Is it possible to implement Multi-Version Concurrency Control (MVCC) on top of MongoDB?

Question

MongoDB is to me a great database. However there are cases where I really need atomic multi-document transactions. For example to transfer things (like money or reputation) between accounts and this needs to either succeed completely or fail completely.

I wonder if it would be possible to interact with MongoDB through a library implementing the MultiVersion Concurrency Control pattern.

How bad would it be concerning performances? Would it be possible and profitable to use a hybrid approach, using the 'mongo-mvcc' library only when necessary and the traditional db connection when working only on a single document or would this break the mvcc stuff ?

Guthrun

Have you tried the atomic flag? – Pytlik

@Jimmy Sawczuk: this only works for a single document – Guthrun

Answer 1

The simplest way is to use locks (two-phase commit), although this is not very efficient in some cases. For higher concurrency some kind of MVCC can be implemented on the top of Mongo. This article provides a good description:

http://highlyscalable.wordpress.com/2012/01/07/mvcc-transactions-key-value/

Angioma

Answer 2

Money transaction can be implemented via two-phase commit : http://www.mongodb.org/display/DOCS/two-phase+commit

Silviasilviculture

Consider improving your post since your answer is essentially a link. See: Are answers that just contain links elsewhere really “good answers”? and Why is linking bad? – Chukar

Answer 3

There is an implementation of MVCC on MongoDB available now on GitHub:

https://github.com/igd-geo/mongomvcc

Povertystricken

Answer 4

MongoDB isn't really designed to work with transactions. There is a really good discussion of how you might be able to implement this over at: http://kylebanker.com/blog/2010/04/30/mongodb-and-ecommerce/

Hunkers

thanks for the link. However the article says: "Certainly, if we needed to literally debit one account and credit another in the same operation, or if we wanted rollback, we’d need full-fledged transactions." This was exactly my question... – Guthrun

Answer 5

You could create a versions collection and have a document for the last committed version.

Atomically update this document with the Read Timestamp (rts) which is not a time based timestamp but a monotonically increasing number when your application code has read a document from a collection.

Before you update a collection, fetch this versions collection document and check if there is a read timestamp below your current transaction, if there is, abort the read or write.

Update the versions document when you want to "publish" with a lastCommit the version of a record and cause it to be visible.

You should only "see" transactions data that are less than or equal to the last committed transaction number.

I implemented MVCC in Java in this repository.

Castiron

Answer 6

Well when you need real TRANSACTIONS you use RDBMS which are designed to support them :) NoSQLs are faster and more scalable mainly because they don't support transactions.

If you need both maybe it's a good idea to have transactional layer to support transactions and NoSQL layer for other purposes? In some cases it shouldn't be difficult to create a hybrid system using for example MongoDB and PostgreSQL

Hoard

In theory, it's not so hard to create system with hybrid persistence, but I guess in practice it is quite a different story... – Brookner

From the couchdb technical overwiew it's written: "If the storage engine is enhanced to allow multi-document update transactions, it is possible to perform Subversion-like 'all or nothing' atomic commits when replicating with an upstream server [...]". I would say that NoSQL engines may drop multi-document transactions, but it's not a requirement... – Guthrun

There are also two open tickets for MongoDB, asking for multi-document transactions: jira.mongodb.org/browse/SERVER-2804 and: jira.mongodb.org/browse/SERVER-2172 – Guthrun

It may be a requirement of RDBMSs to support transactions, but not every DBMS that supports transactions is relational. – Herrod