The Promises of DevOps

A promise theory perspective on DevOps

DevOps is evolving from what started as little more than a rally cry, into a culture for continuous delivery of IT services. As more pieces of the puzzle fall into place, we understand that DevOps is primarily a matter of human collaboration -- assisted, but not encompassed by, certain tools and technologies. This essay is my take on DevOps, and why I think it is about embracing distributed, autonomous operational methods that support scalable human-computer processes.

Cooperation within human-computer systems is an issue that has occupied me intensely since around 2002, when discussions at the USENIX LISA Configuration Management Workshop uncovered very different intuitions about how system administrators should work together with tools to build and maintain functioning IT systems.

For some reason, the discussion about cooperation became tangled with that of configuration management -- perhaps this was because configuration management was where the hard questions about operations were being discussed. But this was really a red herring. To anyone who studies cooperation, it becomes quickly clear that it has little to do with computers or programming or configuration at its heart. The principles of cooperation are the universal matters of availability of information and intent. Computers might be the agents through which intentions are expressed, but the major part of cooperation is a human story.

DevOps and Service Delivery

The term DevOps emerged (some would say it converged) from the world of web operations to highlight one of several weak-links in IT infrastructure engineering: transducing factory product (Dev) into consumable product through a delivery mechanism (Ops). The increased speed craved by the demand for the new `E-commerce' (as it was known then), and exemplified by the scale of operations at companies like Google, Amazon, and Microsoft, made old methods of infrastructure engineering an actual hindrance to doing business. The business imperative to innovate in the high tempo world of the Internet, and the supply chain to deliver those innovations had an operations bottleneck: the outdated methods of operations could not get the changes out fast enough.

As many of the key orators of DevOps have emphasized, the problem was not just one of technology, it was (and still is, for most shops) one of culture. The long-standing demolition approach to management: destroy and reload a golden images should have died out years ago as a method for change, but it persists even now. Such methods are static approaches to a dynamic problem. Initially the only alternative in town was CFEngine 2, as used by many companies with high levels of operational expertise like Amazon, EBay, Facebook, and LinkedIn, who were able to get ahead and achieve the speed they needed to scale up operations. This helped to define new ways of thinking about deployment. Later, CFEngine would be joined by Puppet and then Chef (and the next-generation CFEngine 3) to add to today's options for speedy deployments.

The culture of scalable, continuous delivery itself spread from these origin companies, as post-employees diffused into other start-ups and the tools and ideas began to gain momentum. But speed to implement change is not enough if it is throttled by something else -- the outstanding issue of cooperative process. Deployment processes were suffering from culture shock.

Impedance mismatch: CAMS

There is basic conflict of interest between the roles of developer and operations engineer that is not typical of the supply chains of other industries. Web developers are motivated to push out new goodness as fast as possible. Infrastructure engineers often have little incentive to change anything. They are used to being ignored until something goes wrong, whereupon they are the first line of blame for the fault. Deploying something without due diligence would only land them in trouble. To developers during the web boom, operational resistance was the problem. To operations, developmental change was the problem. There was an impedance mismatch in the cooperation circuit that needed fixing: enter DevOps.

How do you fix the problem? The Theory Of Cooperation says it's straightforward: arrange mutual benefit, that enriches the lives of both. John Willis and Damon Edwards elegantly put this into four categories of initiative, which were labelled CAMS:

  • Culture (of communication and discipline)
  • Automation (superhuman scale and speed)
  • Measurement (bringing verification and certainty)
  • Sharing (to enable a rapidly growing community to raise the bar quickly)

These cornerstones mirror the so-called four barriers to progress and offer strategies to mitigate:

  • Don't know what to do (share knowledge)
  • Don't know how to do it (share culture)
  • Can't gauge progress (measure it)
  • Can't see who is responsible (automate it)

From the perspective of Promise Theory (PT), this makes some sense, as I hope to suggest below. PT says that all cooperation stems from intent (a human quality). How do you fix human intent? Well, the cornerstones are reasonable strategies to get started. Let's see why.

Modelling intent

The arguments begun at the Configuration Management Workshops around cooperative automation in 2002, motivated me to cut through the opinions that were based mainly on unsubstantiated belief, and open the way for a new approach based on ideas that could be substantiated with science. In other words, ideas that could be explained and verified. So I turned back to research and spent 5 years developing a fundamental model for distributed configuration and cooperation.

My background in mathematical physics led me to delve into the wealth of experience accumulated by physicists and mathematicians over the centuries: after all, modelling "stuff that happens" is the domain of physics. The model I eventually came up with (in collaboration with my then research student Siri Fagernes and colleague Jan Bergstra) was an algebraic approach that unified a number of different ideas from the scientific literature into a useful language -- using a formalized notion of `promises'.

After a presentation and discussion at the international conference of Distributed Systems, Operations and Management in 2005, this got the nickname Promise Theory. The idea of promises turns out to be a great metaphor for expressing intentions about both animate and inanimate objects (if you are tenacious reader, with a bent for formal languages, you can see the details of this reasoning in our book Promise Theory: Principles and Applications). The issue of intent turned out to be a much larger one than I had anticipated, but the details revealed not only truths about configuration management, but all interacting systems.

Finding the impedance mismatch

One of the things that a physics approach does right is to start with the basic engineering questions: what are the important scales (of time, for instance) as changes happen in your system (how fast are different things changing)? Addressing such questions allows you to see where systems are mismatched, e.g. under-utilized or over-utilized (to use the language of queues). This is useful given that such a mismatch of expectations had already landed in the laps of the largest growing part of the Web-IT industry.

So what were the timescales?

As the story of promises was developing through 2004-2006, Web 2.0 was undergoing something of a big bang event: the demand for IT systems was multiplying and growing at incredible speed, and the pressures placed on a much slower-growing number of system administrators were mounting. In the past, new computers came once every few years, but this was about to jump a gear: to grow into a web giant, new computers were coming every week. Only top companies had the resources, tools and expertise to do this. Companies like banks, EBay, LinkedIn and Facebook, etc were famously made possible with the help of CFEngine 2 (back then) and a level of expert knowledge combined with intent to scale. The deeper problem was now surfacing -- the human, business issue of the mismatch between the time to market and the time to build the infrastructure to support it.

Promise Theory

In what sense is DevOps a possible answer to this problem? Let's look at this from the viewpoint of promises. To get there, I need to summarize some basics.

Like any scientific method, PT is not a solution to anything, it is a language to describe and discuss cooperative behaviour. The idea is that, if you start with a highly expressive language that allows to you describe any scenario with necessary resolution, then you can compare any two things in a common framework and decide between them. PT is, if you like, an approach to modelling cooperative systems that allows you to ask: "how sure can we be that this will work"? And "at what rate"?

PT is also a kind of `atomic theory'. It gives a table of elements (basic promises) from which any substance can be put together, like a chemistry of intentions (once an intention about self is made public, it becomes a promise). SOA or Service Oriented Architecture is an example of a promise-oriented model, that is based on Web Services and APIs, because it defines autonomous services (agents) with interfaces (APIs) each of which keeps well-documented promises.

The principles that PT cites exist to maintain generality, and to ensure that as few assumptions as possible are made. It also takes care of the idea that every agent's world-view is incomplete, i.e. there are different viewpoints that are limited by what the different parties in a cooperative environment can see and know.

What is unusual about PT, compared to other scientific models is that it models human intentions, and it does it in a way that is completely impersonal. By allowing contact with game theory models, we can also see how cooperative models ultimately have an economic explanation (often referred to as bounded rationality). Why should I keep my promises? What will I get out of it?

Applying PT starts like this:

  1. Identify the key players (agents of intent)

    An agent in PT is any part of a human-computer system that can intend or promise something independently. In this case, these will be people, computers, policy documents, etc -- i.e. anything that can document intent (regardless of its original source), and therefore express a promise that someone or something might be required to keep.

    To get this part of the modelling right, we need to be careful not to confuse intentions with actions or messages. We are not talking about the moving parts of a clock, or even an HTTP request, but the motivations that play a role in making everything happen. Actions may or may not be necessary to fulfill intentions. Maybe inaction is necessary!

    To be independent an agent only needs to think differently or have a different perspective, access to different information, etc. This is about the separation of concerns. If we want agents that reason differently to work together, they need to promise to behave in a mutually beneficial way. These agents can be humans (as in the business-IT bridge) or computers (as in a multi-tier server queue).

  2. Deal with the Uncertainty

    Want absolute certainty? Forget it! In the real world there is no absolute certainty. If you want to live in this fairy tale world, return to CS 101 and prove theorems until you hit Hume, DeMoivre, Godel and Kripke, and see that information is always either incomplete, subject to unknown error, or unverifiable. Then mourn the collapse of your world view in private -- you are no use as an engineer. Dealing with uncertainty is what science is really for.

    Promises might or might not be kept (for a hundred different reasons). We don't know precisely, but we can evaluate the likelihood. Even a machine can break down and fail to keep a promise, so we need to model this. Each promise will have a probability associated with it, based on our trust or belief in its future behaviour.

    In the simplistic two-state model of faults, where manufacturer's like to talk of all their 9's, the expressions Mean Time Before Failure (MTBF) and Mean Time To Repair (MTTR) are coined. These are probabilistic measures, so they have to be multiplied by the number of instances we have. In today's massive scale environments, what used to be a small chance of failure or MTBF gets amplified to a large one -- to counter this, we need speedy repair, if we are going to keep our promises.

    Agents only keep promises about their own behaviour, however. If we try to make promises on others' behalf, they will most likely be rejected, impossible to implement, or the other agent might not even know about them. So it is a "pull" rather than a "push" model, because it assumes that agents only bend to external influence if they want to -- that control cannot be pushed by force. We look more realistically upon illusions like forcible military command structures as cases where there is consensus to voluntarily follow orders -- even these sometimes fail.

  3. From requirements to promises

    In PT we assume that agents cannot be forced to follow a plan, because something might prevent that. One reason is that they are independently governed -- because ultimately there is a stubborn person behind every agent, whether its face is human or machine.

    The goal in PT cooperation is to ensure that agents make all the promises necessary so that some magical on-looker, with access to all the information, would be able to say that an entire cooperative operation could be seen as if it were a single entity making a single service-promise (the algebra tells you if the set of promises is complete or not). How we coax the agents to make promises depends on what kinds of agents they are. If they are human, economic incentives are the generic answer. If the agents are programmable, then they need to be programmed to keep the promises. We call this voluntary cooperation. For humans, the economics are social, professional and economic, and the former are captured in CAMS.

    Is this crazy? Why not just force everyone to comply, like clockwork? Because that makes no sense. A computer follows instructions because it was constructed voluntarily to do so. If we change that promise by pulling out its input wire, it no longer does. And, as for humans, ... Cooperation is voluntary in the sense that it cannot be forced by an external agent (without possibly actually attacking the system to compromise its independence).

  4. Mismatches of intent

    If all agents share the same intentions, there would be no need for promises. Everyone would get along and sing in perfect harmony, working towards a common purpose. The fact that the initial state of a system has unknown intentions means we have to set up things like `agreements', where agents promise to behave in a certain way. This is what we call orchestration.

    For every promise made to serve or provide something (call it X) by one agent in a supply chain, the next agent has to promise to use the result X in order to promise Y to the next agent and so on. Someone has to receive/use the service (promises labelled "-") that is offered or given (promises labelled "+"). So there is a simply symmetry of intent, with polarity like a battery driving activity in a system. Chains, however are fragile: every agent is a single point of failure, so we try for short tiered cooperatives. In truth, most organizations are not chains, but networks of promises.

    This does not necessarily imply serial ordering. Agents can keep their promises in parallel. In an actual orchestra, cooperation happens by promising everyone a copy of the music (so that they can work autonomously). Each player promises to read the music and play this. The conductor promises to provide a service of certain clock ticks and volume settings provided by the conductor service, and the players promise to follow these. The cooperation is pull-based and entirely voluntary. Signals play a part in coordinating the cooperation but the main intent is communicated through the documented score.

The main findings of promise theory

PT seems upside down to some people. They want to think in terms of obligations. A should do B, C must do D, etc. But, apart from riling human beings' sense of dignity, that approach leads quickly to provable inconsistencies. The problem is that the source of any obligation (the obliger) is external to the agent that it being obliged. Thus if the agent is either unable or unwilling to cooperate (perhaps because it never received a message), the problem cannot be resolved without solving another distributed cooperation problem to figure out what went wrong! And so on, ad nauseum. (One begins to see the fallacy of trusting centralized push models and separate monitoring systems.)

Promise theory assumes that an agent can only make promises about its own behaviour (because that is all it is in control of), and this cuts through the distributed issue, ensuring that the information and resources needed to resolve any problem are local and available to the agent, so it can autonomously repair (self-heal) a process.

DevOps cooperation: Dev promises things that Ops like, Ops promise things that Dev likes. Both of them promise to keep the supply chain working at a certain rate, i.e. Dev supplies at a rate that Ops can promise to deploy.

By choosing to express this as promises, we know the estimates were made with accurate information by the agent responsible, not by external wishful thinkers without a clue.

In a human world, this is obvious. If humans or computers command the resources they use, without caveats, they can make clear promises about what they can and can't deliver.

Strong couplings (hard dependencies) are fictitious couplings (obligations and demands), that make systems brittle and fragile under unrealistic assumptions, whether they are human, computer or human-computer in nature. Expecting only best-effort promises, leads to more weakly coupled systems that therefore are more plastic and resilient. They lead to robust systems, with "slack", because one expects incomplete information and can design to expect failure.

If we encourage a system based on autonomous parts, cooperating loosely, we have the maximum chance of being able to behave according to expectations.

The Dev-Ops relationship is not special. The same cooperation applies to all other business developers and implementers.

  • Management promises goals, staff promise to listen and implement goals at a certain rate.
  • Parts manufacturer promises to deliver parts at a certain rate. System integrator promises to use these parts at a rate less than or equal to this, and to produce its derivative product at a rate less than or equal to that (this is a simplification of the queue).

Several proponents of DevOps have rediscovered the Deming cycle for keeping promises and its descendants via Toyota,

  • Kanban continuous, incremental change and delivery.
  • Kaizen continuous improvement in the service chain.

These capture the same kinds of cooperative impedance matchings.

Finally, although everyone talks of supply chains -- these things are really much more complicated networks of cause and effect. By applying graph theory to the network of promise, we can also find likely problem areas and bottlenecks, to achieve full business-unit alignments.

A simplified promise model for an IT service company
+ is a promise given, - is a promise to use

In the figure above, I've sketched just a few of the many promises/roles that make an organization look like a single service entity. I focused on a primarily "Ops" perspective to keep the picture uncluttered (clearly there are many more promises needed to make a company!). In the picture, an architect promises to turn management goals into a design. Policy writers promise to turn the design into Design Sketches (CFEngine 3), Cookbooks (Chef), or Modules (Puppet) perhaps with the help of community repositories. Ops integrates policy with application changes and uses config tools to transduce this into working infrastructure. The role of DevOps is one of several cooperative relationships that makes the organization function.

Why won't the DevOps promise relationship break down? Because there is an `economic' motivation to keep it there, provided by the "C" in CAMS.

NOTE: I based the role of OPS here on CFEngine 3: OPS is that agency that matches policy methods to infrastructure using only a configuration management tool as an intermediary (hands-free). This means that the config tool (CFEngine) keeps the promise of correct infrastructure and OPS is "hands-free" to manage on a continuous basis. Other tools and approaches take different views. For instance, if you used a command to deploy policy once only after building something, then the promise is severed and the infrastructure is basically floating freely (unmanaged). Then we would have to recapture it by getting it to promise monitoring, and add crisis response, etc. CFEngine 3's pure promise implementation says: let the promise about infrastructure design stand, and the config tool will keep the promise by automating continuous verification and repair.

Splicing Dev onto Ops (timescale impedance matching)

Models fail when they can no longer survive the assumptions on which they were built. The old centralized management of the 1990s, sandblasting golden images out to hosts, broke down when the time to market for product improvements fell below the time to deploy for the change.

If we want a model that can last, we need to make it general. This is why I built CFEngine 3 (as a tool in this chain) to be a fairly pure implementation of PT atoms, so that anything built on top of those atoms would be able to keep cooperative promise robustly.

If the delivery of a product holds back the business, business will find a way around the blockage. This is a business imperative, not really an IT one. If a shoe producer can't get its shoes into shops in time for the Christmas sale, because the delivery system is too slow, it will find a new one, or carry the shoes by itself if it has to. The survival of a business depends on this.

The only thing that is special about IT developers (programmers) is that they are close enough to IT operations to be able to understand each other fairly well. DevOps, is not only cultural, it is practically social.

The culture of command-line: throw it over the wall

You knew it, the heroic sysadmin is not going down that easily. The legendary legerdemain of the one-liner, shell command extraordinaire, to fix an apparent problem (after the fact, mind you), is still considered to be a dragon slain by many sysadmins.

But the "do it once and walk away" (until you're called back) approach to management does not cut it in a system of continuous service delivery, or large size, for the MTBF reasons mentioned above. You're probability of error is getting multiplied by a large number, so you might think you don't have time to think about maintenance, but in fact you need to think about it even more. You can't just build and deploy, and throw it over the wall, hoping for the best -- you need to maintain systems and prevent them from failing in the first place. That means your automation needs to be self-monitoring and self-healing.

The ease with which machines can be built today is distorting our view of reliability to some extent, by diverted attention from resilient strategies. The pendulum will swing back, as more start-ups mature to become mission critical. Just because web services can hide their faults behind load balancers, and rebuild a machine on the fly, does not make this a defensible practice for many other mission critical services (e.g. banking), where the diversity of applications is at the level of thousands of different interacting parts, not just a few tiers.

In an orchestra, the conductor does not wait for a player to pass out, then drag him or her off stage, to replace with a new one. A better strategy would be to prevent the issue in the first place, to avoid an embarrassing spectacle. If the wing falls off a plane...

Here is a danger of technology. Our culture of "make it easy" can draw out the professional laziness in operations to the point where high profile web companies can claim that fault prevention doesn't matter. They don't mean this, so why pretend? Maintenance is the key to keeping any ship afloat. The four pillars of CAMS imply maintenance in measurement and automation.


DevOps is a fascinating cultural movement, that has grown from business need into a revival of engineering pride. Promise Theory suggests that the practices of CAMS are fundamental in the sense that they encourage the continuity of autonomy with weak coupling, and maintain the incentive for cooperation to match the timescales of process.

The need for companies to be aligned with a `sense of purpose' (goals) is more acute than ever before. IT is no longer just `nice to have' -- it is a business imperative, a societal imperative. It is infrastructure.

More than that, by encouraging pride in a job well done, DevOps contributes to our human dignity and elevates operations from the role of firefighting to that of infrastructure engineering.