Useful conditions for logging

In addition to the standard logging directives, the following conditions can be used for logging when you set up remote log streaming.

Logging errors only

You can log errors only if you want a general purpose log that catches everything and a more detailed log if there’s an error:

fastly_info.state == "ERROR"

You can also log only 500 errors:

resp.status >= 500 && resp.status < 600`

Logging only specific URLs using a dictionary

Using a dictionary (e.g., urls_to_log), you can log specific URLs having issues:

table.lookup(urls_to_log, req.url.path) == "log"

If a URL becomes a problem, you can start logging it by using the API to add the URL's path to the dictionary as a key with the value "log".

Logging samples

If you have a high-volume service, you might want to log only a proportion of requests by using the randombool VCL function in a condition. The following example will log only one percent of all requests:

randombool(1,100)

You could combine that with a dictionary to change the percentage of requests logged without having to deploy a new version of your service. The following example uses a dictionary named service_variables:

randombool(std.atoi(table.lookup(service_variables, "logging_percentage", "0")), 100)

In the example above, if the key logging_percentage doesn't exist, nothing will be logged.

Using false to construct a log string in custom VCL

To construct a log string in custom VCL, you can use the false condition. This condition makes sure nothing is sent to Fastly logging objects.