limits of number of collections in databases

Question

Can anyone say are there any practical limits for the number of collections in mongodb? They write here https://docs.mongodb.com/manual/core/data-model-operations/#large-number-of-collections:

Generally, having a large number of collections has no significant performance penalty, and results in very good performance.

But for some reason mongodb set limit 24000 for the number of namespaces in the database, it looks like it can be increased, but I wonder why it has some the limit in default configuration if having many collections in the database doesn't cause any performance penalty?

Does it mean that it's a viable solution to have a practically unlimited number of collections in one database, for example, to have one collection of data of one account in a database for the multitenant application, having, for example, hundreds of thousands of collections in the database? If it's the viable solution to have a very large number of collections for a database for every tenant, what's the benefits of it for example versus having documents of each tenant in one collection? Thank you very much for your answers.

Answer 1

This answer is late however the other answers seem a bit...weak in terms of reliability and factual information so I will attempt to remedy that a little.

But for some reason mongodb set limit 24000 for the number of namespaces in the database,

That is merely the default setting. Yes, there is a default setting.

It does say on the limits page that 24000 is the limit ( http://docs.mongodb.org/manual/reference/limits/#Number%20of%20Namespaces ), as though there is no way to expand that but there is.

However there is a maximum limit on how big a namespace file can be ( http://docs.mongodb.org/manual/reference/limits/#Size%20of%20Namespace%20File ) which is 2GB. That gives you roughly 3 million namespaces to play with in most cases which is quite impressive and I am unsure if many people will hit that limit quickly.

You can modify the default value to go higher than 16MB by using the nssize parameter either within the configuration ( http://docs.mongodb.org/manual/reference/configuration-options/#nssize ) or at runtime by manipulating the command used to run MongoDB ( http://docs.mongodb.org/manual/reference/mongod/#cmdoption-mongod--nssize ).

There is no real reason for why MongoDB implements 16MB by default for its nssize as far as I know, I have never heard about the motto of "not bother the user with every single detail" so I don't buy that one.

I think, in my opinion, the main reason why MongoDB hides this is because even though, as the documentation states:

Distinct collections are very important for high-throughput batch processing.

Using multiple collections as a means to scale vertically rather than horizontally through a cluster, as MongoDB is designed to, is considered (quite often) bad practice for large scale websites; as such 12K collections is normally considered something that people will never, and should never, ascertain.

Answer 2

No More Limits!

As other answers have stated - this is determined by the size of the namespace file. This was previously an issue, because it had a default limit of 16mb and a max of 2gb. However with the release of MongoDB 3.0 and the WiredTiger storage engine, it looks like this limit has been removed. WiredTiger seems to be better in almost every way, so I see little reason for anyone to use the old engine, except for legacy support reasons. From the site:

For the MMAPv1 storage engine, namespace files can be no larger than 2047 megabytes.

By default namespace files are 16 megabytes. You can configure the size using the nsSize option.

The WiredTiger storage engine is not subject to this limitation.

http://docs.mongodb.org/manual/reference/limits/