4 ms·

But RDBMS'es are often also distributed. So what is mongo now?

marcosdumay 2y ago
People don't usually distribute Postgres (unless you count read replicas and cold HA replicas). But well, people don't usually distribute MongoDB either, so no difference. In principle, a cluster of something like Mongo can scale much further than Postgres. In practice, Mongo is full of issues even before you replicate it, and you are better with something that abstracts a set if incoherent Postgres (or sqlite) instances.
- zozbot234 2y ago
  Postgres supports foreign data wrapper (FDW), which is the basic building block for a distributed DB. It doesn't support strong consistency in distributed settings as of yet, although it does provide two-phase commit which could be used for such.
  - williamdclt 2y ago
    > strong consistency in distributed settings I doubt it ever will. The point of distributing a data store is latency and availability, both of which would go down the drain with distributed strong consistency
- hibikir 2y ago
  I think of the Stripe Mongo install, as it was a decade or so ago. It really was sharded quite wide, and relied on all shards having multiple replicas, as to tolerate cycling through them on a regular basis. It worked well enough to run as a source of truth for a financial company, but the database team wasn't small, dedicated to keeping all that machinery working well. Ultimately anyone doing things at that scale is going to run a small priesthood doing custom things to keep the persistence payer humming, regardless of what the underlying database is. I recall a project abstracting over the Mongo API, as to allow for swapping the storage layer if they ever needed to
brabel 2y ago
Often?? In my experience it's really hard to do it and still maintain similar performance, which kind of voids any benefit you may be looking for.
otabdeveloper4 2y ago
Postgres doesn't do replication correctly. Mongo does. (In fact, replication is about the only thing Mongo does correctly.) If you actually want a replicated log then Mongo is a very good choice. Postgres isn't.

People don't usually distribute Postgres (unless you count read replicas and cold HA replicas). But well, people don't usually distribute MongoDB either, so no difference. In principle, a cluster of something like Mongo can scale much further than Postgres. In practice, Mongo is full of issues even before you replicate it, and you are better with something that abstracts a set if incoherent Postgres (or sqlite) instances.

zozbot234 2y ago
Postgres supports foreign data wrapper (FDW), which is the basic building block for a distributed DB. It doesn't support strong consistency in distributed settings as of yet, although it does provide two-phase commit which could be used for such.
- williamdclt 2y ago
  > strong consistency in distributed settings I doubt it ever will. The point of distributing a data store is latency and availability, both of which would go down the drain with distributed strong consistency
hibikir 2y ago
I think of the Stripe Mongo install, as it was a decade or so ago. It really was sharded quite wide, and relied on all shards having multiple replicas, as to tolerate cycling through them on a regular basis. It worked well enough to run as a source of truth for a financial company, but the database team wasn't small, dedicated to keeping all that machinery working well. Ultimately anyone doing things at that scale is going to run a small priesthood doing custom things to keep the persistence payer humming, regardless of what the underlying database is. I recall a project abstracting over the Mongo API, as to allow for swapping the storage layer if they ever needed to

brabel 2y ago

Often?? In my experience it's really hard to do it and still maintain similar performance, which kind of voids any benefit you may be looking for.

otabdeveloper4 2y ago

Postgres doesn't do replication correctly. Mongo does. (In fact, replication is about the only thing Mongo does correctly.) If you actually want a replicated log then Mongo is a very good choice. Postgres isn't.