There's a bug when configuring sampling rules.

Using the following configuration:

const api = Api( logger: { level: 'error', sampling: { target: 1, rate: 0.2, period: 30, rules: [ { route: '/report', target: 0, rate: 0, period: 1 }, ] }, } );

requests for the /report route would still end up using the default sampling configuration, instead of the route specific one.

The bug lies in the lines

The code attempts to check whether there is a specific sampling rule specified for the request's route, by looking it up in the rulesMap. The rulesMap is an object, in the example above is { __ANY: 0 }. 0 is the index into the rules array.

However, the code in the lines above evaluates to -1 because 0 is falsy.

ref is then -1, which is treated as no specific rule matching, so you get the default configuration.

As a work around, one can simply specify a dummy rule before any import rules in the sample configuration, which will then happily be ignored.