kristina

For 2.0, there is some major work on locking being done (concurrency will be an ongoing theme in 2011) and online compaction has been implemented (although you have to kick it off, it's not yet automatic).

Ah, sorry, I assumed people would understand that from the post. I added (preview, not production-ready), is that better?

I'm reluctant to put such technical info on the documentation homepage, but I've updated it to very clearly point people to Production Notes. How does it look now?

Actually, it's more like saying, "we HIGHLY recommend having a backup parachute" (as, I'd imagine, most skydiving instructors would).

Also, I think I've always mention it in my talks.

I'm not sure how VoltDB escaped your wrath, it's an in-memory db that claims to be durable.

Writing to disk and transaction logs are nice, but they aren't magic bullets. What if a data center catches fire? More mundanely, I've heard ~6% of hard drive fail/year. Only replication can help you there.

I'd argue that durability is a sliding scale. You have to figure out how much risk you're willing to take and you cannot have a perfectly durable system.

Response (awaiting moderation, I think it's the links?):

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> haha, that’s funny. i regularly use non-CouchDB databases and I get along great with > all the people from other databases at conferences.

Oh, I tend to bite people when I find out they use another database . Maybe I should stop that?

> even if i did feel like we were competing, i wouldn’t care. this post really is about > reliability issues i don’t think your users are fully aware of and i honestly hope that > you fix.

You must be thrilled to learn that single server durability is coming. I look forward to a followup post extolling MongoDB’s virtues this fall.

> I responded earlier to the complexity of actually keeping something available that > depends on [multiple servers]. so i won’t cover it again.

Yes, it is a difficult, but not unsolvable, problem. Mongo’s made a bunch of tradeoffs in the awesome vs. easy to program area. For instance, remember last year when CouchDB was saying MongoDB sucked because of its lack of concurrency? That it was too complicated to do concurrency in C++ and that Erlang was the way? Well, now Mongo has concurrency, so on to the next “must have” thing.

>You make it sounds like this is all just a matter of bugs, it’s not, and i find blaming > it on users who don’t use JIRA or get on IRC a little distasteful.

People discuss everything from bugs to architecture to lunch on our various forums. I was trying to say, possibly badly, that we have a lot of ways for people with questions, problems, and suggestions to reach out.

Eliminating the methods I outlined, I’m not sure how people with suggestions could reach the developers, other than telepathy.

Also, the user you cite is far from typical. It sucks that some people don’t like Mongo, but there’s are a lot more out there from those who do: http://codeascraft.etsy.com/2010/07/03/mongodb-at-etsy-part-..., http://blog.eventbrite.com/guest-post-why-you-should-track-p..., http://blog.wordnik.com/what-has-technology-done-for-words-l..., http://www.engineyard.com/blog/2009/mongodb-a-light-in-the-d... and so on.

We really, really want people to know they should run on multiple servers. Do you have any suggestions on making it clearer? Where did you look for information about running it in production (so I can add stuff about multiple servers to that page)?

> Having said that, I must ask. Do the "unchecked" writes, > configurable fsyncs and multi-node writes exist in the > currently stable version of MongoDB or are these features > still in alpha or beta?

They're all available in stable! This might be a documentation fail. It is on the wiki, but suggestions are welcome if you looked at page X and didn't see it.

I've tried to post this as a comment on the blog, but it's not showing up (moderated?):

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Full disclosure: I work for 10gen.

You strategically posted this when my air conditioning was broken, so here are a few thoughts before I go find somewhere cooler. Since CouchDB is "not a competitor" to MongoDB, it's nice of you to put all this time into a public service.

> MongoDB, <b>by default</b>, doesn’t actually have a response for writes.

Whoopsy, got your emphasis wrong there. We did this to make MongoDB look good in stupid benchmarks (http://www.snailinaturtleneck.com/blog/2009/06/29/couchdb-vs...).

Seriously, though, this "unchecked" type of write is just supposed to be for stuff like analytics or sensor data, when you're getting a zillion a second and don't really care some get lost if the server crashes. <b>You can do an insert that not only waits for a database response, but waits for N slaves (user configurable) to have replicated that insert.</b> Note that this is very similar to Cassandra's "write to multiple nodes" promise. You can also fsync after every write.

> MongoDB writes to a mem-mapped file and lets the kernel fsync it whenever > the kernel feels like it.

fsyncs are configurable. You can fsync once a second, never, or after every single insert, remove, and update if you wish.

> When you look at MongoDB more critically I don’t see how you could actually > justify using it for anything resembling the traditional role of a database.

This is because you assume you'll run it on single server. MongoDB's documentation clearly, repeatedly, and earnestly tells people to run MongoDB on multiple servers.

Also, as another commenter mentioned, full single-server durability is scheduled for the fall.

> Stories like this (http://www.korokithakis.net/node/119) are dubious not > because they expose a few bugs in MongoDB but because they show inherent > architectural problems you cannot overcome long term without something > append-only.

Stories "like this" show that MongoDB doesn't work for everyone, particularly people who give no specifics about their architecture, setup, what happened, or anything else. Isn't it irritating how people will write, "MongoDB lost my data" or "CouchDB is really slow" and provide no specifics?

That's not to say that things never go wrong, MongoDB is definitely not perfect and has lots of room for improvement. I hope that users with questions and problems will contact us on the list, our wiki, the bug tracker, or IRC (or, heck, write a snarky blog post). Anything to contact the community and let us try to help. I wish every person who tried MongoDB had a great experience with it.

Lots of users, hopefully most, love MongoDB and are using it happily and successfully in production.

...which is why you have a slave. Master corrupt? Promote the slave to master, get it its own slave.

Repairing a corrupted database, even a relational database, often takes too long for a production app.

They go into it in the actual paper, if you're interested. They wanted a document store because their data is flexible (all different sizes and shapes), complex, and they needed secondary indexes. They didn't need transactions and MongoDB was very fast.