The Write Stuff

Carlos Bueno
Carlos Bueno

Tell me if this sounds familiar. Once upon a time a company ran its operations on The Database Server, a single machine that talked SQL. It was tricked out with fast hard drives and cool blue lights. As the business grew it became harder for The Database to keep up. So they bought an identical server as a hot spare and set up replication, at first only for backups and failover. That machine was too tempting to leave sitting idle, of course. The business analysts asked for access so they could run reports on live data. Soon the hot spare was just as busy and mission-critical as the master.

The business grew some more. The cost of hardware to handle the load went way up. Caching reads only helped so much, and don’t get me started about maintaining cache consistency. It was beginning to look like it would be impossible for The Database to handle the volume of writes coming in. The operations people weren’t happy either. The latest semi-annual schema change had been so traumatic and caused so much downtime that they were still twitching.

It was then that the company took a deep breath, catalogued all their troubles & heartache and decided to ditch SQL altogether. It was not an easy choice but these were desperate times. Six months later the company was humming along on a cluster of “NoSQL” machines acting in concert. It scaled horizontally. The schemas were fluid. Life was good.

For a while, anyway. It turned out that when scaled up, the NoSQL cluster worked fine except for two minor things: reading data and writing data. Reading data (“finding documents”) could be sped up by adding indexes. But each new index slowed down write throughput. The business analysts weren’t about to learn how to program just to make their reports. That task fell back onto the engineers. They told themselves all this was just the price of graduating to Big Data.

The business grew a little more, and the cracks suddenly widened. They discovered that “global write lock” essentially means “good luck doing more than a few thousand writes per second.”

A few thousand sounds like a lot, but there are only 86,400 seconds in a day, and the peak-hour of traffic is generally two or three times the average because people sleep. A limit of 3,000 writes per second translates to roughly 90 million writes a day. And let’s not talk about reads. Flirting with these limits became as painful as the database platform they’d just abandoned.

Tell me if this sounds familiar. I’ve seen a lot of companies suddenly find themselves stuck up a tree like this. It’s not a fun place to be. Hiring performance experts to twiddle with the existing system may or may not help. Moving to a different platform may or may not help either. A startup you’ve definitely heard of runs four, count ‘em, four separate NoSQL systems because each one had some indispensable feature (eg, sharding or replication) that the others didn’t. That way lies madness.

Hypothetical Corp

Let’s look at the kinds of hardware running Hypothetical Corp’s business.

50 Application Servers (Lots of CPU)

10 Memcached Servers (Lots of RAM)

4 NoSQL Servers (Lots of DIsk)

The interesting thing is that Hypothetical has several times more RAM in its fleet than the size of their database. If you ask them why, they’ll tell you “because accessing data from RAM is much faster than from disk.” This is, of course, absolutely true.  Accessing  a random piece of data in RAM is 100,000 times faster than a spinning hard disk, and 1,000 times faster than from SSDs.

Here’s a crazy idea: instead of throwing a bunch of memory cache around a disk-based NoSQL database that has only half the features you want, what if you cut to the chase and used an in-memory database instead? One that talks SQL? And has replication? And sharding that actually works? And failover? And massive write throughput via lock-free data structures? And transactions? And flexible schemas with JSON & non-blocking ALTER TABLE support…

“After every insert, update, or delete operation, MongoDB must update every index associated with the collection in addition to the data itself. Therefore, every index on a collection adds some amount of overhead for the performance of write operations.”
“…one performance issue that impacted us was MongoDB’s database-level write lock. The amount of time Mailbox’s backends were waiting for the write lock was resulting in user-perceived latency.”
“The partitioning granuliary [sic] is the key, so it is not possible to shard a dataset with a single huge key like a very big sorted set.”


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