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Backend Development/typescript/lib.ts

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+import * as nats from "https://deno.land/x/nats@v1.13.0/src/mod.ts";
+import { parse } from "https://deno.land/std@0.183.0/flags/mod.ts";
+import { parse as parseJSON } from "https://deno.land/std@0.183.0/jsonc/mod.ts";
+import { z } from "https://deno.land/x/zod@v3.21.4/mod.ts";
+import { resolve } from "https://deno.land/std@0.183.0/path/mod.ts";
+
+export const JSONCodec = nats.JSONCodec;
+
+const flags = parse(Deno.args);
+
+const StartServiceConfigSchema = z.object({
+  nats: z.boolean().default(true),
+});
+
+export const MinimalConfigSchema = z.object({
+  NATS_URL: z.string(),
+  NATS_USER: z.string().optional(),
+  NATS_PASSWORD: z.string().optional(),
+  SERVICE_NAME: z.string(),
+  DATA_DIR: z.string(),
+}).passthrough();
+
+export type MinimalConfig = z.infer<typeof MinimalConfigSchema>;
+
+// Development mode => use default values
+
+export type StartServiceConfig = z.infer<typeof StartServiceConfigSchema>;
+
+/**
+ * Starts the service and returns the APIs available to the Telestion service.
+ * @param rawOptions The configuration for the service.
+ * @param rawOptions.nats Whether to enable NATS or not. Defaults to `true`.
+ *
+ * @returns The APIs available to the Telestion service.
+ *
+ * @throws If the service couldn't be started.
+ *
+ * @example
+ * import {startService, JSONCodec} from "./lib.ts";
+ *
+ * const {nc, serviceName} = await startService();
+ * console.log(`Service ${serviceName} started!`);
+ *
+ * nc.publish("my-topic", JSONCodec().encode({foo: "bar"}));
+ * console.log("Message published!");
+ *
+ * await nc.drain();
+ */
+export async function startService(
+    rawOptions: StartServiceConfig = StartServiceConfigSchema.parse({}),
+) {
+  const options = StartServiceConfigSchema.parse(rawOptions);
+
+  const config = assembleConfig();
+
+  return {
+    // Non-NATS APIs
+    get nc(): never {
+      throw new Error("NATS is not enabled");
+    },
+    get messageBus(): never {
+      throw new Error("NATS is not enabled");
+    },
+    // NATS APIs
+    ...(options.nats ? await initializeNats(config) : {}),
+    // Other APIs
+    dataDir: resolve(config.DATA_DIR),
+    serviceName: config.SERVICE_NAME,
+    config,
+  };
+}
+
+/**
+ * Assembles the configuration for the service.
+ */
+function assembleConfig() {
+  /**
+   * Configuration parameters that are passed via environment variables or command line arguments.
+   */
+  const withoutConfigFile = z.object({
+    CONFIG_FILE: z.string().optional(),
+    CONFIG_KEY: z.string().optional(),
+  }).passthrough().parse({
+    ...getDefaultConfig(),
+    ...Deno.env.toObject(),
+    ...flags,
+  });
+
+  // No config file => return the parsed config
+  if (!withoutConfigFile.CONFIG_FILE) {
+    return ensureMinimalConfig(withoutConfigFile);
+  }
+
+  const config = parseJSON(
+      Deno.readTextFileSync(withoutConfigFile.CONFIG_FILE),
+  );
+
+  // Config file doesn't contain an object => throw error
+  if (typeof config !== "object" || config === null || Array.isArray(config)) {
+    throw new Error("Invalid config file");
+  }
+
+  // No config key => return the parsed config combined with the root config file object
+  if (!withoutConfigFile.CONFIG_KEY) {
+    return ensureMinimalConfig({ ...config, ...withoutConfigFile });
+  }
+
+  const childConfig = config[withoutConfigFile.CONFIG_KEY];
+
+  // Config key doesn't exist or doesn't lead to an object => throw error
+  if (
+      typeof childConfig !== "object" || childConfig === null ||
+      Array.isArray(childConfig)
+  ) {
+    throw new Error("Invalid config file");
+  }
+
+  // Return the parsed config combined with the child config object
+  return ensureMinimalConfig({ ...childConfig, ...withoutConfigFile });
+}
+
+function ensureMinimalConfig(rawConfig: unknown) {
+  try {
+    const config = MinimalConfigSchema.parse(rawConfig);
+    return config;
+  } catch (e) {
+    console.error("Missing required configuration parameters.");
+    console.info(`Details: ${e.message}`);
+
+    if (!flags["dev"]) {
+      console.info(
+          'Run with "--dev" to use default values for missing environment variables during development.',
+      );
+    }
+    Deno.exit(1);
+  }
+}
+
+function getDefaultConfig() {
+  if (!flags["dev"]) {
+    return {};
+  }
+
+  console.log(
+      "Running in development mode. Using default values for missing environment variables.",
+  );
+  return {
+    NATS_URL: "localhost:4222",
+    SERVICE_NAME: `dev-${Deno.gid}`,
+    DATA_DIR: resolve("./data"),
+  };
+}
+
+async function initializeNats(config: MinimalConfig) {
+  try {
+    const nc = await nats.connect({
+      servers: config.NATS_URL,
+      user: config.NATS_USER,
+      pass: config.NATS_PASSWORD,
+    });
+
+    // Register health check
+    nc.subscribe("__telestion__.health", {
+      callback: (err, msg) => {
+        if (err) {
+          return;
+        }
+        msg.respond(
+            JSONCodec().encode({
+              name: config.SERVICE_NAME,
+            }),
+        );
+      },
+    });
+    return { nc, messageBus: nc };
+  } catch (e) {
+    console.error(
+        `Error! Couldn't connect to the message bus. Details: ${e.message}`,
+    );
+    throw e;
+  }
+}