const documentdbCluster = new DocumentdbClusterLambdaConnection(
  this,
  "MyDocumentDbConnection",
  masterUser: { username: "yourUsername", secretName: "documentdbSecretName" }
);

const lambda1 = new NodejsFunction(this, "MyFirstLambda", {
  vpc: documentdbCluster.vpc,
  ...restOfLambdaConfiguration,
});
const lambda2 = new NodejsFunction(this, "MySecondLambda", {
  vpc: documentdbCluster.vpc,
  ...restOfLambdaConfiguration,
});

documentdbCluster.allowCommunication(lambda1, lambda2);

import { SecretsManager } from "aws-sdk";
import { MongoClient } from "mongodb";
export const main = async () => {
  const dbClient = await connectToDatabase();
  const db = dbClient.db("db");
  await db.collection("collection").insertOne({ property: "value" });
  return {
    statusCode: 200,
    body: "...",
  };
};
const connectToDatabase = async (): Promise<MongoClient> => {
  if (cachedDb) {
    return Promise.resolve(cachedDb);
  }
  const secretsManager = new SecretsManager({ region: "eu-west-1" });
  const secret = await secretsManager
    .getSecretValue({ SecretId: "documentdbSecretName" })
    .promise();
  const secretString = secret.SecretString;
  if (secretString === undefined) {
    throw new Error();
  }
  const docdbsecret = JSON.parse(secretString) as {
    password: string;
    host: string;
    username: string;
  };
  const { password, host, username } = docdbsecret;
  db = await MongoClient.connect(
    `mongodb://${encodeURIComponent(username)}:${encodeURIComponent(
      password
    )}@${host}`,
    {
      ssl: true,
      sslCA: "/opt/rds-combined-ca-bundle.pem", // https://docs.aws.amazon.com/documentdb/latest/developerguide/ca_cert_rotation.html#ca_cert_rotation-updating_application_step1
      retryWrites: false,
      connectTimeoutMS: 3000,
    }
  );
  cachedDb = db;
  return cachedDb;
};