Where database blog posts get flame-broiled to perfection
Alright, settle down, everyone. Fresh off the press, we have another masterpiece of marketing-driven engineering, titled: “How We Re-invented Data Loss and Called it a Feature.”
I’ve just read this love letter to MongoDB’s replication, and I have to say, it’s beautiful. It has all the right buzzwords. We’ve got “strong consistency,” “durability,” and my personal favorite, a system so transparent that it fails “without raising errors to the application.” Oh, fantastic. I love a good mystery. It’s not a bug, it’s a surprise challenge for the operations team. My pager already feels heavier just reading that sentence.
They talk about Raft like it’s some magic pixie dust you sprinkle on a database to solve all of life’s problems. “Consensus is used to elect one replica as primary.” Great. Wonderful. But then they get to the good part, the part that always gets me. The part where they admit their perfect, consistent, durable system is too slow for the real world.
So what do they offer? A little knob you can turn, w:1, which is business-speak for the “YOLO setting.” You get to “prioritize availability and latency over immediate global consistency.” This is the enterprise version of turning off your smoke detectors because the chirping is annoying. What could possibly go wrong?
The demo is my favorite part. It’s so clean. So sterile.
docker network disconnect lab m1
Ah, if only network partitions were that polite. If only they announced themselves with a tidy shell command. In my world, a partition looks more like a faulty switch in a colo facility starting to drop 30% of packets, but only for VLANs with prime numbers, and only when the ambient temperature exceeds 73 degrees Fahrenheit. But sure, let’s pretend it’s a clean slice.
And then comes the punchline. The absolute gem of the entire article. After your little cluster has a "brief split-brain window"—a phrase that should send a chill down any SRE's spine—what happens to the data written to the old primary?
MongoDB stores them as BSON files in a rollback directory so you can inspect them and perform manual conflict resolution if needed.
Let me translate that for you. At 3 AM on the Sunday of a long holiday weekend, after the alarms have been screaming for an hour and the application team is swearing blind that “nothing changed on our end,” my job is to SSH into a production node, navigate to a directory with a GUID for a name, and start running bsondump on some binary files to manually piece together lost customer transactions.
This isn’t a feature. This is a digital archaeology expedition with the CEO holding the flashlight. “Fully auditable and recoverable,” they say. Sure. It’s recoverable in the same way a scrambled egg is recoverable if you have a Ph.D. in molecular gastronomy and a time machine.
They’re so proud of this. They even say it’s where they “intentionally diverge from vanilla Raft.” You didn't "diverge." You drove the car off the cliff because you thought you could fly. This isn’t a “real-world distributed application pattern.” This is a real-world ticket escalation that ends with my name on it. We're supposed to build an entire conflict resolution and data reconciliation pipeline because their monitoring—oh wait, they didn't mention monitoring, did they? Of course not. That’s always a “Phase 2” problem.
I can just see it now. The post-mortem will have a whole section on how we need better alerts for BSON files suddenly appearing in a rollback directory. The action item will be a hastily written Python script that I have to maintain for the rest of my tenure.
You know, I have a drawer full of vendor stickers. Riak, CouchDB, Aerospike. All of them promised me the moon. All of them had a clever little solution for when the laws of physics and distributed computing inconveniently got in their way. This article has a very familiar energy. I’ll make sure to save a spot for the MongoDB sticker. It’ll go right next to the one for the database that promised “eventual consistency” and delivered “eventual data loss.”
Anyway, I’ve got to go. I need to figure out how to put a bsondump command into a PagerDuty alert. This is the future, I guess.