# What a Service Mesh Does — API Gateway and Service Mesh

Source: https://www.geekswithgeeks.com/en/api-gateway-service-mesh/c-mesh-role

> Describe sidecar proxies, the data plane and the control plane.

## A proxy beside every service

A **service mesh** is infrastructure for service-to-service communication. The **data plane** is a set of **proxies**, classically deployed as a **sidecar** container in each pod, that intercept all traffic in and out of a service and apply policy: **mutual TLS**, **retries and timeouts**, **load balancing**, **circuit breaking**, traffic splitting and telemetry. The **control plane** (the mesh brain) pushes configuration and certificates to those proxies. Because proxies do the work, services written in any language get the same behaviour without library changes. Popular meshes include **Istio** (Envoy proxies; a newer "ambient" mode avoids per-pod sidecars), **Linkerd** (its own lightweight Rust proxy), **Consul** and **Cilium** (eBPF-based). The cost: extra latency per hop, more moving parts and real operational complexity.

## Control plane and data plane

Read it left to right: config flows down from the control plane; traffic flows through the proxies.

```text
            [ control plane ]  <- policies, routes, certificates
              |   config / certs (xDS-style APIs)
   +----------+-----------+
   v                      v
[ proxy ] <== mTLS ==> [ proxy ]
[ orders ]            [ payments ]
  (service)             (service)

data plane = the proxies that carry traffic; services see plain local calls
```

## Measure the overhead

Each sidecar hop adds a small amount of latency and CPU/memory. Measure p99 latency and resource cost with the mesh on and off before rolling it out widely.

**Quiz:** What is the data plane in a service mesh?

- [ ] The Git repository
- [ ] The dashboard
- [ ] The database
- [x] The proxies that carry and police service traffic

*Answer:* The proxies that carry and police service traffic. Data plane = traffic path; control plane = configuration brain.