Introduction

The purpose of this library is to help developers with dependency injection per HTTP request context. This is currently not available via the default dependency injection infrastructure. A secondary goal is provide easy access to common infrastructure services such as security, logging, etc.

DI Usage

In order to leverage this library, there are 3 steps. You would want to setup your DI, apply the ExecutionContextDependencyInjection attribute, and inject the ExecutionContext as a parameter in your function.

Step 1: Setup DI

The first step is to setup your DI via the Autofac Module.

using Autofac;

namespace FunctionApp1
{
    public class MyModuleConfig : Module
    {
        protected override void Load(ContainerBuilder builder)
        {
            builder.RegisterType<MyDomain>().As<IMyDomain>();
        }
    }
}

Step 2/3: Setup ExecutionContextDependencyInjection attribute on said function and inject ExecutionContext.

The second step is to apply the ExecutionContextDependencyInjection on your function and tell it which Module type you would like. Next, you can inject the ExecutionContext which internally carries the function instance Id.

public static class Function1
{
    [ExecutionContextDependencyInjection(typeof(MyModuleConfig))]
    [FunctionName("Function1")]
    public static async Task<IActionResult> Run(
        [HttpTrigger(AuthorizationLevel.Function, "get", "post", Route = null)] HttpRequest req,
        ILogger log,
        ExecutionContext executionContext)
    {

Dependency Resolution Usage:

Now, there are 2 extension methods you can use against the ExecutionContext. You can choose the Resolve method to resolve your dependency.

var myDomain = executionContext.Resolve<IMyDomain>();
myDomain.DoWork();

Or you can get the Resolver.

var resolver = executionContext.GetResolver();
var myDomain = resolver.Get<IMyDomain>();
myDomain.DoWork();

Azure Function Logger Usage:

Note that we have already captured ILogger as well as the actual HttpRequest. So, if you can inject IHttpRequestContext to access the logger or Http request which is hanging off IHttpRequestContext.

public class MyDomain : IMyDomain
{
    private readonly IHttpRequestContext _requestContext;

    public MyDomain(IHttpRequestContext requestContext)
    {
        _requestContext = requestContext;
    }

    public void DoWork()
    {
        _requestContext.Logger.LogInformation("FOOBAR");
    }
}

Exception Handling Usage:

To translate an exception-type to an HTTP response like Bad Request, you can implement the IExceptionHandler which will provide a way for the infrastructure code to recognize an exception type and return the correct HTTP response.

public class MyArgumentExceptionHandler : IExceptionHandler
{
    public ExceptionHandlerResult Handle(Exception ex)
    {
        if (ex.GetType() == typeof(ArgumentException))
        {
            return new ExceptionHandlerResult(true, new BadRequestObjectResult(new ErrorMessage { Message = ex.Message }));
        }

        return null;
    }
}

Remember to register the handler in your Module. You can register more than one.

builder.RegisterType<MyArgumentExceptionHandler>().As<IExceptionHandler>();
builder.RegisterType<MyCustomExceptionHandler>().As<IExceptionHandler>();

To leverage the handlers, you will need to use the extension method Run.

[ExecutionContextDependencyInjection(typeof(MyModule))]
[FunctionName("Function3")]
public static async Task<IActionResult> Run3(
    [HttpTrigger(AuthorizationLevel.Anonymous, "post", Route = null)]
    HttpRequest req, ILogger log, ExecutionContext executionContext)
{
    // Example of how we can directly resolve a dependency.
    return await executionContext.Run<IDtoDomain, DtoResponse>(domain => Task.FromResult(domain.DoWork()));
}

Azure AD integration Usage:

We can access user context from the Azure AD integration. Inject IHttpRequestContext into your domain and access the Security property.

See the following link for more information on these special headers: https://docs.microsoft.com/en-us/azure/app-service/app-service-authentication-how-to#access-user-claims

_httpRequestContext.Security.Principal.Name
...
_httpRequestContext.Security.Principal.Id

Microsoft.Extensions.Configuration.IConfiguration Usage:

We can leverage IConfiguration directly to get settings stored locally in local.settings.json or get settings stored in Azure Web App's Application settings.

Here's an example of using IConfiguration to determine if your Azure Function is running locally by using a common configuration value stored in local.settings.json.

using Microsoft.Extensions.Configuration;

namespace Eklee.Azure.Functions.Http.Example
{
    public class ConfigDomain : IConfigDomain
    {
        private readonly IConfiguration _configuration;

        public ConfigDomain(IConfiguration configuration)
        {
            _configuration = configuration;
        }

        public bool IsLocalEnvironment()
        {
            var value = _configuration.GetValue<string>("AzureWebJobsStorage");
            return value == "UseDevelopmentStorage=true";
        }
    }
}

Microsoft.Extensions.Logging.ILogger Usage:

We can leverage ILogger directly to perform logging activities.

Here's an example of using ILogger to log an info message.

using Microsoft.Extensions.Logging;

namespace Eklee.Azure.Functions.Http.Example
{
    public class MyLogDomain : IMyLogDomain
    {
        private readonly ILogger _logger;

        public MyLogDomain(ILogger logger)
        {
            _logger = logger;
        }

        public void DoWork()
        {
            _logger.LogInformation("MyLogDomain DoWork invoked.");
        }
    }
}

ICacheManager Usage:

We can leverage ICacheManager to perform caching work. First, choose your preferred DistributedCache as you are setting up your Module. Note that MemoryDistributedCache is just an example. In a production senario, you may choose something like Azure Redis.

builder.UseDistributedCache<MemoryDistributedCache>();

Next, you may inject ICacheManager for usage. Here's an example of using ICacheManager to cache a value from repository for a duration of time (5 seconds in the example below) if it does not already exist. CacheResult is returned where we can determine if the result is from Cache or from the repository query.

public DomainWithCache(ICacheManager cacheManager, IRepository repository)
{
    _cacheManager = cacheManager;
    _repository = repository;
    ...
}

public async Task<CacheResult<KeyValueDto>> GetAsync(string key)
{
    return await _cacheManager.TryGetOrSetIfNotExistAsync(() => _repository.Single(x => x.Key == key), key,
        new DistributedCacheEntryOptions
        {
            AbsoluteExpiration = DateTimeOffset.UtcNow.AddSeconds(5)
        });
}