I had to create indices for a collection. I created it but it affected all the requests on the other collections in the other database.
From doc, I read that index creation blocks all read/write locks on all the databases. What is the reason behind such behavior of mongo DB?
There are two ways to build indexes on MongoDB -
1) Foreground : If you're using MongoDB version <4.2 all indexes are by default build Foreground, Which has better data structures & fast in process but major downside would be while indexes getting build it blocks all operations on database. Which has to be avoided on usual prod servers.
2) Background : So for all versions below 4.2 to overcome con of foreground build process of blocking database activities, there is an option {background :true} to build indexes in background, here while MongoDB is building indexes you can still do reads & writes but cons with this approach is the index structure is least performant & index build time is more.
For MongoDB v >= 4.2, building indexes on background is deprecated, So you can no more specify it & take advantage of background indexes, So MongoDB itself internally builds indexes in background fashion but there would be a lock on the particular collection at start of index build & at end of the build, rest apart mostly you can go ahead with your reads & writes, but somehow there will be 'intent exclusive IX lock' which will intermittently lock the collection & after certain stages there will be lock on writes & then a complete lock, which will finally be released at the end. But why ? So this is to take advantage of fastness & better data structures of foreground + non locking mechanism of background build process.
Ref : .createIndex() & index-creation-process
