Stream Logs from Fastly

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Configuration

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Sampling with VCL

1. A Logging rule, which only forwards logs for requests if they are included in the sampled data.
2. Varnish Configuration Language (VCL) snippet(s), which determines if the request should be sampled and at what rate.

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Configure Sampling

1. Update your Fastly configuration to create a logging rule, which forwards requests to Honeycomb only if the req.http.log_request local variable is set to "1".
   
2. Create two VCL snippets:
   1. A table of sample rates for status codes For example, this table describes the number of events which flow through per sampled event based on status code. 1 does not sample at all, 20 samples every 20th event, and so on. To use, copy and adjust the rates in this table based on your projection traffic:
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      table codes {
          "200s": "20",
          "300s": "5",
          "400s": "3",
          "500s": "1",
      }
      
   2. Code that sets the sample rate based on HTTP status Use the following code to set the req.http.log_request variable (as mentioned in the logging rule) if sampling should be applied:
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      set req.http.samplerate = table.lookup(codes, regsub(resp.status, "^([1-5])..", "\100s"), "1");
      if (randombool(1, std.atoi(req.http.samplerate))) {
        set req.http.log_request = "1";
      } else {
        set req.http.log_request = "0";
      }
      
      
3. To ensure that the sample rate is included as a property of the JSON event and sent to Honeycomb, add this line at the same level of the time and data keys:
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    "samplerate": %{req.http.samplerate}V,
   
   This line encodes samplerate as a top0-level key sent to the Honeycomb API and causes all visualizations rendered by Honeycomb to appear as if all of the events, even ones which were sampled out, were sent.