Something that has struck me funny recently surrounds the traditional notion of availability of web applications. With respect to its relationship to revenue, to infrastructure and application behavior, and fault protection and tolerance, I’m thinking it may be time to get a broader upgrade adjustment to the industry’s perception on the topic.

These nuances in the definition and affects of availability aren’t groundbreaking. They’ve been spoken about before, but for some reason I’m not yet convinced that they’re widely known or understood.

Impact On Business

What is laid out here in this article is something that’s been parroted for decades: downtime costs companies money, and lost value. Generally speaking, this is obviously correct, and by all means you should strive to design and operate your site with high availability and fault tolerance in mind.

But underneath the binary idea that uptime = good and downtime = bad, the reality is that there’s a lot more detail that deserves exploring.

This irritatingly-designed site has a post about a common equation to help those that are arithmetically challenged:

LOST REVENUE	=	(GR/TH) x I x H
GR	=	gross yearly revenue
TH	=	total yearly business hours
I	=	percentage impact
H	=	number of hours of outage

In my mind, this is an unnecessarily blunt measure. I see the intention behind this approach, because it’s not meant to be anywhere close to being accurate. But modern web operations is now a field where gathering metrics in the hundreds of thousands per second is becoming more common-place, fault-tolerance/protection is a thing we do increasingly well, and graceful degradation techniques are the norm.

In other words: there are a lot more considerations than outage minutes = lost revenue, even if you did have a decent way to calculate it (which, you don’t). Companies selling monitoring and provisioning services will want you to subscribe to this notion.

We can do better than this blunt measure, and I thought it’s worth digging in a bit deeper.

“Loss”

Thought experiment: if Amazon.com has a full and global outage for 30 minutes, how much revenue did it “lose”? Using the above rough equation, you can certainly come up with a number, let’s say N million dollars. But how accurate is N, really? Discussions that surround revenue loss are normally designed to motivate organizations to invest in availability efforts, so N only needs to be big and scary enough to provide that motivation. So let’s just say that goal has been achieved: you’re convinced! Availability is important, and you’re a firm believer that You Own Your Own Availability.

Outside of the “let this big number N convince you to invest in availability efforts” I have some questions that surround N:

• How many potential customers did Amazon.com lose forever, during that outage? Meaning: they tried to get to Amazon.com, with some nonzero intent/probability of buying something, found it to be offline, and will never return there again, for reasons of impatience, loss of confidence, the fact that it was an impulse-to-buy click whose time has passed, etc.
• How much revenue did Amazon lose during that 30 minute window, versus how the revenue that it simply postponed when it was down, only to be executed later? In other words: upon finding the site down, they’ll return sometime later to do what they originally intended, which may or may not include buying something or participate in some other valuable activity.
• How much did that 30 minutes of downtime affect the strength of the Amazon brand, in a way that could be viewed as revenue-affecting? Meaning: are users and potential users now swayed to having less confidence in Amazon because they came to the site only to be disappointed that it’s down, enough to consider alternatives the next time they would attempt to go to the site in the future?

I don’t know the answers to these questions about Amazon, but I do know that at Etsy, those answers depend on some variables:

• the type of outage or degradation (more on that in a minute),
• the time of day/week/year
• how we actually calculate/forecast how those metrics would have behaved during the outage

So, let’s crack those open a bit, and see what might be inside…

Temporal Concerns

Not all time periods can be considered equal when it comes to availability, and the idea of lost revenue. For commerce sites (or really any site whose usage varies with some seasonality) this is hopefully glaringly obvious. In other words:

X minutes of full downtime during the peak hour of the peak day of the year can be worlds apart from Y minutes of full downtime during the lowest hour of the lowest day of the year, traffic-wise.

Take for example a full outage that happens during a period of the peak day of the year, and contrast it with one that happens during a lower-period of the year. Let’s say that this graph of purchases is of those 24-hour periods, indicating when the outages happen:

The impact time of the outage during the lower-traffic day is actually longer than the peak day, affecting the precious Nines math by a decent margin. But yet: which outage would you rather have, if you had to have one of those? 🙂

Another temporal concern is: across space and time, distribution and volume of any level degradation could be viewed as perfect uptime as the length of the outage approaches zero.

Dig, if you will, these two outage profiles, across a 24-hour period. The first one has many small outages across the day:

and the other has the same amount of impact time, in a single go:

So here we have the same amount of time, but spread out throughout the day. Hopefully, folks will think a bit more beyond the clear “they’re both bad! don’t have outages!” and could investigate how they could be different. Some considerations in this simplified example:

• Hour of day. Note that the single large outage is “earlier” in the day. Maybe this will affect EU or other non-US users more broadly, depending on the timezone of the original graph. Do EU users have a different expectation or tolerance for outages in a US-based company’s website?
• Which outage scenario has a greater affect on the user population: if the ‘normal’ behavior is “get in, buy your thing, and get out” quickly, I could see the many-small-outages more preferable to the single large one. If the status quo is some mix of searching, browsing, favoriting/sharing, and then purchase, I could see the singular constrained outage being preferable.

Regardless, this underscores the idea that not all outages are created equal with respect to impact timing.

Performance

Loss of “availability” can also be seen as an extreme loss of performance. At a particular threshold, given the type of feedback to the user (a fast-failed 404 or browser error, versus a hanging white page and spinning “loading…”) the severity of an event being slow can effectively be the same as a full outage.

Some concerns/thought exercises around this:

• Where is this latency threshold for your site, for the functionality that is critical for the business?
• Is this threshold a cliff, or is it a continuous/predictable relationship between performance and abandonment?

There’s been much more work on performance’s effects on revenue than availability. The Velocity Conference in 2009 brought the first real production-scale numbers (in the form of a Bing/Google joint presentation as well as Shopzilla and Mozilla talks) behind how performance affects businesses, and if you haven’t read about it, please do.

Graceful Degradation

Will Amazon (or Etsy) lose sales if all or a portion of its functionality is gone (or sufficiently slow) for a period of time? Almost certainly. But that question is somewhat boring without further detail.

In many cases, modern web sites don’t simply live in a “everything works perfectly” or “nothing works at all” boolean world. (To be sure, neither does the Internet as a whole.) Instead, fault-tolerance and resilience approaches allow for features and operations degrade under a spectrum of failure conditions. Many companies build their applications to have both in-flight fault tolerance to degrade the experience in the face of singular failures, as well as making use of “feature flags” (Martin and Jez call them “feature toggles“) which allow for specific features to be shut off if they’re causing problems.

I’m hoping that most organizations are familiar with this approach at this point. Just because user registration is broken at the moment, you don’t want to prevent  already logged-in users from using the otherwise healthy site, do you? 🙂

But these graceful degradation approaches further complicates the notion of availability, as well as its impact on the business as a whole.

For example: if Etsy’s favoriting feature is not working (because the site’s architecture allows it to gracefully fail without affecting other critical functionality), but checkout is working fine…what is the result? Certainly you might paused before marking down your blunt Nines record.

You might also think: “so what? as long as people can buy things, then favoriting listings on the site shouldn’t be considered in scope of availability.”

But consider these possibilities:

• What if Favoriting listings was a significant driver of conversions?
• If Favoriting was a behavior that led to conversions at a rate of X%, what value should X be before ‘availability’ ought to be influenced by such a degradation?
• What if Favoriting was technically working, but was severely degraded (see above) in performance?

Availability can be a useful metric, but when abused as a silver bullet to inform or even dictate architectural, business priority, and product decisions, there’s a real danger of oversimplifying what are really nuanced concerns.

Bounce-Back and Postponement

As I mentioned above, what is more likely for sites that have an established community or brand, outages (even full ones) don’t mark an instantaneous amount of ‘lost’ revenue or activity. For a nonzero amount, they’re simply postponed. This is the area that I think could use a lot more data and research in the industry, much in the same way that latency/conversion relationship has been investigated.

The over-simplified scenario involves something that looks like this. Instead of the blunt math of “X minutes of downtime = Y dollars of lost revenue”, we can be a bit more accurate, if we tried just a bit harder. The red is the outage:

So we have some more detail, which is that if we can make a reasonable forecast about what purchases did during the time of the outage, then we could make a better-inform estimate of purchases “lost” during that time period.

But is that actually the case?

What we see at Etsy is something different, a bit more like this:

What this line of thinking doesn’t capture is how many of those users postponed their activity not for immediately after the outage, but maybe the next day because they needed to leave their computer for a meeting at work, or leaving work to commute home?

Intention isn’t entirely straightforward to figure out, but in the cases where you have a ‘fail-over’ page that many CDNs will provide when the origin servers aren’t available, you can get some more detail about what requests (add to cart? submit payment?) came in during that time.

Regardless, availability and its affect on business metrics isn’t as easy as service providers and monitoring-as-a-service companies will have you believe. To be sure, a good amount of this investigation will vary wildly from company to company, but I think it’s well worth taking a look into.