const Mongo = require("mongodb");

const DB_NAME = 'FILL_DB_NAME';
const COLLECTION_ONE = 'tx_collection_one';
const COLLECTION_TWO = 'tx_collection_two';
const CONNECTION_STRING = "FILL_CONNECTION_STRING"

const connect = async () => {
  const client = new Mongo.MongoClient(CONNECTION_STRING);

  await client.connect();

  console.log('connected');

  return client;
};

const withTransaction = async (session, cb) => {
  const commit = async () => {
    try {
      session.startTransaction({
        // readPreference: Mongo.ReadPreference.primary,
        // readConcern: { level: 'local' },
        writeConcern: { w: 'majority', journal: true },
        maxCommitTimeMS: 500,
      });

      await cb();

      await session.commitTransaction();
    } catch (e) {
      await session.abortTransaction();

      throw e;
    }
  };

  try {
    // The cb is expected to fail, this commits the transaction 15 times
    for (let i = 0; i < 15; i += 1) {
      await commit().catch(e => {
        if (e instanceof Mongo.MongoError) {
          console.log(`mongo error -> ${e.constructor.name}: ${e.message} (${e.code})`);
        }
      });

      await new Promise((r) => setTimeout(r, 100));
    }
  }
  finally {
    await session.endSession();
  }
};

const createDocuments = async () => {
  const documents = [];

  for (let i = 0; i < 3; i += 1) {
    documents.push({ id: i });
  }

  return documents;
};

const run = async () => {
  try {
    const client = await connect();
    const session = client.startSession();
    const db = client.db(DB_NAME);
    const collectionOne = db.collection(COLLECTION_ONE);
    const collectionTwo = db.collection(COLLECTION_TWO);

    // Create empty collections (without using the session)
    await Promise.all([
      db.createCollection(COLLECTION_ONE),
      db.createCollection(COLLECTION_TWO),
    ]);

    // Simulate a failure writing to three places
    await withTransaction(session, async () => {
      await Promise.all([
        collectionOne.insertMany([createDocuments()], { session }),
        new Promise((_, reject) => reject(new Error())),
        collectionTwo.insertMany([createDocuments()], { session })
      ]);
    });

    await Promise.all([
      collectionOne.countDocuments().then((count) => {
        console.log("collection_one count -> %d", count);
      }),
      collectionTwo.countDocuments().then((count) => {
        console.log("collection_two count -> %d", count);
      }),
    ]);

    await Promise.all([
      db.dropCollection(COLLECTION_ONE),
      db.dropCollection(COLLECTION_TWO),
    ]);

    await client.close();
  }
  catch (e) {
    console.error(e);
  }
};

return run();