What is Site Reliability Engineering?

Site Reliability Engineering is an incredibly interesting field, one which has straddled the interesting line of both being relatively clearly defined and being hugely open to interpretation. For many familiar with it, the idea of SRE brings thoughts of SLI/SLO/SLAs, incident response, on-call, and an obsession with system architecture and failure modes. For others, the line between concepts like DevOps and SRE is hard to make out, and to add yet another perspective to the pile - it can simply mean "we want to hire an ops team, but nobody wants to work for us unless we call it something else".

Recently, Niall Murphy (one of the original authors of the (in)famous Google SRE Book) wrote a thought provoking pieceopen in new window about the future of SRE and the assumptions that underpin it. It does a great job of articulating some of the problems that have consistently worn a blister in my experience of SRE - especially when it comes to articulating what it is we do to leadership.

I'd like to present what Site Reliability Engineering means to me and a hypothesis for what SRE may be when we take a step back from some of the implementation details.

The Evolution of Software Engineering

Over the decades our industry has progressively divested itself of many specialist roles within the Software Engineering space. QA teams are in many cases a thing of the past, Operations teams are being phased out in favour of developers being on-call, and there is a not uncommon thought that SRE will follow the same path as a specific discipline in the future.

The trouble here, as it has been with every prior discipline, is that generalization inherently reduces the emphasis placed on any specific aspect of the field. This certainly can work, as proven by the large number of successful companies employing this strategy, but the lack of specific incentives places a heavy burden on the supporting system to ensure adequate investment in all critical domains. It is not uncommon to see a service built and operated by developers who are drowning under the burden of on-call and have an insurmountable backlog of security debt to work through.

On the flip side, the local optima of high resource utilization (i.e. our engineers are constantly working hard) is easily met with the above approach, even if the global optima may well be missed. This is the problem highlighted in books like The Goalopen in new window, where efforts to maximize the efficiency of each stage of the production pipeline result in catastrophic inefficiencies in the end output.

The solution is not simply to hire experts in these fields - there is after all, a reason why we've started to shift how we approach these problem spaces - but instead to embrace the need for structures which better support our end to end success as an industry.

With that in mind, let's talk about the role that I see SRE as an approach filling in the future and how I see it filling this role in organizations today.

Modelling Success

When we talk about the role of SRE within organizations, we are often subject to three distinct visions of what it represents. Development teams unfamiliar with SRE will often see the role as "Ops Re-branded". Business leadership will often look to SRE to take the organization to "Google Scale" and solve all of their reliability woes. When it comes to the SREs themselves, they often envisage their role as the only team capable of fixing the systemic problems with the services they support.

All of these visions are self serving and based on the value that the observer wishes to bring to the table. After all, the dev team wants to own the code they work on, business leadership wants to be the head of a global monopoly, and SREs want to feel that they are indispensable and are the source of all reliability in the system.

In reality, I've found that SRE's greatest ability to drive positive outcomes is when they act as a repository of knowledge and are able to effectively convey this knowledge to the people who require it at the time they need it. In this capacity, SRE is an inherently horizontal facility within the organization and tightly coupled with the organization's ability to learn and apply those lessons.

Complexity in Large Systems

Anyone who has been cursed, or privileged, to work on large systems will recognize the central role that complexity plays in everything we do. We strive to reduce complexity, and are constantly dealing with the emergent behaviours that result from its inescapable presence in our systems.

This complexity can be most harmful when it obscures the behaviour of these systems, or the impact that a proposed change will have on the system's behaviour. In any sufficiently complex system, it becomes impossible to fully reason through the impact of every change and this leads to well intentioned changes surprising us with unintended negative outcomes.

This problem is most profoundly felt by new hires and early-career engineers, but impacts the ability of everyone to safely contribute towards the system's development. It also represents a crossroads in how your teams approach the problem: are they proud of the complexity they can wrangle, or do they aggressively attempt to strip it away? Too many teams take pride in their ability to operate in a complex environment, and their on-boarding is a hazing into this exclusive club.

Simplicity as a Role

I believe that the role of SRE is to strip away complexity in all its forms, exposing the truth of the systems we work on in a manner which democratizes the ability to make changes safely. We have several tools at our disposal to accomplish this, but without an ability to reason about the systems we operate on it is impossible for us to achieve the long-term outcomes we seek.

In this space, an SRE team attempts to decompose a complex system into its simplest possible model(s) and then validates these models against the behaviour of the system. We then maintain the corpus of behaviours and risks associated with these models, and use them to predict the possible outcomes of change.


A service team is attempting to optimize the performance of a specific workload, and notices that their system is reading data from a separate geographic location. Reducing this latency by reading from the local region has the potential to reduce the workload's end-to-end runtime by up to 10%.

By simplifying this to the PACELCopen in new window model, we can theorize that reducing latency will reduce consistency - even if we do not know anything else about the system in question.

And indeed, the change would have resulted in consistency problems by reading from an asynchronously replicated read replica instead of the primary write location.

This approach of applying simplified models to the complex systems we deal with, and using this to inform our approach to reviewing changes has repeatedly proven its ability to predict problems and inform design improvements without the need for perfect familiarity with the system in question.

Da Vinci

In many ways, I think the perfect exemplar of what SRE may represent in the modern world is found by looking to the ancient world. Leonardo Da Vinci was as quoted from Wikipedia..

A painter, draughtsman, engineer, scientist, theorist, sculptor, and architect.

He helped inspire and guide generations in every one of these fields and did so as a consequence of his pursuit of learning and efforts to share his knowledge with others. His ideas, theories, and approaches have lived on even as our technical capabilities to implement them have far outstripped anything he dreamt of. It is in his example that I find a vision for where we may end up, if we so choose.

A picture of Benjamin Pannell

Benjamin Pannell

Site Reliability Engineer, Microsoft

Dublin, Ireland