The Correlation library allows web applications to correlate events through several tires communicating over HTTP(s). It allows to:

• extract correlation-ids from incoming request
• inject correlation-ids to outgoing request
• keep correlation context though request lifecycle

Library provides functionality to works with three identifiers

• correlation id common identifier of the whole operation (or flow) potentially involving multiple HTTP calls. By default is extracted from x-ms-request-root-id header and generated if not found.
• request id: identifier of this particular request. By default is extracted from x-ms-request-id, ASP.NET TraceIdentifier is used if not set.
• child request id identifies outgoing request id, passed to downstream service in x-ms-request-id and becomes request-id there. Generated by the library.

Notes: Use CorrelationHeaderInfo to change header names

• Microsoft.Diagnostics.Correlation.AspNetCore Supported on NET 4.X. and .NET Core. This is standalone package which should be used for ASP.NET Core apps.
• Microsoft.Diagnostics.Correlation.AspNet Supported on .NET 4.X. Provides ActionFilters for MVC and WebApi on top of Correlation package (below).
• Microsoft.Diagnostics.Correlation Supported on .NET 4.X This package alone should be used for non ASP.NET apps or ASP.NET apps hosted with OWIN self-host.

Notes:

• You may use Microsoft.Diagnostics.Mvc and/or Microsoft.Diagnostics.WebApi packages instead of Microsoft.Diagnostics.Correlation.AspNet
• Microsoft.Diagnostics.Instrumentation inplements profiler instrumentation. Currently does not ship as nuget package.

• Microsoft.Diagnostics.Correlation.Common.Owin.CorrelationContextTracingMiddleware Middleware extracts correlation-id (string) and request-id (string) from HTTP request headers and stores them in CorrelationContext
• Microsoft.Diagnostics.Correlation.Common.Owin.ContextTracingMiddleware<TContext> Generic middleware allowing to specify IContextFactory to generate any context from the request headers. Note: In OWIN self-hosted applications, middleware provides all the necessary capabilities to parse incoming request and store it's context. It may be substituted with MVC or WebApi ActionFilter provided by the suite, however it would mean that other Owin middleware(s) used by application, e.g. for logging, will not be able to use advantages of the correlation context. We strongly recommend to have CorrelationContextTracingMiddleware or ContextTracingMiddleware the first middleware invoked.

Due to ASP.NET thread agility model, context filled by the OWIN middleware will not be retained, so the suit provides ActionFilter for WebApi and MVC to set the context or restore the lost one. So, application can still use middleware to parse request and log it, however ActionFilter should also be configured to use correlation context in application.

• Microsoft.Diagnostics.Correlation.WebApi.CorrelationTracingFilter ActionFilter for WebApi controllers
• Microsoft.Diagnostics.Correlation.Mvc.CorrelationTracingFilter ActionFilter for MVC controllers

We recommend to add above filters (as appropriate) to the global filters collection rather than adding attribute to every controller.

Note: Please note, filters work only with CorrelationContext.

Correlation listens to diagnostics events from ASP.NET Core. Use app.UseCorrelationInstrumentation method to enable it.

When one service calls another service in the multi-tier application, the suite allows to inject header to outgoing request. The suite provides several approaches for it:

Library provides: Microsoft.Diagnostics.Correlation.Common.Http.CorrelationContextRequestHandler DelegatingHandler which could be integrated in existing pipeline. Developers also could use builders provided by the library to construct HttpClient with CorrelationContextRequestHandler.

HttpClientBuilder and CorrelationHttpClientBuilder allow to provide inner handler along with other delegating handlers to build custom pipeline, however ContextRequestHandler<TContext> will be called first.

• Microsoft.Diagnostics.Correlation.Http.CorrelationHttpClientBuilder provides builder for CorrelationContext
• Microsoft.Diagnostics.Correlation.Http.HttpClientBuilder provides generic way to specify how context should be injected in the request

Note:

• CorrelationContextRequestHandler only works with CorrelationContext. Use ContextRequestHandler<TContext> for other context types.
• We recommend creating HttpClient for every dependency you have and use dependency injection framework to make HttpClient instances available to controllers and retain them though the application lifecycle

It's important to log every outgoing request. Usually, for HttpClient it's done with DelegatingHandler, however library does not provide such handler and expect users to add it to the pipeline. Outgoing request includes child request id passed as header for the downstream service, so when the request is logged, child request id should also be logged as a part of the context. However, CorrelationContext is tight to incoming request rather than to outgoing request and there could be multiple outgoing requests having different child request ids. To get the child context, use one of 2 APIs:

• CorrelationContext.GetChildRequestContext. Please note, it will clone 'parent' context
• Log 'parent' context and child request id separately with extension method HttpRequestMessage.GetChildRequestId()

Notes: Child request id is stored in the request, so as long as you have reference to HttpRequestMessage, you may properly log it.

HttpClient is instrumented in .NET Core with DiagnosticSource. It allows to intercept HttpClient calls and instrument outgoing requests. You may enable it with: app.UseCorrelationInstrumentation(Configuration.GetSection("Correlation")), making sure InstrumentOutgoingRequests is set to false Note: You may pass your own IContextFactory and list of IContextInjector by using low level Microsoft.Diagnostics.Correlation.AspNetCore.ContextTracingInstrumentation.Enable<TContext> method

Note: Make sure your implementations of IContextFactory and IContextInjector do not have complex logic, or do any time-consuming operations. They will be called for any outgoing and incoming request and therefore may have heavy performance impact.

Unlike HttpClient handler pipeline, DiagnosticListener instrumentation does not allow to hook into it, so the library notifies about outgoing request events with IOutgoingRequestNotifier Context received in IOutgoingRequestNotifier already contains child request id You may implement IOutgoingRequestNotifier and register it in application services.

Library provides capability to instrument WebRequest.BeginGetResponse (which would be called for HttpClient and WebRequest methods). You may enable it with

• Microsoft.Diagnostics.Correlation.Instrumentation.ContextTracingInstrumentation.Enable
• Microsoft.Diagnostics.Correlation.Instrumentation.ContextTracingInstrumentation.Enable<TContext> - allows to specify IContextFactory to generate any context from the request and collection of IContextInjector to inject correlation context to outgoing request. Profiler requires some configuration to be done with application: agent should be installed on the host and app should run with some environment variables.

1. For Azure Web Applications, just install ApplicationInsights Extension on the portal
2. For CloudServices, please follow AI help: https://azure.microsoft.com/en-us/documentation/articles/app-insights-cloudservices/

Note: To use profiler together with ApplicationInsights, please diable runtime instrumentation in the applicationInsights.config:

     <DisableRuntimeInstrumentation>true</DisableRuntimeInstrumentation>
</Add>```

#### Logging outgoing requests
Unlike ```HttpClient``` handler pipeline, Profiler instrumentation does not allow to hook into it, so the library notifies about outgoing request events with ```IOutgoingRequestNotifier``` 
Context received in ```IOutgoingRequestNotifier```  already contains ```child request id``` 
You may configure ```IOutgoingRequestNotifier``` in ```Configuration``` class and pass to ```Enable``` methods.

## Accessing context
```CorrelationContext``` or other custom context implemented by developer may be accessed while request is still being processed with ```ContextResolver``` class
```ContextResolver``` is based on [```AsyncLocal```](https://msdn.microsoft.com/en-us/library/dn906268%28v=vs.110%29.aspx) or [```CallContext```](https://msdn.microsoft.com/en-us/library/system.runtime.remoting.messaging.callcontext%28v=vs.110%29.aspx) depending on the target framework.
> **Note:**
Context is stored as ```object```

Correlation library provides implementation of ```CorrelationContext``` and several ```IContextFactory```  and ```IContextInjector``` which work with it.
```CorrelationContext``` is dictionary, containing correlation-id and request-id and any other custom field which could be added or removed.


## Samples
### ASP.NET Core
You can define correlation configuration in the settings file or construct `AspNetCoreCorrelationConfiguration`.
Sample configuration:

"Correlation" : { "InstrumentOutgoingRequests" : true, "Headers" : { "CorrelationIdHeaderName" : "x-custom-correlation-id", "RequestIdHeaderName" : "x-custom-request-id", }, "EndpointFilter" : { "Allow" : false, "Endpoints" : ["core.windows.net", "dc.services.visualstudio.com"] } }

All parameters are optional.
- `InstrumentOutgoingRequests` - enables/disables outgoing requests instrumentation, true by default,
- `Headers` - allows to set correlaiton header names. Default values are `x-ms-request-root-id` (correlation-id) and `x-ms-request-id` (request id)
- `EndpointFilter` - filter for outgoing request `Uri`, allows to control which doutgoing requests are instrumented. By default rejects instrumentation for Azure Storage and ApplicationInsights endpoints,


#### HttpClient handler
In ```Startup.cs```

>- Create necessary ```HttpClient``` instances and register them:

services.AddSingleton(CorrelationHttpClientBuilder.CreateClient());

>- Enable correlation support:

app.UseCorrelationInstrumentation(Configuration.GetSection("Correlation"));

#### DiagnosticSource Instrumentation
>- Register `IOutgoingRequestNotifier` to receive outgoing requests notifications:

services.AddRequestNotifier(new OutgoingRequestNotifier());

>- Enable correlation support:

app.UseCorrelationInstrumentation(Configuration.GetSection("Correlation"));

### ASP.NET
You may use OWIN correlation middleware or HttpModule implementation to handle incoming requests,
however MVC or WebAPI filter should be configured in ```Global.asax```
##### **ActionFilters**
>- **MVC**:

GlobalFilters.Filters.Add(new Microsoft.Diagnostics.Correlation.Mvc.CorrelationTracingFilter()); FilterConfig.RegisterGlobalFilters(GlobalFilters.Filters);

>- **WebAPI**

GlobalConfiguration.Configuration.Filters.Add(new Microsoft.Diagnostics.Correlation.WebApi.CorrelationTracingFilter());

##### **HttpModule**
Add ```CorrelationTracingHttpModule```