Exploring Architectural Concepts Building a Card Game

Key Takeaways

  • An actual time card sport involving a couple of participant represents a good playground to train some architectural ideas: flexibility in cloud deployment fashions, reactivity within the entrance finish, testability of advanced eventualities.
  • The first idea to check is flexibility in relation to deploying within the cloud. The finest deployment mannequin to undertake relies on the context, so you will need to keep versatile and reduce the hassle wanted to maneuver from one deployment mannequin to a different if circumstances require a change.
  • The second idea is on the Front End. We need to use a reactive mannequin and examine as much as which level we are able to transfer our logic to a pure Javascript / Typescript layer minimizing the dependencies on the precise library / framework we select to make use of. 
  • The third idea is about testing, particularly testing interactive multi consumer eventualities. In these eventualities a couple of consumer is linked to the app and makes use of it concurrently with different shoppers, which makes testing tougher. But, even in such instances, it’s attainable for builders to run BDD take a look at suites on their laptop computer, with out advanced setups and, on the identical time, granting a excessive stage of confidence. 
  • In addition to bringing some enjoyable, constructing a card sport app is a chance to see these architectural ideas in motion with actual code and improve the extent of confidence on the opportunity of making use of such ideas to actual enterprise eventualities.

One of the issues I missed through the pandemic have been my buddies, the chance to satisfy them, talk about with them and, why not, play some playing cards with them.

Zoom may partially work as a substitute for bodily presence, however what about playing cards? What about our video games of Scopone*?

So I made a decision to implement an app to play Scopone with my buddies and, on the identical time, take a look at “within the code” some architectural ideas which had been intriguing me for a while.

All the supply code of the app can be found in this repo.

What I needed to search out out

Freedom in relation to deploy a server

An interactive card sport app involving a couple of participant has to have a consumer half and a server half. The server half has to reside someplace within the Cloud. But the place within the Cloud? As a element operating on a devoted server? As a Docker picture on a managed Kubernetes? As a serverless perform?

I didn’t know which was the most suitable choice however I needed to examine whether or not it was attainable to take care of the core of the logic of the sport impartial from the deployment mannequin I might have finally chosen.

Independence from the framework or library chosen to develop the UI

“Angular is finest”. “No, React is way superior and quicker”. I learn an excessive amount of of this. But is it actually related? Shouldn’t we now have many of the Front End logic as pure Javascript or Typescript code fully impartial from the UI framework or library which we’ll finally find yourself utilizing? I had the sensation this was attainable, however needed to strive it for actual.

Possibility to check mechanically interactive multi-users eventualities

A card sport, as different interactive functions these days, has a number of customers interacting with one another in actual time by way of a central server. For occasion, when one performs a card, all others must see in actual time the performed card. At the start it was not clear to me tips on how to take a look at this sort of software. Would it have been attainable to check it mechanically with easy Javascript testing libraries like Mocha and customary testing practices?

The Scopone app: a good playground to reply my questions

The Scopone app represented a good floor to attempt to reply in a concrete means the questions I had. So I made a decision to attempt to implement it and see which classes I may draw from it.

The huge image

The guidelines of the Scopone sport

Scopone is a conventional italian card sport which is performed by 4 gamers, cut up in 2 groups of two, with a 40 playing cards deck.

At the beginning of the sport all gamers are given 10 playing cards every and the primary participant performs the primary card which is placed on the desk face up. The second participant then performs its card. If this card has the identical rank as the cardboard on the desk, the second participant “takes” the cardboard from the desk. If no playing cards are left on the desk, the participant taking the playing cards scores a “scopa”. Then the third participant performs its card and so forth and so forth till all playing cards have been performed.

Enough of guidelines. The key level to recollect right here is that when a participant performs a card, they alter the state of the sport, as an illustration when it comes to “which playing cards are face up on the desk” or “which participant can play the following card”. 

Structure of the app and tech stack

The Scopone app requires one server occasion and 4 consumer cases which might be launched by the 4 gamers from their gadgets.

If we take a look at the interactions among the many numerous parts of the sport, we observe that

  • Players carry out actions, as an illustration a participant play a card
  • As a consequence of a participant’s motion, all gamers have to be up to date with the brand new state of the sport

This implies that the shoppers and the server want a two-way communication protocol, for the reason that shoppers need to ship instructions and the server must push the up to date state. WebSockets is a standard protocol appropriate to this goal and out there in numerous languages.

The server is applied in Go because it has good help for WebSockets and matches effectively with the completely different deployment fashions at hand, in different phrases it may be deployed as a devoted server, as Docker picture, or as a Lambda. 

The consumer is a browser-based software applied in two completely different flavors: one with Angular and one with React. Both variations use Typescript and leverage a reactive design applied by way of RxJs.

The following diagram represents the overall structure of our sport app.

Commands and occasions

In a nutshell, the app works like this:

  • A consumer sends a command by a message to the server
  • The server updates the state of  the sport
  • The server pushes the brand new state of the sport to the shoppers by a message to the shoppers
  • When a consumer receives a message from the server it treats it like an occasion that triggers the replace of the state for that particular consumer.

This cycle is repeated till the sport is over.

Freedom in relation to deploying the server a part of the app

The server receives messages representing instructions despatched by shoppers. Based on these instructions, it updates the state of the sport and sends messages to shoppers with the brand new up to date state.

A command is a message despatched by a consumer by way of a websocket channel which is reworked into the invocation of a particular API of the server.

The response produced by the invocation of the API is a image of the brand new state which is reworked into a set of messages that need to be despatched to every consumer over the websocket channel.

So, within the server implementation there are two distinct layers with distinct tasks: the sport logic layer and the websockets mechanics layer.

Game logic layer

This layer is chargeable for implementing the sport logic, that’s to replace the state of the sport primarily based on the command acquired and return a image of the brand new state to be despatched to every consumer.

This layer due to this fact may be applied with an inside state and a set of APIs implementing the command logic. The APIs return the brand new state that must be communicated to the shoppers.

Websocket mechanics layer

This layer is chargeable for reworking a message acquired over the websocket channel into the invocation of the corresponding API with the anticipated parameters. Additionally,  it transforms the up to date state, acquired as a response from the API invocation, into the set of messages that need to be pushed to the respective consumer.

Dependency relationships between layers

Based on the earlier discussione,  the sport logic layer is impartial from the idea of websocket. It is simply a set of APIs returning a state.

The websockets mechanics layer, then again, is the place the websockets specificities are applied. This layer will depend upon the precise deployment mannequin chosen. 

For occasion, if we resolve to deploy as a devoted server, we must cope with the precise package deal chosen to implement the websocket protocol (in our case the Gorilla package), whereas if we resolve to deploy as an AWS Lambda perform, we now have to depend on the Lambda implementation of the websocket protocol.

If we maintain the sport logic layer strictly separated from the websockets mechanics layer, with the latter importing the previous (and never vice versa) we’re certain that we are able to use the sport logic layer whatever the particular deployment mannequin chosen.

Applying this technique, it was attainable to develop a single model of the sport logic with the liberty to deploy the server the place most handy.

This introduced a number of benefits. For occasion, through the improvement of the consumer it is rather handy to run towards a native Gorilla websocket implementation, perhaps even launched in debug mode from inside VSCode. This makes it attainable to put breakpoints within the server code and step by the logic triggered by the assorted instructions despatched by the shoppers whereas enjoying a actual sport.

When the time got here to deploy the server for manufacturing, which was extra handy for actual play with my buddies, it was attainable to deploy the identical sport logic to the Cloud, as an illustration to Google Application Engine (GAE).

Furthermore, once I found that Google was charging a minimal price no matter whether or not we performed or not (GAE all the time retains at the very least one server on), I made a decision to maneuver the server to AWS Lambda for a full “on demand” mannequin with no change within the sport logic code.

Independence from the framework or library chosen to develop the UI

And then got here the massive query: Angular or React?

But then I additionally requested myself one other query: is it attainable to code many of the consumer logic as pure Typescript, impartial of which framework or library will probably be used to handle the view a part of the Front End?

It turned out that it’s attainable, at the very least on this case, with some fascinating advantages as negative effects.

The design of the Front End of the app: view layer and repair layer

At the bottom of the design of the Front End a part of the app there are three easy concepts:

  • The consumer in cut up into two layers:
    • The view layer, applied as composable parts (sure, each Angular and React share the identical primary idea of making UIs as composition of parts) to implement the pure presentation logic.
    • The service layer, applied in Typescript with no dependency in any way on any a part of Angular or React, to carry state and implement any logic that manages such state, together with the invocation of instructions on the distant server and the interpretation of the responses when it comes to state modifications (in different phrases a home-made bespoke retailer).
  • The service layer exposes two varieties of APIs to the view layer:
    • Public strategies that may be referred to as to invoke instructions on the distant server or, extra usually, to vary the state of the consumer.
    • Public streams of occasions, applied as RxJs Observable, which may be subscribed by whichever UI element desires to be notified of state modifications.
  • The view layer retains solely two easy tasks:
    • Intercept UI occasions and switch them into invocations of the service layer public API strategies.
    • Subscribe to the general public API Observables and react to the notifications acquired with the suitable modifications within the presentation.

An instance of View-Service-Server interplay

A participant can play a card by clicking on it (fits within the image are conventional north-eastern italian)

To make it extra concrete, let’s think about what it means to play a card.

Let’s assume Player_X is the participant that’s to play the following card. Player_X clicks on the cardboard “Ace of Hearts” and this UI occasion triggers the motion “Player_X has performed Ace of Hearts”.

These are the steps the applying goes by:

  1. The view layer intercepts the consumer generated occasion and calls the service layer invoking the strategy playCard passing Ace of Hearts as parameter.
  2. The service layer sends the distant server the message “Player_X has performed Ace of Hearts”.
  3. The distant server updates the state of the sport and informs all shoppers of the state modifications. For occasion it tells all of the shoppers which card Player_X performed and who’s the following participant that may play a card.
  4. The messages with the state updates despatched by the distant server are acquired by the service layer of every consumer and changed into notifications of particular occasions over Observable streams made public for shoppers consumption. For occasion, the service layer within the consumer occasion of Player_X will notify false on the stream isMyTurnToPlay$ since Player_X is definitely not the following participant. On the opposite hand, if the opposite participant is Player_Y, the service layer on the consumer of Player_Y will notify a true on the stream isMyTurnToPlay$.
  5. The view layer of every consumer has subscribed to the streams of occasions printed by the service layer and can react to the occasions notified updating the UI as required. For occasion, the view layer of Player_Y (the following participant) will allow its consumer to play a card whereas all different shoppers serving the opposite gamers will disable such a chance.

View layer and repair layer interactions

Light parts and heavy companies

Following these guidelines we find yourself constructing “mild parts”, which handle solely the UI issues (presentation and UI occasion dealing with) and “heavy companies” the place the entire logic is stored.

The most vital consequence although is that the “heavy companies”, which include many of the logic, are fully impartial from the UI framework or library used. There is not any dependency on both Angular or React.

More particulars on the best way the UI layer works may be discovered on the finish of the article.

The advantages

Which are the advantages of such an method?

Certainly not the portability between completely different frameworks and libraries. Once Angular is chosen it’s unlikely that somebody desires to change to React and vice versa. But there are nonetheless benefits.

A primary benefit of such an method is that, if applied totally, it standardizes the best way we develop the Front End and makes it simpler to cause about it. At the tip of the day, it’s simply one other method to design a unidirectional move of knowledge utilizing a bespoke retailer (the service layer is simply a bespoke retailer). Being bespoke has the benefit of a decrease stage of abstraction and larger simplicity at, perhaps, the price of a little bit of “reinventing the wheel” feeling.

The greatest profit although lies in higher and simpler testability of the applying. 

Testing UI is advanced, irrespective of which framework or library you utilize. 

But if we transfer many of the code to a pure Typescript implementation, testing turns into simpler. We can take a look at the core logic of the applying utilizing customary testing frameworks (in our instance we use Mocha) and we are able to additionally method advanced testing eventualities in a comparatively easy means, as we talk about within the subsequent and final part.

Automatically testing interactive actual time multi-users eventualities

We have seen that Scopone is a sport performed by 4 gamers.

Four shoppers need to be linked concurrently to a central server by way of WebSockets. Actions carried out by one consumer, as an illustration “play a card”, set off updates (negative effects) on all shoppers.

This is an interactive actual time multi-user state of affairs. This implies that we have to have a number of shoppers and a server operating on the identical time if we need to take a look at the conduct of the app in its entirety.

How can we take a look at such eventualities mechanically? Can we take a look at them with customary testing Javascript libraries? Can we take a look at them on a standalone developer workstation? These have been the inquiries to reply subsequent. It seems that each one this stuff are attainable, at the very least as much as a giant extent.

What is a take a look at in an interactive real-time multi-user state of affairs 

Let’s think about, with a easy instance, that we need to take a look at the proper distribution of the playing cards amongst all gamers originally of a sport. As quickly as a new sport is began, all shoppers obtain ten playing cards every  from the server (the Scopone deck consists of 40 playing cards of which every participant will get ten).

If we need to take a look at this conduct mechanically from a single standalone machine (say, the developer’s machine) we’d like a native server. This is feasible for the reason that server can run domestically as a Container or a WebSockets server. So we assume to have a native server up and operating on our machine.

But, for the take a look at to run, we’d like additionally to search out a method to create the best context assumed by the take a look at and launch the motion that triggers the negative effects we need to take a look at (the distribution of the playing cards to the gamers is a facet impact of the truth that one participant has began the sport). In different phrases we have to discover a method to simulate the next:

  • Four gamers launch the app and be part of the identical sport (create the best context)
  • One participant begins the sport (set off the negative effects we need to take a look at)

Only then can we examine whether or not the server sends the anticipated playing cards to all gamers.

A take a look at in a multi consumer state of affairs

How to simulate a number of shoppers

Each consumer consists of a view layer and a service layer.

The APIs of the service layer (strategies and Observable streams) are outlined in a class (referred to as ScoponeServerService within the implementation code).

Each consumer creates an occasion of this service class and connects it to the server. The view layer interacts with its occasion of the service class.

Therefore, if we need to simulate 4 shoppers, we now have to first create 4 completely different cases of the service class and join all of them to our native server.

Create 4 service class cases representing 4 shoppers

How to create the context for the take a look at

Now that we now have 4 shoppers out there and linked, we have to construct the best context for the take a look at. We want 4 gamers and we’d like that every of them joins the identical sport. 

Setting the context for the take a look at

Finally, tips on how to take a look at

After we now have created the 4 shoppers and constructed the best context, we are able to run the core of the take a look at. We can have one participant sending the command to start out the sport after which we are able to examine that every participant receives the anticipated quantity of playing cards.

Running the take a look at

Wrapping it up

The take a look at of an interactive multi-user state of affairs is a perform that:

  • Creates one service occasion per consumer
  • Creates the context of the take a look at by sending a sequence of instructions to the companies in the best order
  • Sends the command that triggers the negative effects we need to examine (we are able to say that is the command below take a look at)
  • Verifies that the notifications emitted by the Observable APIs of every service, as consequence of this command (the negative effects), include the info anticipated

It is BDD run at APIs boundaries of the service layer

We can see this method as a type of behavior-driven development (BDD) carried out towards the APIs provided by the service layer

The behavioral specs are offered, in keeping with the BDD method, reminiscent of:

  • Given the preliminary context: 4 gamers becoming a member of a sport
  • When: a participant begins a sport
  • Then: we anticipate every participant to obtain 10 playing cards. 

The perform representing the take a look at is written with a type of DSL which consists of ad-hoc helper features whose mixture units up the context (an instance of helper perform is playersJoinTheGame). 

It will not be end-to-end, however it may be very highly effective

This will not be a full end-to-end take a look at. We will not be testing the view layer

But it might probably nonetheless be a very highly effective instrument, particularly if we follow the rule “Light parts and heavy companies”.

If the view layer is made up of sunshine parts and many of the logic is concentrated within the service layer, then this method permits us to cowl the core of software conduct, each on the consumer and on the server facet, with a comparatively easy setup, fairly customary instruments (we use Mocha as testing library, undoubtedly not the most recent shiniest factor) and on a standalone developer’s machine. 

The internet profit is that builders can create take a look at suites which might be quick to run and due to this fact may be executed typically. At the identical time, such take a look at suites are actually testing all the software logic, from consumer to server, giving a excessive stage of confidence even with a multi-user actual time software.  


Building a card sport app has been an fascinating expertise. 

Apart from bringing some aid through the darkest days of the pandemic, it gave me the chance to discover some architectural ideas with some code.

We typically use architectural ideas as abstractions to precise our level of views. I discover that taking a look at these ideas in motion, even in easy proof-of-concept eventualities, will increase our understanding of them and our stage of confidence after we finally use them in a actual mission.

Appendix: The view layer mechanics

The parts of the View layer do two issues:

  • Handle UI occasions and remodel them into instructions for the service.
  • Subscribe to streams uncovered by the service and react to incoming occasions by updating the UI.

To be extra concrete about what the final level means, we are able to take a look at one instance of logic: tips on how to decide who’s the participant that may play the following card.

As we stated, one rule of the sport is that gamers can play playing cards one after the opposite. So, as an illustration, if Player_X is the primary participant and Player_Y is the second, after Player_X performs a card solely Player_Y can play the following card. All different gamers cannot play any playing cards. This info is a part of the state which is stored by the server.

Any time a card is performed the server sends a message to all shoppers specifying which is the following participant.

The service layer turns this message into a notification over an Observable stream referred to as enablePlay$. If the message says that the participant can play the following card, the service layer will notify true over enablePlay$ in any other case false.

The element that allows for a participant the chance to play a card has to subscribe to the enablePlay$ stream and react accordingly to the info notified.

In our React implementation that is the useful element Hand. This element defines a state variable, enablePlay, that governs the opportunity of enjoying a card. The Hand element subscribes to the enablePlay$ Observable in an impact hook and, any time it receives a notification from enablePlay$, it units the worth of enablePlay triggering the redraw of the UI.

The related code for this explicit performance applied by the  Hand element utilizing React is the next.

export const Hand: FC = () => {
 const server = useContext(ServerContext);
  . . .
 const [handReactState, setHandReactState] = useState({
   . . .
   enablePlay: false,
  . . .
 useEffect(() => {
  . . .
  . . .
   const enablePlay$ = server.enablePlay$.pipe(
     faucet((enablePlay) => {
       setHandReactState((prevState) => ({ ...prevState, enablePlay }));

   const subscription = merge(
       . . .

   return () => {
     console.log("Unsubscribe Hand subscription");
 }, [server]);
  . . .

 return (
       . . .
       . . .

The Angular counterpart to this instance  is logically an identical and is applied in HandElement. The solely distinction is that the subscription to the enablePlay$ Observable is made instantly within the template by way of the async pipe.


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