Photo by on Madison Grooms Unsplash Support for transactions in MongoDB has been something long desired by many. With MongoDB v4.0, the wait is now over. Welcome to multi-document transactions for replica sets. In a application when you have parent-child relationship between two entities, such as , you typically embed the child documents ( ) in a parent document ( ). Such schema design way will not only help with faster reads, but also help meet the atomicity. A write operations on a single document is atomic. However, MongoDB database orders & order details order details order ‘How do you maintain the atomicity while working with multiple documents?’ This is one of the many articles in multi-part series, , solely created for you to master by learning . In a few series of articles, I would like to give various tips to help you answer the above question. This article discusses , a new feature in MongoDB v4.0 — its applications, use case scenarios and finally some hands-on lab exercises. Mastering MongoDB — One tip a day MongoDB ‘one tip a day’ multi-document transactions Mastering — Multi-document transactions What are Transactions In MongoDB, a write operations on a single document is , even if the operation modifies multiple embedded documents within a single document. When a single write operation, like , modifies multiple documents, the modification of each document is atomic, but the operation as a whole is not atomic. Some of the use cases may also require you to modify multiple write operations as part of single operation. In such scenarios you do need to enforce atomicity across multiple write operations. atomic updateMany transactions Why use Transactions Until MongoDB v4.0 the only way you could emulate is by implementing a in your application. wrote an article, , on two-phase commits using module for applications. Please be aware that the two-phase commits can only offer transaction-like semantics. While using two-phase commit, it is possible for applications to return intermediate data during the two-phase commit or rollback. transactions two-phase commit Emmanuel Olaojo “Fawn: in MongoDB” transactions Fawn Node.js The MongoDB v4.0 introduces for replica sets and can be used across multiple operations, collections, and documents. The multi-document provide a globally consistent view of data, and enforce all-or-nothing execution to maintain data integrity. The will have following properties multi-document transactions transactions transactions When a transaction is committed, all data changes made in the transaction are saved. If any operation in the transaction fails, the transaction aborts When a transaction aborts/aborted, all data changes made in the transaction are discarded. Until a transaction is committed, no write operations in the transaction are visible outside the transaction. Hands-On lab exercises This lab exercise helps you understand how to make use of within a MongoDB shell. Since the Multi-document are available only for replica sets, please ensure that you do have at least 1 member replicaset rather than standalone . transactions transactions mongod Before we get started, I want you to be aware of a few points You can only specify read/write (CRUD) operations on existing collections. A multi-document transaction cannot include an insert operation that would result in the creation of a new collection. Transactions are associated with a session. At any given time, you can have at most one open transaction for a session. To associate read and write operations with an open transaction, you pass the session to the operations. Setup environment First, you would need an environment to play around. If you already have a MongoDB v4.0 replicaset environment, you may skip this step. A bash script with download MongoDB v4.0 release candidate and create a 1 member replica set The next set of exercises will illustrate how the commitTransaction / abortTransaction work and most importantly also covers how multiple write operations with lead to . To help you understand better I have included the output of these commands right below them as comments. write conflicts abortTransaction Saving data changes with commitTransaction The below MongoDB shell commands shows a collection with some sample data in it. Notice that the newly added document in the is not visible on the collection object until the is committed. person transaction db.person session1 MongoDB commands illustrating the data changes in transaction are save when the transaction is committed. Discarding data changes with abortTransaction The below MongoDB shell commands shows a collection with some sample data in it. Notice that the newly added document in the is not visible on the collection object until the is aborted. person transaction db.person session1 MongoDB commands illustrating the data changes in transaction are discarded when the transaction is aborted on the session. Transactions with no write conflicts can be committed In the below code, you may notice that there are multiple write operations ( , and ) are invoked from multiple scopes, inside/outside of transaction. As long as there is no , you can successfully commit these transaction. insert update delete WriteConflict MongoDB commands illustrating transactions with no write conflicts can successfully commit Transactions with write conflicts are aborted If two or more write operations modify the same document from different scopes, then the data changes in one transaction impact the data changes from the other transactions. When such exists, the operations would result in and . These writes could be any combination of insert/update/delete operations. Below example shows two delete operations on same document invoked in two different transactions. WriteConflict TransientTransactionError abortTransaction MongoDB commands illustrating multiple transactions with write conflicts are aborted Summary The support for the Multi-document for replica sets is just the beginning. The future release may address the across the sharded deployment and various isolation levels that you may have exposure from other relational databases. I want to remind an important point — transactions transactions “Just because you now have support for transactions, you must not design the data model around 3rd normal form. You must always have an effective MongoDB schema design to ensure your application is highly performant. Multi-document transaction incurs a greater performance cost when compared to single document writes. So, “What is the performance cost of using transactions?”. Great question! But that’s a topic for another day. Hopefully, you learned something new today on you scale the path to “ ”. Mastering MongoDB — One tip a day

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