kilgore

A very basic event sourcing library.

Installation

Add the following to your :dependencies:

[kilgore "0.2.1"]

Usage

Setup a store and a stream; a store is responsible for keeping your events safe and a stream aggregates events closely related to each other.

(require '[kilgore.store :as store])
(require '[kilgore.stream :as stream])

(def store (store/acquire {:store-atom (atom nil)}))
(def stream (stream/acquire store "Slate Rock and Gravel Company"))

Next we’ll define some events. Please notice the use of past tense; we will be recording events which have already happened.

(require '[kilgore.event :refer [defevent]])

(defevent employee-hired [id name])
(defevent employee-promoted [id])
(defevent employee-demoted [id])
(defevent employee-left [id])

Now we can record events.

(require '[kilgore.stream :refer [record-event!]])

(record-event! stream (->EmployeeHired "fred" "Fred Flintstone"))
(record-event! stream (->EmployeeHired "barney" "Barney Rubble"))
(record-event! stream (->EmployeePromoted "fred"))
(record-event! stream (->EmployeeLeft "barney"))

To access our our current data set, we’ll write an event handler. In this case we’re interested in making a map of employees, record their employment level and count how many times employees where hired.

(require '[kilgore.stream :refer [event-reducer]])

(defmulti handle-event (fn [_ event] (type event)))
(defmethod handle-event :default [state _] state)

(defmethod handle-event employee-hired
   [state {:keys [id name]}]
   (-> state
       (assoc-in [:employees id] {:name name})
       (update-in [:total-hires] (fnil inc 0))))

(defmethod handle-event employee-promoted
   [state {:keys [id]}]
   (update-in state [:employees id :level] (fnil inc 0)))

(defmethod handle-event employee-demoted
   [state {:keys [id]}]
   (update-in state [:employees id :level] (fnil dec 0)))

(defmethod handle-event employee-left
   [state {:keys [id]}]
   (update-in state [:employees] dissoc id))

(def current (event-reducer handle-event))

Let’s see what the current state of our company is given the events we recorded earlier.

(current stream)
;; => {:employees {"fred" {:name "Fred Flintstone", :level 1}}, :total-hires 2}

That’s it!

Issues

This library does not provide a locking mechanism to ensure a consistent data set for things like uniqueness constraints.

License

Copyright © 2015 R.W. van ‘t Veer

Distributed under the Eclipse Public License either version 1.0 or (at your option) any later version.