IMPLEMENT_POSTPROCESSOR.md

Implement a PostProcessor module

In this guide we will learn how to implement a PostProcessor module which is a type of module that allows us to modify an HTTP response after it was handled.

IPostProcessorModule interface

Let's look at the definition of the IPostProcessorModule interface.

class IPostProcessorModule : public IModule {
public:
    virtual void PostProcess(http::Context &ctx, http::Response &res) = 0;

    [[nodiscard]] virtual double GetPostProcessorPriority() const noexcept = 0;

    [[nodiscard]] virtual bool ShouldPostProcess(const http::Context &ctx, const http::Request &req, const http::Response &res) const = 0;
};

We have three methods to implement, let's go!

❗ Don't forget to implement the IModule methods

Create a post-processor

Okay, let's create our own class which inherits from IPostProcessorModule.

#include "ziapi/Module.hpp"

class MyPostProcessor : public ziapi::IPostProcessorModule {
public:
    void PostProcess(http::Context &ctx, http::Response &res) override;

    [[nodiscard]] double GetPostProcessorPriority() const noexcept override;

    [[nodiscard]] bool ShouldPostProcess(const http::Context &ctx, const http::Request &req, const http::Response &res) const override;
};

Then, let's implement the GetPostProcessorPriority(). Our module doesn't have specific priority requirements so we'll put it at 0.5f.

[[nodiscard]] double MyPostProcessor::GetPostProcessorPriority() const noexcept
{
    return 0.5f;
}

Then, let's implement the ShouldPostProcess(). This method is invoked to know if our post-processor should be called for a specific request. We can return true if we want all requests to go through this post-processor but let's just say our post-processor only handles status code inferior to 400 for the sake of the example.

[[nodiscard]] bool ShouldPostProcess(const http::Context &ctx, const http::Request &req, const http::Response &res) const
{
    return res.status_code < 400;
}

Great! Now our post-processor will be called on all responses with a status code lower than 400! Now let's add the PostProcess().

void MyPostProcessor::PostProcess(http::Context &ctx, http::Response &res)
{
    std::cout << "New " << res.status_code << " response!" << std::endl;
}

And that's it! You now have a module which logs all responses with a status code < 400.