CODEWITHSUNDEEP
mongodbaciddatabasenosql
fmchan
fmchan

Reputation: 760

NO Guarantee of ACID for NOSql then how to ensure consistency?

For example, for find and save document in MongoDB:

Article article1 = mongoOperation.findOne(new Query(Criteria.where("_id").is(1)), Article.class);
mongoOperation.save(article.setAttrA("A"));
Article article2 = mongoOperation.findOne(new Query(Criteria.where("_id").is(1)), Article.class);
mongoOperation.save(article.setAttrB("B"));

By default, _id is primary key so the document of article1 and article2 are the same.

Will it be possible to get a outdated document for article2 which attrA has not been set to A yet because in theoretically MongoDB does not follow the ACID properties.

Upvotes: 0

Views: 289

Answers (1)

Philipp
Philipp

Reputation: 69663

Yes, the second query might return the same document before applying the save.

This is in fact quite likely if you are using an asynchronous API, a synchronous API with unsafe write concern or have a read-preference which prefers the secondary of a replica-set (which usually lacks behind a few seconds).

What you can do to detect and handle edit conflicts is to add a revision-id to each article which you increment by one on each edit. When you update an article, you use update instead of save, like this:

article.revision++;
writeResult = db.articles.update(
    { _id: article._id, revision: article.revision -1 }, 
    article, 
    { writeConcern: WriteConcern.ReplicaAcknowledged}
);

When someone edited the article in the meantime, the article with the old revision-id will be no longer in the database and you will get writeResult.nModified == 0.

Upvotes: 2

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