As I mentioned in my earlier post, there are 2 options available to you out of the box for logging. You can either use the `TraceWriter` or the `ILogger`. While this is fine when you are doing some small projects or Functions, it can become a problem if you want your Azure Functions to reuse earlier developed logic or modules used in different projects, a Web API for example.

In these shared class libraries you are probably leveraging the power of a ‘full-blown’ logging library. While it is possible to wire up a secondary logging instance in your Azure Function, it’s better to use something which is already available to you, like the `ILogger` or the `TraceWriter`.

I’m a big fan of the log4net logging library, so this post is about using log4net with Azure Functions. As it goes, you can apply the same principle for any other logging framework just the implementation will be a bit different.

Creating an appender

One way to extend the logging capabilities of log4net is by creating your own logging appender. You are probably already using some default file appender or console appender in your projects. Because there isn’t an out-of-the-box appender for the `ILogger`, yet, you have to create one yourself.

Creating an appender isn’t very hard. Make sure you have log4net added to your project and create a new class which derives from `AppenderSkeleton`. Having done so you are notified the `Append`-method should be implemented, which makes sense. The most basic implementation of an appender which is using the `ILogger` looks pretty much like the following.

internal class FunctionLoggerAppender : AppenderSkeleton
{
    private readonly ILogger logger;

    public FunctionLoggerAppender(ILogger logger)
    {
        this.logger = logger;
    }
    protected override void Append(LoggingEvent loggingEvent)
    {
        switch (loggingEvent.Level.Name)
        {
            case "DEBUG":
                this.logger.LogDebug($"{loggingEvent.LoggerName} - {loggingEvent.RenderedMessage}");
                break;
            case "INFO":
                this.logger.LogInformation($"{loggingEvent.LoggerName} - {loggingEvent.RenderedMessage}");
                break;
            case "WARN":
                this.logger.LogWarning($"{loggingEvent.LoggerName} - {loggingEvent.RenderedMessage}");
                break;
            case "ERROR":
                this.logger.LogError($"{loggingEvent.LoggerName} - {loggingEvent.RenderedMessage}");
                break;
            case "FATAL":
                this.logger.LogCritical($"{loggingEvent.LoggerName} - {loggingEvent.RenderedMessage}");
                break;
            default:
                this.logger.LogTrace($"{loggingEvent.LoggerName} - {loggingEvent.RenderedMessage}");
                break;
        }
    }
}

Easy, right?

You probably notice the injected `ILogger` in the constructor of this appender. That’s actually the ‘hardest’ part of setting up this thing, because it means you can only add this appender in a context where the ILogger has been instantiated!

Using the appender

Not only am I a big fan of log4net, but Autofac is also on my shortlist of favorite libraries.
In order to use Autofac and log4net together you can use the LoggingModule from the Autofac documentation page. I’m using this module all the time in my projects, with some changes if necessary.

Azure Functions doesn’t support the default app.config and web.configfiles, which means you can’t use the default XML configuration block which is used in a ‘normal’ scenario. It is possible to load some configuration file by yourself and providing it to log4net, but there are easier (& cleaner) implementations.

What I’ve done is pass along the Azure Functions `ILogger` to the module I mentioned earlier and configure log4net to use this newly created appender.

public class LoggingModule : Autofac.Module
{
    public LoggingModule(ILogger logger)
    {
        log4net.Config.BasicConfigurator.Configure(new FunctionLoggerAppender(logger));
    }
// All the other (default) LoggingModule stuff
}

// And for setting up the dependency container

internal class Dependency
{
    internal static IContainer Container { get; private set; }
    public static void CreateContainer(ILogger logger)
    {
        if (Container == null)
        {
            var builder = new ContainerBuilder();
            builder.RegisterType<Do>().As<IDo>();
            builder.RegisterModule(new LoggingModule(logger));
            Container = builder.Build();
        }
    }
}

I do find it a bit dirty to pass along the `ILogger` throughout the code. If you want to use this in a production system, please make the appropriate changes to make this a bit more clean.

You probably notice I’m storing the Autofac container in a static variable. This is to make sure the wiring of my dependencies is only done once, per instance of my Azure Function. Azure Functions are reused quite often and it’s a waste of resources to spin up a complete dependency container per invocation (IMO).

Once you’re done setting up your IoC and logging, you can use any piece of code which is using the log4net `ILog` implementations and still see the results in your Azure Functions tooling!

If you are running locally, you might not see anything being logged in your local Azure Functions emulator. This is a known issue of the currentprevious tooling, there is an openclosed issue on GitHub. Install the latest version of the tooling (1.0.12 at this time) and you’ll be able to see your log messages from the class library.

image

Of course, you can also check the logging in the Azure Portal if you want to. There are multiple ways to find the log messages, but the easiest option is probably the Log-window inside your Function.

image


Well, that’s all there is to it!

By using an easy to write appender you can reuse your class libraries between multiple projects and still have all the necessary logging available to you. I know this’ll help me in some of my projects!
If you want to see all of the source code on this demo project, it’s available on my GitHub page: https://github.com/Jandev/log4netfunction

So, one of my previous customers reached out to me a couple of weeks ago. They had a question concerning on how to use dependency injection in their AutoMapper profiles. For this project we were using profiles which were dynamically loaded inside the application using MEF and were using Autofac for dependency injection.

The way you would normally load all of these profiles is by using the `AddProfiles` method when initializing AutoMapper. The code would look similar to the following excerpt.

private static void RegisterAutomapperDefault(IEnumerable<Assembly> assemblies)
{
    AutoMapper.Mapper.Initialize(cfg =>
    {
        cfg.AddProfiles(assemblies);
    });
}

This works fine on most occasions and is the recommended approach, to my knowledge.

When you start thinking about using dependency injection (constructor injection in this case), you might want to rethink your mapping profile. Because, if you have the need for dependencies when mapping object properties to the properties of a different object it probably means there’s too much logic going on over here.

Of course, if you need this, one thing you might want to consider is using the custom type convertors or custom value resolvers. You can use dependency injection (constructor injection) using these convertors and resolvers by adding a single line in the `Initialize` method of AutoMapper.

private static void RegisterAutomapperDefault(IContainer container, IEnumerable<Assembly> assemblies)
{
    AutoMapper.Mapper.Initialize(cfg =>
    {
        cfg.ConstructServicesUsing(container.Resolve);

        cfg.AddProfiles(assemblies);
    });
}

Now if you still feel like you need to do constructor injection inside your mapping `Profile` classes, that’s also quite possible, but please think about it in before doing so.

In order to get this working, I first created a new `Profile` class which injects an `IConvertor`, like below.

public class MyProfile : Profile
{
    public MyProfile(IConvertor convertor)
    {
        CreateMap<Model, ViewModel>()
            .ForMember(dest => dest.Id, opt => opt.MapFrom(src => src.Identifier))
            .ForMember(dest => dest.Name, opt => opt.MapFrom(src => convertor.Execute(src.SomeText)))
            ;
    }
}

What you need to do now is register all of the `Profile` implementations to your IoC-framework, like Autofac. To do this, you have to do some reflection magic. The code used below retrieves all `Profile` implementations in the assemblies which have their name starting with “Some”.

public static IContainer Autofac()
{
    var containerBuilder = new ContainerBuilder();

    // Register the dependencies...
    containerBuilder.RegisterType<Convertor>().As<IConvertor>();


    var loadedProfiles = RetrieveProfiles();
    containerBuilder.RegisterTypes(loadedProfiles.ToArray());

    var container = containerBuilder.Build();

    RegisterAutoMapper(container, loadedProfiles);

    return container;
}

/// <summary>
/// Scan all referenced assemblies to retrieve all `Profile` types.
/// </summary>
/// <returns>A collection of <see cref="AutoMapper.Profile"/> types.</returns>
private static List<Type> RetrieveProfiles()
{
    var assemblyNames = Assembly.GetExecutingAssembly().GetReferencedAssemblies()
        .Where(a => a.Name.StartsWith("Some"));
    var assemblies = assemblyNames.Select(an => Assembly.Load(an));
    var loadedProfiles = ExtractProfiles(assemblies);
    return loadedProfiles;
}

private static List<Type> ExtractProfiles(IEnumerable<Assembly> assemblies)
{
    var profiles = new List<Type>();
    foreach (var assembly in assemblies)
    {
        var assemblyProfiles = assembly.ExportedTypes.Where(type => type.IsSubclassOf(typeof(Profile)));
        profiles.AddRange(assemblyProfiles);
    }
    return profiles;
}

All of this code is just to register your mapping profiles to Autofac. This way Autofac can resolve them when initializing AutoMapper. To register your mapping profiles in AutoMapper you need to use a specific overload of the `AddProfile` method which takes a `Profile` instance, instead of a type.

/// <summary>
/// Over here we iterate over all <see cref="Profile"/> types and resolve them via the <see cref="IContainer"/>.
/// This way the `AddProfile` method will receive an instance of the found <see cref="Profile"/> type, which means
/// all dependencies will be resolved via the <see cref="IContainer"/>.
/// </summary>
private static void RegisterAutoMapper(IContainer container, IEnumerable<Type> loadedProfiles)
{
    AutoMapper.Mapper.Initialize(cfg =>
    {
        cfg.ConstructServicesUsing(container.Resolve);
                
        foreach (var profile in loadedProfiles)
        {
            var resolvedProfile = container.Resolve(profile) as Profile;
            cfg.AddProfile(resolvedProfile);
        }
                
    });
}

You can see I’m resolving all of the loaded profiles via Autofac and add each resolved instance to AutoMapper.

This takes quite a bit of effort, but resolving your profiles like this will give you the possibility to do any kind of dependency injection inside your AutoMapper code.


Just remember, as I’ve written before: “Just because you can, doesn’t mean you should!”
Still I wanted to show you how this can be done as it’s kind of cool. If you want to check out the complete solution, check out my GitHub repository for this project.

For years we (a lot of people I know and myself included) have been using the Unit of Work and Repository pattern combined with each other. This makes quite a lot of sense as, in most cases, they both have something to do with your database calls.

When searching for both of these patterns you’ll often be directed to a popular article on the Microsoft documentation site. The sample code over there has a very detailed implementation on how you can implement both of these patterns for accessing and working with your database. I kind of like this post as it goes in great length to describe both the unit of work- and repository pattern and the advantages of using them. I see a lot of projects/companies having implemented the pattern combo like described in the Microsoft article. I can’t really blame them as it’s one of the top hits when you search for it in any search engine.

There is a downside to this sample though. It violates the Open/Closed principle which states “software entities (classes, modules, functions, etc.) should be open for extension, but closed for modification”. Whenever you need to add a new repository to your database context, you also need to add this repository to your unit of work, therefore violating the open/closed principle.

It also violates the Single Responsibility Principle, which states “everymoduleorclassshould have responsibility over a single part of thefunctionalityprovided by thesoftware, and that responsibility should be entirelyencapsulatedby the class. All itsservicesshould be narrowly aligned with that responsibility.” or in short “A class should have only one reason to change.”. The reason why the sample implementation violates this principle is because it is handling multiple responsibilities. The unit of work’s purpose should be to encapsulate and commit or rollback transactions of atomic operations. However, it’s also creating and managing the several repository objects, therefore having multiple responsibilities.

Implementing the unit of work and repository pattern can be done in multiple ways. Derek Greer goes on about this at great length about this in an old post of him. As always there are several ways to improve the design. You might even want to keep the mentioned design in the Microsoft example, because ‘it-just-works’. For the sake of cleaner code I’ll describe one of the ways, which I personally like very much, to improve the software design. By adding a decorator to the project the functional code will be much cleaner.

First thing you have to consider is implementing some form of CQRS in your software design. This will make your live much easier when splitting the command, unit of work and repository functionality. You can perfectly implement the described solution without implementing CQRS, but why would you want to do this?

I’ll just assume you have a command handler in your application. The interface will probably look similar to the following piece of code.

public interface IIncomingFileHandler<in TCommand>
	where TCommand : IncomingFileCommand
{
	void Handle(TCommand command);
}

The actual command handler can be implemented like the following piece of code.

public class IncomingFileHandler<TCommand> : IIncomingFileHandler<TCommand>
    where TCommand : IncomingFileCommand
{
    private readonly IRepository<Customer> customerRepository;
    private readonly IRepository<File> fileRepository;
    
    protected IncomingFileHandler(IRepository<Customer> customerRepository, IRepository<File> fileRepository)
    {
        this.customerRepository = customerRepository;
        this.fileRepository = fileRepository;
    }

    public void Handle(TCommand command)
    {
        //Implement your logic over here.
        var customer = customerRepository.Get(command.CustomerId);
        customer.LatestUpdate = command.Request;
        customerRepository.Update(customer);
        var file = CreateNewIncomingFileDto(command);
        fileRepository.Add(file);

        return;
    }
}

All of the necessary repositories are injected over here so we can implement the logic for this functional area. The implementation doesn’t make much sense, but keep in mind it’s just an example. This piece of code wants to write to the database multiple times. We could implement the call to the SaveChanges() method inside the Update- and Add-methods, but that’s a waste of database requests and you’ll sacrifice transactional consistency.

At this time nothing is actually written back to the database, because the SaveChanges isn’t called anywhere and we aren’t committing (or rolling back) any transaction either. The functionality for persisting the data will be implemented in a transaction handler, which will be added as a decorator. The transaction handler will create a new TransactionScope, invoke the Handle-method of the actual IIncomingFileHandler<TCommand> implementation (in our case the IncomingFileHandler<TCommand>), save the changes and commit the transaction (or roll back).

A simple version of this transaction decorator is shown in the following code block.

public class IncomingFileHandlerTransactionDecorator<TCommand> : IIncomingFileHandler<TCommand> 
    where TCommand : IncomingFileCommand
{
    private readonly IIncomingFileHandler<TCommand> decorated;
    private readonly IDbContext context;

    public IncomingFileHandlerTransactionDecorator(IIncomingFileHandler<TCommand> decorated, IDbContext context)
    {
        this.decorated = decorated;
        this.context = context;
    }

    public void Handle(TCommand command)
    {
        using (var transaction = context.BeginTransaction())
        {
            try
            {
                decorated.Handle(command)

                context.SaveChanges();
                context.Commit(transaction);
            }
            catch (Exception ex)
            {
                context.Rollback(transaction);
                throw;
            }
        }
    }
}

This piece of code is only responsible for creating a transaction and persisting the changes made into the database.

We are still using the repository pattern and making use of the unit-of-work, but each piece of code now has its own responsibility. Therefore making the code much cleaner. You also aren’t violating the open/closed principle as you can still add dozens of repositories, without affecting anything else in your codebase.

The setup for this separation is a bit more complex compared to just hacking everything together in one big file/class. Luckily Autofac has some awesome built-in functionality to add decorators. The following two lines are all you need to make the magic happen.

builder.RegisterGeneric(typeof(IncomingFileHandler<>)).Named("commandHandler", typeof(IIncomingFileHandler<>));
builder.RegisterGenericDecorator(typeof(IncomingFileHandlerTransactionDecorator<>), typeof(IIncomingFileHandler<>), fromKey: "commandHandler");

This tells Autofac to use the IncomingFileHandlerTransactionDecorator as a decorator for the IncomingFileHandler.

After having implemented the setup you are good to go. So, whenever you think of implementing the unit-of-work and repository pattern in your project, keep in mind the suggestions in this post.

On a recent project I had to implement the decorator pattern to add some functionality to the existing code flow.

Not a big problem of course. However, on this project we were using Autofac for our dependency injection framework so I had to check how to implement this pattern using the framework built-in capabilities. One of the reasons I always resort to Autofac is the awesome and comprehensive documentation. It’s very complete and most of the time easy to understand. The advanced topics also have a chapter dedicated to the Adapter- and Decorator pattern which was very useful for implementing the decorator pattern in this project.

I wanted to use the decorator pattern to add some logic to determine if a command should be handled and for persisting database transactions of my commands and queries. You can also use it for things like security, additional logging, enriching the original command, etc.

As the documentation already states, you’ll have to register your original command handler as a Named service. The Autofac extensions for registering a decorator will use this named instance to add the decorators on to. One thing to remember when you need to add several decorators to your command, you’ll have to register each decorator as a named service also, except for the last one!

The command handlers we were using were accepting a generic argument to instantiate a class. Therefore, we also had to use the open generic version for registering the implementations and decorators.

The implementation of the actual command handler looks very much like the follwing code block.

public class ProcessedItemHandler<TCommand> : IProcessedMessageHandler<TCommand> 
		where TCommand : ProcessedMessageCommand
{
	public ProcessedItemHandler(
		IBackendSystemFormatter<TModel> formatter, 
		IQueueItemWriter<TModel> writer, 
		IRepository<ProcessQueue> processQueueRepository)
	{
	}
	
	public void Handle(TCommand command)
	{
		/* Implementation logic */
	}		
}

It implements the IProcessedMessageHandler<TCommand> interface and contains the logic to execute the command.

The decorator has to implement the same interface and one of the injected dependencies is the same interface. This tells Autofac to inject an IProcessedMessageHandler<TCommand> which is ‘linked’ in the registration of our application.

public class ProcessedMessageTransactionDecorator<TCommand> : IProcessedMessageHandler<TCommand>
		where TCommand : ProcessedMessageCommand
{
	private readonly IProcessedMessageHandler<TCommand> decorated;
	private readonly ITransactionHandler transactionHandler;

	public ProcessedMessageTransactionDecorator(
		IProcessedMessageHandler<TCommand> decorated,
		ITransactionHandler transactionHandler)
	{
		this.decorated = decorated;
		this.transactionHandler = transactionHandler;
	}

	public void Handle(TCommand command)
	{
		/* Decorator logic */

		decorated.Handle(command);

		/* Decorator logic */
	}
}

As you can see, you will be able to do all kinds of stuff in the Handle-method before or after invoking the decorated object.

The registration in our application looks very much like the following code block.

var storeProcessedMessageCommandHandlers = GetAllStoreProcessedMessageCommandHandlerImplementationsFromAssemblies();

foreach (var commandHandler in storeProcessedMessageCommandHandlers)
{
	builder.RegisterGeneric(commandHandler).Named("storeProcessedMessageHandler", typeof(IProcessedMessageHandler<>));
}

builder.RegisterGenericDecorator(typeof(ProcessedMessageTransactionDecorator<>), typeof(IProcessedMessageHandler<>),
										fromKey: "storeProcessedMessageHandler");

First we need to collect all implementations of the IProcessedMessageHandler<TCommand> and register them within the Autofac container. As you can see, all these implementations are registered as a named service with an index called storeProcessedMessageHandler. If you only have 1 implementation of the command handler, you can just register this one implementation of course.

After having registered all of the command handlers, the decorator(s) can be registered. The helper method RegisterGenericDecorator helps with this. This method also works with open generics and registration looks very similar to registering a ‘normal’ class and interface. The main difference is the addition of the fromKey argument. This argument is used to determine to which named service the decorator should be added to.

If you want to hook up multiple decorators you can also add the toKey argument to your RegisterGenericDecorator method. By adding the toKey argument, the decorator is also added as a named service to Autofac and you will be able to hook up another decorator to the earlier decorator by using the name defined in the toKey in the fromKey of the new decorator. This might be a bit abstract, so let me just write up a small example.

builder.RegisterGeneric(typeof(IncomingHandler<>)).Named("commandHandler", typeof(ICommandHandler<>));
builder.RegisterGenericDecorator(typeof(TransactionRequestHandlerDecorator<>), typeof(ICommandHandler<>), fromKey: "commandHandler", toKey: "transactionHandler");
builder.RegisterGenericDecorator(typeof(ShouldHandleCommandHandlerDecorator<>), typeof(ICommandHandler<>), fromKey: "transactionHandler");

Makes more sense right?

Just remember, not to add a toKey argument to the last decorator of your flow. Otherwise you will not be able to inject the interface, because everything is added to the IIndex<T> collection and there isn’t a defined entrypoint. Ask me how I know……

 

Hope this helps you in future projects. Knowing about this functionality surely has helped me to keep the code clean.