prometheus-net

This is a .NET library for instrumenting your applications and exporting metrics to Prometheus.

The library targets .NET Standard 2.0 which supports the following runtimes (and newer):

.NET Framework 4.6.1
.NET Core 2.0
Mono 5.4

The ASP.NET Core specific functionality requires ASP.NET Core 2.1 or newer.

Best practices and usage

This library allows you to instrument your code with custom metrics and provides some built-in metric collection integrations for ASP.NET Core.

The documentation here is only a minimal quick start. For detailed guidance on using Prometheus in your solutions, refer to the prometheus-users discussion group. You are also expected to be familiar with the Prometheus user guide.

Four types of metrics are available: Counter, Gauge, Summary and Histogram. See the documentation on metric types and instrumentation best practices to learn what each is good for.

The Metrics class is the main entry point to the API of this library. The most common practice in C# code is to have a static readonly field for each metric that you wish to export from a given class.

More complex patterns may also be used (e.g. combining with dependency injection). The library is quite tolerant of different usage models - if the API allows it, it will generally work fine and provide satisfactory performance. The library is thread-safe.

Installation

Nuget package for general use and metrics export via HttpListener or to Pushgateway: prometheus-net

Install-Package prometheus-net

Nuget package for ASP.NET Core middleware and stand-alone Kestrel metrics server: prometheus-net.AspNetCore

Install-Package prometheus-net.AspNetCore

Counters

Counters only increase in value and reset to zero when the process restarts.

private static readonly Counter ProcessedJobCount = Metrics
	.CreateCounter("myapp_jobs_processed_total", "Number of processed jobs.");

...

ProcessJob();
ProcessedJobCount.Inc();

Gauges

Gauges can have any numeric value and change arbitrarily.

private static readonly Gauge JobsInQueue = Metrics
	.CreateGauge("myapp_jobs_queued", "Number of jobs waiting for processing in the queue.");

...

jobQueue.Enqueue(job);
JobsInQueue.Inc();

...

var job = jobQueue.Dequeue();
JobsInQueue.Dec();

Summary

Summaries track the trends in events over time (10 minutes by default).

private static readonly Summary RequestSizeSummary = Metrics
	.CreateSummary("myapp_request_size_bytes", "Summary of request sizes (in bytes) over last 10 minutes.");

...

RequestSizeSummary.Observe(request.Length);

By default, only the sum and total count are reported. You may also specify quantiles to measure:

private static readonly Summary RequestSizeSummary = Metrics
	.CreateSummary("myapp_request_size_bytes", "Summary of request sizes (in bytes) over last 10 minutes.",
		new SummaryConfiguration
		{
			Objectives = new[]
			{
				new QuantileEpsilonPair(0.5, 0.05),
				new QuantileEpsilonPair(0.9, 0.05),
				new QuantileEpsilonPair(0.95, 0.01),
				new QuantileEpsilonPair(0.99, 0.01),
			}
		});

Histogram

Histograms track the size and number of events in buckets. This allows for aggregatable calculation of quantiles.

private static readonly Histogram OrderValueHistogram = Metrics
	.CreateHistogram("myapp_order_value_usd", "Histogram of received order values (in USD).",
		new HistogramConfiguration
		{
			// We divide measurements in 10 buckets of $100 each, up to $1000.
			Buckets = Histogram.LinearBuckets(start: 100, width: 100, count: 10)
		});

...

OrderValueHistogram.Observe(order.TotalValueUsd);

Measuring operation duration

Timers can be used to report the duration of an operation (in seconds) to a Summary, Histogram or Gauge. Wrap the operation you want to measure in a using block.

private static readonly Histogram LoginDuration = Metrics
	.CreateHistogram("myapp_login_duration_seconds", "Histogram of login call processing durations.");

...

using (LoginDuration.NewTimer())
{
    IdentityManager.AuthenticateUser(Request.Credentials);
}

Tracking in-progress operations

You can use Gauge.TrackInProgress() to track how many concurrent operations are taking place. Wrap the operation you want to track in a using block.

private static readonly Gauge DocumentImportsInProgress = Metrics
	.CreateGauge("myapp_document_imports_in_progress", "Number of import operations ongoing.");

...

using (DocumentImportsInProgress.TrackInProgress())
{
	DocumentRepository.ImportDocument(path);
}

Counting exceptions

You can use Counter.CountExceptions() to count the number of exceptions that occur while executing some code.

private static readonly Counter FailedDocumentImports = Metrics
	.CreateCounter("myapp_document_imports_failed_total", "Number of import operations that failed.");

...

FailedDocumentImports.CountExceptions(() => DocumentRepository.ImportDocument(path));

You can also filter the exception types to observe:

FailedDocumentImports.CountExceptions(() => DocumentRepository.ImportDocument(path), IsImportRelatedException);

bool IsImportRelatedException(Exception ex)
{
	// Do not count "access denied" exceptions - those are user error for pointing us to a forbidden file.
	if (ex is UnauthorizedAccessException)
		return false;

	return true;
}

Labels

All metrics can have labels, allowing grouping of related time series.

See the best practices on naming and labels.

Taking a counter as an example:

private static readonly Counter RequestCountByMethod = Metrics
	.CreateCounter("myapp_requests_total", "Number of requests received, by HTTP method.",
		new CounterConfiguration
		{
			// Here you specify only the names of the labels.
			LabelNames = new[] { "method" }
		});

...

// You can specify the values for the labels later, once you know the right values (e.g in your request handler code).
counter.WithLabels("GET").Inc();

NB! Best practices of metric design is to minimize the number of different label values. HTTP request method is good - there are not many values. However, URL would be a bad choice for labeling - it has too many possible values and would lead to significant data processing inefficiency. Try to minimize the possible number of label values in your metric model.

When are metrics published?

Metrics without labels are published immediately after the Metrics.CreateX() call. Metrics that use labels are published when you provide the label values for the first time.

Sometimes you want to delay publishing a metric until you have loaded some data and have a meaningful value to supply for it. The API allows you to suppress publishing of the initial value until you decide the time is right.

private static readonly Gauge UsersLoggedIn = Metrics
	.CreateGauge("myapp_users_logged_in", "Number of active user sessions",
		new GaugeConfiguration
		{
			SuppressInitialValue = true
		});

...

// After setting the value for the first time, the metric becomes published.
UsersLoggedIn.Set(LoadSessions().Count);

You can also use .Publish() on a metric to mark it as ready to be published without modifying the initial value (e.g. to publish a zero).

ASP.NET Core exporter middleware

For projects built with ASP.NET Core, a middleware plugin is provided.

If you use the default Visual Studio project template, modify Startup.cs as follows:

public void Configure(IApplicationBuilder app, IHostingEnvironment env)
{
    // ...

    app.UseMetricServer();

    app.Run(async (context) =>
    {
        // ...
    });
}

Alternatively, if you use a custom project startup cycle, you can add this directly to the WebHostBuilder instance:

WebHost.CreateDefaultBuilder()
	.Configure(app => app.UseMetricServer())
	.Build()
	.Run();

The default configuration will publish metrics on the /metrics URL.

This functionality is delivered in the prometheus-net.AspNetCore NuGet package.

ASP.NET Core HTTP request metrics

The library provides some metrics for ASP.NET Core applications:

Number of HTTP requests in progress.
Total number of received HTTP requests.
Duration of HTTP requests.

These metrics include labels for status code, HTTP method, ASP.NET Core Controller and ASP.NET Core Action.

You can register all of the metrics using the default labels and names as follows:

// In your Startup.cs Configure() method
app.UseHttpMetrics();

If you wish to provide a custom metric instance or disable certain metrics you can configure the HTTP metrics like this:

app.UseHttpMetrics(options =>
{
	options.RequestCount.Enabled = false;

	options.RequestDuration.Histogram = Metrics.CreateHistogram("myapp_http_request_duration_seconds", "Some help text",
		new HistogramConfiguration
		{
			Buckets = Histogram.LinearBuckets(start: 1, width: 1, count: 64)
			Labels = new[] { "code", "method" }
		});
});

The labels for the custom metric you provide must be a subset of the following:

"code" - Status Code
"method" - HTTP method
"controller" - ASP.NET Core Controller
"action" - ASP.NET Core Action

ASP.NET Core with basic authentication

You may wish to restrict access to the metrics export URL. This can be accomplished using any ASP.NET Core authentication mechanism, as prometheus-net integrates directly into the composable ASP.NET Core request processing pipeline.

For a simple example we can take BasicAuthMiddleware by Johan Boström which can be integrated by replacing the app.UseMetricServer() line with the following code block:

app.Map("/metrics", metricsApp =>
{
    metricsApp.UseMiddleware<BasicAuthMiddleware>("Contoso Corporation");

    // We already specified URL prefix in .Map() above, no need to specify it again here.
    metricsApp.UseMetricServer("");
});

Kestrel stand-alone server

In some situation, you may wish to start a stand-alone metric server using Kestrel (e.g. if your app has no other HTTP-accessible functionality).

var metricServer = new KestrelMetricServer(port: 1234);
metricServer.Start();

The default configuration will publish metrics on the /metrics URL.

Publishing to Pushgateway

Metrics can be posted to a Pushgateway server.

var metricServer = new MetricPusher(endpoint: "https://pushgateway.example.org:9091/metrics", job: "some_job");
metricServer.Start();

Publishing via standalone HTTP handler

As a fallback option for scenarios where Kestrel or ASP.NET Core hosting is unsuitable, an HttpListener based metrics server implementation is also available.

var metricServer = new MetricServer(port: 1234);
metricServer.Start();

The default configuration will publish metrics on the /metrics URL.

MetricServer.Start() may throw an access denied exception on Windows if your user does not have the right to open a web server on the specified port. You can use the netsh command to grant yourself the required permissions:

netsh http add urlacl url=http://+:1234/metrics user=DOMAIN\user

Just-in-time updates

In some scenarios you may want to only collect data when it is requested by Prometheus. To easily implement this scenario prometheus-net enables you to register a callback before every collection occurs. Register your callback using Metrics.DefaultRegistry.AddBeforeCollectCallback().

Note that all callbacks will be called synchronously before each collection. They should not take more than a few milliseconds in order to ensure that the scrape does not time out. Do not read data from remote systems in these callbacks.

Suppressing default metrics

The library provides some sample metrics about the current process out of the box, simply to ensure that some output is produced in a default configuration. If these metrics are not desirable you may remove them by calling Metrics.SuppressDefaultMetrics() before registering any of your own metrics.

Related projects

prometheus-net.DotNetRuntime instruments .NET Core 2.2 apps to export metrics on .NET Core performance.

cbloechle/prometheus-net

prometheus-net

Best practices and usage

Installation

Counters

Gauges

Summary

Histogram

Measuring operation duration

Tracking in-progress operations

Counting exceptions

Labels

When are metrics published?

ASP.NET Core exporter middleware

ASP.NET Core HTTP request metrics

ASP.NET Core with basic authentication

Kestrel stand-alone server

Publishing to Pushgateway

Publishing via standalone HTTP handler

Just-in-time updates

Suppressing default metrics

Related projects