Promise Theory—What Is It?

During the past 20 years, there has been a growing sense of inadequacy about the "command and control" model for managing IT systems. Years in front of the television with a remote control have left us hard pressed to think of any other way of making machines work for us. But, the truth is that point-and-click, imperative scripting and remote execution do not scale very well when you are trying to govern the behavior of a large number of things.

IT installations grow to massive size in data centers, and the idea of remote command and control, by an external manager, struggles to keep pace, because it is an essentially manual human-centric activity. Thankfully, a simple way out of this dilemma was proposed in 2005 and has acquired a growing band of disciples in computing and networking. This involves the harnessing of autonomous distributed agents.

From Imposing to Obtaining

Imagine that we want to implement a software design or enact a business process. For cultural reasons, our first thought is to want to issue a list of instructions to all the parts involved to carry out some kind of an algorithm, but that might not be the best choice at scale. An algorithm is just a story we tell ourselves about how we envisage the process in our minds. Often we try to tell the same story to a computer or to a team of people, in the form of a flow chart or scripted language. This is one way of describing, building or processing the design. The algorithm might involve a lot of detail and take the form of step-by-step instructions, but other approaches use parallelism or non-linear stories to lay out the job more efficiently. In either case, the instructor assumes that all of the parts involved simply will do exactly as they are instructed. Alas, this is not necessarily true. Remote parts of a system might be unable to comply with these instructions, or even be unwilling to comply, depending on their nature and their circumstances.

This command and control model is called an obligation model in computer science. It has many problems. One of those problems is that it separates intent from implementation, creating uncertainty of outcome. The place where the instructions are determined is not the place where they must be carried out, so the instructor does not have the information about conditions where the local work needs to take place. If two different instructors start issuing conflicting instructions to the same set of parts, there would start a conflict, which no part of the system would be able to resolve, because none of the parts would have the information in one place: intent is not local to the point of action. The instructors might not even be aware of each other to resolve the conflict.

Figure 1. Obligation Model vs. Promise Theory

Luckily, there is a complementary approach to looking at design that fixes these deficiencies, not in terms of obligations, but in terms of promises.

In a promise-based design, each part behaves only according to the promises it makes to others. Instead of instructions from without, we have behavior promised from within. Since the promises are made by "self" (human self or machine self), it means that the decision is always made with knowledge of the same circumstances under which implementation will take place. Moreover, if two promises conflict with one another, the agent has complete information about those circumstances and conflicting intentions to be able to resolve them without having to ask for external help.

A promise-oriented view is somewhat like a service view. Instead of trying to remote-control things with strings and levers, one makes use of an ecosystem of promised services that advertise intent and offer a basic level of certainty about how they will behave. Promises are about expectation management, and knowing the services and their properties that will help us to compose a working system. It doesn't matter here how we get components in a system to make the kinds of promises we need—that is a separate problem.

Electronics are built in this way, as is plumbing and other commoditized construction methods. You buy components (from a suitable supplier) that promise certain properties (resistance, capacitance, voltage-current relationships), and you combine them based on those expectations into a circuit that keeps a greater promise (like being a radio transmitter or a computer).

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Mark Burgess is the founder, CTO and original author of CFEngine.