I recently wrote a Twitter thread about cloud risk and resilience that drew a lot of interest, so I figured I’d expand on it in a blog post. I’ve been thinking about cloud resilience a lot recently, given that clients have been asking about how they manage their risks.

Inquiries about this historically come in waves, almost always triggered by incidents that raise awareness (unfortunately often because the customer has been directly impacted). A wave generally spans a multi-week period, causing waves to bleed into one another. Three distinct sets come to mind over the course of 2021:

• The Azure AD outages earlier this year had a huge impact on client thinking about concentration risks and critical service dependencies — often more related to M365 than Azure, though (and exacerbated by the critical dependency that many organizations have on Teams during this pandemic). Azure AD is core to SSO for many organizations, making its resilience enormously impactful. These impacts are still very top of mind for many clients, months later.
• The Akamai outage (and other CDN outages with hidden dependencies) this summer raised application and infrastructure dependency awareness, and came as a shock to many customers, as Akamai has generally been seen as a bedrock of dependability.
• The near-daily IBM Cloud “Severity 1” outages over the last month have drawn selective client mentions, rather than a wave, but add to the broader pattern of cloud risk concerns. (To my knowledge, there has been no public communication from IBM regarding root cause of these issues. Notifications indicate the outages are multi-service and multi-regional, often impacting all Gen 2 multizone regions. Kubernetes may be something of a common factor, to guess from the impact scope.)

Media amplification of outage awareness appears to have a lot to do with how seriously they’re taken by customers — or non-customers. Affecting stuff that’s consumed by end-users — i.e. office suites, consumer websites, etc. — gets vastly more attention than things that are “just” a really bad day for enterprise ops people. And there’s a negative halo effect — i.e. if Provider X fails, it tends to raise worries about all their competitors too. But even good media explanations and excellent RCAs tend to be misunderstood by readers — and even by smart IT people. This leads, in turn, to misunderstanding why cloud services fail and what the real risks are.

I recently completed my writing on a note about HA and failover (DR) patterns in cloud IaaS and PaaS, with a light touch on application design patterns for resilience. However, concerns about cloud resilience applies just as much — if not more so — to SaaS, especially API SaaS, which creates complicated and deep webs of dependencies.

You can buy T-shirts, stickers, and all manner of swag that says, “The cloud is just somebody else’s computer.” Cute slogan, but not true. Cloud services — especially at massive scale — are incredibly complex software systems. Complex software systems don’t fail the way a “computer” fails. The cloud exemplifies the failure principles laid out by Richard Cook in his classic “How Complex Systems Fail“.

As humans, we are really bad at figuring out the risk of complex systems, especially because the good ones are heavily defended against failure. And we tend to over-index on rare but dramatic risks (a plane crash) versus more commonplace risks (a car crash).

If you think about “my application hosted on AWS” as “well, it’s just sitting on a server in an AWS data center rather than mine”, then at some point in time, the nature of a failure is going to shock you, because you are wrong.

Cloud services fail after all of the resiliency mechanisms have failed (or sometimes, gone wrong in ways that contribute to the failure). Cloud services tend to go boom because of one or more software bugs, likely combined with either a configuration error or some kind of human error (often related to the deployment process for new configs and software versions). They are only rarely related to a physical failure — and generally the physical failure only became apparent to customers because the software intended to provide resilience against it failed in some fashion.

Far too many customers still think about cloud failure as a simple, fundamentally physical thing. Servers fail, so we should use more than one. Data centers fail, so we should be able to DR into another. Etc. But that model is wrong for cloud and for the digital age. We want to strive for continuous availability and resilience (including graceful degradation and other ways to continue business functionality when the application fails). And we have to plan for individual services failures rather than total cloud failure (whether in an AZ, region, or globally). Such failures can be small-scale, and effectively merely “instability”, rather than an “outage” — and therefore demands apps that are resilient to service errors.

So as cloud buyers, we have to think about our risks differently, and we need to architect and operate differently. But we also need to trust our providers — and trust smartly. To that end, cloud providers need to support us with transparency, so we can make more informed decisions. Key elements of that include:

• Publicly-documented engineering service-level objectives (SLOs), which are usually distinct from the financially-backed SLAs. This is what cloud providers design to internally and measure themselves against, and knowing that helps inform our own designs and internal SLOs for our apps.
• Service architecture documentation that helps us understand the ways a service is and isn’t resilient, so we can design accordingly.
• Documented service dependency maps, which allow us to see the chain of dependencies for each of the services we use, allowing us to think about if Service X is really the best fallback alternative if Service Y goes down, as well as inform our troubleshooting.
• Public status dashboards, clearly indicating the status of services, with solid historical data that allows us to see the track record of service operations. This helps with our troubleshooting and user communication.
• Public outage root-cause analysis (RCA), which allow us to understand why outages occurred, and receive a public pledge as to what will be done to prevent similar failures in the future. A historical archive of these is also a valuable resource.
• Change transparency that could help predict stability concerns. Because so many outages end up being related to new deployments / config changes, and the use of SRE principles, including error budgets, is pretty pervasive amongst cloud providers, there is often an interesting pattern to outages. Changes tend to freeze when the error budget is exceeded, leading to an on-and-off pattern of outages; instability can resume at intervals unpredictable to the customer.

Mission-critical cloud applications are becoming commonplace — both in the pervasive use of SaaS, along with widespread production use of IaaS and PaaS. It’s past time to modernize thinking about cloud operations, cloud resilience, and cloud BC/DR. Cloud risk management needs to be about intelligent mitigation and not avoidance, as forward-thinking businesses are will not accept simply avoiding the cloud at this point.

I am interested in your experiences with resilience as well as cloud instability and outages. Feel free to DM me on Twitter to chat about it.