use singleton pattern for sampler

Author: quinnmil

src/scout_apm/core/tracked_request.py

@@ -24,6 +24,14 @@ class TrackedRequest(object):
     their keyname
     """
 
+    _sampler = None
+
+    @classmethod
+    def get_sampler(cls):
+        if cls._sampler is None:
+            cls._sampler = Sampler(scout_config)
+        return cls._sampler
+
     __slots__ = (
         "sampler",
         "request_id",
@@ -50,7 +58,6 @@ def instance(cls):
         return context.get_tracked_request()
 
     def __init__(self):
-        self.sampler = Sampler(scout_config)
         self.request_id = "req-" + str(uuid4())
         self.start_time = dt.datetime.now(dt.timezone.utc)
         self.end_time = None
@@ -153,12 +160,10 @@ def finish(self):
             self.end_time = dt.datetime.now(dt.timezone.utc)
 
         if self.is_real_request:
-            # Use the sampler to determine if the request should be sampled
-            logger.debug("Checking if request should be sampled")
             if (
                 not self.is_ignored()
                 and not self.sent
-                and self.sampler.should_sample(self.operation)
+                and self.get_sampler().should_sample(self.operation)
             ):
                 self.tag("mem_delta", self._get_mem_delta())
                 self.sent = True

tests/unit/core/test_sampler.py

@@ -77,7 +77,7 @@ def test_should_sample_unknown_operation(sampler):
 
 def test_should_sample_no_sampling_enabled(config):
     config.set(
-        sample_rate=100,  # 100% global sampling (effectively no sampling)
+        sample_rate=100,  # Return config to defaults
         sample_endpoints={},
         sample_jobs={},
         ignore_endpoints=[],