CODEWITHSUNDEEP
mongodblocking
sparrovv
sparrovv

Reputation: 7784

MongoDB collection locking - how does it work?

I have not so big collection, that has about 500k records, but it's mission critical.

I want to add one field and remove another one. I was wondering would it lock that collection from inserting/updating (I really don't want any downtime).

I've made an experiment, and it looks that it doesn't block it:

// mongo-console 1
 use "my_db"

// add new field
db.my_col.update(
    {},
    { $set:
      { foobar : "bizfoo"}
    },
    { multi: true}
    );

// mongo-console 2
use "my_db"
db.my_col.insert({_id: 1, foobar: 'Im in'});
db.my_col.findOne({_id: 1});

=>{ "_id" : 1, "foo" : "bar" }

Although I don't really understand why, because db.currentOp() shows that there are Write locks on it.

Also on the production system I have replica set, and I was curious how does it impact the migration. Can someone answer these questions, or point me to some article where it's nicely explained. Thanks!

(MongoDB version I use is 2.4)

Upvotes: 0

Views: 1912

Answers (1)

Brody
Brody

Reputation: 86

MongoDB 2.4 locks on the database level per shard. You mentioned you have a replica set. Replica sets have no impact on the locking. Shards do. If you have your data sharded, when you perform an update, the lock will only lock the database on the shard where the data lives. If you don't have your data sharded, then the database is locked during the write operation.

In order to see impact, you'll need a test that does a significant amount of work.

You can read more at: http://www.mongodb.org/display/DOCS/How+does+concurrency+work

Upvotes: 2

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