Redis
The open source, in-memory data store used by millions of developers as a database, cache, streaming engine, and message broker.
Use Cases
1. Cache
For frequently accessed data, store in redis to reduce load on persistant database and improve response time of application (e.g. web server).
2. Session Store
Share session data among stateless servers. Session-based authentication has a drawback. When a user connects to another server instance, the user will be asked to login again as the other server doesn't have login session data in memory, making servers non-stateless. JWT based token authentication doesn't have this issue as servers don't store any data; instead user sends a token with required information (payload) with cryptography to prove the credibility of the token. Redis can be used as a global variable to contain all session data from all web server instances.
3. Distributed Lock
Like mutex. In a distributed syste, when multiple instances want to access a shared mutable resource, Redis can be used as a mutex lock. Instances need to acquire the lock before making changes, and release the lock once finished. There are many existing distributed lock libraries for redis.
4. Rate Limiter
Limit how frequent user can send requests to server to prevent DoS attack and robots. For a time window (e.g. 1 minute), each request will increment a counter in redis by 1. If the number of request is below the allowed value, then keep going, reject request with Status Code: 429 Too Many Requests otherwise. The key has a TTL (time to live) of 1 minute. A counter is created if a counter for the user doesn't already exist. Implementation of this can be done in web server middleware, such as express-rate-limit - npm (npmjs.com)
5. Leaderboard
Gaming Leaderboard.
6. Publisher Subscriber for Distributed Syncing
Example: Socketio Redis Adapter SocketIO is a popular library to websocket (realtime syncing). For example it can be used to build a realtime message app. When a user sends a message, server broadcast to everyone else in the chat room. However, in a distributed system where users in the same room may connect to different servers, how to achieve syncing/broadcasting? Redis has a publisher subscriber mechanism that can be used to sync data across servers with socketio. When user A sends a message to room R to server A, server A will make a broadcast to room R while also publish this message to redis. Other SocketIO instances receive this message and broadcast to room R users connected to them. For example, user B connected to server B will also receive the message from user A.