## Flexible React-Redux Hook Mocks in jest & React Testing Library

Best practices in mocking React components aren’t entirely unheard of, even in connection with a Redux state, and even not in connection with the quite convenient Hooks description ({ useSelector, useDispatch}).

So, of course the knowledge of a proper approach is at hand. And in many scenarios, it makes total sense to follow their principle of exactly arranging your Redux state in your test as you would in your real-world app.

Nevertheless, there are reasons why one wants to introduce a quick, non-overwhelming unit test of a particular component, e.g. when your system is in a state of high fluctuation because multiple parties are still converging on their interfaces and requirements; and a complete mock would be quite premature, more of a major distraction, less of being any help.

Proponents of strict TDD would now object, of course. Anyway – Fortunately, the combination of jest with React Testing Library is flexible enough to give you the tools to drill into any of your state-connected components without much knowledge of the rest of your React architecture*

(*) of course, these tests presume knowledge of your current Redux store structure. In the long run, I’d also consider this bad style, but during highly fluctuatig phases of develpment, I’d favour the explicit “this is how the store is intended to look” as safety by documentation.

On a basic test frame, I want to show you three things:

1. Mocking useSelector in a way that allows for multiple calls
2. Mocking useDispatch in a way that allows expecting a specific action creator to be called.
3. Mocking useSelector in a way that allows for mocking a custom selector without its actual implementation

(Upcoming in a future blog post: Mocking useDispatch in a way to allow for async dispatch-chaining as known from Thunk / Redux Toolkit. But I’m still figuring out how to exactly do it…)

So consider your component e.g. as a simple as:

import {useDispatch, useSelector} from "react-redux";
import {importantAction} from "./place_where_these_are_defined";

const TargetComponent = () => {
const dispatch = useDispatch();
const simpleThing1 = useSelector(store => store.thing1);
const simpleThing2 = useSelector(store => store.somewhere.thing2);

return <>
<div>{simpleThing1}</div>
<div>{simpleThing2}</div>
<button title={"button title!"} onClick={() => dispatch(importantAction())}>Do Important Action!</button>
</>;
};

## Multiple useSelector() calls

If we had a single call to useSelector, we’d be as easily done as making useSelector a jest.fn() with a mockReturnValue(). But we don’t want to constrain ourselves to that. So, what works, in our example, to actually construct a mockin store as plain object, and give our mocked useSelector a mockImplementation that applies its argument (which, as selector, is a function of the store)) to that store.

Note that for this simple example, I did not concern myself with useDispatch() that much. It just returns a dispatch function of () => {}, i.e. it won’t throw an error but also doesn’t do anything else.

import React from 'react';
import { render, screen, fireEvent } from '@testing-library/react';
import TargetComponent from './TargetComponent;
import * as reactRedux from 'react-redux';
import * as ourActions from './actions';

jest.mock("react-redux", () => ({
useSelector: jest.fn(),
useDispatch: jest.fn(),
}));

describe('Test TargetComponent', () => {

beforeEach(() => {
useDispatchMock.mockImplementation(() => () => {});
useSelectorMock.mockImplementation(selector => selector(mockStore));
})
afterEach(() => {
useDispatchMock.mockClear();
useSelectorMock.mockClear();
})

const useSelectorMock = reactRedux.useSelector;
const useDispatchMock = reactRedux.useDispatch;

const mockStore = {
thing1: 'this is thing1',
somewhere: {
thing2: 'and I am thing2!',
}
};

it('shows thing1 and thing2', () => {
render(<TargetComponent/>);
expect(screen.getByText('this is thing1').toBeInTheDocument();
expect(screen.getByText('and I am thing2!').toBeInTheDocument();
});

});

This is surprisingly simple considering that one doesn’t find this example scattered all over the internet. If, for some reason, one would require more stuff from react-redux, you can always spread it in there,

jest.mock("react-redux", () => ({
...jest.requireActual("react-redux"),
useSelector: jest.fn(),
useDispatch: jest.fn(),
}));

but remember that in case you want to build full-fledged test suites, why not go the extra mile to construct your own Test store (cf. link above)? Let’s stay simple here.

## Assert execution of a specific action

We don’t even have to change much to look for the call of a specific action. Remember, we presume that our action creator is doing the right thing already, for this example we just want to know that our button actually dispatches it. E.g. you could have connected that to various conditions, the button might be disabled or whatever, … so that could be less trivial than our example.

We just need to know how the original action creator looked like. In jest language, this is known as spying. We add the blue parts:

// ... next to the other imports...
import * as ourActions from './actions';

//... and below this block
const useSelectorMock = reactRedux.useSelector;
const useDispatchMock = reactRedux.useDispatch;

const importantAction = jest.spyOn(ourActions, 'importantAction');

//...

//... other tests...

it('dispatches importantAction', () => {
render(<TargetComponent/>);
const button = screen.getByTitle("button title!"); // there are many ways to get the Button itself. i.e. screen.getByRole('button') if there is only one button, or in order to be really safe, with screen.getByTestId() and the data-testid="..." attribute.
fireEvent.click(button);
expect(importantAction).toHaveBeenCalled();
});


That’s basically it. Remember, that we really disfigured our dispatch() function. What we can not do this way, is a form of

// arrangement
const mockDispatch = jest.fn();
useDispatchMock.mockImplementation(() => mockDispatch);

// test case:
expect(mockDispatch).toHaveBeenCalledWith(importantAction()); // won't work

Because even if we get a mocked version of dispatch() that way, the spyed-on importantAction() call is not the same as the one that happened inside render(). So again. In our limited sense, we just don’t do it. Dispatch() doesn’t do anything, importantAction just gets called once inside.

## Mock a custom selector

Consider now that there are custom selectors which we don’t care about much, we just need them to not throw any error. I.e. below the definition of simpleThing2, this could look like

import {useDispatch, useSelector} from "react-redux";
import {importantAction, ourSuperComplexCustomSelector} from "./place_where_these_are_defined";

const TargetComponent = () => {
const dispatch = useDispatch();
const simpleThing1 = useSelector(store => store.thing1);
const simpleThing2 = useSelector(store => store.somewhere.thing2);
const complexThing = useSelector(ourSuperComplexCustomSelector);

//... whathever you want to do with it....
};

Here, we want to keep it open how exactly complexThing is gained. This selector is considered to already be tested in its own unit test, we just want its value to not-fail and we can really do it like this, blue parts added / changed:

import React from 'react';
import { render, screen, fireEvent } from '@testing-library/react';
import TargetComponent from './TargetComponent;
import * as reactRedux from 'react-redux';
import * as ourActions from './actions';
import {ourSuperComplexCustomSelector} from "./place_where_these_are_defined";

jest.mock("react-redux", () => ({
useSelector: jest.fn(),
useDispatch: jest.fn(),
}));

const mockSelectors = (selector, store) => {
if (selector === ourSuperComplexCustomSelector) {
return true;  // or what we want to
}
return selector(store);
}

describe('Test TargetComponent', () => {

beforeEach(() => {
useDispatchMock.mockImplementation(() => () => {});
useSelectorMock.mockImplementation(selector => mockSelectors(selector, mockStore));
})
afterEach(() => {
useDispatchMock.mockClear();
useSelectorMock.mockClear();
})

const useSelectorMock = reactRedux.useSelector;
const useDispatchMock = reactRedux.useDispatch;

const mockStore = {
thing1: 'this is thing1',
somewhere: {
thing2: 'and I am thing2!',
}
};

// ... rest stays as it is
});

This wasn’t as obvious to me as you never know what jest is doing behind the scenes. But indeed, you don’t have to spy on anything for this simple test, there is really functional identity of ourSuperComplexCustomSelector inside the TargetComponent and the argument of useSelector.

## So, yeah.

The combination of jest with React Testing Library is obviously quite flexible in allowing you to choose what you actually want to test. This was good news for me, as testing frameworks in general might try to impose their opinions on your style, which isn’t always bad – but in a highly changing environment as is anything that involves React and Redux, sometimes you just want to have a very basic test case in order to concern yourself with other stuff.

So, without wanting that you lower your style to such basic constructs, I hope this was of some insight for you. In a more production-ready state, I would still go the way as that krawaller.se blog post state above, it makes sense. I was just here to get you started 😉

A few weeks ago, I heard a nice story about the hidden cost of new features. Imagine a website, driven by a content management system, consisting of text, pictures and fancy styling. When the content management system gets an update, the website developer takes a look at the release notes and finds that a lot of new and cool features are included that you’ll get for free once you update.

So he updates the site, tries it out and publishes it onto the web. A few days later, the customer and owner of the website sends a bug report of some arbitrarily flipped images. There are just short of a hundred images on the website and a handful of them now show up upside down.

Who would update a website and randomly rotate some images?

Why would a content management system decide that excatly these images need a spin?

The answer is not as obvious as one might think.

The latest cause of the effect was a change of the imaging library the content management system uses to deliver the image content. It got upgraded to a new engine that essentially does the same thing as the old one: Take the image file content and put it on the web. But, it does it more thoroughly.

One feature of JPEG images are the EXIF metadata properties. Examples of useful properties are the photography time, the geolocation or the camera model. Some cameras add even more information into the metadata, like exposure time or the camera’s orientation (rotation) during the photographing process. There are cameras that notice if you hold them upside down and store this circumstance into the picture.

Then, there are imaging libraries that just take the pixels and put them on the screen. And there are libraries that know about their domain and read the EXIF metadata, interpret the rotation data and accomodate for that fact. Because, who would like to look at pictures that are displayed totally wrong?

The first version of the content management system’s imaging library didn’t care much about metadata. The new version takes rotation into account.

So, the cause of the suddenly rotated pictures originates with the photographer that happened to work during a workout session or in australia. This fact was registered and stored by the camera and promply ignored by the picture editing software and the earlier content management system. It was rediscovered only when the new version went live.

For the customer, this is a random regression. It worked just fine all those years! For the developer, this is a minefield. Every picture could contain an evil rotation information that gets applied someday.

For a security engineer, this is a harmless but perfect example of a persistence attack. You embed malicious payloads into data that do nothing for a long time, but are activated suddenly, without outside intervention, by an unrelated change of system parts towards a “lucky” constellation.

Guess what you can embed into EXIF metadata, too? Javascript or any other form of executable code. And then you wait.

To end this blog entry on a light note, sometimes the payload may just happen to be your last name – True!

## React for the algebra enthusiast – Part 2

In Part 1, I explained how algebra can shed some light on a quite restricted class of react-apps. Today, I will lift one of the restrictions. This step needs a new kind of algebraic structure:

## Categories

Category theory is a large branch of pure mathematics, with many facets and applications. Most of the latter are internal to pure mathematics. Since I have a very special application in mind, I will give you a definition which is less general than the most common ones.

Categories can be thought of as generalized monoids. At the same time, a Category is a labelled, directed multigraph with some extra structure. Here is a picture of a labelled directed multigraph – its nodes are labelled with upper case letters and its edges are labelled with lower case letters:

If such a graph happens to be a category, the nodes are called objects and the edges morphisms. The idea is, that the objects are changed or morphed into other objects by the morphisms. We will write $h:A\to B$ for a morphism from object $A$ to object $B$.

But I said something about extra structure and that categories generalize monoids. This extra structure is essentially a monoid structure on the morphisms of a category, except that there is a unit for each object called identity and the operation “$\_\circ\_$” can only be applied to morphisms, if they form “a line”. For example, if we have morphisms like k and i in the picture below, in a category, there will be a new morphism “$k\circ i$“:

Note that “$i$” is on the right in “$k\circ i$” but it is the first morphism if you follow the direction indicated by the arrows. This comes from function composition in mathematics, which suffers from the same weirdness by some historical accident. For now that just means that chains of morphisms have to be read from right to left to make sense of them.

For the indentities and the operation “$\_\circ\_$“, we can ask for the same laws to hold as in a monoid, which will complete the definition of a category:

## Definition (not as general as it could be…)

A category consists of the following data:

• A set of objects A,B,…
• A set of morphisms $f : A_1\to B_1, g:A_2\to B_2,\dots$
• An operation “$\_\circ\_$” which for all (consecutive) pairs of morphisms $f:A\to B$ and $g:B\to C$ returns a morphism $g \circ f : A \to C$
• For any object a morphism $\mathrm{id}_A : A\to A$

Such that the following laws hold:

• $\_\circ\_$ ” is associative: For all morphisms $f : A \to B$, $g : B\to C$ and $h : C\to D$, we have: $h \circ (g \circ f) = (h \circ g) \circ f$
• The identities are left and right neutral: For all morphisms $f: A\to B$ we have: $f \circ \mathrm{id}_A=\mathrm{id}_B \circ f$

## Examples

Before we go to our example of interest, let us look at some examples:

• Any monoid is a category with one object O and for each element m of the monoid a morphism $m:O\to O$. “$m\circ n$” is defined to be $m\cdot n$.
• The graph below can be extended to a category by adding the morhpisms $ef: B\to B, fe: A\to A, efe: A\to B, fef: B\to A, \dots$ and an identity for $A$ and $B$. The operation “$\_\circ\_$” is defined as juxtaposition, where we treat the identities as empty sequences. So for example, $ef\circ efe$ is $efefe: A\to B$.
• More generally: Let $G$ be a labelled directed graph with edges $e_1,\dots,e_r$ and nodes $n_1,\dots,n_l$. Then there is a category $C_G$ with objects $n_1,\dots,n_l$ and morphisms all sequences of consectutive edges – including the empty sequence for any node.

## Action Categories

So let’s generalize Part 1 with our new tool. Our new scope are react-apps, which have actions without parameters, but now, action can not neccessarily be applied in any order. If an action can be fired, may now depend on the state of the app.

The smallest example I can think of, where we can see whats new, is an app with two states, let’s call them ON and OFF and two actions, let’s say SWITCH_ON and SWITCH_OFF:

Let us also say, that the action SWITCH_ON can only be fired in state OFF and SWITCH_OFF only in state ON. The category for that graph has as its morphims the possible sequences of actions. Now, if we follow the path of part 1, the obvious next step is to say that SWITCH_ON after SWITCH_OFF (and the other way around) is the same as the empty action-sequence — which leads us to…

## Quotients

We made a pretty hefty generalization from monoids to categories, but the theory for quotients remains essentially the same. As we defined equivalence relations on the elements of a monoid, we can define equivalence relations on the morphisms of a category. As last time, this is problematic in general, but turns out to just work if we replace sequences of morphisms in the action category with matching source and target.

So in the example above, it is ok to say that SWITCH_ON SWITCH_OFF is the empty sequence on ON and SWITCH_OFF SWITCH_ON is the empty sequence on OFF (keep in mind that the first action to be executed is on the right). Then any action sequence can be reduced to simply SWITCH_ON, SWITCH_OFF or an empty sequence (not the empty sequence, because we have two of them with different source and target). And in this case, the quotient category will be what we drew above, but as a category.

Of course, this is not an example where any high-powered math is needed to get any insights. So far, these posts where just about understanding how the math works. For the next part of this series, my plan is to show how existing tools can be used to calculate larger examples.

## Bridging Eons in Web Dev with Polyfills

Indeed, web development is kind of peculiar. On the one hand, there‘s seldom a field in which new technologies overturn each other at that pace, creating very exciting opportunities ranging from quickly sketching out proof-of-concepts to the efficient construction of real-world applications. On the other hand, there is this strange air of browser dependency and with any new technology one acquires, there‘s always the question of whether this is just some temporary fashion or here to stay.

Which is why it hapens, that one would like to quickly scaffold a web application on the base of React and its ecosystem, but has the requirement that the customer is – either voluntarily or forced by higher powers – using some legacy browser like Internet Explorer 11, for which Microsoft has recently announced its end of life support for 30th November this year. Which doesn’t sound nice for the… *searching quickly* … 5% of desktop/laptop users that still use this old horse, but then again, how long can you cling to an outdated thing?

For the daily life of a web developer, his mind full of peculiarities that the evolution of the ECMAScript standard which basically is JavaScript, there is the practical helper of caniuse.com, telling you for every item of your code you want to know about, which browser / device has support and which doesn’t.

But what about whole frameworks? When I recently had my quest for a IE11-comptabile React app, I already feared that at every corner, I needed to double-check all my doing, especially given that for the development itself, one is certainly advised to instead use one of the browsers that come with a quite some helpful developer tools, like extensions for React, Redux, etc. — but also the features in the built-in Console, where it makes your life a lot easier whether you can just log a certain state as a string of “[Object object]” or a fully interactive display of object properties. Sorry IE11, there are reasons why you have to go.

But actually, then, I figured, that my request is maybe not that far outside the range of rather widespread use cases. Thus, the chance that someone already tried to tackle the problem, aren’t so hopeless. And so this works pretty straightforward:

• Install “react-app-polyfill”, e.g. via npm:
npm install react-app-polyfill

• At the very top of your index.js, add for good measure:
import "react-app-polyfill/ie11";
import "react-app-polyfill/stable";

• Include “IE 11” (with quotes) in your package.json under the “browserlist” as a new entry under “production” and “development”

That should do it. There are people on the internet that advise removing the “node_modules/.cache” directory when doing this in an existing project.

The term of a polyfill is actually derived from some kind of putty, which is actually a nice picture. It’s all about allowing a developer to use accustomed features while maintaining the actual production environment.

Another very useful polyfill in this undertaking was…

// install
npm install --save-dev @babel/plugin-transform-arrow-functions

// then add to the "babel" > "plugins" config array:
"babel": {
"plugins": [
"@babel/plugin-transform-arrow-functions"
]
}


… as I find the new-fashioned arrow function notation quite useful.

So, this seems to bridge (most of) the worries one encounters in this web dev world where use cases span eons of technology evolution. Now, do you know any more useful polyfills that make your life easier?

## React for the algebra enthusiast – Part 1

When I learned to use the react framework, I always had the feeling that it is written in a very mathy way. Since simple googling did not give me any hints if this was a consideration in the design, I thought it might be worth sharing my thoughts on that. I should mention that I am sure others have made the same observations, but it might help algebraist to understand react faster and mathy computer scientiests to remember some algebra.

## Free monoids

In abstract algebra, a monoid is a set M together with a binary operation “$\cdot$” satisfying these two laws:

• There is a neutral element “e”, such that: $\forall x \in M: x \cdot e = e \cdot x = e$
• The operation is associative, i.e. $\forall x,y,z \in M: x \cdot (y\cdot z) = (x\cdot y) \cdot z$

Here are some examples:

• Any set with exactly one element together with the unique choice of operation on it.
• The natural numbers $\mathbb{N}=\{0,1,2,\dots \}$ with addition.
• The one-based natural numbers $\mathbb{N}_1=\{1,2,3,\dots\}$ with multiplication.
• The Integers $\mathbb Z$ with addition.
• For any set M, the set of maps from M to M is a monoid with composition of maps.
• For any set A, we can construct the set List(A), consisting of all finite lists of elements of A. List(A) is a monoid with concatenation of lists. We will denote lists like this: $[1,2,3,\dots]$

Monoids of the form List(A) are called free. With “of the form” I mean that the elements of the sets can be renamed so that sets and operations are the same. For example, the monoid $\mathbb{N}$ with addition and List({1}) are of the same form, witnessed by the following renaming scheme:

$0 \mapsto []$

$1 \mapsto [1]$

$2 \mapsto [1,1]$

$3 \mapsto [1,1,1]$

$\dots$

— so addition and appending lists are the same operation under this identification.

With the exception of $\mathbb{N}_1$, the integers and the monoid of maps on a set, all of the examples above are free monoids. There is also a nice abstract definition of “free”, but for the purpose at hand to describe a special kind of monoid, it is good enough to say, that a monoid M is free, if there is a set A such that M is of the form List(A).

## Action monoids

A react-app (and by that I really mean a react+redux app) has a set of actions. An action always has a type, which is usally a string and a possibly empty list of arguments.

Let us stick to a simple app for now, where each action just has a type and nothing else. And let us further assume, that actions can appear in arbirtrary sequences. That means any action can be fired in any state. The latter simplification will keep us clear from more advanced algebra for now.

For a react-app, sequences of actions form a free monoid. Let us look at a simple example: Suppose our app is a counter which starts with “0” and has an increment (I) and decrement (D) action. Then the sequences of action can be represented by strings like

ID, IIDID, DDD, IDI, …

which form a free monoid with juxtaposition of strings. I have to admit, so far this is not very helpful for a practitioner – but I am pretty sure the next step has at least some potential to help in a complicated situation:

## Quotients

Quotients of sets by an equivalence relation are a very basic tool of modern math. For a monoid, it is not clear if a quotient of its underlying set will still be a monoid with the “same” operations.

Let us look at an example, where everything goes well. In the example from above, the counter should show the same integer if we decrement and then increment (or the other way around). So we could say that the two action sequences

• ID and
• DI

do really nothing and should be considered equivalent to the empty action sequence. So let’s say that any sequence of actions is equivalent to the same sequence with any occurence of “DI” or “ID” deleted. So for example we get:

IIDIIDD $\sim$ I

With this rule, we can reduce any sequence to an equivalent one that is a sequence of Is, a sequence of Ds or empty. So the quotient monoid can be identified with the integers (in two different ways, but that’s ok) and addition corresponds to juxtaposition of action sequences.

The point of this example and the moral of this post is, that we can take a syntactic description (the monoid of action sequences), which is easy to derive from the source code and look at a quotient of the action monoid by a reasonable relation to arrive at some algebraic structure which has a lot to do with the semantic of the app.

So the question remains, if this works just well for an example or if we have a general recipe.

Here is a problem in the general situation: Let $x,y,z\in M$ be elements of a monoid $M$ with operation “$\cdot$” and $\sim$ be an equivalence relation such that $x$ is identified with $y$. Then, denoting equivalence classes with $[\_]$ it is not clear if $[x] \cdot [y]$ should be defined to be $[x\cdot z]$ or $[y\cdot z]$.

Fortunately problems like that disappear for free monoids like our action monoid and equivalence relations constructed in a specific way. As you can see on wikipedia, it is always ok to take the equivalence relation generated by the same kind of identifications we made above: Pick some pairs of sequences which are known “to do the same” from a semantic point of view (like “ID” and “DI” did the same as the empty sequence) and declare sequences to be equivalent, if they arise by replacing sequences known to be the same.

So the approach is that general: It works for apps, where actions do not have parameters and can be fired in any order and for equivalence relations generated by defining finitely many action sequences to do the same. The “any order” is a real restriction, but this post also has a “Part 1” in the title…

## Changing the keyboard navigation behaviour of form inputs

The default behaviour in HTML forms is that you can move the focus from one input element to the next via the tab key and submit the form via the enter key. This is also how dialogs work on most operating systems when using the native UI components. This behaviour is consistent across all browsers, and changing it messes with the user’s expectations and reduces accessibility. So I would normally advise against changing this behaviour without good reasons.

However, one of our customers wanted a different behaviour for an application developed by us. This application replaced an older application where the enter key did not submit the form, but moved the focus to the next input element. The ‘muscle memory’ effect made users accidentally submit the form by hitting the enter key, causing frustration. Since this application is not a public web site, but merely a web technology based intranet application with a small and specialized user base, changing the default behaviour is acceptable if the users want it.

So here’s how to do it. The following JavaScript function focusNextInputOnEnter takes a form element as a parameter and changes the focus behaviour on the input elements within this form.

function focusNextInputOnEnter(form) {
var inputs = form.querySelectorAll('input, select, textarea');
for (var i = 0; i < inputs.length; i++) {
var input = inputs[i];
return function(event) {
if (!isEnter(event.which)) {
return;
}
var nextIndex = index + 1;
while (nextIndex < inputs.length) {
var nextInput = inputs[nextIndex];
if (nextInput.disabled) {
nextIndex++;
continue;
}
nextInput.focus();
break;
}
};
})(i));
}

function isEnter(keyCode) {
return keyCode === 13;
}
}


It works by handling the keypress events on the input elements and checking the key code for the enter key (code 13). It has an additional check so that disabled input elements are skipped.

To apply this change in behaviour to a form we have to call the function when the DOM content is loaded:

<form id="demo-form">
<input type="text">
<input type="text" disabled="disabled">
<input type="checkbox">
<select>
<option>A</option>
<option>B</option>
</select>
<textarea></textarea>
<input type="text">
<input type="text">
</form>

<script>
focusNextInputOnEnter(document.getElementById('demo-form'));
});
</script>


I want to reiterate my warning that you should definitely not do this for public web sites, and elsewhere only if you know that this is what your users want.

## Using (elastic)search with .NET Core

Many modern applications require powerful search mechanisms to become useful and make their users more productive. That is in large part due to the amount of data available to work with. Thankfully there are already powerful tools to index your data and make it searchable.

One of the most well known state-of-the-art solutions is ElasticSearch and it has an API to be used from .NET called NEST. While the documentation is ok I want to give a quick rundown on how to add searching capabilities to your .NET Core application. Some ideas are borrowed from the great post “Using Elasticsearch with ASP.NET Core and Docker“.

## Getting ElasticSearch running

The easiest way to get up and running with elasicsearch is to use their docker images and just run the container on your development machine. I like to a docker compose file like the following to get elasticsearch and its tooling application kibana up and running fast:

version: '3.8'
services:
elasticsearch:
image: docker.elastic.co/elasticsearch/elasticsearch:7.8.0
container_name: elastic
environment:
- node.name=elastic
- cluster.initial_master_nodes=elastic
ports:
- "9200:9200"
- "9300:9300"
volumes:
- type: bind
source: ./esdata
target: /usr/share/elasticsearch/data
networks:
- esnetwork
kibana:
image: docker.elastic.co/kibana/kibana:7.8.0
ports:
- "5601:5601"
networks:
- esnetwork
depends_on:
- elasticsearch
volumes:
esdata:
networks:
esnetwork:
driver: bridge


After you run it with docker compose you can talk to the search service on http://localhost:9200/ and the kibana management GUI on http://localhost:5601/. On the Kibana UI especially the Dev Tools and its console are interesting for experimenting with search queries.

## Access ElasticSearch from your .NET Core app

I find it quite elegant to write a extension method for the IServiceCollection to configure the ElasticClient and register it as a Singleton to the dependency injection framework of .NET Core like so

    public static class ElasticSearchExtension
{
this IServiceCollection services, IConfiguration configuration)
{
var url = configuration["elasticsearch:url"];
var settings = new ConnectionSettings(new Uri(url))
.DefaultMappingFor<SearchableDevice>(deviceMapping => deviceMapping
.IndexName("devices")
.IdProperty(dev => dev.Id)
)
;
var client = new ElasticClient(settings);
var response = client.Indices.Create("devices", creator => creator
.Map<SearchableDevice>(device => device
.AutoMap()
)
);
// maybe check response to be safe...
}
}


The configuration block looks like following

  "ElasticSearch": {
"url": "http://localhost:9200/"
},


and allows for future extension.

Our search service has to be registered in Startup of course:

public void ConfigureServices(IServiceCollection services)
{
...
}


## Indexing our objects

In the above example we built a SearchableDevice class whose public properties are going to be indexed by ElasticSearch. The API allows for a much more fined grained control about what and how to index but we want to keep things simple without having to worry about excluding Properties and so on. If you have set it up that way indexing a SearchableDevice is merely one simple call:

// SearchClient is the injected IElasticClient
// mySearchableDevice is an instance of SearchableDevice
SearchClient.IndexDocument(mySearchableDevice);


## Searching for objects

When developing a search query I like to try it in the Kibana Dev Tools and then transform it to a NEST call. A simple query to look in all device properties if they start with “needle” looks like this:

{
"query": {
"multi_match": {
"type": "phrase_prefix",
"fields": ["*"],
"query": "needle"
}
}
}


The nice thing about the Kibana Dev Tools console is that it can display and complete the possible values for fields like “type” in your multi_match query.

That search query can then be translated to a NEST call in a straightforward way and looks this way:

var response = SearchClient.Search<SearchableDevice>(sd => sd
.Query(q => q
.MultiMatch(query => query
.Type(TextQueryType.PhrasePrefix)
.Fields("*")
.Query("needle")
)
)
);


The search response contains the hits with their source objects and some metadata like the score and result count.

Bear in mind that ElasticSearch only returns the first/best 10 matches by default, so specifying the result size often might be closer to what you want.

## Wrapping it up

Getting started with ElasticSearch in .NET Core does not require too much boiler plate an setup work if you use tools like docker and the NEST library. Making it usable and tuning the indexing and querying may require a lot of work to achieve the best results. On the other hand smaller applications can start-off with a simple search setup like shown above and simply evolve it when need be.

## State Management for emotionally overwhelmed React rookies

State Management for overwhelmed React rookies

The topic of React state management is nowhere near new. Just to be safe what we‘re talking about here: Consider a multitude of components, which, in nice React-fashion, are finely interleaved into each other, each one with a Single Responsibility, each one only holding as much information as it needs. Depending on the complexity of your application, there can now be a lot of complex dependencies, where one small component somewhere might cause another small component totally-elsewhere to update (re-render), without these having to really know much about each other, because we strive for Low Coupling. In front-end development, this is not only done in terms of „cleaner code“, but also in the performance problem of having to re-render stuff that is not actually changing.

So, just a few months back, a new competitor appeared in the question of React state management, which was open-sourced by Facebook and is called Recoil. The older top dog in this field is the widely-used Redux, with smaller interventions of libraries like MobX, that also aimed to offer an alternative of managing state in smaller applications, and when React in version 16.3 opened up a new standard of Context API, it already officially advanced quite a step into the direction of an official React solution to these questions.

There‘s probably not a single web developer on earth who wouldn‘t agree that in our field, one of the most fun…fundamental challenges is the effort of staying afloat on top of the turbulent JavaScript-osphere. If you are the type of person who doesn‘t want to jump on every bandwagon, but still don‘t want to miss out on all the amazing opportunities that all this progress could give you, you better start a bunch of side projects (call them „recreational“ if you like) and give yourself a chance to dive into particular technologies with confined scope, for some research time.

This is what I‘ve done now and I try to focus completely on the issues that an ambitious developer can experience when having all these choices. This is what I want to outline for you now, because as usual – if you have lots of time studying a single technology, you can succeed in spite of many limitations, and you also get used to certain kinds of things you might have to do that you originally didn‘t want to do, and so on and so on.

So with Redux, nobody really appeared to talk a lot of bad things about it and there even are some Mariuses who seem to be absolutely in love with the official Redux documentation, that are actually more of a guide to time-tested Best Practices, giving you the opportunity to do things right and have a scaleable state container which supports you even if your application grows to large dimensions. Then there‘s stuff like a time-travelling state debugger and the flexible middleware integration which I didn‘t even touch yet. When your project has a number of unrelated data structures, there‘s the Ducks pattern that advises you to organize your required Reducers, Actions and Action Creators in a coherently arranged files. Which, however, turned complicated in my one project in which the types of data objects aren‘t as unrelated, and I had to remove all the combineReducer() logic and ended up with one large global state object; I now have one source file that just consists of everything-related-with-redux and for my purpose, this seems fine, but I still have to write rather cumbersome connect(mapStateToProps, mapDispatchToProps) structure in every component in which I want to access the state. I would prefer to have smaller state containers, but maybe it‘s due to the structure of my project that makes these complicated.

It really is that way: Due to the everchanging recommendations that come with the evolution of React, the question of how to do things best (read: best for your specific purpose), always stays fresh. Since React 16.8 and the arrival of Hooks there is a procession towards less boilerplate code, favoring functional components with a leaner appearance. In this spirit, I strived for something less Redux-y. E.g. if I want some text in my state to be set; I would have to do something like

// ./ducks/TextDucks.js
// avoid having to rely on a magical string, therefore save the string to a constant
const SET_TEXT = 'SET_TEXT';

// action creator
export const setTextCreator = (text) ==> ({type: CLEAR_TEXT, payload: {text}});

const Reducer = (state = initialState, {type, payload}) => {
//... other state stuff
if (type === SET_TEXT) {
}
}

================
// Component file
import {setTextCreator} from './ducks/TextDucks.js';
const mapDispatchToProps = (dispatch) => ({
setText: text => dispatch(setTextCreator(text));
});
const Component = ({setText, ...}) => {
// here can I actually use setText()
};
export default connect(..., mapDispatchToProps)(Component);


Which is more organized than passing along some setText(‘‘) function through my whole component tree, but still feels a bit overhead.

Then there was MobX. Which seemed to be quite lightweight and clearly laid out a coherent use of the Observable pattern, implemented with its own JavaScript decorators that follow a simple structure. Still, however, the lookandfeel of this code would appear to differ quite a lot from my usual coding style, which kept me from actually using it. This decision was then advanced by certain statements online, who, some years ago, actually predicted that the advancement of React’s own Context API would make any third-party library redundant. Now to be fair, React’s current Context API, with its useReducer() and useContext() also makes it possible to imiate a Redux-like structure already, but consider it as ways of thinking: If you write your code in the same style as you would with Redux’ recommendations, why not use it directly? Clearly, the point of avoiding Redux should go towards the direction of thinking differently.

The Context API actually supplies the underlying structure on which Redux’ own <Provider> builds. Insofar, it is not a big astonishment that you can accomplish similar things with it. Using the Context API, you wrap your whole Application like

// myContext.js
import React from "react";
const TextContext = React.createContext();
export default TextContext;

// App.js
import TextContext from './myContext';
const App = () => <TextContext.Provider value={"initial text"}>{/* actual app components here */}</TextContext.Provider>;

// some subComponent.js
import React from 'react';
import TextContext fom './myContext';
const SubComponent = (props) => {
const [text, setText] = React.useContext(TextContext);
// now use setText() as you would with a local React useState dispatch function..
}


Personally, I found that arrangement a bit clearer than the Redux structure, if you ‘re not used to Redux’ way of thinking anyway. However, if your state grows and is more than just a text, you would either keep state information in one large object again, or wrap your <App/> in a ton of different Contexts which I personally disdained already when I just had three different global state variables to manage.

Having all these possbilities at hand, one might wonder why Facebook felt the need to implement a new way of state management with Recoil. Recoil is still in its experimental phase, however, it didn’t take long to find one aspect very promising: The coding style of managing state in Recoil feels a lot like writing React code itself, which itself makes it very smooth to operate, as you don’t have to treat global state much different than local state. Our example would look like this

// textState.js
import * as Recoil from 'recoil';
export const text = Recoil.atom({key: 'someUniqueKey', default: 'inital text'});

// App.js
import {RecoilRoot} from 'recoil';
const App = () => <RecoilRoot>{/* here the actual app components */}</RecoilRoot>

// some Component.js
import * as TextState from './textState';
const [text, setText] = Recoil.useRecoilState(TextState.text);
// from then on, you can use setText() like you would as a React useState dispatch function


Even more simple, with Recoil you directly have access to the single useRecoilValue() function to just read the text value, and the single useSetRecoilState() function to just dispatch a new text. This avoids the complication of having to re-think your treating of whatever-in-your-state-is-global differently from what is local. Your <App/> component doesn’t grow to ugly levels of intendation, and you can neatly organize everything state-related in a separate file.

As someone who considers himself quite eager to learn new technologies, but also wants to quickly see some results without having to learn a lot of fresh basic understanding first, I had the most fun trying out Recoil in my projects, I have to admit. However, I totally believe that the demise of Redux is not closing in that soon at all, due to its focus on sustainability. For the future, I would aim to see my one Recoil project grow, and I keep you updated on how well this grows…

## Updating Grails 3.3.x to 4.0.x

We have a long history of maintaining a fairly large grails application which we took from Grails 1.0 to 4.0. We sometimes decided to skip some intermediate releases of the framework because of problems or missing incentives to upgrade. If your are interested in our experiences of the past, feel free to have a look our stories:

This is the next installment of our journey to the latest and greatest version of the Grails framework. This time the changes do not seem as intimidating like going from 2.x to 3.x. There are less moving parts, at least from the perspective of an application developer where almost everything stayed the same (gradle build system, YAML configuration, Geb functional tests etc.). Under the hood there are of course some bigger changes like new major versions of GORM/Hibernate and Spring Boot and the switch to Micronaut as the parent application context.

# The hurdles we faced

• For historical reasons our application uses flush mode “auto”. This does not work until today, see https://github.com/grails/grails-core/issues/11376
• The most work intensive change is that Hibernate 5 requires you to perform your work in transactions. So we have dozens of places where we need to add missing @Transactional annotations to make especially saving domain objects work. Therefore we have to essentially test the whole application.
• The handling of HibernateProxies again became more intransparent which led to numerous IllegalArgumentExceptions (“object ist not an instance of declaring type”). Sometimes we could move from generated hashCode()/equals() implementations to the groovy-Annotation @EqualsAndHashCode (actually a good thing) whereas in other places we did manual unwrapping or switched to eager fetching to avoid these problems.

In addition we faced minor gotchas like changed configuration entries. The one that cost us some hours was the subtle change of server.contextPath to server.servlet.context-path but nothing major or blocking.

We also had to transform many of our unit and integration tests to Spock and the new Grails Testing Support framework. It makes the tests more readable anyway and feels more fruitful than trying to debug the old-style Grails Test Mixins based tests.

# Improvements

One major improvement for us in the Grails ecosystem is the good news that the shiro plugin is again officially available, maintained and cleaned up (see https://github.com/nerdErg/grails-shiro). Now we do not need to use our own poor man’s port anymore.

# Open questions

Regarding the proclaimed performance improvements and reduced memory consumptions we do not have final numbers or impressions yet. We will deliver results on this front in the future.

More important is an incovenience we are still facing regarding hot-code-reloading. It does not work for us even using OpenJDK 8 with the old spring-loaded mechanism. The new restart-style of micronaut/spring-boot is not really productive for us because the startup times can easily reach the minute range even on fast hardware.

# Pro-Tip

My hottest advice for you is this one:

Create a fresh Grails 4 app and compare central files like application.yml and build.gradle to get up to the state-of-the-art.

# Conclusion

While this upgrade still was a lot of work and meant many places had to be touched it was a lot smoother than many of the previous ones. We hope that things improve further in the future as the technological stack is up-to-date and much more mature than in the early days…

## Adding a dynamic React page to your classic grails multi-page application

We are developing and maintaining a more than 10 years old classic multi-page application based on the Grails web framework. With the advent of HTML 5 and modern browsers with faster JavaScript engines user expect more and more dynamic and pleasant user experience (UX) from web applications. Our application is used by hundreds of users and our customer expects a stable, familiar and feature-rich experience that continues to improve over time. Something like a complete rewrite of the UI is way out of scope time- and budget-wise.

One of the new feature requests would benefit highly from a client-side JavaScript implementation so we looked at our options. Fortunately it is quite easy to integrate a react app with grails and the gradle build system. So we implemented the new page almost completely as a react app while leaving all the other pages as normal server-side rendered Groovy Server Pages (GSP). The result is quite convincing and opens up a transition path to more and more dynamic client-side pages and perhaps even to the complete transformation to a single-page-application (SPA) in a distant future.

# Integrating a React-App into Grails build process

The Grails react-webpack profile can serve as a great starting point to integrate a react app into an existing grails project. First you create the react app for the new page in the folder src/main/webapp, using the create-react-app scripts for example. Then you need to add a $GRAILS_PROJECT/webpack.config.js to configure webpack appropriately like so: var path = require('path'); module.exports = { entry: './src/main/webapp/index.js', output: { path: path.join(__dirname, 'grails-app/assets/javascripts'), publicPath: '/assets/', filename: 'bundle.js' }, module: { rules: [ { test: /\.js$/,
include: path.join(__dirname, 'src/main/webapp'),
use: {
options: {
presets: ["@babel/preset-env", "@babel/preset-react"],
plugins: ["transform-class-properties"]
}
}
},
{
test: /\.css$/, use: [ 'style-loader', 'css-loader' ] }, { test: /\.(jpe?g|png|gif|svg)$/i,
use: {
}
}
]
}
};


The next step is to move the package.json to the \$GRAILS_PROJECT directory because we want gradle tasks to take care of building and bundling it as a grails asset. To make this convenient we add some gradle tasks employing yarn to our build.gradle:

buildscript {
dependencies {
...
}
}

...

apply plugin:"com.moowork.node"

...

node {
version = '12.15.0'
yarnVersion = '1.22.0'
distBaseUrl = 'https://nodejs.org/dist'
}

group = 'build'
description = 'Build the client bundle'
args = ['run', 'bundle']
}

group = 'application'
description = 'Build the client bundle in watch mode'
args = ['run', 'start']
}

bootRun.dependsOn(['bundle'])
assetCompile.dependsOn(['bundle'])

...


Now we have integrated our new react app with the grails build system and packaging. The webpack task allows updating the javascript bundle on the fly so that we have almost the same hot reloading support when developing as with the rest of grails.

# Delivering the react app as a page

Now that we have integrated the react app in the build and packaging process of our grails application we need to deliver it when the new page is requested by the browser. This is quite simple and straightforward and can be achieved with a GSP like so:

<html>
<meta name="layout" content="main"/>
<title>
</title>
<body>
<div id="react-content">
</div>
<asset:javascript src="bundle.js"/>
</body>
</html>


Now you just have to develop the endpoints for the javascript app in form of normal grails controllers rendering JSON instead of GSP views. This is extremely easy using groovy maps and the grails JSON converters:

import grails.converters.JSON

class DataApiController {

def getData = {
def responseData = [
name: 'John',
age: 37
]
render responseData as JSON
}
}


# Conclusion

Grails and its build infrastructure is flexible enough to easily integrate SPA pages into an existing traditional web application. This allows you to deliver modern UX and features expected by nowadays users without completely rewriting your trusty and proven grails application. The process can be gradually and individual pages/views can be renewed when needed. That way you can continually add value to your customer while incrementally modernizing your application.