前言

OpenTracing是一個鏈路跟蹤的開放協議，已經有開源的.net實現：opentracing-csharp，同時支援.net framework和.net core，Github地址：https://github.com/opentracing/opentracing-csharp。

這個庫支援多種鏈路跟蹤模式，不過僅提供了最基礎的功能，想用在實際專案中還需要做很多增強，還好也有人做了開源專案：opentracing-contrib，Github地址：https://github.com/opentracing-contrib/csharp-netcore。

opentracing-contrib中集成了一個名為Jaeger的類庫，這個庫實現了鏈路跟蹤資料的取樣和上報，支援將資料上傳到Jaeger進行分析統計。

為了同時保障效能和跟蹤關鍵資料，能夠遠端調整取樣率是很重要的，Jaeger本身也提供了遠端配置取樣率的支援。

不過我這裡用的阿里雲鏈路跟蹤不支援，配置的設計也和想要的不同，所以自己做了一個取樣和上報配置的動態更新，也才有了這篇文章。

思路

使用Jaeger初始化Tracer大概是這樣的：

var tracer = new Tracer.Builder(serviceName)
                       .WithSampler(sampler)
                       .WithReporter(reporter)
                       .Build();
GlobalTracer.Register(tracer);

首先是提供當前服務的名字，然後需要提供一個取樣器，再提供一個上報器，Build下生成ITracer的一個例項，最後註冊到全域性。

可以分析得出，取樣和上報配置的更新就是更新取樣器和上報器。

不過Tracer並沒有提供UpdateSampler和UdapteReporter的方法，被卡住了，怎麼辦呢？

前文提到Jaeger是支援取樣率的動態調整的，看看它怎麼做的：

        private RemoteControlledSampler(Builder builder)
        {
           ...

            _pollTimer = new Timer(_ => UpdateSampler(), null, TimeSpan.Zero, builder.PollingInterval);
        }

        /// <summary>
        /// Updates <see cref="Sampler"/> to a new sampler when it is different.
        /// </summary>
        internal void UpdateSampler()
        {
            try
            {
                SamplingStrategyResponse response = _samplingManager.GetSamplingStrategyAsync(_serviceName)
                    .ConfigureAwait(false).GetAwaiter().GetResult();

                ...

                    UpdateRateLimitingOrProbabilisticSampler(response);
            }
            catch (Exception ex)
            {
                ...
            }
        }

        private void UpdateRateLimitingOrProbabilisticSampler(SamplingStrategyResponse response)
        {
          ...
            lock (_lock)
            {
                if (!Sampler.Equals(sampler))
                {
                    Sampler.Close();
                    Sampler = sampler;
                    ...
                }
            }
        }

這裡只留下關鍵程式碼，可以看到核心就是：通過一個Timer定時獲取取樣策略，然後替換原來的Sampler。

這是一個很好理解的辦法，下邊就按照這個思路來搞。

方案

分別提供一個可更新的Sampler和可更新的Reporter，Build Tracer時使用這兩個可更新的類。這裡延續開源專案中Samper和Reporter的建立方式，給出這兩個類。

可更新的Sampler：

internal class UpdatableSampler : ValueObject, ISampler
    {
        public const string Type = "updatable";

        private readonly ReaderWriterLockSlim _lock = new ReaderWriterLockSlim();
        private readonly string _serviceName;
        private readonly ILoggerFactory _loggerFactory;
        private readonly ILogger _logger;
        private readonly IMetrics _metrics;

        internal ISampler Sampler { get; private set; }

        private UpdatableSampler(Builder builder)
        {
            _serviceName = builder.ServiceName;
            _loggerFactory = builder.LoggerFactory;
            _logger = _loggerFactory.CreateLogger<UpdatableSampler>();
            _metrics = builder.Metrics;
            Sampler = builder.InitialSampler;
        }

        /// <summary>
        /// Updates <see cref="Sampler"/> to a new sampler when it is different.
        /// </summary>
        public void UpdateSampler(ISampler sampler)
        {
            try
            {
                _lock.EnterWriteLock();
                if (!Sampler.Equals(sampler))
                {
                    Sampler.Close();
                    Sampler = sampler;
                    _metrics.SamplerUpdated.Inc(1);
                }
            }
            catch (System.Exception ex)
            {
                _logger.LogWarning(ex, "Updating sampler failed");
                _metrics.SamplerQueryFailure.Inc(1);
            }
            finally
            {
                _lock.ExitWriteLock();
            }
        }

        public SamplingStatus Sample(string operation, TraceId id)
        {
            try
            {
                _lock.EnterReadLock();
                var status=  Sampler.Sample(operation, id);
                return status;
            }
            finally
            {
                _lock.ExitReadLock();
            }
        }

        public override string ToString()
        {
            try
            {
                _lock.EnterReadLock();
                return $"{nameof(UpdatableSampler)}(Sampler={Sampler})";
            }
            finally
            {
                _lock.ExitReadLock();
            }
        }

        public void Close()
        {
            try
            {
                _lock.EnterWriteLock();
                Sampler.Close();
            }
            finally
            {
                _lock.ExitWriteLock();
            }
        }

        protected override IEnumerable<object> GetAtomicValues()
        {
            yield return Sampler;
        }

        public sealed class Builder
        {
            internal string ServiceName { get; }
            internal ILoggerFactory LoggerFactory { get; private set; }
            internal ISampler InitialSampler { get; private set; }
            internal IMetrics Metrics { get; private set; }

            public Builder(string serviceName)
            {
                ServiceName = serviceName ?? throw new ArgumentNullException(nameof(serviceName));
            }

            public Builder WithLoggerFactory(ILoggerFactory loggerFactory)
            {
                LoggerFactory = loggerFactory ?? throw new ArgumentNullException(nameof(loggerFactory));
                return this;
            }

            public Builder WithInitialSampler(ISampler initialSampler)
            {
                InitialSampler = initialSampler ?? throw new ArgumentNullException(nameof(initialSampler));
                return this;
            }

            public Builder WithMetrics(IMetrics metrics)
            {
                Metrics = metrics ?? throw new ArgumentNullException(nameof(metrics));
                return this;
            }

            public UpdatableSampler Build()
            {
                if (LoggerFactory == null)
                {
                    LoggerFactory = NullLoggerFactory.Instance;
                }
                if (InitialSampler == null)
                {
                    InitialSampler = new ProbabilisticSampler();
                }
                if (Metrics == null)
                {
                    Metrics = new MetricsImpl(NoopMetricsFactory.Instance);
                }

                return new UpdatableSampler(this);
            }
        }
    }

可更新的Reporter：

internal class UpdatableReporter : IReporter
    {
        public const string Type = "updatable";

        private readonly string _serviceName;
        private readonly ILoggerFactory _loggerFactory;
        private readonly ILogger _logger;
        private readonly IMetrics _metrics;
        private readonly ReaderWriterLockSlim _lock = new ReaderWriterLockSlim();

        internal IReporter Reporter { get; private set; }

        private UpdatableReporter(Builder builder)
        {
            _serviceName = builder.ServiceName;
            _loggerFactory = builder.LoggerFactory;
            _logger = _loggerFactory.CreateLogger<UpdatableReporter>();
            _metrics = builder.Metrics;
            Reporter = builder.InitialReporter;
        }

        /// <summary>
        /// Updates <see cref="Reporter"/> to a new reporter when it is different.
        /// </summary>
        public void UpdateReporter(IReporter reporter)
        {
            try
            {
                _lock.EnterWriteLock();

                if (!Reporter.Equals(reporter))
                {
                    Reporter.CloseAsync(CancellationToken.None).ConfigureAwait(false).GetAwaiter().GetResult();
                    Reporter = reporter;
                    _metrics.SamplerUpdated.Inc(1);
                }
            }
            catch (System.Exception ex)
            {
                _logger.LogWarning(ex, "Updating reporter failed");
                _metrics.ReporterFailure.Inc(1);
            }
            finally
            {
                _lock.ExitWriteLock();
            }
        }

        public void Report(Span span)
        {
            try
            {
                _lock.EnterReadLock();
                Reporter.Report(span);
            }
            finally
            {
                _lock.ExitReadLock();
            }
        }

        public override string ToString()
        {
            try
            {
                _lock.EnterReadLock();
                return $"{nameof(UpdatableReporter)}(Reporter={Reporter})";
            }
            finally
            {
                _lock.ExitReadLock();
            }
        }

        public async Task CloseAsync(CancellationToken cancellationToken)
        {
            try
            {
                _lock.EnterWriteLock();
                await Reporter.CloseAsync(cancellationToken);
            }
            finally
            {
                _lock.ExitWriteLock();
            }
        }

        public sealed class Builder
        {
            internal string ServiceName { get; }
            internal ILoggerFactory LoggerFactory { get; private set; }
            internal IReporter InitialReporter { get; private set; }
            internal IMetrics Metrics { get; private set; }

            public Builder(string serviceName)
            {
                ServiceName = serviceName ?? throw new ArgumentNullException(nameof(serviceName));
            }

            public Builder WithLoggerFactory(ILoggerFactory loggerFactory)
            {
                LoggerFactory = loggerFactory ?? throw new ArgumentNullException(nameof(loggerFactory));
                return this;
            }

            public Builder WithInitialReporter(IReporter initialReporter)
            {
                InitialReporter = initialReporter ?? throw new ArgumentNullException(nameof(initialReporter));
                return this;
            }

            public Builder WithMetrics(IMetrics metrics)
            {
                Metrics = metrics ?? throw new ArgumentNullException(nameof(metrics));
                return this;
            }

            public UpdatableReporter Build()
            {
                if (LoggerFactory == null)
                {
                    LoggerFactory = NullLoggerFactory.Instance;
                }
                if (InitialReporter == null)
                {
                    InitialReporter = new NoopReporter();
                }
                if (Metrics == null)
                {
                    Metrics = new MetricsImpl(NoopMetricsFactory.Instance);
                }

                return new UpdatableReporter(this);
            }
        }
    }

注意這裡邊用到了讀寫鎖，因為要做到不停止服務的更新，而且大部分情況下都是讀，使用lock就有點大柴小用了。

現在初始化Tracer大概是這樣的：

sampler = new UpdatableSampler.Builder(serviceName)
                            .WithInitialSampler(BuildSampler(configuration))
                            .Build();

reporter = new UpdatableReporter.Builder(serviceName)
                            .WithInitialReporter(BuildReporter(configuration))
                            .Build();

var tracer = new Tracer.Builder(serviceName)
                       .WithSampler(sampler)
                       .WithReporter(reporter)
                       .Build();

當配置發生改變時，呼叫sampler和reporter的更新方法：

        private void OnTracingConfigurationChanged(TracingConfiguration newConfiguration, TracingConfigurationChangedInfo changedInfo)
        {
            ...
                    ((UpdatableReporter)_reporter).UpdateReporter(BuildReporter(newConfiguration));
                    ((UpdatableSampler)_sampler).UpdateSampler(BuildSampler(newConfiguration));
            ...
        }

這裡就不寫如何監聽配置的改變了，使用Timer或者阻塞查詢等等都可以。

後記

opentracing-contrib這個專案只支援.net core，如果想用在.net framwork中還需要自己搞，這個方法會單獨寫一篇文章，這裡就不做介紹了。

&n