Many indicator metrics are most naturally gathered on the server side, using a monitoring system such as Borgmon (see Chapter 10) or Prometheus, or with periodic log analysis — for instance, HTTP 500 responses as a fraction of all requests. However, some systems should be instrumented with client-side collection, because not measuring behavior at the client can miss a range of problems that affect users but don’t affect server-side metrics. For example, concentrating on the response latency of the Shakespeare search backend might miss poor user latency due to problems with the page’s
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