be used to implement a message queue for asynchronous delivery of messages between different applications.
3. Ranking list: Redis's Sorted Sets are very suitable for implementing the ranking function, which can quickly obtain rankings and scores and update them in real time.
4. Session cache: Redis can be used to store session information of web applications, such as user login status, shopping cart information, etc.
5. Real-time analysis: Redis's fast reading and writing and rich data structure make it very suitable for real-time analysis, such as statistics of website visits, user behavior, etc.
6. Geospatial data: Redis's geospatial index can be used to store geographical location information and perform location queries and distance calculations.
7. Counter: Redis's INCR and DECR commands can be used to implement counter functions, such as counting website clicks, downloads, etc.
8. Distributed lock: Redis can be used to implement distributed locks for mutually exclusive access to resources in a distributed environment.

The above are only some of the uses of Redis. Due to the flexibility and high performance of Redis, it can be used in more scenarios.

Redis has a wealth of application scenarios, and ensuring its high availability is a top priority.
Next, we will introduce Redis high availability.

3.High availability of Redis

There are two main types of high availability for Redis: Sentinel mode and Cluster mode.

3.1 Sentinel mode (Sentinel)

Redis Sentinel is the officially recommended high-availability solution for Redis. It can monitor Redis master-slave servers, perform failover, and send notifications.

Redis master-slave replication can copy data from one Redis server to one or more Redis servers. In the configuration file, use the slaveof directive to specify the IP address and port of the master node.
slaveof <masterip> <masterport>

The Redis Sentinel configuration process is as follows:

First, you need to create a configuration file on each Sentinel node, such as sentinel.conf, and set the monitored Redis server and other options.

sentinel monitor mymaster 127.0.0.1 6379 2
sentinel down-after-milliseconds mymaster 5000
sentinel failover-timeout mymaster 10000

Among them, mymaster is the name of the main server,
127.0.0.1 6379 is the address and port of the main server,
2 is the number of Sentinels required to judge the failure of the main server, 5000 is the timeout time to judge the failure of the main server, 10000 is the timeout time for failover .

Then start each Sentinel node using the configuration file.

redis-sentinel ./sentinel.conf

3.2 Cluster mode (Cluster)

Redis Cluster is a distributed solution for Redis that can automatically shard data, perform failover, and redistribute shards.

1. Data fragmentation: Redis Cluster divides all data into 16384 slots, each slot corresponding to a part of the key. Each Redis node is responsible for a part of the slot, that is, a part of the data. When you need to get or set the value of a key, Redis Cluster will calculate the corresponding slot based on the hash value of the key, and then find the node responsible for the slot. This method can evenly distribute data on all nodes and improve storage and processing efficiency.
2. Fault detection: Each node of Redis Cluster regularly communicates with other nodes to check the status of other nodes. This communication is implemented through the Gossip protocol. Each node will periodically send ping messages to other nodes. If no response is received within the specified time, the node will be marked as invalid.
3. Failover: When a node is marked as failed, Redis Cluster will select a slave node to take over the data of the failed node. This slave node will be promoted to the master node and take over all slots of the failed node. During the failover process, Redis Cluster may reject some write operations to ensure data consistency. This process is automated and requires no human intervention.
4. Data consistency: Redis Cluster uses asynchronous replication, but during failover, data consistency will be guaranteed as much as possible. When a master node fails, failover will only occur when at least one slave node has completed data synchronization. This ensures that after the master node fails, at least one slave node has the latest data.

The Gossip protocol is mentioned above. Here is a brief introduction.

In Redis Cluster, the gossip protocol is mainly used for information exchange between nodes, including node discovery, fault detection and dissemination of configuration information.

1. Node discovery: Newly added nodes can be quickly discovered by other nodes in the cluster through the Gossip protocol. At the same time, existing nodes can also discover newly added nodes through the Gossip protocol.
2. Fault detection: Nodes detect the status of other nodes by periodically sending ping messages to other nodes and waiting for pong replies. If no pong reply is received within a certain period of time, the node will be marked as invalid.
3. Configuration information dissemination: Configuration information can be disseminated between nodes through the Gossip protocol, such as the role of the node (master node or slave node), the allocation of data slots, etc.
   The Gossip protocol is an information exchange protocol based on the spread of "gossip". It can achieve efficient information exchange in large-scale distributed systems. In Redis Cluster, the gossip protocol is an important mechanism for node management and failover.

The Redis Cluster configuration process is as follows:

Create a configuration file on each Redis node, such as redis.conf, and set cluster-enabled to yes, as well as other options.

cluster-enabled yes
cluster-config-file nodes.conf
cluster-node-timeout 5000

Then use the redis-cli tool to create a Cluster.

redis-cli --cluster create 127.0.0.1:7000 127.0.0.1:7001 127.0.0.1:7002 127.0.0.1:7003 127.0.0.1:7004 127.0.0.1:7005 --cluster-replicas 1

The above are the basic methods to ensure high availability of Redis. The specific configuration and use can be adjusted according to actual needs. In actual use, other factors may need to be considered, such as network partitions, data consistency, etc.

4.Reference

Redis documentation

All official documentation for Redis, including quick start guide, command reference, theme guide, etc. You can find all information about Redis here, including installation, configuration, usage, etc.