实现.Net程序中OpenTracing采样和上报配置的自动更新

前言

OpenTracing是一个链路跟踪的开放协议，已经有开源的.net实现：opentracing-csharp，同时支持.net framework和.net core，Github地址：https://github.com/opentracing/opentracing-csharp。

这个库支持多种链路跟踪模式，不过仅提供了最基础的功能，想用在实际项目中还需要做很多增强，还好也有人做了开源项目：opentracing-contrib，Github地址：https://github.com/opentracing-contrib/csharp-netcore。

opentracing-contrib中集成了一个名为Jaeger的类库，这个库实现了链路跟踪数据的采样和上报，支持将数据上传到Jaeger进行分析统计。

为了同时保障性能和跟踪关键数据，能够远程调整采样率是很重要的，Jaeger本身也提供了远程配置采样率的支持。

不过我这里用的阿里云链路跟踪不支持，配置的设计也和想要的不同，所以自己做了一个采样和上报配置的动态更新，也才有了这篇文章。

思路

使用Jaeger初始化Tracer大概是这样的：

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

首先是提供当前服务的名字，然后需要提供一个采样器，再提供一个上报器，Build下生成ITracer的一个实例，最后注册到全局。

可以分析得出，采样和上报配置的更新就是更新采样器和上报器。

不过Tracer并没有提供UpdateSampler和UdapteReporter的方法，被卡住了，怎么办呢？

前文提到Jaeger是支持采样率的动态调整的，看看它怎么做的：

        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;
                    ...
                }
            }
        }

这里只留下关键代码，可以看到核心就是：通过一个Timer定时获取采样策略，然后替换原来的Sampler。

这是一个很好理解的办法，下边就按照这个思路来搞。

方案

分别提供一个可更新的Sampler和可更新的Reporter，Build Tracer时使用这两个可更新的类。这里延续开源项目中Samper和Reporter的创建方式，给出这两个类。

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);
            }
        }
    }

注意这里边用到了读写锁，因为要做到不停止服务的更新，而且大部分情况下都是读，使用lock就有点大柴小用了。

现在初始化Tracer大概是这样的：

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();

当配置发生改变时，调用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));
            ...
        }

这里就不写如何监听配置的改变了，使用Timer或者阻塞查询等等都可以。

后记

opentracing-contrib这个项目只支持.net core，如果想用在.net framwork中还需要自己搞，这个方法会单独写一篇文章，这里就不做介绍了。