Extending KrakenD, the big picture.

Document updated on Jan 15, 2019

Before starting to dive into the KrakenD code, spend a few minutes understanding the big pieces of the system, how it works, and the philosophy behind it.

The KrakenD rules

Let’s start with the rules followed to code KrakenD (shared with The Lura Project), as they answer to architectural design questions:

• Reactive is key
• Reactive is key (yes, it is very very important)
• Failing fast is better than succeeding slow (say it one more time!)
• The simpler, the better
• Everything is pluggable
• Each request must be processed in its request-scoped context

KrakenD internal states

When you start KrakenD, the system goes through two different internal states: building and working. Let’s see what happens in every state.

Building state

The building state administers the service start-up and prepares the system before it can start receiving traffic. During the building state, three things happen:

• Parsing of the configuration to fix the system behavior
• Preparation of the middlewares
• Construction of the pipes

A pipe is a function that receives a request message, processes it, and produces the response message and an error. The KrakenD router binds the pipes to the selected transport layer (e.g., HTTP, gRPC).

When the building state finishes, the KrakenD service is not going to need to calculate any route or lookup for the associated handler function, as all the mapping is direct in-memory.

Working state

The working state is when the system is ready and can process the requests. When they arrive, the router already has the mapping of the request with the handler function and triggers the pipe execution. The proxy is the step of the pipe that manipulates, aggregates, and does other data handling for the rest of the process.

As the handler functions are in the previous step, KrakenD doesn’t penalize the performance depending on the number of endpoints or the possible cardinality of the URIs requested by the users.

The important packages

The Lura Project (KrakenD’s engine) is composed of a set of packages designed as building blocks for creating pipes and processors between an exposed endpoint and one or several API resources served by your backends.

The most important packages are:

1. the config package defines the service.
2. the router package sets up the endpoints exposed to the clients.
3. the proxy package adds the required middlewares and components for further processing of the requests to send and the received responses sent by the backends, and also to manage the connections against those backends.

The rest of the packages of the framework contain some helpers and adapters for additional tasks, like encoding, logging or service discovery.

Additionally, the KrakenD-CE bundles a lot of middleware and components that are in its scope and package. These packages and others are listed in our KrakenD Contrib repository.

The config package

The config package contains the structs required for the service description.

The ServiceConfig struct defines the entire service. Initialize it before using it to be sure that all parameters are normalized and default values are applied.

The config package also defines an interface for a file config parser and a parser based on the Viper library.

The router package

The router package contains an interface and several implementations for the KrakenD router layer using the mux router from the net/http and the httprouter wrapped in the gin framework.

The router layer is responsible for setting up the HTTP(S) services, binding the endpoints defined at the ServiceConfig struct and transforming the HTTP request into proxy requests before delegating the task to the inner layer (proxy). Once the internal proxy layer returns a proxy response, the router layer converts it into a proper HTTP response and sends it to the user.

This layer can be easily extended to use any HTTP router, framework or middleware of your choice. Adding transport layer adapters for other protocols (Thrift, gRPC, AMQP, NATS, and others) is in the roadmap. As always, PRs are welcome!

The proxy package

The proxy package is where most of the KrakenD components and features are. It defines two important interfaces, designed to be stacked:

• Proxy is a function that converts a given context and request into a response.
• Middleware is a function that accepts one or more proxies and returns a single proxy wrapping them.

This layer transforms the request received from the outer layer (router) into a single or several requests to your backend services, processes the responses and returns a single response.

Middlewares generates custom proxies that are chained depending on the workflow defined in the configuration until each possible branch ends in a transport-related proxy. Every one of these generated proxies can transform the input or even clone it several times and pass it or them to the next element in the chain. Finally, they can also modify the received response or responses adding all kinds of features to the generated pipe.

The Lura Project provides a default implementation of the proxy stack factory.

Middlewares available

• The balancing middleware uses some strategy for selecting a backend host to query.
• The concurrent middleware improves the QoS by sending several concurrent requests to the next step of the chain and returning the first successful response using a timeout for canceling the generated workload.
• The logging middleware logs the received request and response and also the duration of the segment execution.
• The merging middleware is a fork-and-join middleware. It is intended to split the process of the request into several concurrent processes, each one against a different backend, and to merge all the received responses from those created pipes into a single one. It applies a timeout, as the concurrent one does.
• The http middleware completes the received proxy request by replacing the parameters extracted from the user request in the defined URLPattern.

Proxies available

• The http proxy translates a proxy request into an HTTP one, sends it to the backend API using an HTTPClientFactory, decodes the returned HTTP response with a Decoder, manipulates the response data with an EntityFormatter and returns it to the caller.

Other components of the proxy package

The proxy package also defines the EntityFormatter, the block responsible for enabling a powerful and fast response manipulation.