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Applied Design Patterns: Rolling with the Punches O S L I D O P R Y S I K N G A Y S I R P D I

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2 Story Time

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3 We want to send JSON data through Kinesis streams.

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4 Pause

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5 Acronyphilia?

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We ♥ Acronyms OOP DRY YAGNI KISS 6

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Keep it Simple? ▪ To Understand? ▪ To Write? ▪ To Instantiate? 7 ▪ To Maintain? ▪ To Test? ▪ To Change?

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Keep it Simple? ▪ To Understand? ▪ To Write? ▪ To Instantiate? 8 ▪ To Maintain? ▪ To Test? ▪ To Change?

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SOLID 9

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SOLID ▪ S ▪ O ▪ L ▪ I ▪ D 10

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SOLID ▪ SRP ▪ OCP ▪ LSP ▪ ISP ▪ DIP 11

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SOLID ▪ Single Responsibility ▪ Open-Closed ▪ Liskov Substitution ▪ Interface Segregation ▪ Dependency Inversion 12 }Principle

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SOLID ▪ Single Responsibility ▪ Open-Closed ▪ Liskov Substitution ▪ Interface Segregation ▪ Dependency Inversion 13 Does one thing

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SOLID ▪ Single Responsibility ▪ Open-Closed ▪ Liskov Substitution ▪ Interface Segregation ▪ Dependency Inversion 14 Extend without modification

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SOLID ▪ Single Responsibility ▪ Open-Closed ▪ Liskov Substitution ▪ Interface Segregation ▪ Dependency Inversion 15 Child can replace a parent

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SOLID ▪ Single Responsibility ▪ Open-Closed ▪ Liskov Substitution ▪ Interface Segregation ▪ Dependency Inversion 16 Small interfaces

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SOLID ▪ Single Responsibility ▪ Open-Closed ▪ Liskov Substitution ▪ Interface Segregation ▪ Dependency Inversion 17 Rely on abstractions

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18 Why?

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Jeremy’s Theorem Good OO Design is like a nice set of Quick-Assembly Shelves 23

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SOLID ▪ Single Responsibility ▪ Open-Closed ▪ Liskov Substitution ▪ Interface Segregation ▪ Dependency Inversion 24 }Principle

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Applied Design Patterns: Rolling with the Punches O S L I D O P R Y S I K N G A Y S I R P D I

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Applied Design Patterns: Rolling with the Punches O S L I D O P R Y S I K N G A Y S I R P D I

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30 Remember Our Story?

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31 We want to send JSON data through Kinesis streams.

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32 // The Simple Approach class KinesisStream { func new(KinesisClient client, string streamName); func write(Message message): void; } class Message { func new(map data); func getData(): map; func toJson(): string; } // Example stream = new KinesisStream(client, "app-events"); stream.write(new Message(["foo": "bar"])); YAGNI SRP

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33 We might be switching to SQS, actually. Can you support both?

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34 // Adding New Functionality? class Stream { func new(KinesisClient kinesisClient, SqsClient sqsClient, string streamName, string streamType ); func write(Message message): void { if (this.streamType === "kinesis") this.kinesisClient.(…) elif (this.streamType === "sqs") this.sqsClient.(…) else throw new Exception("…"); } } SRP OCP

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35 // Adding New Functionality? class Stream { func new(KinesisClient kinesisClient, SqsClient sqsClient, string streamName, string streamType ); func write(Message message): void { if (this.streamType === "kinesis") this.kinesisClient.(…) elif (this.streamType === "sqs") this.sqsClient.(…) else throw new Exception("…"); } } SRP OCP

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36 // Interfaces And Multiple Implementations interface Stream { func write(Message message): void; } class KinesisStream implements Stream { func new(KinesisClient client, string streamName); func write(Message message): void; } class SqsStream implements Stream { func new(SqsClient client, string queueUrl); func write(Message message): void; } SRP ISP

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37 // Factories Are Your Friends class EventsFactory { func createEventStream(): Stream { switch (this.config.get("events.stream.type")) { case "kinesis": return this.createKinesisStream(); case "sqs": return this.createSqsStream(); // ... } func createKinesisStream(): KinesisStream; func createSqsStream(): SqsStream; } SRP LSP

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38 // Polymorphism Can Be Easy class OrdersController { func new(Stream stream, OrdersRepository orders) {/* ... */} // ... func doSomethingWithAnOrder(string orderId) { // ... order = this.orders.get(orderId); this.stream.write(new Message([ "order_id": order.id, "item_count": order.items.count(), ])); // ... } } DIP

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39 Can you log messages that you send?

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40 // Decorators Are Cool class LoggingStream implements Stream { func new(Stream stream, LoggerInterface logger); func write(Message message): void { this.stream.write(message); this.logger.info('Message sent', message.getData()); }; } // Example stream = new LoggingStream(kinesisStream, logger); stream.write(message); SRP OCP LSP DIP

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41 We are moving to SQS, but some systems haven’t yet. Can you send to both for now?

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42 // Aggregates Can Work Too class MultiStream implements Stream { func new(Stream ...streams); func write(Message message): void; } // Example stream = new LoggingStream( new MultiStream(kinesisStream, sqsStream), logger ); stream.write(message); SRP OCP LSP DIP

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44 We’ve come up with a specific format for messages that contains metadata.

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45 // Messages Get a Structure class Message extends JsonSerializable { func new(Header header, array data); func getHeader(): Header; func getData(): map; func toJson(): string; } class Header extends JsonSerializable { func new(string id, …, DateTime timestamp); func getId(): string; func getSource(): string; func getTopic(): string; func getAction(): string; func getTimestamp(): DateTime; func toJson(): string; }

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46 // A New Layer of Abstraction class EventPublisher { func new(Stream stream, string source); func publish(Event event): void; } interface Event { func getTopic(): string; func getAction(): string; func getData(): map; } publisher.publish(event); SRP DIP

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47 // A New Layer of Abstraction class OrderEvent implements Event { func new(Order order, string action); // ... } class OrderCreatedEvent extends OrderEvent { func new(Order order); // ... } publisher.publish(new OrderCreatedEvent(order)); OCP LSP

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48 We’d like to be able to turn events off via configuration.

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49 // Polymorphism Instead of Branching class EventPublisher { func new(Stream stream, string source, bool on = true) { // … } func publish(Event event): void { if (this.on) { // Do stuff. } }; } OCP

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50 // Polymorphism Instead of Branching interface Publisher { func publish(Event event): void; } class EventPublisher { func new(Stream stream, string source); func publish(Event event): void; } class NullPublisher { func publish(Event event): void { // Do nothing. }; } LSP DIP

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51 // Conditionalize Your Factory Instead of Code class EventsFactory { // ... func createEventPublisher(): Publisher { if (!this.config.get("events.enabled")) { return new NullPublisher(); } return new EventPublisher( this.createEventStream(), this.config.get("events.source") ); } } SRP LSP

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52 We want different streams for some topics, so they can scale independently.

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53 // Under the Hood Changes interface TopicMapper { func mapTopic(string topic): string; } class KinesisStream implements Stream { func new(KinesisClient client, TopicMapper tm); func write(Message message): void; } class SqsStream implements Stream { func new(SqsClient client, TopicMapper tm); func write(Message message): void; } SRP LSP ISP

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54 We’re moving to a custom streaming service now. Can you support that?

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55 // New Streams? No Biggie! class HttpStream implements Stream { func new(HttpClient client, string url); func write(Message message): void; } class CustomServiceStream implements Stream { func new(CustomServiceClient client); func write(Message message): void; }

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56 We want to send lots of messages at a time. Can we send them in batches?

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57 You’ve Got to Be Kidding Me!

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58 // Is There a Good Way? class CustomServiceBatchStream extends CustomServiceStream { func write(Message message): void func flush(): void; } OCP

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59 // Is There a Good Way? interface Stream { func write(Message message): void; func flush(): void; } class CustomServiceBatchStream implements Stream { func write(Message message): void func flush(): void; } ISP

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60 // Multiple Interfaces & Additive Changes interface Stream { func write(Message message): void; } interface BatchStream extends Stream { func flush(): void; } class CustomServiceBatchStream implements BatchStream { func new(CustomServiceClient client); func write(Message message): void; func flush(): void; } ISP

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Final Thoughts ▪ Software is complicated, because life is complicated. ▪ You can’t predict the future, but you can prepare for it. ▪ Learn from others’ experiences, but experience it for yourself. 61 O S L I D I P P P P P R C S S

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62 class MultiStream implements Stream { func new(Stream ...streams); func write(Message message): void { }; } // Example stream = new LoggingStream( new MultiStream(kinesisStream, sqsStream), logger ); stream.write(message); Can anyone guess the programming language?

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63 class MultiStream implements Stream { public function __construct(Stream ...$streams) { }; public function write(Message $message): void { }; } // Example $stream = new LoggingStream( new MultiStream($kinesisStream, $sqsStream), $logger ); $stream->write($message); It’s PHP!

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64 Thanks! You can find me at ▪ @jeremeamia ▪ jeremeamia@gmail.com

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Credits ▪ Presentation template by SlidesCarnival ▪ Photographs by Unsplash 65