consul/website/content/docs/ecs/architecture.mdx

---
layout: docs
page_title: Architecture - AWS ECS
description: >-
  Architecture of Consul Service Mesh on AWS ECS (Elastic Container Service).
---

# Architecture

![Consul on ECS Architecture](/img/consul-ecs-arch.png)

As shown above there are two main components to the architecture.

1. **Consul Server task:** Runs the Consul server.
1. **Application tasks:** Runs user application containers along with two helper containers:
   1. **Consul Client:** The Consul client container runs Consul. The Consul client communicates
      with the Consul server and configures the Envoy proxy sidecar. This communication
      is called _control plane_ communication.
   1. **Sidecar Proxy:** The sidecar proxy container runs [Envoy](https://envoyproxy.io/). All requests
      to and from the application container(s) run through the sidecar proxy. This communication
      is called _data plane_ communication.

For more information about how Consul works in general, see Consul's [Architecture Overview](/docs/architecture).

In addition to the long-running Consul Client and Sidecar Proxy containers, there
are also two initialization containers that run:

1. `discover-servers`: This container runs at startup and uses the AWS API to determine the IP address of the Consul server task.
1. `mesh-init`: This container runs at startup and sets up initial configuration for Consul and Envoy.

### Task Startup

This diagram shows the timeline of a task starting up and all its containers:

![Task Startup Timeline](/img/ecs-task-startup.png)

- **T0:** ECS starts the task. The `discover-servers` container starts looking for the Consul server task’s IP.
  It waits for the Consul server task to be running on ECS, looks up its IP and then writes the address to a file.
  Then the container exits.
- **T1:** Both the `consul-client` and `mesh-init` containers start:
  - `consul-client` starts up and uses the server IP to join the cluster.
  - `mesh-init` registers the service for this task and its sidecar proxy into Consul. It runs `consul connect envoy -bootstrap` to generate Envoy’s bootstrap JSON file and write it to a shared volume. After registration and bootstrapping, `mesh-init` exits.
- **T2:** The `sidecar-proxy` container starts. It runs Envoy by executing `envoy -c <path-to-bootstrap-json>`.
- **T3:** The `sidecar-proxy` container is marked as healthy by ECS. It uses a health check that detects if its public listener port is open. At this time, the user’s application containers are started since all the Consul machinery is ready to service requests.
- **T4:** Consul marks the service as healthy by running the health checks specified in the task Terraform. The service will now receive traffic. At this time the only running containers are `consul-client`, `sidecar-proxy` and the user’s application container(s).