Clojars Project

Domino is a data flow engine that helps you organize the interactions between your data model and events. Domino allows you to declare your business logic using a directed acyclic graph of events and effects. Whenever an external change is transacted to the data model, the graph determines the chain of events that will be executed, and side effects triggered as a result of the computation.

Without a way to formalize the interactions between different parts of the application, relationships in code become implicit. This results in code that's difficult to maintain because of the mental overhead involved in tracking these relationships. Domino makes the interactions between pieces of business logic explicit and centralized.

Domino explicitly separates logic that makes changes to the data model from side effectful functions. Business logic functions in Domino explicitly declare how they interact with the data model by declaring their inputs and outputs. Domino then uses these declarations to build a graphs of related events. This approach handles cascading business logic out of the box, and provides a data specification for relationships in code. Once the changes are transacted, the effectful functions are called against the new state.


Domino consists of three main concepts:

1. Model

The model represents the paths within an EDN data structure. These paths will typically represent fields within a document. Each path entry is a tuple where the first value is the path segment, and the second value is the metadata associated with it. If the path is to be used for effects and/or events, the metadata must contain the :id key.

For example, [:amount {:id :amount}] is the path entry to the :amount key within the data model and can be referenced in your events and effects as :amount (defined by the :id). You can nest paths within each other, such as the following model definition:

[[:patient [:first-name {:id :fname}]]]

2. Events

The events define the business logic associated with the changes of the model. Whenever a value is transacted, associated events are computed. Events are defined by three keys; an :inputs vector, an :outputs vector, and a :handler function.

The handler accepts three arguments: a context containing the current state of the engine, a list of the input values, and a list of the output values. The function should produce a vector of outputs matching the declared :outputs key. For example:

{:inputs  [:amount]
 :outputs [:total]
 :handler (fn [ctx {:keys [amount]} {:keys [total]}]
            {:total (+ total amount)})}

3. Effects

Effects are executed after events have been transacted and the new context is produced. Effects are defined as a map of :inputs and a :handler function.

The handler accepts two arguments: a context containing the current state of the engine, and a list of input values. The effects do not cascade. For example:

{:inputs [:total]
 :handler (fn [ctx {:keys [total]}]
            (when (> total 1337)
              (println "Woah. That's a lot.")))}


1. Require domino.core

(require '[domino.core :as domino]) 

2. Declare your schema

Let's take a look at a simple engine that accumulates a total. Whenever an amount is set, this value is added to the current value of the total. If the total exceeds 1337 at any point, it prints out a statement that says "Woah. That's a lot."

(def schema
  {:model   [[:amount {:id :amount}]
             [:total {:id :total}]]
   :events  [{:id      :update-total
              :inputs  [:amount]
              :outputs [:total]
              :handler (fn [ctx {:keys [amount]} {:keys [total]}]
                         {:total (+ total amount)})}]
   :effects [{:inputs [:total]
              :handler (fn [ctx {:keys [total]}]
                         (when (> total 1337)
                           (js/alert "Woah. That's a lot.")))}]})

This schema declaration is a map containing three keys:

Using a unified model referenced by the event functions allows us to easily tell how a particular piece of business logic is triggered.

The event engine generates a direct acyclic graph (DAG) based on the :input keys declared by each event that's used to compute the new state in a transaction. This approach removes any ambiguity regarding when and how business logic is executed.

Domino explicitly separates the code that modifies the state of the data from the code that causes side effects. This encourages keeping business logic pure and keeping the effects at the edges of the application.

3. Initialize the engine

The schema that we declared above provides a specification for the internal data model and the code that operates on it. Once we've created a schema, we will need to initialize the data flow engine. This is done by calling the domino/initialize function. This function can be called by providing a schema along with an optional initial state map. In our example, we will give it the schema that we defined above, and an initial value for the state with the :total set to 0.

(def ctx (atom (domino/initialize schema {:total 0})))

Calling the initialize function creates a context ctx that's used as the initial state for the engine. The context will contain the model, events, effects, event graph, and db (state). In our example we use an atom in order to easily update the state of the engine.

4. Transact your external data changes

We can update the state of the data by calling domino/transact that accepts the current ctx along with an inputs vector, returning the updated ctx. The input vector is a collection of path-value pairs. For example, to set the value of :amount to 10, you would pass in the following input vector [[[:amount] 10]].

(swap! ctx domino/transact [[[:amount] 10]])

The updated ctx contains the :change-history which is a simple vector of all the changes as they were applied to the data in exectution order of the events that were triggered.

(:change-history @ctx)

We can see the new context contains the updated total amount and the change history shows the order in which the changes were applied.

The :domino.core/db key in the context will contain the updated state reflecting the changes applied by running the events.

(:domino.core/db @ctx)

Finally, let's update the :amount to a value that triggers an effect.

(require '[reagent.core :as reagent])

(defn button []
    {:on-click #(swap! ctx domino/transact [[[:amount] 2000]])}
    "trigger effect"])

(reagent/render-component [button] js/klipse-container)


Domino provides the ability to add interceptors pre and post event execution. Interceptors are defined in the schema's model. If there are multiple interceptors applicable, they are composed together.

In the metadata map for a model key, you can add a :pre and :post key to define these interceptors. Below are some examples

{:model  [[:foo {:id  :foo
           :pre [(fn [handler]
                   (fn [ctx inputs outputs]
                     (handler ctx
                              (assoc inputs :bar 5)
           :post [(fn [handler]
                    (fn [result]
                      (handler (update result :bar inc))))]}]]}

With interceptors, you can also short circuit an event, wherein you prevent handler execution by returning nil. For example

{:model  [[:foo {:id  :foo
           :pre [(fn [handler]
                   (fn [ctx {:keys [baz] :as inputs} outputs]
                     (when (> baz 2)
                       (handler ctx inputs outputs))
                     ;; returning nil prevents handler execution

Triggering Events

Occasionally, there may be a use case where you would want to trigger an event directly without transacting updated state. Events, optionally, can have an id key associated with them. This is how you would be able to reference the event from trigger-events.

For example, this might happen when a button is clicked and you want a value to increment. This can be accomplished with a call to trigger-events.

trigger-events takes a list of events that you would like trigger and calls transact with the current state of the data from all the inputs of the events. For example:

(domino.core/trigger-events ctx [:increment-total])

This wraps up everything you need to know to start using Domino. You can see a more detailed example using Domino with re-frame here.

Possible Use Cases


Example applications using Domino can be found here.



Copyright © 2019

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