Async Control Flow In re-frame

This re-frame library coordinates a set of asynchronous, stateful tasks which have dependencies (and consequently need to be ordered).

Using this library, you can coordinate the kind of asynchronous control flow which is often necessary to successfully boot a re-frame application "up" into a functioning state, while also gracefully handling failures.

As an example, imagine an application which, during startup, has to connect with a backend server via a web socket, load some user settings data, and then a user portfolio, while also connecting to Intercom, but not until the user settings were loaded, while remembering that any of these processes might fail because of network problems.

So, it has a similar intent to mount or component, but it dovetails with, and leverages the event driven nature of re-frame's architecture.

Technically, this library implements an Effect Handler, keyed :async-flow. It has a declarative, data oriented design.

TOC

• Quick Start Guide
• Testing
• Problem Definition
• The Solution
• Design Philosophy

Quick Start Guide

Step 1. Add Dependency

Add the following project dependency:

Requires re-frame >= 0.8.0

Step 2. Initiate Boot

In your app's main entry function, we want to initiate the boot process:

(defn ^:export main
  []
  (dispatch-sync [:boot])            ;; <--- boot process is started
  (reagent/render
    [this-app.views/main]
    (.getElementById js/document "app")))

Why the use of dispatch-sync, rather than dispatch?

Well, dispatch-sync is convenient here because it ensures that app-db is synchronously initialised before we start mounting components/views (which subscribe to state). Using dispatch would work too, except it runs the handler later. So, we'd have to then code defensively in our subscriptions and views, guarding against having an uninitialised app-db.

This is the only known case where you should use dispatch-sync over dispatch (other than in tests).

Step 3. Registration And Use

In the namespace where you register your event handlers, perhaps called events.cljs, you have 3 things to do.

First, add this require to the ns:

(ns app.events
  (:require
    ...
    [day8.re-frame.async-flow-fx]   ;; <-- add this
    ...))

Because we never subsequently use this require, it appears redundant. But its existence will cause the :async-flow effect handler to self-register with re-frame, which is important to everything that follows.

Second, write a function which returns a declarative description (as a data structure) of the async flow required, like this:

(defn boot-flow
  []
  {:first-dispatch [:do-X]              ;; what event kicks things off ?
   :rules [                             ;; a set of rules describing the required flow
     {:when :seen? :events :success-X  :dispatch [:do-Y]}
     {:when :seen? :events :success-Y  :dispatch [:do-Z]}
     {:when :seen? :events :success-Z  :halt? true}
     {:when :seen-any-of? :events [:fail-X :fail-Y :fail-Z] :dispatch  [:app-failed-state] :halt? true}]})

Try to read each rule line as an English sentence. When this event happens, dispatch another event. The rules above combine to run tasks X, Y and Z serially, like dominoes. Much more complicated scenarios are possible. Full particulars of this data structure are provided below in the Tutorial section.

Third, write the event handler for :boot:

Remember that (dispatch-sync [:boot]) in step 2. We are now writing and registering the associated event handler.

This event handler will do two things:

1. It goes though an initial synchronous series of tasks which get app-db into the right state
2. It kicks off a multistep asynchronous flow

(reg-event-fx                    ;; note the -fx
  :boot                          ;; usage:  (dispatch [:boot])  See step 3
  (fn [_ _]
    {:db (-> {}                  ;;  do whatever synchronous work needs to be done
            task1-fn             ;; ?? set state to show "loading" twirly for user??
            task2-fn)            ;; ?? do some other simple initialising of state
     :async-flow  (boot-flow)})) ;; kick off the async process

Notice at that last line. This library provides the "effect handler" which implements :async-flow. It reads and actions the data structure returned by (boot-flow).

Testing

Unit tests use standard cljs.test

To run tests with the test.html test runner in a browser

lein test-once # or
lein test-auto

To run the tests with Karma

npm install karma-cli -g # install the global CLI Karma tool
lein npm install # install Karma NPM dependencies

lein karma-once # or
lein karma-auto
# and then
karma start --single

Problem Definition

When an App boots, it performs a set of tasks to initialise itself.

Invariably, there are dependencies between these tasks, like task1 has to run before task2.

Because of these dependencies, "something" has to coordinate how tasks are run. Within the clojure community, a library like Component or mount is often turned to in these moments, but we won't be doing that here. We'll be using an approach which is more re-frame friendly.

Easy

If the tasks are all synchronous, then the coordination can be done in code.

Each task is a function, and we satisfy the task dependencies by correctly ordering how they are called. In a re-frame context, we'd have this:

(reg-event-db
  :boot
  (fn [db]
    (-> {}
        task1-fn
        task2-fn
        task3-fn)))

and in our app's main function we'd (dispatch [:boot])

Time

But, of course, it is never that easy because some of the tasks will be asynchronous.

A booting app will invariably have to coordinate asynchronous tasks like "open a websocket", "establish a database connections", "load from LocalStore", "GET configuration from an S3 bucket" and "querying the database for the user profile".

Coordinating asynchronous tasks means finding ways to represent and manage time, and time is a programming menace. In Greek mythology, Cronus was the much feared Titan of Time, believed to bring cruelty and tempestuous disorder, which surely makes him the patron saint of asynchronous programming.

Solutions like promises and futures attempt to make time disappear and allow you to program with the illusion of synchronous computation. But time has a tendency to act like a liquid under pressure, finding the cracks and leaking through the abstractions.

Something like CSP (core.async) is more of an event oriented treatment. Less pretending. But... unfortunately more complicated. core.async builds a little state machine for you, under the covers, so that you can build your own state machine on top of that again via deft use of go loops, channels, gets and puts. Both layers try to model/control time.

In our solution, we'll be using a re-frame variation which hides (most of) the state machine complexity.

Failures

There will also be failures and errors!

Nothing messes up tight, elegant code quite like error handling. Did the Ancient Greeks have a terrifying Titan for the unhappy path too? Ernos? They should have.

When one of the asynchronous startup tasks fails, we must be able to stop the normal boot sequence and put the application in a satisfactory failed state, sending necessary logs and explaining to the user what went wrong - eg: "No Internet connection" or "Couldn't load user portfolio".

Efficiency

And then, of course, there's the familiar pull of efficiency.

We want our app to boot in the shortest possible amount of time. So any asynchronous tasks which can be done in parallel, must be done in parallel.

The boot process is seldom linear, one task after an another. Instead, it involves dependencies like: when task1 has finished, start all of task2, task3 and task4 in parallel. And task5 can be started only when both task2 and task3 has completed successfully. And task6 can start when task3 alone has completed, but we really don't care if it finishes properly - it is non essential to a working app.

So, we need to coordinate asynchronous flows, with complex dependencies, while handling failures. Not easy, but that's why they pay us the big bucks.

As Data Please

Because we program in Clojure, we spend time in hammocks dreamily re-watching Rich Hickey videos and meditating on essential truths like "data is the ultimate in late binding".

So, our solution must involve "programming with data" and be, at once, all synonyms of easy.

In One Place

The control flow should be described in just one place, and easily grokable as a unit.

To put that another way: we do not want a programmer to have to look in multiple places to reconstruct a mental model of the overall control flow.

The Solution

re-frame has events. That's how we roll.

A re-frame application can't step forward in time unless an event happens; unless something does a dispatch. Events will be the organising principle in our solution exactly because events are an organising principle within re-frame itself.

Tasks and Events

As you'll soon see, our solution assumes the following about tasks...

If we take an X-ray of an async task, we'll see this event skeleton:

• an event is used to start the task
• if the task succeeds, an event is dispatched
• if the task fails, an event is dispatched

So that's three events: one to start and two ways to finish. Please read that again

• its importance is sometimes missed on first reading. Your tasks must conform to this 3 event structure (which is not hard).

Of course, re-frame will route all three events to their registered handler. The actual WORK of starting the task, or handling the errors, will be done in the event handler that you write.

But, here, none of that menial labour concerns us. Here we care only about the coordination of tasks. We care only that task2 is started when task1 finishes successfully, and we don't need to know what task1 or task2 actually do.

To distill that: we care only that the dispatch to start task2 is fired correctly when we have seen an event saying that task1 finished successfully.

When-E1-Then-E2

Read that last paragraph again. It distills further to: when event E1 happens then dispatch event E2. Or, more pithily again, When-E1-Then-E2.

When-E1-Then-E2 is the simple case, with more complicated variations like:

• when both events E1 and E2 have happened, then dispatch E3
• when either events E1 or E2 happens, then dispatch E3
• when event E1 happens, then dispatch both E2 and E3

We call these "rules". A collection of such rules defines a "flow".

Flow As Data

Collectively, a set of When-E1-then-E2 rules can describe the entire async boot flow of an app.

Here's how that might look in data:

[{:when :seen?        :events :success-db-connect   :dispatch-n '([:do-query-user] [:do-query-site-prefs])}
 {:when :seen-both?   :events [:success-user-query :success-site-prefs-query] :dispatch [:success-boot] :halt? true}
 {:when :seen-any-of? :events [:fail-user-query :fail-site-prefs-query :fail-db-connect] :dispatch [:fail-boot] :halt? true}
 {:when :seen?        :events :success-user-query   :dispatch [:do-intercom]}]

That's a vector of 4 maps (one per line), where each represents a single rule. Try reading each line as if it was an English sentence and something like this should emerge: when we have seen all of events E1 and E2, then dispatch this other event

The structure of each rule (map) is:

{:when       W     ;; one of:  :seen?, :seen-both?, :seen-all-of?, :seen-any-of?
 :events     X     ;; either a single keyword or a seq of keywords representing event ids
 :dispatch   Y     ;; (optional) single vector (to dispatch)
 :dispatch-n Z     ;; (optional) list of vectors (to dispatch)
 :halt?    true}   ;; optional, will teardown the flow after the last event is dispatched

Although optional, only one of :dispatch or :dispatch-n can be specified

In our mythical app, we can't issue a database query until we have a database connection, so the 1st rule (above) says:

1. When :success-db-connect is dispatched, presumably signalling that we have a database connection...
2. then (dispatch [:query-user]) and (dispatch [:query-site-prefs])

We have successfully booted when both database queries succeed, so the 2nd rule says:

1. When both success events have been seen (they may arrive in any order),
2. then (dispatch [:success-queries]) and cleanup because the boot process is done.

If any task fails, then the boot fails, and the app can't start which means go into a failure mode, so the 3rd rules says:

1. If any one of the various tasks fail...
2. then (dispatch [:fail-boot]) and cleanup because the boot process is done.

Once we have user data (from the user-query), we can start the intercom process, so the 4th rules days:

1. When :success-user-query is dispatched
2. then (dispatch [:do-intercom])

Further Notes:

1. The 4th rule starts "Intercom" once we have user data. But notice that nowhere do we wait for a :success-intercom. We want this process started, but it is not essential for the app's function, so we don't wait for it to complete.

2. The coordination processes never actively participate in handling any events. Event handlers themselves do all that work. They know how to handle success or failure - what state to record so that the twirly thing is shown to users, or not. What messages are shown. Etc.

3. A naming convention for events is adopted. Each task can have 3 associated events which are named as follows: :do-* is for starting tasks. Task completion is either :success-* or :fail-*

4. The :halt? value of true means the boot flow is completed. Clean up the flow coordinator. It will have some state somewhere. So get rid of that. And it will have been "sniffing events", so stop doing that, too. You should provide at least one of these in your rules.

5. There's nothing in here about the teardown process as the application is closing. Here we're only helping the boot process.

6. There will need to be something that kicks off the whole flow. In the case above, presumably a (dispatch [:do-connect-db]) is how it all starts.

7. A word on Retries. XXX

The Flow Specification

The :async-flow data structure has the following fields:

• :id - optional - an identifier, typically a namespaced keyword. Each flow should have a unique id. Must not clash with the identifier for any event handler (because internally an event handler is registered using this id). If absent, :async/flow is used. If this default is used then two flows can't be running at once because they'd be using the same id.

• db-path - optional - the path within app-db where the coordination logic should store state. Two pieces of state are stored: the set of seen events, and the set of started tasks. If absent, then state is not stored in app-db and is instead held in an internal atom. We prefer to store state in app-db because we like the philosophy of having all the data in the one place, but it is not essential.

• first-dispatch - optional - the event which initiates the async flow. This is often something like the event which will open a websocket or HTTP GET configuration from the server. If omitted, it is up to you to organise the dispatch of any initial event(s).

• rules - mandatory - a vector of maps. Each map is a rule.

A rule is a map with the following fields:

• :when one of :seen?, :seen-both?. :seen-all-of?, :seen-any-of? :seen?, :seen-both? and :seen-all-of? are interchangeable.
• :events either a single keyword, or a collection of keywords, presumably event ids. a collection can also contain whole event vectors that will be matched, or event predicates that return true or false when passed an event vector.
• :dispatch can be a single vector representing one event to dispatch.
• :dispatch-n to dispatch multiple events, must be a coll where each elem represents one event to dispatch.
• :dispatch-fn can be a function that accepts the seen event, and returns a coll where each elem represents one event to dispatch.
• :halt? optional boolean. If true, the flow enters teardown and stops.

Under The Covers

How does async-flow work? It does the following:

1. It dynamically creates an event handler to perform the flow coordination.
2. It registers this event handler using the supplied :id
3. It requests that all :events mentioned in flow rules should be "forwarded" to this event handler, after they have been handled by their normal handlers. So, if the event :abc was part of flow spec, then after [:abc 1] was handled by its normal handler there would be an additional (dispatch [:async/flow [:abc 1]]) which would be handled the coordinator created in steps 1 and 2.
4. the event handler keeps track of what events have occurred, and what tasks have already been started. It keeps this state at the path nominated in :db-path.
5. the event handler uses your flow specification and the state it internally maintains to work out how it should respond to each newly forwarded event.
6. At some point, the flow finishes (failed or succeeded) and the event handler from step 1 is dispatched a :halt-flow. It de-registers itself, and stops all event sniffing.

Notes:

1. This pattern is flexible. You could use it to implement a more complex FSM coordinator.
2. All the work is done in a normal event handler (dynamically created for you). And it is all organised around events which this event handler processes. So this solution is aligned on re-frame fundamentals.

Advanced use

In some circumstances, it is necessary to hook into not just the event itself, but the data carried in the event. In these cases, functions can be used as an event predicate, or a dispatch rule.

For example, when uploading a file, a success event may return an id which needs to be passed on to a subsequent event.

{:when :seen? :events :upload/success
 :dispatch-fn (fn [[e id]] [[:remote/file-uploaded id]])}

Or, to to dispatch a server error event if a status of 500 or above has been seen

{:when :seen? :events (fn [[e status]] (and (= e :http/response-received) (>= status 500)))
 :dispatch [:server/error]))

Design Philosophy

Managing async task flow means managing time, and managing time requires a state machine. You need:

• some retained state (describing where we have got to)
• events which announce that something has happened or not happened (aka FSM triggers)
• a set of rules about transitioning app state and triggering further activity when events arrive.

One way or another you'll be implementing a state machine. There's no getting away from that.

Although there are ways of hiding it!! Redux-saga uses ES6 generator functions to provide the illusion of a synchronous control flow. The "state machine" is encoded directly into the generator function's statements (sequentially, or via if then else logic, or via loops). And that's a nice and simple abstraction for many cases.

But, as always, there are trade-offs.

First, the state machine is encoded in javascript "code" (the generator function implements the state machine). In clojure, we have a preference for "programming in data" where possible.

Second, coding (in javascript) a more complicated state machine with a bunch of failure states and cascades will ultimately get messy. Time is like a liquid under pressure and it will force its way out through the cracks in the abstraction. A long history with FSM encourages us to implement state machines in a data driven way (a table driven way).

So we choose data and reject the redux-saga approach (while being mindful of the takeoffs).

But it would be quite possible to create a re-frame version of redux-saga. In ClosureScript we have core.async instead of generator functions. That is left as an exercise for the motivated reader.

A motivated user might also produce a fully general FSM version of this effects handler.