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Insight on MongoDB Transactions
By AKumar   |   July 10, 2019   |   Posted in : Mongodb

Before version 4.0, MongoDB uses $isolated operator to update multiple documents but it does not provide “all-or-nothing” atomicity for write operations.

MongoDB resolved this issue in version 4.0 and provides the support of Multi-Document transactions. 
Prior to starting a transaction, a session must be created. A transaction cannot be run outside a session.

At any given time you may have multiple running sessions in the system, but each session may run only a single transaction at a time. You can run transactions in parallel according to how many open sessions you have.

The following mongo shell methods are available for Transactions to Start & Commit or Abort :

How it works?

Below example clearly explains how transactions work in MongoDB. Here we will be implementing transactions by creating two different sessions.

Session 1 :

In this session 1, I am starting a transaction and inserting a document without commiting the transaction.

Session 2 :

Lets find the inserted document.



Before Commiting the transaction: No result set, as Session 1 did not commit the transaction.

Session 1:

Now again in the session 1, lets commit the transaction.

Session 2 :

Now, when we find the document, we could see the same data that we have inserted already in session1.



Note : By default, trananaction uses Read Commited isolation and we can't change the isolation level. 

Limitations of Transactions :
  1. A collection cannot be dropped or created inside a transaction.

  2. It is not possible to read and write into a system.* collection.

  3. An index cannot be created or dropped inside a transaction.

  4. We can’t use transactions on a standalone server, we need to configure it as a replica set.

  5. By default, a transaction that executes for longer than 60 seconds will automatically expire.

  6. A transaction cannot read or write in config, admin, and local databases.

  7. Size of a transaction is limited to 16MB.
     

Note: From upcoming MongoDB version(4.2), transaction will get supported in shared cluster as well.




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