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Things I wish I knew about MongoDB a year ago

snmaynard.com

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Re: Things I wish I knew about MongoDB a year ago

#2

Post by nickzoic »

The count({condition}) one is a worry. I'm guessing it is slow in the case where it has to page the index in in order to count it. I wonder if it is still a problem where the index is used a lot anyway. A fix in MongoDB would seem a lot better solution than having everyone implement their own hacky count-caching solution.

EDIT: Actually, looking at the bug reports, sounds like maybe lock contention on the index?

The master/slave replication problem seems bad but I think it can be worked around (for my particular project) with a flag on the user session ... if they've performed a write in the last 30 seconds, set slaveOkay = false. Users who are just browsing may experience a slight delay in seeing new documents but users who are editing stuff will see their edits immediately.

Re: Things I wish I knew about MongoDB a year ago

#5

Post by dia80 »

Genuine question:

In what use cases does mongo kick mysql's ass?

I've used it a couple of times in hobby projects and enjoyed not maintaining a schema. I read so many of these 'gotcha' style articles and for example one commenter here wants to have a manual "recently dirty" flag to combat the master / slave lag mentioned in the article. I know it's faster (tm) but once you have to take in to account all this low level stuff you have to worry about yourself wouldn't it just be better to rent/buy another rack of mysql servers and not worry about it?

Look forward to learning something...

Re: Things I wish I knew about MongoDB a year ago

#7

Post by alexro »

dia80 wrote: post #5

Genuine question: In what use cases does mongo kick mysql's ass? I've used it a couple of times in hobby projects and enjoyed not maintaining a schema. I read so many of these 'gotcha' style articles and for example one commenter here wants to have a manual "recently dirty" flag to combat the master / slave lag mentioned in the article. I know it's faster (tm) but once you have to take in to account all this low leve…

IMO there is no rational explanation to this phenomena other than: people are different. Some get bored with stored procs and want same hassle but in another form.

Re: Things I wish I knew about MongoDB a year ago

#9

Post by morsch »

alexro wrote: post #7
dia80 wrote: post #5

Genuine question: In what use cases does mongo kick mysql's ass? I've used it a couple of times in hobby projects and enjoyed not maintaining a schema. I read so many of these 'gotcha' style articles and for example one commenter here wants to have a manual "recently dirty" flag to combat the master / slave lag mentioned in the article. I know it's faster (tm) but once you have to take in to account all this low leve…

IMO there is no rational explanation to this phenomena other than: people are different. Some get bored with stored procs and want same hassle but in another form.

A cynical but insightful description of many kinds of progress and change. ;)

Re: Things I wish I knew about MongoDB a year ago

#10

Post by jbert »

lars512 wrote: post #8

The inconsistent reads in replica sets is something we've come across with MySQL read slaves as well. I think it's a gotcha of that whole model of replication, rather than a MongoDB-specific issue.

One way to resolve it is to mark that user or session (or even just request) "sticky to the master" for long enough to cover your normal replication delay.

When we saw it before, ensuring that a given request which issued a write also read from the master was sufficient. (sub-second replication delay).

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