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Necessary complexity

DBMS2 - Sat, 2014-04-19 02:17

When I’m asked to talk to academics, the requested subject is usually a version of “What should we know about what’s happening in the actual market/real world?” I then try to figure out what the scholars could stand to hear that they perhaps don’t already know.

In the current case (Berkeley next Tuesday), I’m using the title “Necessary complexity”. I actually mean three different but related things by that, namely:

  1. No matter how cool an improvement you have in some particular area of technology, it’s not very useful until you add a whole bunch of me-too features and capabilities as well.
  2. Even beyond that, however, the simple(r) stuff has already been built. Most new opportunities are in the creation of complex integrated stacks, in part because …
  3. … users are doing ever more complex things.

While everybody on some level already knows all this, I think it bears calling out even so.

I previously encapsulated the first point in the cardinal rules of DBMS development:

Rule 1: Developing a good DBMS requires 5-7 years and tens of millions of dollars.

That’s if things go extremely well.

Rule 2: You aren’t an exception to Rule 1. 

In particular:

  • Concurrent workloads benchmarked in the lab are poor predictors of concurrent performance in real life.
  • Mixed workload management is harder than you’re assuming it is.
  • Those minor edge cases in which your Version 1 product works poorly aren’t minor after all.

My recent post about MongoDB is just one example of same.

Examples of the second point include but are hardly limited to:

BDAS and Spark make a splendid example as well. :)

As to the third point:

Bottom line: Serious software has been built for over 50 years. Very little of it is simple any more.

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MongoDB is growing up

DBMS2 - Thu, 2014-04-17 02:56

I caught up with my clients at MongoDB to discuss the recent MongoDB 2.6, along with some new statements of direction. The biggest takeaway is that the MongoDB product, along with the associated MMS (MongoDB Management Service), is growing up. Aspects include:

  • An actual automation and management user interface, as opposed to the current management style, which is almost entirely via scripts (except for the monitoring UI).
    • That’s scheduled for public beta in May, and general availability later this year.
    • It will include some kind of integrated provisioning with VMware, OpenStack, et al.
    • One goal is to let you apply database changes, software upgrades, etc. without taking the cluster down.
  • A reasonable backup strategy.
    • A snapshot copy is made of the database.
    • A copy of the log is streamed somewhere.
    • Periodically — the default seems to be 6 hours — the log is applied to create a new current snapshot.
    • For point-in-time recovery, you take the last snapshot prior to the point, and roll forward to the desired point.
  • A reasonable locking strategy!
    • Document-level locking is all-but-promised for MongoDB 2.8.
    • That means what it sounds like. (I mention this because sometimes an XML database winds up being one big document, which leads to confusing conversations about what’s going on.)
  • Security. My eyes glaze over at the details, but several major buzzwords have been checked off.
  • A general code rewrite to allow for (more) rapid addition of future features.

Of course, when a DBMS vendor rewrites its code, that’s a multi-year process. (I think of it at Oracle as spanning 6 years and 2 main-number releases.) With that caveat, the MongoDB rewrite story is something like:

  • Updating has been reworked. Most of the benefits are coming later.
  • Query optimization and execution have been reworked. Most of the benefits are coming later, except that …
  • … you can now directly filter on multiple indexes in one query; previously you could only simulate doing that by pre-building a compound index.
  • One of those future benefits is more index types, for example R-trees or inverted lists.
  • Concurrency improvements are down the road.
  • So are rewrites of the storage layer, including the introduction of compression.

Also, you can now straightforwardly transform data in a MongoDB database and write it into new datasets, something that evidently wasn’t easy to do before.

One thing that MongoDB is not doing is offer any ODBC/JDBC or other SQL interfaces. Rather, there’s some other API — I don’t know the details — whereby business intelligence tools or other systems can extract views, and a few BI vendors evidently are doing just that. In particular, MicroStrategy and QlikView were named, as well as a couple of open source usual-suspects.

As of 2.6, MongoDB seems to have a basic integrated text search capability — which however does not rise to the search functionality level that was in Oracle 7.3.2. In particular:

  • 15 Western languages are supported with stopwords, tokenization, etc.
  • Search predicates can be mixed into MongoDB queries.
  • The search language isn’t very rich; for example, it lacks WHERE NEAR semantics.
  • You can’t tweak the lexicon yourself.

And finally, some business and pricing notes:

  • Two big aspects of the paid-versus-free version of MongoDB (the product line) are:
    • Security.
    • Management tools.
  • Well, actually, you can get the management tools for free, but only on a SaaS basis from MongoDB (the company).
    • If you want them on premises or in your part of the cloud, you need to pay.
    • If you want MongoDB (the company) to maintain your backups for you, you need to pay.
  • Customer counts include:
    • At least 1000 or so subscribers (counting by organization).
    • Over 500 (additional?) customers for remote backup.
    • 30 of the Fortune 100.

And finally, MongoDB did something many companies should, which is aggregate user success stories for which they may not be allowed to publish full details. Tidbits include:

  • Over 100 organizations run clusters with more than 100 nodes. Some clusters exceed 1,000 nodes.
  • Many clusters deliver hundreds of thousands of operations per second (combined read and write).
  • MongoDB clusters routinely store hundreds of terabytes, and some store multiple petabytes of data. Over 150 clusters exceed 1 billion documents in size. Many manage more than 100 billion documents.
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The worst database developers in the world?

DBMS2 - Wed, 2014-04-16 00:45

If the makers of MMO RPGs (Massive Multi-Player Online Role-Playing Games) aren’t quite the worst database application developers in the world, they’re at least on the short list for consideration. The makers of Guild Wars didn’t even try to have decent database functionality. A decade later, when they introduced Guild Wars 2, the database-oriented functionality (auction house, real-money store, etc.) would crash for days at a time. Lord of the Rings Online evidently had multiple issues with database functionality. Now I’m playing Elder Scrolls Online, which on the whole is a great game, but which may have the most database screw-ups of all.

ESO has been live for less than 3 weeks, and in that time:

1. There’s been a major bug in which players’ “banks” shrank, losing items and so on. Days later, the data still hasn’t been recovered. After a patch, the problem if anything worsened.

2. Guild functionality has at times been taken down while the rest of the game functioned.

3. Those problems aside, bank and guild bank functionality are broken, via what might be considered performance bugs. Problems I repeatedly encounter include:

  • If you deposit a few items, the bank soon goes into a wait state where you can’t use it for a minute or more.
  • Similarly, when you try to access a guild — i.e. group — bank, you often find it in an unresponsive state.
  • If you make a series of updates a second apart, the game tells you you’re doing things too quickly, and insists that you slow down a lot.
  • Items that are supposed to “stack” appear in 2 or more stacks; i.e., a very simple kind of aggregation is failing. There are also several other related recurring errors, which I conjecture have the same underlying cause.

In general, it seems like that what should be a collection of database records is really just a list, parsed each time an update occurs, periodically flushed in its entirety to disk, with all the performance problems you’d expect from that kind of choice.

4. Even stupider are the in-game stores, where fictional items are sold for fictional money. They have an e-commerce interface that is literally 15+ years out of date — items are listed with VERY few filtering options, and there is no way to change the sort. But even that super-primitive interface doesn’t work; in particular, filter queries frequently return incorrect empty-set responses.

5. Much as in other games, over 10 minutes of state changes can be lost.

Except perhaps for #5, these are all functions that surely are only loosely coupled to the rest of the game. Hence the other difficulties of game scaling and performance should have no bearing on them. Hence there’s no excuse for doing such a terrible job of development on large portions of gameplay functionality.

Based on job listings, ESO developer Zenimax doesn’t see database functionality as a major area to fix. This makes me sad.

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