Database Connection Multiplexing: The Perfect Companion to Connection Pooling

In my previous post on connection pooling, I discussed why connection pools are ideal for large-scale applications. In my team, ProxySQL is used for implementing connection pooling at the time of writing this. While I was exploring ProxySQL, I came across another technique caleld Multiplexing. I knew about this concept, but I wanted to explain how each techniques - Connection Pooilng and Multiplexing - interleaves for each other in plain English.

Today, I want to explore database multiplexing - a powerful technique that, when combined with connection pooling, can dramatically improve database performance and resource efficiency.

Understanding the Basics

I love analogy. Overdosing comes with risk, but using analogy often helps me understanding real world examples in simple ways, and helps me building mental models in my mind. Let's explore Connection Pooling and Multiplexing using the same analogy - phone lines.

Think of it this way:

Connection Pooling: This is like having a set of pre-established phone lines (network sockets) ready to be used. When an application needs to talk to the database, it picks up one of these ready lines instead of dialing a new number every time. This saves the overhead of establishing a new connection for each request. ProxySQL maintains these idle connections to the backend MySQL servers.

Multiplexing: This is like having a sophisticated switchboard operator who can efficiently route multiple conversations (queries) through the same active phone line. Instead of dedicating a single phone line to each ongoing conversation, the operator can rapidly switch between different conversations, making it appear as if they are all happening simultaneously on that single line.

Problems Solved by Each Technique

Feature	Connection Pooling	Multiplexing
Primary Goal	✅ Reduce the overhead of establishing new connections.	✅ Increase concurrency and reduce idle connection usage on the backend.
Mechanism	Maintaining a cache of idle, ready-to-use connections.	Interleaving and rapidly switching multiple client requests over fewer backend connections.
Resource Usage	Reduces the number of new connections created.	Reduces the number of active connections to the backend at any given time.
Analogy	pre-established phone lines	skillfull switchboard operators

How They Work Together

ProxySQL showcases how these techniques complement each other perfectly:

1. The Connection Pool Layer: ProxySQL maintains a pool of pre-established connections to backend MySQL servers, ready to be used instantly.

2. The Multiplexing Layer: When client requests arrive, instead of dedicating one backend connection per client, ProxySQL cleverly interleaves multiple client requests over the same backend connections.

The process works like this:

1. Multiple application instances connect to ProxySQL
2. ProxySQL maintains a much smaller pool of connections to actual MySQL servers
3. When queries arrive from different clients, ProxySQL sends them through the available backend connections in rapid succession
4. As results return from the database, ProxySQL routes them back to the appropriate client connections

This two-layer approach gives you the best of both worlds: applications can maintain their own connection pools for quick access, while the database sees a much smaller, optimized connection load.

+-------------------+       +-------------------+       +-------------------+
|   Application A   |       |   Application B   |       |   Application C   |
+-------------------+       +-------------------+       +-------------------+
          |                           |                           |
          |                           |                           |
          +---------------------------+---------------------------+
                                      |
                                      v
                          +-----------------------+
                          |      ProxySQL         |
                          |  (Connection Pooling) |
                          +-----------------------+
                                      |
          +---------------------------+---------------------------+
          |                           |                           |
          v                           v                           v
+-------------------+       +-------------------+       +-------------------+
| Backend Conn 1    |       | Backend Conn 2    |       | Backend Conn 3    |
| (Multiplexing)    |       | (Multiplexing)    |       | (Multiplexing)    |
+-------------------+       +-------------------+       +-------------------+

Conclusion

Connection pooling and multiplexing are like the perfect duo. While connection pooling ensures your application has quick access to pre-established connections, multiplexing swoops in to make sure those connections are used as efficiently as possible. Together, they create a harmonious system that keeps your database infrastructure running smoothly, even under heavy load.