CODEWITHSUNDEEP
mongodbtransactionsselect-for-update
Mike
Mike

Reputation: 699

MongoDB - select document for update - without another operation modifying the same document after the select

I've got a document that needs to be read and updated. Meanwhile, it's quite likely that another process is doing the same which would break the document update.

For example, if Process A reads document d and adds field 'a' to it and writes the document, and Process B reads document d before Process A writes it, and adds field b and writes the document, then whichever process writes the changes out will get their change because it clobbers the change by the one that wrote first.

I've read this article and some other very complicated transaction articles around mongo. Can someone describe a simple solution to this - I have not come across something that makes me comfortable with this yet.

https://www.mongodb.com/blog/post/how-to-select--for-update-inside-mongodb-transactions

[UPDATE]- In addition, I'm trying to augment a document that might not yet exist. I need to create the document if it doesn't exist. I also need to read it to analyze it. One key is "relatedIds" (an array). I push to that array if the id is not found in it. Another method I have that needs to create the document if it doesn't exist adds to a separate collection of objects.

[ANOTHER UPDATE x2] --> From what I've been reading and getting from various sources - is that the only way to properly create a transaction for this - is to "findOneAndModify" the document to mark it as dirty with some field that will definitely update, such as "lock" with an objectId (since that will never result in a NO-OP - ie, it definitely causes a change).

If another operation tries to write to it, Mongo can now detect that this record is already part of a transaction.

Thus anything that writes to it will cause a writeError on that other operation. My transaction can then slowly work on that record and have a lock on it. When it writes it out and commits, that record is definitely not touched by anything else. If there's no way to do this without a transaction for some reason, then am I creating the transaction in the easiest way here?

Upvotes: 1

Views: 1189

Answers (1)

Tom Slabbaert
Tom Slabbaert

Reputation: 22296

Using Mongo's transactions is the "proper" way to go but i'll offer a simple solution that is sufficient ( with some caveats ).

The simplest solution would be to use findOneAndUpdate to read the document and update a new field, let's call it status, since it is atomic this is possible.

the query would look like so:

const doc = await db.collection.findOneAndUpdate(
    {
        _id: docId,
        status: { $ne: 'processing' }
    },
    {
        $set: {
            status: 'processing'
        }
    }
);

so if dov.value is null then it means (assuming the document exists) that another process is processing it. When you finish processing you just need to reset status to be any other value.

Now because you are inherently locking this document from being read until the process finishes you have to make sure that you handle cases like an error thrown throughout the process, update failure, db connection issue's, etc .

Overall I would be cautious about using this approach as it will only "lock" the document for the "proper" queries ( every single process needs to be updated to use the status field ), which is a little problematic, depending on your usecase.

Upvotes: 1

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