Service Level Indicators

Service level indicators (SLIs) are metrics used to measure the performance of a service. They are typically used in the context of service level agreements (SLAs), which are contracts between a service provider and its customers that define the expected level of service. SLIs are used to track the actual performance of the service against the agreed upon SLA.

There are many different types of SLIs that can be used to measure the performance of a service. Some common examples include:

• Availability: This measures the percentage of time that a service is available and functioning properly.
• Response time: This measures the amount of time it takes for a service to respond to a request.
• Throughput: This measures the amount of work that a service can handle in a given period of time.
• Error rate: This measures the percentage of requests that result in errors.

SLIs are important because they provide a way to objectively measure the performance of a service. By tracking SLIs over time, service providers can identify trends and make improvements to the service to ensure that it meets the needs of its customers.

Service Level Indicator Library

ServiceLevelIndicators library will help emit latency metrics for each API operation to help monitor the service performance over time. The metrics are emitted via the standard .NET Meter Class.

By default, a meter named ServiceLevelIndicator with instrument name operation.duration is added to the service metrics. The metrics are emitted with the following attributes.

• CustomerResourceId - A value that helps identity the customer, customer group or calling service.
• LocationId - The location where the service running. eg. Public cloud, West US 3 region. Azure Core
• Operation - The name of the operation.
• activity.status.code - The activity status code is set based on the success or failure of the operation. ActivityStatusCode.

ServiceLevelIndicators.Asp adds the following dimensions.

• Operation - In ASP.NET the operation name defaults to AttributeRouteInfo.Template information like GET Weatherforecast.
• The activity status code will be "Ok" when the http response status code is in the 2xx range, "Error" when the http response status code is in the 5xx range, "Unset" for any other status code.
• http.response.status.code - The http status code.
• http.request.method (Optional)- The http request method (GET, POST, etc) is added.

Difference between ServiceLevelIndicator and http.server.request.duration

	ServiceLevelIndicator	http.server.request.duration
Resolution	milliseconds	seconds
Customer	CustomerResourceId	N/A
Error check	Activity or HTTP status.code	HTTP status code

ServiceLevelIndicators.Asp.Versioning adds the following dimensions.

• http.api.version - The API Version when used in conjunction with API Versioning package.

NuGet Packages

• ServiceLevelIndicators

  This library can be used to emit SLI for all .net core applications, where each operation is measured.

  

• ServiceLevelIndicators.Asp

  For measuring SLI for ASP.NET Core applications use this library that will automatically measure each API operation.

  

• ServiceLevelIndicators.Asp.ApiVersioning

  If API Versioning package is used, this library will add the API version as a metric dimension.

  

Installation

dotnet add package ServiceLevelIndicators

For ASP.NET Core:

dotnet add package ServiceLevelIndicators.Asp

For API Versioning support:

dotnet add package ServiceLevelIndicators.Asp.ApiVersioning

Usage for Web API MVC

1. Register SLI with open telemetry by calling AddServiceLevelIndicatorInstrumentation.

   Example:

   builder.Services.AddOpenTelemetry()
       .ConfigureResource(configureResource)
       .WithMetrics(builder =>
       {
           builder.AddServiceLevelIndicatorInstrumentation();
           builder.AddOtlpExporter();
       });

2. Add ServiceLevelIndicator into the dependency injection. AddMvc() is required for overrides present in SLI attributes to take effect.

   Example:

   builder.Services.AddServiceLevelIndicator(options =>
   {
       options.LocationId = ServiceLevelIndicator.CreateLocationId("public", AzureLocation.WestUS3.Name);
   })
   .AddMvc();

3. Add the middleware to the pipeline.

   app.UseServiceLevelIndicator();

Usage for Minimal API

1. Register SLI with open telemetry by calling AddServiceLevelIndicatorInstrumentation.

   Example:

   builder.Services.AddOpenTelemetry()
       .ConfigureResource(configureResource)
       .WithMetrics(builder =>
       {
           builder.AddServiceLevelIndicatorInstrumentation();
           builder.AddOtlpExporter();
       });

2. Add ServiceLevelIndicator into the dependency injection.

   Example:

   builder.Services.AddServiceLevelIndicator(options =>
   {
       options.LocationId = ServiceLevelIndicator.CreateLocationId("public", AzureLocation.WestUS3.Name);
   });

3. Add the middleware to the ASP.NET Core pipeline.

   Example:

   app.UseServiceLevelIndicator();

4. To each API route mapping, add AddServiceLevelIndicator().

   Example:

   app.MapGet("/hello", () => "Hello World!")
      .AddServiceLevelIndicator();

Usage for background jobs

You can measure a block of code by wrapping it in a using clause of MeasuredOperation.

Example:

async Task MeasureCodeBlock(ServiceLevelIndicator serviceLevelIndicator)
{
    using var measuredOperation = serviceLevelIndicator.StartMeasuring("OperationName");
    // Do Work.
    measuredOperation.SetActivityStatusCode(System.Diagnostics.ActivityStatusCode.Ok);
}

Customizations

Once the Prerequisites are done, all controllers will emit SLI information. The default operation name is in the format <HTTP Method> <Controller>/<Action>. eg GET WeatherForecast/Action1

• To add API versioning as a dimension use package ServiceLevelIndicators.Asp.ApiVersioning and enrich the metrics with AddApiVersion.

  Example:

  builder.Services.AddServiceLevelIndicator(options =>
  {
      /// Options
  })
  .AddMvc()
  .AddApiVersion();

• To add HTTP method as a dimension, add AddHttpMethod to Service Level Indicator.

  Example:

  builder.Services.AddServiceLevelIndicator(options =>
  {
      /// Options
  })
  .AddMvc()
  .AddHttpMethod();

• Enrich SLI with the Enrich callback. The callback receives a MeasuredOperation as context that can be used to set CustomerResourceId or additional attributes. An async version EnrichAsync is also available.

  Example:

  builder.Services.AddServiceLevelIndicator(options =>
  {
      options.LocationId = ServiceLevelIndicator.CreateLocationId(Cloud, Region);
  })
  .AddMvc()
  .Enrich(context =>
  {
      var upn = context.HttpContext.User.Claims
          .FirstOrDefault(c => c.Type == "upn")?.Value ?? "Unknown";
      context.SetCustomerResourceId(upn); // Set CustomerResourceId
      context.AddAttribute("UserPrincipalName", upn); // Add custom attribute
  });

• To override the default operation name, add the attribute [ServiceLevelIndicator] and specify the operation name.

  Example:

  [HttpGet("MyAction2")]
  [ServiceLevelIndicator(Operation = "MyNewOperationName")]
  public IEnumerable<WeatherForecast> GetOperation() => GetWeather();

• To set the CustomerResourceId within an API method, mark the parameter with the attribute [CustomerResourceId]

  [HttpGet("get-by-zip-code/{zipCode}")]
  public IEnumerable<WeatherForecast> GetByZipcode([CustomerResourceId] string zipCode)
     => GetWeather();

  Or use GetMeasuredOperation extension method.

  [HttpGet("{customerResourceId}")]
  public IEnumerable<WeatherForecast> Get(string customerResourceId)
  {
      HttpContext.GetMeasuredOperation().CustomerResourceId = customerResourceId;
      return GetWeather();
  }

• To add custom Open Telemetry attributes.

  HttpContext.GetMeasuredOperation().AddAttribute(attribute, value);

  GetMeasuredOperation will throw if the route is not configured to emit SLI.

  When used in a middleware or scenarios where a route may not be configured to emit SLI.

  if (HttpContext.TryGetMeasuredOperation(out var measuredOperation))
      measuredOperation.AddAttribute("CustomAttribute", value);

  You can add additional dimensions to the SLI data by using the Measure attribute. Parameters decorated with [Measure] are automatically added as metric attributes (dimensions) using the parameter name as the attribute key.

  [HttpGet("name/{first}/{surname}")]
  public IActionResult GetCustomerResourceId(
      [Measure] string first,
      [CustomerResourceId] string surname)
        => Ok(first + " " + surname);

• To prevent automatically emitting SLI information on all controllers, set the option,

  ServiceLevelIndicatorOptions.AutomaticallyEmitted = false;

  In this case, add the attribute [ServiceLevelIndicator] on the controllers that should emit SLI.

• To measure a process, run it within a using StartMeasuring block.

  Example:

  public void StoreItem(MyDomainEvent domainEvent)
  {
      var attribute = new KeyValuePair<string, object?>("Event", domainEvent.GetType().Name);
      using var measuredOperation = _serviceLevelIndicator.StartMeasuring("StoreItem", attribute);
      DoTheWork();
  }

Sample

Try out the sample weather forecast Web API.

To view the metrics locally using the .NET Aspire Dashboard:

1. Start the Aspire dashboard:

   docker run --rm -it -d -p 18888:18888 -p 4317:18889 -e DOTNET_DASHBOARD_UNSECURED_ALLOW_ANONYMOUS=true -e DASHBOARD__OTLP__AUTHMODE=Unsecured --name aspire-dashboard mcr.microsoft.com/dotnet/aspire-dashboard:latest
   

2. Run the sample web API project and call the GET WeatherForecast using the Open API UI.
3. Open http://localhost:18888 to view the dashboard. You should see the SLI metrics under the instrument operation.duration where Operation = "GET WeatherForecast", http.response.status.code = 200, LocationId = "ms-loc://az/public/westus2", activity.status.code = Ok.
4. If you run the sample with API Versioning, you will see something similar to the following.