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New Relic's Open-Source Elixir Agent
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lib/new_relic/distributed_trace/backoff_sampler.ex
defmodule NewRelic.DistributedTrace.BackoffSampler do
use GenServer
alias NewRelic.Harvest.Collector.AgentRun
# This GenServer tracks the sampling rate across sampling periods,
# which is used to determine when to sample a Distributed Trace
@moduledoc false
def start_link() do
GenServer.start_link(__MODULE__, :ok, name: __MODULE__)
end
def init(:ok) do
NewRelic.sample_process()
trigger_next_cycle()
{:ok, init_state()}
end
def init_state() do
%{
sampling_target: AgentRun.lookup(:sampling_target) || 10,
cycle_number: 0,
sampled_true_count: 0,
decided_count: 0,
decided_count_last: 0
}
end
def sample?, do: GenServer.call(__MODULE__, :sample?)
def handle_call(:sample?, _from, state) do
{sampled, state} = calculate(state)
{:reply, sampled, state}
end
def handle_info(:cycle, state) do
trigger_next_cycle()
{:noreply, cycle(state)}
end
def handle_info(:reset, _state) do
{:noreply, init_state()}
end
def cycle(state) do
%{
state
| cycle_number: state.cycle_number + 1,
sampled_true_count: 0,
decided_count: 0,
decided_count_last: state.decided_count
}
end
def calculate(state) do
sampled = do_sample?(state)
state = update_state(sampled, state)
{sampled, state}
end
def do_sample?(%{
cycle_number: 0,
sampled_true_count: sampled_true_count,
sampling_target: sampling_target
}) do
sampled_true_count < sampling_target
end
def do_sample?(%{
sampled_true_count: sampled_true_count,
sampling_target: sampling_target,
decided_count_last: decided_count_last
})
when sampled_true_count < sampling_target do
random(decided_count_last) < sampling_target
end
def do_sample?(%{
sampled_true_count: sampled_true_count,
sampling_target: sampling_target,
decided_count: decided_count
}) do
random(decided_count) <
:math.pow(sampling_target, sampling_target / sampled_true_count) -
:math.pow(sampling_target, 0.5)
end
def trigger_next_cycle() do
cycle_period = AgentRun.lookup(:sampling_target_period) || 60_000
Process.send_after(self(), :cycle, cycle_period)
end
def update_state(false = _sampled?, state) do
%{state | decided_count: state.decided_count + 1}
end
def update_state(true = _sampled?, state) do
%{
state
| decided_count: state.decided_count + 1,
sampled_true_count: state.sampled_true_count + 1
}
end
def random(0), do: 0
def random(n), do: :rand.uniform(n)
end