README.md

Service Level Indicators

ServiceLevelIndicators is a .NET library for emitting service-level latency metrics in milliseconds using the standard System.Diagnostics.Metrics and OpenTelemetry pipeline.

It is designed for teams that need more than generic request timing. The library helps measure meaningful operations, attach service-specific dimensions such as customer, location, operation name, and status, and build SLO or SLA-oriented dashboards and alerts from those metrics.

Service level indicators (SLIs) are metrics used to track how a service is performing against expected reliability and responsiveness goals. Common examples include availability, response time, throughput, and error rate. This library focuses on latency SLIs so you can consistently measure operation duration across background work, ASP.NET Core APIs, and versioned endpoints.

Service Level Indicator Library

Trellis.ServiceLevelIndicators emits operation latency metrics in milliseconds so service owners can monitor performance over time using dimensions that matter to their system. 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 - The target resource of the operation — the noun in the URL path being read or modified, normalized to a stable identifier (tenant, subscription, account, work item). NOT the caller, NOT a per-request GUID, NOT a user ID or email. Example: for GET /teams/{teamId} called by user xa1 for team team1, the value is "team1", not "xa1". See the ASP.NET Core package README for the full mental model.
• 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.

Trellis.ServiceLevelIndicators.Asp adds the following dimensions.

• Operation - For ASP.NET endpoints, the operation name is the HTTP method plus the route template, resolved in this order: (1) [ServiceLevelIndicator(Operation = "...")] attribute or .AddServiceLevelIndicator("op") override, (2) MVC AttributeRouteInfo.Template, (3) the endpoint's RouteEndpoint.RoutePattern.RawText (Minimal APIs / conventional routing). Route placeholders such as {id} are preserved, never substituted with the concrete request value. If no bounded template is available, the middleware emits the sentinel "<METHOD> <unrouted>" and logs a warning — see that value in your metrics as a signal to add a route template.
• 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

This makes the library useful when generic HTTP server metrics are not enough, especially for multi-tenant services, APIs with customer-specific objectives, or workloads that need the same SLI model outside HTTP request handling.

Trellis.ServiceLevelIndicators.Asp.ApiVersioning adds the following dimensions.

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

NuGet Packages

• Trellis.ServiceLevelIndicators

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

  

• Trellis.ServiceLevelIndicators.Asp

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

  

• Trellis.ServiceLevelIndicators.Asp.ApiVersioning

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

  

Installation

dotnet add package Trellis.ServiceLevelIndicators

For a concise package-selection and integration guide, see docs/usage-reference.md.

API references:

• docs/api_reference/trellis-api-sli.md — Trellis.ServiceLevelIndicators (core)
• docs/api_reference/trellis-api-sli-asp.md — Trellis.ServiceLevelIndicators.Asp (middleware + attributes)
• docs/api_reference/trellis-api-sli-apiversioning.md — Trellis.ServiceLevelIndicators.Asp.ApiVersioning

For ASP.NET Core:

dotnet add package Trellis.ServiceLevelIndicators.Asp

For API Versioning support:

dotnet add package Trellis.ServiceLevelIndicators.Asp.ApiVersioning

Usage for ASP.NET Core MVC

1. Register SLI with open telemetry by calling AddServiceLevelIndicatorInstrumentation.

   Example:

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

   If you configure ServiceLevelIndicatorOptions.Meter with a custom meter, register that same meter with OpenTelemetry:

   var sliMeter = new Meter("MyCompany.ServiceLevelIndicator");
   
   builder.Services.AddOpenTelemetry()
       .ConfigureResource(configureResource)
       .WithMetrics(metrics =>
       {
           metrics.AddServiceLevelIndicatorInstrumentation(sliMeter);
           metrics.AddOtlpExporter();
       });
   
   builder.Services.AddServiceLevelIndicator(options =>
   {
       options.Meter = sliMeter;
       options.LocationId = ServiceLevelIndicator.CreateLocationId("public", AzureLocation.WestUS3.Name);
   });

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 APIs

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);
}

Operational Guidance

Cardinality Guidance

All three required tags — Operation, LocationId, and CustomerResourceId — must be low-cardinality and bounded. The library bounds Operation for you via the route-template resolver and the <unrouted> sentinel; you are responsible for LocationId (set once from configuration) and CustomerResourceId (stable tenant / subscription / resource identifier).

The same discipline applies to [Measure] parameters and any custom attributes added via AddAttribute(...). Avoid email addresses, request IDs, timestamps, or unconstrained free text unless your metrics backend is explicitly designed for high-cardinality telemetry.

Disposal

ServiceLevelIndicator is a sealed IDisposable registered as a singleton; the DI container disposes it (and the Meter it created) at host shutdown — no manual cleanup needed. A Meter you supply via ServiceLevelIndicatorOptions.Meter is owned by you and is never disposed by SLI.

ASP.NET Core Customizations

Once the Prerequisites are done, all controllers will emit SLI information. The default operation name is the HTTP method plus the route template (placeholders such as {id} are preserved). The full resolution order is described under Trellis.ServiceLevelIndicators.Asp above.

• To add API versioning as a dimension use package Trellis.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 =>
  {
      // Pull a STABLE tenant/subscription identifier — NOT the caller's UPN/user ID.
      var tenantId = context.HttpContext.User.Claims
          .FirstOrDefault(c => c.Type == "tid")?.Value ?? "unknown";
      context.SetCustomerResourceId(tenantId);
  
      // Caller identity belongs in a separate (still bounded) dimension, not in CustomerResourceId.
      var tier = context.HttpContext.User.Claims
          .FirstOrDefault(c => c.Type == "tier")?.Value ?? "free";
      context.AddAttribute("CallerTier", tier);
  });

• 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,

  builder.Services.AddServiceLevelIndicator(options =>
  {
      options.AutomaticallyEmitted = false;
  })
  .AddMvc();

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

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.