add proxy design pattern real world example

Author: samuel.casanova

src/Proxy/RealWorld/Output.txt

@@ -0,0 +1,16 @@
+Client: Executing the client code with a real weather service:
+The weather forecast is 4.539002575927763º average temperature and 54.50427119342287% max precipitation probability.
+The weather forecast is 12.87183981383º average temperature and 60.041907992745514% max precipitation probability.
+The weather forecast is 27.63639710822103º average temperature and 2.2777239912981972% max precipitation probability.
+
+Client: Executing the same client code with a proxied weather service:
+Requesting forecast on date 2023-06-01T07:45:08.143Z.
+Invalid cache. Calling real weather service...
+Request processed in 1002 milliseconds
+The weather forecast is 27.65024492643842º average temperature and 46.87027596334876% max precipitation probability.
+Requesting forecast on date 2023-06-01T07:45:09.146Z.
+Request processed in 0 milliseconds
+The weather forecast is 27.65024492643842º average temperature and 46.87027596334876% max precipitation probability.
+Requesting forecast on date 2023-06-01T07:45:09.147Z.
+Request processed in 0 milliseconds
+The weather forecast is 27.65024492643842º average temperature and 46.87027596334876% max precipitation probability.

src/Proxy/RealWorld/index.ts

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+/**
+ * EN: Real World Example for the Proxy Design Pattern
+ *
+ * Need: Cache and log access to an external weather service SDK
+ *
+ * Solution: Create a proxy class to cache the SDK calls and log the requests
+ */
+
+/**
+ * EN: The WeatherService defines the SDK interface and response
+ */
+interface WeatherService {
+    request(): Promise<WeatherForecast>;
+}
+
+interface WeatherForecast {
+    avgTemperature: number;
+    maxPrecipitationProbability: number;
+}
+
+/**
+ * EN: The real service emulates a network request to an external service with
+ * a 1 second delay
+ */
+class RealWeatherServiceSDK implements WeatherService {
+    public async request(): Promise<WeatherForecast> {
+        const randomWeatherForecast = {
+            avgTemperature: Math.random() * 35,
+            maxPrecipitationProbability: Math.random() * 100,
+        };
+
+        return new Promise((resolve) => {
+            setTimeout(() => resolve(randomWeatherForecast), 1000);
+        });
+    }
+}
+
+/**
+ * EN: The Proxy implements the same interface than the real service. However
+ * the proxy uses an internal cache to store responses. Subsequent calls to the
+ * proxied service will go faster as they won't call the real slow service. The
+ * proxy also logs useful information about the cache usage and timmings.
+ */
+class ProxyWeatherService implements WeatherService {
+    private cachedResponse : WeatherForecast;
+    private cacheDate : Date;
+    private expirationTimeInMillis = 24 * 60  * 60 * 1000;
+
+    constructor(private realWeatherService: WeatherService) {
+    }
+
+    public async request(): Promise<WeatherForecast> {
+        console.log(`Requesting forecast on date ${new Date().toISOString()}.`);
+        const initialTime = Date.now();
+        if (this.isCacheExpired()) {
+            console.log('Invalid cache. Calling real weather service...');
+            this.setCache(await this.realWeatherService.request());
+        }
+        const requestTimeInMillis = Date.now() - initialTime;
+        console.log(`Request processed in ${requestTimeInMillis} milliseconds`);
+        return this.cachedResponse;
+    }
+
+    private isCacheExpired() : boolean {
+        return this.cachedResponse ?
+            Date.now() > this.cacheDate.getTime() + this.expirationTimeInMillis :
+            true;
+    }
+
+    private setCache(weatherForecast: WeatherForecast) {
+        this.cachedResponse = weatherForecast;
+        this.cacheDate = new Date();
+    }
+}
+
+/**
+ * EN: The client code works with real and proxied services
+ */
+async function clientCode(weatherService: WeatherService) {
+    for (let i = 0; i < 3; i += 1) {
+        const weatherForecast = await weatherService.request();
+        console.log(`The weather forecast is ${weatherForecast.avgTemperature}º average temperature and ${weatherForecast.maxPrecipitationProbability}% max precipitation probability.`);
+    }
+}
+
+async function main() {
+    console.log('Client: Executing the client code with a real weather service:');
+    const realWeatherService = new RealWeatherServiceSDK();
+    await clientCode(realWeatherService);
+
+    console.log('');
+
+    console.log('Client: Executing the same client code with a proxied weather service:');
+    const proxy = new ProxyWeatherService(realWeatherService);
+    await clientCode(proxy);
+}
+
+main();