TCP Load Balancing for Robust Backend Connections

TL;DR

TCP Load Balancing distributes connections at the transport layer without evaluating HTTP content. It is suitable for protocols and services where transparency, protocol fidelity, and low processing depth are more important than URL or header-based routing. Key factors include appropriate backend pools, health checks, and a clear understanding of existing TCP connections.

Introduction

TCP Load Balancing is not a simplified version of HTTP routing but a deliberate architectural decision. Where a service does not provide HTTP information or it should not be evaluated, Layer 4 Load Balancing distributes incoming TCP connections based on network and transport information. This reduces processing depth and keeps the edge open for various TCP-based protocols. At the same time, functions like routing by hostname, paths, or HTTP headers are omitted. Companies must therefore clarify in advance whether they merely want to distribute connections or understand and control application traffic. This distinction determines how backend pools, failover, and operational processes are designed.

1. Suitable Use Cases for Layer 4 Load Balancing

Layer 4 Load Balancing operates based on IP addresses and ports as well as the state of the TCP connection. The edge accepts a connection, selects a backend from a defined pool, and forwards the TCP data stream. The application content is not required for this decision.

This is useful for services that do not communicate over HTTP or HTTPS. These include, for example, database accesses, messaging protocols, proprietary TCP services, or applications where a front-end proxy should not change the protocol semantics. HTTP services can also be distributed on Layer 4 if TLS or HTTP should run unchanged to the backend.

The advantage lies in protocol fidelity: The edge does not need to interpret application-related rules. This keeps the architecture relatively transparent for clients and backends. In return, the selection of the backend instance is not dependent on URL, cookie, or header. All connections to a virtual service therefore follow the same L4 distribution logic unless separate listeners or ports are used.

2. The Limits of Missing HTTP Information

The lack of HTTP inspection is also the most significant limitation. A Layer-4 system cannot recognize whether a request calls /login or /checkout, which hostname a client uses, or which tenant is in an HTTP header. Routing decisions based on URL path, content type, or session cookie are not possible.

This limitation also affects security and operational functions. A Web Application Firewall requires HTTP or HTTPS context. It can examine HTTP requests on Layer 7, while pure TCP forwarding does not evaluate the data stream at the application level. Similarly, application-related metrics such as requests per route or status codes are not automatically available.

For the architecture, this results in a clear separation: L4 Load Balancing distributes transport connections, L7 Load Balancing interprets application traffic. Those with both requirements can use different listeners or services. With TLS passthrough, encryption remains intact to the backend; evaluation at the HTTP level, on the other hand, requires appropriate termination and processing at the edge or in the backend.

3. Backend Pools and Stable TCP Connections

A backend pool bundles reachable instances of a service. For each new TCP connection, an instance is selected. During the connection, the data stream must remain consistent to this backend because TCP state and application session cannot be arbitrarily moved between servers. A change of backend is typically only possible with a new connection.

For failover to work, not only IP addresses and ports must match. Health checks must reliably determine whether a backend can accept connections and provide the intended service. A reachable port alone does not prove that the application is operational. If an instance fails, it should be removed from the pool for new connections. Established connections, on the other hand, require separate consideration: Their behavior depends on the error pattern, TCP state, and the application’s retry mechanisms.

The ayedo Edge Cloud supports backend health checks and failover for such distribution models. For operations, this means planning pools, check intervals, and error responses together with the timeout and retry strategies of the backends. Too aggressive retries can additionally burden an already overloaded pool.

4. Transparency, Network Path, and Operational Model

L4 Load Balancing is transparent but not automatically invisible. The edge forms the public entrance and connects clients with private or shielded backend pools. Backend cloaking can prevent the actual backend addresses from being publicly exposed. The application remains accessible through a stable entry point while the infrastructure behind this point can be changed.

The choice of backend location is provider-independent: The target systems can be operated in ayedo Managed Kubernetes , in own Kubernetes clusters, or with other providers. Kubernetes-native integration is therefore not limited to a single compute operation. The Edge Cloud takes over the public TCP entrance and distribution; the compute infrastructure remains responsible for application and workload.

Anycast-based Layer-4 distribution, own network infrastructure, and an own Autonomous System form the network context of the ayedo Edge Cloud. The distributed multi-PoP architecture and the active-active principle are relevant if the public entry should not depend on a single location. Economically, this reduces the need to operate the same public accessibility separately at each compute location. For this, routing, health checks, and responsibilities between edge and compute teams must be clearly documented.

Practical Scenario: Distributing a TCP Service Across Multiple Clusters

A company operates a TCP-based service in two Kubernetes clusters: one cluster in its own data center and another with an external provider. Both instance groups are set up as a backend pool behind a common public service. The ayedo Edge Cloud distributes new TCP connections via Layer 4 to the available backends and checks their reachability through health checks.

It is important that both clusters support the same protocol and compatibility level. An existing TCP stream is not moved to the other cluster during runtime. In case of a failure, clients or the application must be able to establish a new connection. However, if different HTTP routes or tenants are distributed across different clusters, L4 is not sufficient. Application-related routing on Layer 7 would be required for that.

FAQ

When is TCP Load Balancing better than HTTP Load Balancing?

When protocol transparency, TLS passthrough, or a non-HTTP-based TCP protocol is required. HTTP Load Balancing is more suitable as soon as routing or security rules require application content.

Can an L4 Load Balancer redistribute individual requests?

No. It distributes TCP connections, not individual requests within an existing connection. The assignment remains relevant for the TCP stream.

Does the client need to know the backend address?

No. The client connects to the public service of the edge. Backend pools and their addresses remain hidden behind the public entrance.

Conclusion

TCP Load Balancing is the right choice when services rely on stable transport connections, protocol fidelity, and low processing depth. The architecture gains transparency but consciously avoids HTTP-based routing and application-related inspection. Crucial are reliable health checks, well-defined backend pools, and realistic expectations for existing connections. The ayedo Edge Cloud integrates these tasks into a provider-independent, distributed edge architecture without mixing edge and compute.