MongoDB commands illustrating multiple transactions with no write conflicts can successfully commitTransactions

transactions-no-writeconflict.js

// v4.0.0-rc0/bin/mongo --port 38000

// ****************************************************
// On a sample person collection with two documents _id 1, 2
// Insert new document, _id 3, inside session1 scope
// Update a document, _id 1, in session2 scope
// Delete a document, _id 2, directly on collection
// Understand how the find operation on these scopes change
// from beginning of transaction till after commit
// ****************************************************

// drop and recreate person collection with 2 documents _id 1, 2
use test;
db.person.drop();
db.person.insert({"_id": 1, "fname": "fname-1", "lname": "lname-1"});
db.person.insert({"_id": 2, "fname": "fname-2", "lname": "lname-2"});

// create session1 and a collection object using session1 and start a transaction on it
var session1 = db.getMongo().startSession();
var session1PersonColl = session1.getDatabase('test').getCollection('person');
session1.startTransaction({readConcern: {level: 'snapshot'}, writeConcern: {w: 'majority'}});

// create session2 and a collection object using session2 and start a transaction on it
var session2 = db.getMongo().startSession();
var session2PersonColl = session2.getDatabase('test').getCollection('person');
session2.startTransaction({readConcern: {level: 'snapshot'}, writeConcern: {w: 'majority'}});

// The find operations on all collections inside/outside transactions show same data
db.person.find()
// { "_id" : 1, "fname" : "fname-1", "lname" : "lname-1" }
// { "_id" : 2, "fname" : "fname-2", "lname" : "lname-2" }

session1PersonColl.find()
// { "_id" : 1, "fname" : "fname-1", "lname" : "lname-1" }
// { "_id" : 2, "fname" : "fname-2", "lname" : "lname-2" }

session2PersonColl.find()
// { "_id" : 1, "fname" : "fname-1", "lname" : "lname-1" }
// { "_id" : 2, "fname" : "fname-2", "lname" : "lname-2" }


// insert a document _id 3, inside a transaction/session1
session1PersonColl.insert({"_id": 3, "fname": "fname-3", "lname": "lname-3"});
// WriteResult({ "nInserted" : 1 })

// update a document _id 1, inside transaction/session2
session2PersonColl.updateOne({"_id": 1}, {"$set": {"fname": "fname-1U", "lname": "lname-1U"}} );
// { "acknowledged" : true, "matchedCount" : 1, "modifiedCount" : 1 }

// delete a new document directly on the collection
db.person.deleteOne({"_id": 2});
// { "acknowledged" : true, "deletedCount" : 1 }

// The find operations on all collections inside/outside transactions show different set of data
// notice that all the uncommitted data changes inserts/updates are not visible on db.person
db.person.find();
// { "_id" : 1, "fname" : "fname-1", "lname" : "lname-1" }

// notice that the insert on session1 is only visible to it.
// the delete operation is not visible to session1
session1PersonColl.find()
// { "_id" : 1, "fname" : "fname-1", "lname" : "lname-1" }
// { "_id" : 2, "fname" : "fname-2", "lname" : "lname-2" }
// { "_id" : 3, "fname" : "fname-3", "lname" : "lname-3" }

// notice that the update on session2 is only visible to it.
// the delete operation is not visible to session2
session2PersonColl.find()
// { "_id" : 1, "fname" : "fname-1U", "lname" : "lname-1U" }
// { "_id" : 2, "fname" : "fname-2", "lname" : "lname-2" }


// commit and end the session1 and session2
session1.commitTransaction()
session1.endSession()
session2.commitTransaction()
session2.endSession()

// The find operation on the collection now shows committed changes
db.person.find()
// { "_id" : 1, "fname" : "fname-1U", "lname" : "lname-1U" }
// { "_id" : 3, "fname" : "fname-3", "lname" : "lname-3" }