What does the event loop do in Redis?

The event loop in Redis is the central mechanism that manages all incoming and outgoing events: network connections, client commands, timers, and internal tasks. It's what gives Redis its asynchronous, non-blocking operation in single-threaded mode.

#1. The event loop's main role

The event loop runs an infinite cycle, in which Redis:

1. waits for events (new connections, commands, readiness to send a response),
2. processes them,
3. goes back to waiting for the next event.

That's how Redis can serve thousands of clients at once without creating separate threads or processes.

#2. What counts as an "event" in Redis

Every action is an event, placed into the loop's queue:

• data arriving from a client (a command);
• readiness to send a response to the client;
• a timer expiring (e.g. a key's TTL);
• internal tasks, replication, publications, lifetime checks.

Redis uses an event-multiplexing mechanism, built on OS system calls:

• epoll (Linux),
• kqueue (BSD/macOS),
• select or poll (fallback options).

#3. How the event loop runs, step by step

Simplified, Redis's event loop works like this:

1. poll(), waits for activity on the sockets;
2. accept(), accepts new connections;
3. read(), reads data from clients;
4. execute(), runs the corresponding Redis command;
5. write(), sends the response to the client;
6. cron(), runs internal periodic tasks (expiring TTLs, replication, statistics).

Then the cycle repeats indefinitely.

#4. Why this is efficient

• Redis doesn't spawn a new thread per connection, unlike traditional servers.
• Every client is served in a single thread through I/O multiplexing.
• While one client is waiting for a response, Redis can process other clients' commands.
• There's no context switching between threads, which makes the system predictable and fast.

#5. How it interacts with other components

• The main event loop handles the networking side and executes commands.
• Background threads handle heavy operations: saving to disk, freeing memory, replication.
• They run independently and don't block the event loop.

#6. Summary

The event loop in Redis is the mechanism that:

1. listens on every connection,
2. dispatches events,
3. executes commands sequentially,
4. manages internal tasks,
5. serves thousands of clients in a single thread.

That's what lets Redis stay a single-threaded, non-blocking, high-performance server with minimal overhead.