Two Kinds of Traffic

North-south vs east-west

In a microservices system, network traffic falls into two categories, and each is solved by a different infrastructure component.

• North-south traffic – requests that cross the boundary between the outside world (browsers, mobile apps, partners) and your cluster. This is handled by an API gateway.
• East-west traffic – requests between your own services inside the cluster (the orders service calling the payments service). This is handled by a service mesh.

The two are complementary, not competing. A common interview mistake is treating them as alternatives. The gateway is the front door; the mesh is the internal nervous system. Both push cross-cutting networking concerns – auth, retries, encryption, observability – out of application code and into infrastructure, but they operate at different layers and on different traffic.

The rest of this module builds up each piece, then shows the combined picture that real production systems use.