We often see request error rates as an indicator for SLA compliance. Reality however shows that this draws a wrong picture.

Let’s start with an example.

We had a meeting with a customer and were talking about their SLA and what it is based on. Like in many other cases the request error rate was used and the actual SLA they agreed on was 0.5%. From the operations team we got the input that at the moment they have a request error rate of 0.1%. So they are far below the agreed value. The assumption from current rate is that every 1000th customer has a problem while using the website. Which really sounds good but is this assumption true or do more customers have problems?

Most people assume that a page load equals a single request, however if you start thinking about it you quickly realizes that this is of course not the case. A typical page consists of multiple resource requests. So from now on we focus on all resource requests.

Let’s take a look at a typical eCommerce example. A customer searches for a certain product and wants to buy it in our store. Typically he will have to walk through multiple pages. Each click will lead to a page load which executes multiple resource requests or execute one or more AJAX requests. In our example the visitor has to go through at least seven steps/pages starting at the product detail page ending up with on the confirmation page

The report shows the total Request Count per page. The shortest possible click path for a successful buy leads to 317 resource requests. To achieve a good user experience we need to deliver the resources fast and without any errors. However if we do the math for the reported error rate:

Customers with Errors = 317 requests * 0.1% = 31.7%

That means that on average every third user will have at least one failing request – and it doesn’t even violate our SLA!

>> Continue reading the rest of the blog from Klaus Enzenhofer

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