FaasUtils 1.0.1

FaasUtils

FaasUtils is a collection of utilities that could be useful for various FaaS systems/providers (FnProject, OpenFaaS, etc). It is designed to be used as a dependency for FaaS SDKs, rather than directly by functions themselves.

It consists of two main components:

Function Input

IInput is an interface representing the input to a FaaS function. This contains several methods to obtain the input:

• AsString: Raw input as a string
• AsJson<T>: Parses the input as a strongly-typed JSON object
• AsJson: Parses the input as a dynamic JSON object
• AsStream Raw input as a stream (ideal for large inputs, or binary content such as image files)

It comes with a Input class that implements IInput by wrapping HttpContext. Additional implementations could be provided in order to implement different trigger types.

Function Calls

The main component of FaasUtils is FunctionExpressionTreeBuilder. This class can produce a lambda function for any arbitrary class containing an Invoke or InvokeAsync method, similar to how ASP.NET middleware classes work. The generated lambda function handles resolving interfaces through an IServiceProvider.

As an example, given a function like this:

class MyFunction
{
    public async Task<string> InvokeAsync(string input, IFoo foo)
    {
        return $"Hello {input}";
    }
}

Calling FunctionExpressionTreeBuilder.Compile will compile a lambda function roughly like this:

(MyFunction instance, IServiceProvider services) => instance.Invoke(
    services.GetRequiredService<IInput>().AsString,
    services.GetRequiredService<IFoo>(),
);

The returned lambda function always has the same signature: Func<T, IServiceProvider, Task<object>>, where T is the type of the function class (MyFunction in this case). For classes that have an Invoke method instead of InvokeAsync, the returned value is wrapped in a task using Task.FromResult.

This allows a very flexible API for FaaS functions, without having to stick to an arbitrary interface.

Argument Resolution

Arguments to the Invoke or InvokeAsync method are resolved using an IArgumentResolver. The default implementation, ArgumentResolver, handles the arguments the following way:

• string arguments named input are treated as raw input, resolved using IInput.AsString()
• Object arguments named input as treated as JSON, resolved using IInput.AsJson<T>()
• IServiceProvider is passed through as-is
• Other interfaces are resolved through the dependency injection container, using services.GetRequiredService<T>()

Usage

When configuring your IServiceCollection, call services.AddFaasUtils() (in the FaasUtils.Extensions namespace) to add the required services:

public void ConfigureServices(IServiceCollection services)
{
    services.AddFaasUtils();
}

To customize the IArgumentResolver used, use services.replace:

services.Replace(ServiceDescriptor.Transient<IArgumentResolver, MyCustomArgumentResolver>();