Real world scenario ETL - Facade pattern

Author: alfonsograziano

src/Facade/RealWorld/index.ts

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+import * as fs from "fs/promises";
+
+/**
+ * Example Facade pattern for a ETL process.
+ * In this example I have created three subsystems.
+ * The first one is the Loader (DataSource), which is a file system.
+ * The second one is the Parser (DataTransformer), which is a string parser.
+ * The third one is the Writer (DataSink), which is a file system.
+ * 
+ * To keep the example simple, in the loader I'm not doing input validation.
+ * In a real world scenario, I would do it creating a validation layer 
+ * on the extractor and passing the parsed result to the transformer.
+ */
+
+type Map = { [key: string]: any };
+
+interface Extractor {
+  extract(): Promise<string>;
+}
+
+interface Transformer {
+  transform(input: string): Map;
+}
+
+interface Loader {
+  load(input: Map): Promise<any>;
+}
+
+class FileExtractor implements Extractor {
+  filepath: string;
+  constructor(filepath: string) {
+    this.filepath = filepath;
+  }
+
+  public async extract() {
+    //load file from this.filepath
+    return fs.readFile(this.filepath, "utf8");
+  }
+}
+
+class FileLoader implements Loader {
+  filepath: string;
+  constructor(filepath: string) {
+    this.filepath = filepath;
+  }
+  public async load(input: Map) {
+    return fs.writeFile(this.filepath, JSON.stringify(input, undefined, 4));
+  }
+}
+
+class FileTransformer implements Transformer {
+  public transform(input: string): Map {
+    let result: Map = {};
+
+    input.split("\n").forEach((line) => {
+      if (line.trim().length === 0) return;
+
+      const [key] = line.split(",");
+      if (typeof result === "undefined") {
+      }
+      if (typeof result[key] === "undefined") {
+        result[key] = 0;
+      }
+      result[key] = result[key] + 1;
+    });
+
+    return result;
+  }
+}
+
+/**
+ * The Facade class is the main class of the Facade pattern.
+ * It's responsible for creating the subsystems and calling their methods.
+ * I'm injecting the subsystems in the constructor.
+ * In the process method I'm calling the extract, transform and load methods of the subsystems.
+ */
+class ETLProcessor {
+  extractor: Extractor;
+  transformer: Transformer;
+  loader: Loader;
+
+  constructor(extractor: Extractor, transformer: Transformer, loader: Loader) {
+    this.extractor = extractor;
+    this.transformer = transformer;
+    this.loader = loader;
+  }
+
+  public async process() {
+    const input = await this.extractor.extract();
+    const transformed = this.transformer.transform(input);
+    return this.loader.load(transformed);
+  }
+}
+
+const processor = new ETLProcessor(
+  new FileExtractor("input.txt"),
+  new FileTransformer(),
+  new FileLoader("output.json")
+);
+
+processor.process().then(() => {
+  console.log("Process completed");
+});

src/Facade/RealWorld/input.txt

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+11/07/2022,[email protected]
+11/07/2022,[email protected]
+11/07/2022,[email protected]
+12/07/2022,[email protected]
+12/07/2022,[email protected]
+13/07/2022,[email protected]
+15/07/2022,[email protected]
+15/07/2022,[email protected]
+15/07/2022,[email protected]
+16/07/2022,[email protected]
+16/07/2022,[email protected]