SCADA in Atlas
SCADA is a service to allow tools to integrate with Atlas. SCADA stands for Supervisory Control And Data Acquisition, and as the name implies it allows Atlas to provide control functions and request data from the tools that integrate. Using SCADA is completely optional and requires an opt-in with every tool that supports it.
For example, when SCADA integration is enabled in Consul the Atlas Web UI can be used to inspect the current cluster state. Providing the Consul dashboard is done by means of data acquisition from the connected instances. Optionally, Consul can also perform an auto-join to other members of it's cluster. This is done by informing the SCADA system that auto-join should be enabled allowing Atlas to orchestrate the cluster joins. This is done by means of supervisory control of the connected instances.
The technical details about how SCADA works are fairly simple. Clients first open a connection to the SCADA service at scada.hashicorp.com on port 7223. This connection is secured by TLS, allowing clients to verify the identity of the servers and to encrypt all communications. Once connected, a handshake is performed where a client provides it's Atlas API credentials so that Atlas can verify the client identity. Once complete, clients keep the connection open in an idle state waiting for commands to be received. Commands map to APIs exposed by the product, and are subject to any ACLs, authentication or authorization mechanisms of the client. More specific details are available in the product documentation.