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Comment by mmt | Hacker News Reader

> It makes zero sense to pull a bunch of records back from the db using multiple network calls to join and then filter. Let the db do its job.

But then it wouldn't be distributed processing! :)

Seriously, though, consider it for a moment.. this pattern has similar features to something like Hadoop. The data comes from storage nodes (database server and, hopefully, their read replicas) and goes to processing nodes (app server) to have the work done and is then new data is written back out over the network to storage nodes and replicated across the network (to the replica/slave database servers).

If the data volume is particularly low or the compute load (CPU and/or RAM) is particularly high, the distributed method would make intuitive sense. I haven't seen it yet, however.

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Doing distributed joins correctly requires an architectural/technical capability that most distributed database engines don't have: decentralized parallel orchestration. If you have this, you can do joins even with very high data volumes efficiently given good parallel scheduling algorithms. Most databases are designed such that there is a single point of control that declaratively schedules all data flows required to execute the query; this scales poorly for operations like joins, never mind recursive joins, which is why you don't see it.

Letting individual database nodes dynamically schedule and orchestrate their own data flows with each other, essentially allowing each node in the parallel system construct its own execution plan in relation to other nodes as it goes along, does not fit within the "giant distributed file system" paradigm that most distributed systems are based on. People who design codes for supercomputers are often familiar with parallel orchestration idioms that work at extremely large scales but it hasn't crossed over into ordinary distributed database engines. (This is also a good litmus test for what makes a database "parallel" as distinct from "distributed".)

Most distributed database architectures are much more centralized than they need to be, particularly around control of execution planning, and this limits their expressiveness. It is quite difficult to hack together a distributed join that performs better than a centralized one without good support for parallel orchestration.

Thanks for that detail. I knew there were more interesting, technical reasons than my oversimplification for why I had never seen it.

> People who design codes for supercomputers are often familiar with parallel orchestration idioms that work at extremely large scales but it hasn't crossed over into ordinary distributed database engines. (This is also a good litmus test for what makes a database "parallel" as distinct from "distributed".)

My understanding was that this is primarily (or also) due to the lack of low-latency (and high-bandwidth) interconnects in the ordinary distributed environments, as those enable that orchestration.

I admit to wanting that to be true, as it supports a mantra I like to say to managers with programming-only (no Ops/sysadmin or even DBA) backgrounds: not all problems can be solved with software.

Could you point out any open source supercomputing / data products that get this right (ie have decentralized data flows?).

I'm obviously not the parent commenter and have no inside knowledge of the HPC world, but my guess is the main open source supercompting projects would be from NASA, the US national labs, and CERN.

You might search HN for the recent announcements about new clusters, especially top500, and look for the comments discussing using MPI (versus something custom, I think?), as my recollection is that those topics would yield further pointers to the actual examples you're looking for.

Take a look at parallel simulation codes (for example, LAMMPS in chemical physics).

Touché. You're right, at some point the data must be joined and filtered. My point was to let the tool do its job :)

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