Add Gateway deployment arch

content/en/docs/collector/deployment/gateway.md → content/en/docs/collector/deployment/gateway/index.md

@@ -4,7 +4,8 @@ description:
   Why and how to send signals to a single OTLP end-point and from there to
   backends
 weight: 3
-cSpell:ignore: hostnames loadbalancer loadbalancing
+# prettier-ignore
+cSpell:ignore: filelogreceiver hostmetricsreceiver hostnames loadbalancer loadbalancing resourcedetectionprocessor
 ---
 
 The gateway collector deployment pattern consists of applications (or other
@@ -198,6 +199,38 @@ The load-balancing exporter emits metrics including
 that you can use for health and performance monitoring of the OTLP endpoint
 collector.
 
+## Combined deployment of Collectors as agents and gateways
+
+Often a deployment of multiple OpenTelemetry collectors involves running both
+Collector as gateways and as [agents](/docs/collector/deployment/agent/).
+
+The following diagram shows an architecture for such a combined deployment:
+
+- We use the Collectors running in the agent deployment pattern (running on each
+  host, similar to Kubernetes daemonsets) to collect telemetry from services
+  running on the host and host telemetry, such as host metrics and scrap logs.
+- We use Collectors running in the gateway deployment pattern to process data,
+  such as filtering, sampling, and exporting to backends etc.
+
+![gateway](otel-gateway-arch.svg)
+
+This combined deployment pattern is necessary, when you use components in your
+Collector that either need to be unique per host or that consume information
+that is only available on the same host as the application is running:
+
+- Receivers like the
+  [`hostmetricsreceiver`](https://github.com/open-telemetry/opentelemetry-collector-contrib/tree/main/receiver/hostmetricsreceiver)
+  or
+  [`filelogreceiver`](https://github.com/open-telemetry/opentelemetry-collector-contrib/tree/main/receiver/filelogreceiver)
+  need to be unique per host instance. Running multiple instances of these
+  receivers will result in duplicated data.
+
+- Processors like the
+  [`resourcedetectionprocessor`](https://github.com/open-telemetry/opentelemetry-collector-contrib/tree/main/processor/resourcedetectionprocessor)
+  are used to add information about the host, the collector and the application
+  are running on. Running them within a Collector on a remote machine will
+  result in incorrect data.
+
 ## Tradeoffs
 
 Pros: