VS Offline? Mid-Month Outage, Explained
An explanation of the Visual Studio Online outage that occurred July 18 was simply Azure SQL Database slowness, but it was a bit more than that.
Visual Studio Online was offline for about 90 minutes on July 18, and the outage was significant enough that Microsoft Technical Fellow Brian Harry blogged an explanation of it. In short, the cause was slowness of an Azure SQL Database.
Harry explained that the outage occurred during non-peak hours so as to affect very few customers. Even so, there were some lessons to be taken away from the cause of the outage.
A simple explanation goes like this: the Visual Studio IDE was calling Shared Platform Services (SPS) "to establish a connection to get notified about updates to roaming settings"; SPS called Azure Service Bus (ASB); and ASB was calling the Azure SQL Database. The cause wasn't the services themselves, but as Harry explained, failure to "handle a transient failure in a secondary service properly and allowed it to cascade into a total service outage." In other words, one thing leads to another and problems tend to pile up rather than resolve themselves of their own accord.
Harry said that several takeaway lessons could be learned from the incident:
- Smaller services are better: Don't pair critical and non-critical services on a shared resource; factor services into as small an atomic unit of work as possible to minimize failure points.
- With services reliant on each other, retries will pile up exponentially. Assume the worst even if the retries seem normal.
- Prioritize traffic and set up thresholds along the Service Bus.
- Synchronous or asynchronous calls, threading matters.
In short: "The key thing is to examine each and every failure, trace the failure all the way to the root cause, generalize the lessons and build defenses for the future."
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