Lambda-kube is a Clojure library for building inputs for Kubernetes.

Usage

Add lambdakube as a dependency to your project.clj, and (preferably), lein-auto as a plugin.

:require the namespace:

(:require [lambdakube.core :as lk]
          [clojure.java.io :as io])

Define a module function, defining the different parts of the system.

;; A module function takes an injector ($) as parameter, and adds rules to it.
(defn module [$]
  (-> $
      ;; A rule defines a resource (:frontend) and dependencies ([:num-fe-replicas]).
      (lk/rule :frontend [:backend-master :backend-slave :num-fe-replicas]
               (fn [master slave num-replicas]
                 (-> (lk/pod :frontend {:app :guesbook
                                        :tier :frontend})
                     (lk/add-container :php-redis "gcr.io/google-samples/gb-frontend:v4"
                                       (lk/add-env {}  {:GET_HOST_FROM :dns}))
                     (lk/deployment num-replicas)
                     (lk/expose-node-port :frontend (lk/port :php-redis 80)))))))

Define configuration.

(def config
  {:num-fe-replicas 3})

Define a -main function.

(defn -main []
  (-> (lk/injector)
      module
      (lk/get-deployable config)
      lk/to-yaml
      (lk/kube-apply (io/file "my-app.yaml"))))

Run it:

$ lein auto run

It will create a YAML file (my-app.yaml) and call kubectl apply on it. Then it will remain to watch your source files for changes, and when changed, will re-apply automatically.

A complete example can be found here.

Documentation

• Core Library
• Utility Functions

Rationale

Kubernetes is a really great tool, which transforms the task of deploying complex systems from an IT task to a software development task. Lambda-kube takes this one step further and turns the generation of the YAML files which describe the deployment declaratievly, from a "configuration" task to a programming task.

Imagine you are developing a new database system. Your database system consists of a few different kinds of nodes. Now imagine you wish to allow your users to deploy this database as part of their software. A good first step in this direction would be to package your software as Docker images, one for each type of node. But a Docker image does not answer the question of how these nodes should be connected to one another.

To answer this question, you could, for example, provide example YAML files for Kubernetes, to give the user a sense of what they need to do in order to deploy your database as part of their system, given they are using Kubernetes. However, the files you provide are no more than an example. Eventually, the user will have to update these files to match their needs.

Lambda-kube allows you to provide them a Clojure library, which generates these YAML files according to the recipe you design, but matching the parameters they provide. Being a Clojure library, it can be then integrated in a library they write, which integrates your database with their software, and potentially other microservices, each coming with its own Lambda-kube-based library.

Why Not Helm?

The above description matches the mission of the Helm project, and Helm charts could indeed stand in place of Lambda-kube libraries. However, the way I see it, Helm has two significant drawbacks.

Text-based Templates

Helm charts use text-based templates to generate YAML files. While this can work properly in simple cases, this can break horribly for others.

For example, the template engine used by Helm does nothing to escape string values. For example, if you provide a string value to a field and that string value contains new-lines, these new-line characters will break the YAML syntax.

Similarly, if you use a template to create a block (e.g., a map of values), it is your responsibility to indent the output properly, or else you will break the YAML syntax.

Lack of Abstraction

One of the problems with Kubernetes YAML files to begin with, is their verbosity. These files make extensive use of names, which are defined in one place, and used in another. These names bloat up the YAML files.

Helm does not provide a real answer for this bloat. It hides the bloat in charts, but the charts are even more verbose than the YAML files they produce. Wanting to account for every possibility, real-life charts are bloated with many esoteric options, making them hard for developers to understand them and maintain them.

The root cause for this is that Go Templates do not provide a good abstraction mechanism. What you want to have is the ability to create small things, each responsible for one thing, and to have the mechanism to compose them together. Go Templates are not good at this, but functional programming is.

What Lambda-Kube Is

Lambda-Kube is a Clojure library. It contains three families of functions:

1. Functions for defining API objects, such as Pods, Deployments, Services, etc.
2. Functions for augmenting API objects, adding things to them or updating their properties.
3. Functions for defining modules, supporting the gradual definition of a complete system, based on Dependency Injection (DI).

Lambda-Kube takes follows a few best practices made to make the systems you build with it maintainable.

1. It is purely functional. It uses no side-effects and even no macros. Just plain old Clojure functions.
2. Functions are simple and cohesive, intended to do one thing and to it well.
3. Functions are composable. Augmentation is always done on the first argument. This makes most functions compatible with Clojure's threading macro (->).

If a function we provide does not do exactly what you are looking for, you can replace it, or better yet, augment it with your own. For example, our pod function creates a very basic pod. It allows you to add additional fields, but if there is a pattern you want in your pods, and the pod function doesn't support it, you can (and should) write an augmentation function to modify the pod in any way you want.