The invoker is the component that is responsible for receiving invocation requests from Kafka topics, message queues that consumers could subscribe to receive messages. After receiving the messages, the invoker executes the functions using a backend runtime. OpenWhisk supports both native and Docker runtimes. The Docker runtime is internally called blackbox.

OpenWhisk also has an option to use Docker's runc directly to improve the function performance. With this architecture, the invoker requires access to /var/run/docker.sock of the local Docker host. This limitation prevents OpenWhisk from scaling efficiently in Swarm mode. We will discuss the new architecture for OpenWhisk over Swarm in a later chapter, when we discuss the deployment of OpenWhisk in a production environment.