IMPLEMENT_PREPROCESSOR.md

Implement a PreProcessor module

In this guide we will learn how to implement a PreProcessor module which is a type of module that allows us to modify an HTTP request before it is handled.

IPreProcessorModule interface

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

class IPreProcessorModule : public IModule {
public:
    virtual void PreProcess(http::Context &ctx, http::Request &req) = 0;

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

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

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

❗ Don't forget to implement the IModule methods

Create a pre-processor

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

#include "ziapi/Module.hpp"

class MyPreProcessor : public ziapi::IPreProcessorModule {
public:
    void PreProcess(http::Context &ctx, http::Request &req) override;

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

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

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

[[nodiscard]] double MyPreProcessor::GetPreProcessorPriority() const noexcept
{
    return 0.5f;
}

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

[[nodiscard]] bool ShouldPreProcess(const http::Context &ctx, const http::Request &req) const
{
    return req.method == http::method::kGet;
}

Great! Now our pre-processor will be called on all GET requests! Now let's add the PreProcess().

void MyPreProcessor::PreProcess(http::Context &ctx, http::Request &req)
{
    req.method = http::method::kPost;
}

And that's it! You now have a module which transforms all GET requests to POST requests in order to completely break your API ^^.