GridFS in transactions

[miroslavvojtus]

Hi Spring masters,

we are trying to use MongoDB 4.0 transactions in our project.
Unfortunatelly there is no out of the box transaction support for GridFS (or am I wrong?)

We attempted to create custom transaction aware MongoDatabaseFactory which enabled transactions in constructed GridFSTemplate but it immediately fails due to listIndexes operation which is not possible to disable in default GridFsBucketImpl.

Stack trace starts with:

com.mongodb.MongoCommandException: Command failed with error 263 (OperationNotSupportedInTransaction): 'Cannot run 'listIndexes' in a multi-document transaction.'

Do you have some hints how to use transactions with current GridFS implementation? Do you plan to implement full fledged transaction support?

Many thanks.

[christophstrobl]

Thanks for bringing this to our attention!

You can use MongoDatabaseUtils to interact with the transaction support and obtain session bound resources. The current GridFsTemplate implementation itself does not use this mechanism, but retrieves the MongoDatabase directly from the factory. Still, even if it did so, any GridFsBucket interaction would need to be called with the corresponding ClientSession which is not the case either.

I do not think there's a reasonable way how this could work with the GridFsTemplate as it is today.

Transaction support for GridFS clearly is something we'll support in the future. I'll take this to the team for further discussion and planning.

[miroslavvojtus]

Could you help me to intorduce transactions prior to your build in support?

My current attempt looks like:

1. Create Transaction aware MongoDatabaseFactory, which simply delegates most of methods to default mongoTemplate or uses MongoDatabaseUtils like MongoDatabaseUtils.getDatabase(dbName, mongoDatabaseFactory)
2. Create a GridFSTemplateProvider which simply instantiates the GridFsTemplate like new GridFsTemplate(TransactionAwareMongoDbFactory.wrap(mongoTemplate.getMongoDatabaseFactory()), mongoTemplate.getConverter(), bucket)

I thought that once the MongodDatabase produced by TransactionAwareMongoDatabasefactory there is no need to propagate the ClientSession's. You said it is still necessary.

How would you suggest to retrieve current ClientSession in an eventual custom implementation of GridFsTemplate?
Unfortunately the MongoTransactionObject is protected (I can not read it from resource holder returned by TransactionSynchronizationManager) thus I did not find a way how to read/use it from somewhere?

[christophstrobl]

Sorry - got distracted by the GridFSBucket API that has some overloads accepting ClientSession. Looking at the implementation you're right, the proxied collection should be used, applying the session for you.

Unfortunately, having another look at the documentation, MongoDB 4.4 does not support multi document transactions for GridFS.

[miroslavvojtus]

Well this seems to be a pity for us then.
I have asked for a reasoning on the mongodb community forum: https://developer.mongodb.com/community/forums/t/gridfs-does-not-support-multi-document-transactions/105984