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Tractor Blade is the term used to describe both a physical host out on the farm as well as the tractor-blade remote execution service. The tractor-blade service program is the "pointy end" of the tractor system, it actually launches the commands specified in jobs, causing renderings or simulations or anything else to occur.
Only one instance of tractor-blade itself is needed on a given host. It can manage several concurrent slots on that host, each running a different command. It also tracks various host metrics such as CPU load and memory usage across the whole machine, and reports them to tractor-engine.
Since a farm may consist of many blade hosts, but only a few variations of machine types or duties, Tractor defines blade behavior in terms of blade profiles that are defined once and then potentially applied to many blade instances out on the farm.