Fonda

An async pipeline approach to functional core - imperative shell from by Gary Bernhardt's Boundaries talk.

Minimal Viable Example

This example illustrates fonda's basic mechanics:

(ns example.simple
  (:require [cljs-http.client :as http]
            [cljs.core.async :as cca :include-macros true]
            [clojure.set :as set]
            [fonda.core :as fonda]))

(defn fetch-user
  [ctx]
  (http/get "http://insecure-endpoint.com"
            {:basic-auth (select-keys ctx [:username :password])}))

(fonda/execute
 {:initial-ctx {:username js/process.USER
                :password js/process.PASSWORD}}

 [{:processor  :example.simple/fetch-user                ;; can be either a function or a keyword
   :path       [:github-response]}

  {:processor  :example.simple/github-response->things   ;; pure function - ctx in -> ctx out
   :path       [:github-things]}]

 ;; on-exception
 (fn [exception]
   (handle-exception exception))

 ;; on-success
 (fn [ctx]
   (handle-success (:github-things ctx))))

HINT: The parameter order makes it easy to partially apply execute for leaner call sites.

---

Fonda sequentially executes a series of steps, one after the other, augmenting a context map. The steps can be synchronous or asyncronous. After the steps are run, the termination callbacks will be executed.

If a js/Error, an exception in fonda parlance, is thrown it will be automatically caught and the chain short circuits and the on-exception function is called with the js/Error.

Exceptions are things we can't and don't want to recover from, like unexpected bugs.

Anomalies

Anomaly is

Deviation or departure from the normal or common order, form, or rule.

There is a stark contrast between anomalies and JavaScript js/Errors, Promise rejections or Java Exceptions: anomalies are data that define a recoverable error, like receiving a 409 (conflict) http status code that you can retry.

Anomalies are first class citizens in fonda and by default they are maps containing the :cognitect.anomalies/anomaly key.

If anomaly? is truthy and an anomaly is returned by a step the whole pipeline short circuits and the on-anomaly function is called.

It is also possible to redefine what an anomaly is by passing a config predicate, anomaly?, so that client code can have its own representation of an anomaly as data.

The following section describes the parameters fonda/execute accepts.

Parameters

• config - static configuration map

  Key	Optional?	Notes
  :anomaly?	Yes	A boolean or a function that gets a map and determines if it is an anomaly.
  :initial-ctx	Yes	The data that initializes the context. Must be a map, {} by default.
  :anomaly-handlers	Yes	A map from step name keyword to function that gets called with a map {:ctx <ctx> :anomaly <anomaly>} when the step returns an anomaly.
  :exception-handlers	Yes	A map from step name keyword to function that gets called with a map {:ctx <ctx> :exception <exception>} when the step triggers an exception.

• steps - each item must be either a Tap or a Processor, or a Injector

  • tap

    Key	Optional?	Notes
    :tap	No	A function that gets the context but doesn't augment it. If it succeeds the result is ignored. If asynchronous it will still block the pipeline and interrupt the execution whenever either an anomaly or an exception happen.
    :name	Yes	The name of the step as string or keyword

  • processor

    Key	Optional?	Notes
    :processor	No	A function that gets the context and returns data. The data is assoced-in at the given path Can be asynchronous. If asynchronous it will still block the pipeline and interrupt the execution whenever either an anomaly or an exception happen.
    :path	No	Path where to assoc the result of the processor
    :name	Yes	The name of the step as string or keyword

  • injector

    Key	Optional?	Notes
    :injector	No	A function that gets the context and returns either a step or a collection of steps. The step(s) returned will be executed right after the injector step and just before the next steps. Can be asynchronous.
    :name	Yes	The name of the injector step as string or keyword

• on-exception Function called with an exception when any of the steps throws one.

• on-success Function called with the context if all steps succeed.

• [Optional] on-anomaly Function called in case of anomaly with the anomaly data itself.

Full Example

(ns example.full
  ...)

(defn print-remote-thing
  [{:keys [remote-thing-response]}]
  (println "the remote thing response was:" remote-thing-response))

(defn get-remote-thing
  [ctx]
  (ajax/GET "http://remote-thing-url.com" {:params (:remote-thing-params ctx)}))

(fonda/execute
  {:initial-ctx {:env-var-xyz "value",
                 :remote-thing-params {:p1 "p1" :p2 "p2"}
                 :other-remote-thing-responses []}
   :anomaly-handlers {:get-remote-thing (fn [{:keys [anomaly]}]
                                           (post-error-to-log-server anomaly))}
   :exception-handlers {:get-remote-thing (fn [{:keys [exception]}]
                                              (js/console.log "An exception retrieving the remote thing occurred:" exception))}}

  [{:processor  :example.full/get-remote-thing
    :name       "get-remote-thing"
    :path       [:remote-thing-response]}

   {:tap        :example.full/print-remote-thing}

   {:processor  :other.namespace/process-remote-thing-response
    :path       [:remote-thing]}

   ;; Injector returns a collection of steps to be added right after the injector step
   {:inject         (fn [{:keys [remote-thing]}]
                      (->> (:side-effect-post-urls remote-thing)
                           (map (fn [side-effect-post-url]
                                  {:tap (fn [{:keys [remote-thing-params]}]
                                          (ajax/POST side-effect-post-url remote-thing-params))}))))}]

  ;; on-exception
  (fn [exception]
   (handle-exception exception))

  ;; on-success
  (fn [{:keys [remote-thing-processed]}]
   (handle-success remote-thing-processed))

  ;; on-anomaly
  (fn [anomaly]
   (handle-anomaly anomaly)))

Thanks

The conception of the library has taken place during early research and development work at Elastic Path Software Inc.. A heart-felt thank you goes especially to Matt Bishop for supporting open source.

Trivia

The name got inspired by Jane Fonda's step very successful fitness programs.

As with the fitness program, fonda consist of well-curated 🚶‍♀️ steps 🚶‍♂️.

Contributing

See CONTRIBUTING.md.

License

Copyright 2018-2019, The Fonda Authors

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.