prometheus-net

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namespace Prometheus;
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/// <summary>
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/// Base class for various metric server implementations that start an independent exporter in the background.
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/// The expoters may either be pull-based (exposing the Prometheus API) or push-based (actively pushing to PushGateway).
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/// </summary>
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public abstract class MetricHandler : IMetricServer, IDisposable
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{
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    // The token is cancelled when the handler is instructed to stop.
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    private CancellationTokenSource? _cts = new CancellationTokenSource();
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    // This is the task started for the purpose of exporting metrics.
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    private Task? _task;
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    protected MetricHandler()
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    {
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    }
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    public IMetricServer Start()
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    {
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        if (_task != null)
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            throw new InvalidOperationException("The metric server has already been started.");
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        if (_cts == null)
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            throw new InvalidOperationException("The metric server has already been started and stopped. Create a new server if you want to start it again.");
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        _task = StartServer(_cts.Token);
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        return this;
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    }
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    public async Task StopAsync()
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    {
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        // Signal the CTS to give a hint to the server thread that it is time to close up shop.
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        _cts?.Cancel();
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        try
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        {
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            if (_task == null)
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                return; // Never started.
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            // This will re-throw any exception that was caught on the StartServerAsync thread.
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            // Perhaps not ideal behavior but hey, if the implementation does not want this to happen
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            // it should have caught it itself in the background processing thread.
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            await _task.ConfigureAwait(false); // Issue #308
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        }
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        catch (OperationCanceledException)
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        {
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            // We'll eat this one, though, since it can easily get thrown by whatever checks the CancellationToken.
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        }
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        finally
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        {
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            _cts?.Dispose();
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            _cts = null;
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        }
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    }
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    public void Stop()
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    {
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        // This method mainly exists for API compatiblity with prometheus-net v1. But it works, so that's fine.
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        StopAsync().GetAwaiter().GetResult();
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    }
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    public void Dispose()
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    {
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        Stop();
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    }
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    protected abstract Task StartServer(CancellationToken cancel);
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}
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