- Command execution flow
Command Execution Extension service (CEX) extends the communication capability of Kaa Protocol (1/KP) by implementing Command Execution Protocol (11/CEP). CEX supports this extension protocol to deliver commands to endpoint and consume endpoint command execution result. As with other Kaa extension services, CEX uses Extension Service Protocol (4/ESP) for integration with a communication service.
CEX supports two modes of command invocation via REST API:
- synchronous mode requires the REST API client to wait for the response while the command is being executed. The maximum allowed response timeout is specified by the client in a request query parameter. This mode is useful in cases when the command execution is expected to be completed relatively fast (e.g. turning on an air conditioner).
- asynchronous mode allows the REST API caller not to wait for the command execution to complete. Rather, immediately following the API call the client would receive a URL to the created command resource. The result will be retained by CEX for the duration specified by the caller via a query parameter. This command resource URL can be further used for checking the command execution result. This mode is useful in cases when the command execution takes relatively long (e.g. updating the firmware). CEX implements a proactive command push—commands are sent to an endpoint as soon as possible, and an explicit endpoint subscription is not required. Note that explicit subscription is still recommended.
CEX supports a number of interfaces to perform its functional role. The key supported interfaces are summarized in the following diagram.
For inter-service communication, Kaa services mainly use REST APIs and messaging protocols that run over NATS messaging system.
Command invocation protocol
Extension service protocol
In some scenarios, endpoint can be inaccessible during command invocation request emission, thus it will not get a message with a command.
CEX persists commands for a configurable amount of time. So endpoint has an ability to request already scheduled commands.
The command store is maintained in Redis.
Command execution flow
The overall command invocation and execution sequence diagram can be illustrated as follows:
CEX is integrated with the Kaa Tekton for centralized application configuration management. It receives configuration update messages from Tekton over 17/SCMP and uses Tekton REST API to retrieve current configs.
See configuration for more information.