1 // Copyright 2023 gRPC authors.
2 //
3 // Licensed under the Apache License, Version 2.0 (the "License");
4 // you may not use this file except in compliance with the License.
5 // You may obtain a copy of the License at
6 //
7 // http://www.apache.org/licenses/LICENSE-2.0
8 //
9 // Unless required by applicable law or agreed to in writing, software
10 // distributed under the License is distributed on an "AS IS" BASIS,
11 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
12 // See the License for the specific language governing permissions and
13 // limitations under the License.
14
15 #include "sampler.h"
16
17 #include <cmath>
18 #include <cstdint>
19
20 #include "absl/strings/escaping.h"
21
22 namespace grpc_observability {
23
24 namespace {
25
26 // Converts a probability in [0, 1] to a threshold in [0, UINT64_MAX].
CalculateThreshold(double probability)27 uint64_t CalculateThreshold(double probability) {
28 if (probability <= 0.0) return 0;
29 if (probability >= 1.0) return UINT64_MAX;
30 // We can't directly return probability * UINT64_MAX.
31 //
32 // UINT64_MAX is (2^64)-1, but as a double rounds up to 2^64.
33 // For probabilities >= 1-(2^-54), the product wraps to zero!
34 // Instead, calculate the high and low 32 bits separately.
35 const double product = UINT32_MAX * probability;
36 double hi_bits, lo_bits = ldexp(modf(product, &hi_bits), 32) + product;
37 return (static_cast<uint64_t>(hi_bits) << 32) +
38 static_cast<uint64_t>(lo_bits);
39 }
40
CalculateThresholdFromBuffer(const std::string & trace_id)41 uint64_t CalculateThresholdFromBuffer(const std::string& trace_id) {
42 const std::string trace_id_bytes = absl::HexStringToBytes(trace_id);
43 const uint8_t* buf = reinterpret_cast<const uint8_t*>(trace_id_bytes.c_str());
44 uint64_t res = 0;
45 for (int i = 0; i < 8; ++i) {
46 res |= (static_cast<uint64_t>(buf[i]) << (i * 8));
47 }
48 return res;
49 }
50 } // namespace
51
Get()52 ProbabilitySampler& ProbabilitySampler::Get() {
53 static ProbabilitySampler* sampler = new ProbabilitySampler;
54 return *sampler;
55 }
56
SetThreshold(double probability)57 void ProbabilitySampler::SetThreshold(double probability) {
58 uint64_t threshold = CalculateThreshold(probability);
59 threshold_ = threshold;
60 }
61
ShouldSample(const std::string & trace_id)62 bool ProbabilitySampler::ShouldSample(const std::string& trace_id) {
63 if (threshold_ == 0 || trace_id.length() < 32) return false;
64 // All Spans within the same Trace will get the same sampling decision, so
65 // full trees of Spans will be sampled.
66 return CalculateThresholdFromBuffer(trace_id) <= threshold_;
67 }
68
69 } // namespace grpc_observability
70