Class: Google::Cloud::Trace::TimeSampler

Inherits:
Object
  • Object
show all
Defined in:
lib/google/cloud/trace/time_sampler.rb

Overview

A sampler determines whether a given request's latency trace should actually be reported. It is usually not necessary to trace every request, especially for an application serving heavy traffic. You may use a sampler to decide, for a given request, whether to report its trace. A sampler is simply a Proc that takes the Rack environment as an argument and returns a boolean indicating whether or not to sample the current request. Alternately, it could be an object that duck-types the Proc interface by implementing the call method.

TimeSampler is the default sampler. It bases its sampling decision on two considerations:

  1. It allows you to blacklist certain URI paths that should never be traced. For example, the Google App Engine health check request path /_ah/health is blacklisted by default.
  2. It spaces samples out by delaying a minimum time between each sample. This enforces a maximum QPS for this Ruby process.

Constant Summary collapse

DEFAULT_PATH_BLACKLIST =

Default list of paths for which to disable traces. Currently includes App Engine Flex health checks.

["/_ah/health"].freeze

Class Method Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(qps: 0.1, path_blacklist: DEFAULT_PATH_BLACKLIST) ⇒ TimeSampler

Create a TimeSampler for the given QPS.

Parameters:

  • qps (Number) (defaults to: 0.1) —

    Samples per second. Default is 0.1.

  • path_blacklist (Array{String,Regex}) (defaults to: DEFAULT_PATH_BLACKLIST) —

    An array of paths or path patterns indicating URIs that should never be traced. Default is DEFAULT_PATH_BLACKLIST.



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# File 'lib/google/cloud/trace/time_sampler.rb', line 51

def initialize qps: 0.1, path_blacklist: DEFAULT_PATH_BLACKLIST
  @delay_secs = 1.0 / qps
  @last_time = ::Time.now.to_f - @delay_secs
  @path_blacklist = path_blacklist
end

Class Method Details

.default ⇒ TimeSampler

Get the default global TimeSampler.

Returns:



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# File 'lib/google/cloud/trace/time_sampler.rb', line 64

def self.default
  @default
end

Instance Method Details

#call(env) ⇒ Boolean

Implements the sampler contract. Checks to see whether a sample should be taken at this time.

Parameters:

  • env (Hash) —

    Rack environment.

Returns:

  • (Boolean) —

    Whether to sample at this time.



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# File 'lib/google/cloud/trace/time_sampler.rb', line 75

def call env
  return false if path_blacklisted? env
  time = ::Time.now.to_f
  delays = (time - @last_time) / @delay_secs
  if delays >= 2.0
    @last_time = time - @delay_secs
    true
  elsif delays >= 1.0
    @last_time += @delay_secs
    true
  else
    false
  end
end