The Distributed Execution Manager (DEM) is a VMware vCloud Automation Center feature designed to implement the business logic of custom models by interacting with the VMware database and external databases and systems such as vCloud Director and vCloud Air. DEM is included in VMware's infrastructure as a service (IaaS) offering along with Model Manager and Manager Service.
The DEM is divided into two parts -- the Worker and the Orchestrator -- both of which play a specific and essential role in the production and monitoring of workflows. The Worker creates and carries out workflows, while the Orchestrator performs a number of functions, including monitoring Worker instances. The Orchestrator does this by pre-processing and scheduling workflows. When pre-processing workflows, the DEM Orchestrator checks the preconditions required to run workflows in order to create an accurate workflow execution history.
The DEM Orchestrator schedules recurring workflows by only creating new workflows at a scheduled time. This, in addition to the use of the RunOneOnly feature, allows Orchestrator to ensure that only instance of any workflow is executed at a given time. In the event that a Worker workflow ceases or loses connection to the Model Manager Web repository, the Orchestrator places workflows in a queue for the next Worker instance to pick up, limiting the need for high availability DEM Workers.
Only one instance of the DEM Orchestrator can function as the active Orchestrator to complete these tasks at a time. Due to this, users should install a second Orchestrator on a separate machine to safeguard against failure. The backup Orchestrator will monitor the active DEM Orchestrator and automatically replace it should it fail.
The VMware Distributed Execution Manager is commonly used by administrators who regularly make use of VMware IaaS resources as part of vCAC 6.0.