Layer 4 vs. Layer 7 Load Balancing

A load balancer is a solution that is integral to server health monitoring. It’s a logical device designed to act as a traffic director, distributing network or application traffic across a number of servers. Load balancers are typically used to increase the capacity and reliability of applications. Without them, it’s highly possible for the servers of an application or site to break down more frequently than they should, resulting in the disruption of operations.

Load balancers improve the overall performance of applications by decreasing the burden on servers in charge of managing and maintaining application and network sessions, and those that perform application-specific tasks.

When it comes to load balancing, solutions are often grouped into two categories: Layer 4 and Layer 7. To help you figure out which one would work best for your operations, here are the pros and cons of each one:

Layer 4 Load Balancer

Layer 4 load balancing operates at the intermediate transport layer, which is responsible for delivering messages regardless of the content. Layer 4 load balancers simply forward network packets to and from the upstream server without bothering to inspect what’s in them. They can only make limited routing decisions by examining the first few packets in the transmission control protocol (TCP) stream.

Pros of Layer 4 Load Balancing

  • Ideal for simple packet-level load balancing
  • Because it doesn’t consider the data, it’s fast and efficient.
  • More secure because packets aren’t looked into. In the event that it gets compromised, no one can see the data.
  • Does not need to decrypt the content—it merely forwards them
  • Uses NAT
  • Maintains only one connection between client and server NATed so your load balancer can serve a maximum number of TCP connections = to (number of servers * max connections per server)

Cons of Layer 4 Load Balancing

  • Not capable of smart load balancing based on the content
  • Can’t do real microservices
  • Needs to be sticky as it is a stateful protocol. Once a connection is established, it goes to one server at the backend. All packets flowing to this connection goes to one server. The next connection will then pick another server based on the algorithm.

Layer 7 Load Balancer

Layer 7 load balancing operates at the high-level application layer, which is responsible for the actual content of the message. Layer 7 load balancers route network traffic in a more complex manner, usually applicable to TCP-based traffic like HTTP. Unlike Layer 4, a Layer 7 load balancer terminates the network traffic and reads the message within. It makes a decision based on the content of the message. Afterwhich, it makes a new TCP connection to the selected upstream server and writes the request to the server. It can also cache, layer 4 isn’t capable of doing so as it has no clue of what’s in the packets.

Pros of Layer 7 Load Balancing

  • Offers smart routing based on the URL
  • Provides caching

Cons of Layer 7 Load Balancing 

  • More expensive
  • Requires decrypting
  • In terms of security, you have to share your certificate with the load balancers. If an attacker gets access to the load balancer, they automatically have access to all your data.
  • Its proxy creates multiple connections—client to proxy/proxy to server—so you are bounded by the max TCP connection on your load balancer.

Conclusion

When it all comes down to it, the choice of Layer 4 or Layer 7 load balancing comes down to your unique needs. It’s wise to consult with an expert when selecting the best load balancer for your applications, as they’ll ensure your applications or servers stay safe and secure while maximizing uptime. If you have any more queries about load balancing or need assistance with application security, get in touch with us.

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