MongoDB :: setWriteConcern默认值？

Question

I have a specific question with the way the PHP Mongo driver handles figuring out setWriteConcern and setReadPreferences. From the documentation I gather that:

1. You can change writeConcern at the individual insert, update, remove level.
2. You can adjust the writeConcern at the connection level.
3. You can use MongoCollection::setWriteConcern to set the default for this collection.
4. You can use MongoDB::setWriteConcern to set the default for this database.

We are testing a sharded, replicated setup and I am wondering if I use MongoDB::setWriteConcern and later do an insert on a collection in that database without specifying any write concern will it default to the concern I did on the DB object itself?

Basically, if I set a default writeConcern, or readPreference for that matter, on a DB/Collection does that info get synced to the config servers such that all subsequent connections will default to those concerns/preferences if none are specified at the connection or query level?

Answer 1

If you use setWriteConcern to set the default, it will affect only the subsequent write operations within the mongo client/shell you are using.

For example, open a mongo shell and type this:

db.setWriteConcern(1)

Now check the default in the same shell:

db.getWriteConcern()
WriteConcern({ "w" : 1 })

Then open another mongo shell and getWriteConcern, you should see null value.

db.getWriteConcern()
null