Distributed postgresql ID collision handling

Question

Let's imagine we have a distributed table with an ID, CONTENT and TIMESTAMP. The ID is hash(CONTENT) and the CONTENT is deterministic enough to be entered in multiple places in the system, shortly after each other.

Let's say a certain real life event happened. Like someone won the Olympics. Then that goes into this database in a record that always looks the same, except for the timestamp. As each machine observes the event at slightly different delays.

So. As the machines sync this distributed table they will wonder "We have this exact ID already! It's also not an identical row! What should we do!?". I want to give them the answer in the form of:bool compare(row a, row b) or, preferably, row merge(row a, row b).

Does anyone know how to do this? I can only find 'merge' things related to merging two different tables while in fact this is the same table, only distributed.

For me this is pretty essential for making my system 'eventually consistent'. I want to leverage postgresql's distributed database mechanics because they are so reliable, I wouldn't want to rewrite them.

Answer 1

PostgreSQL has no "distributed database" features. You can't rewrite them or avoid rewriting them because they don't exist, and I'm quite curious about where you got your reliability information from.

The closest tihng I can think of is a 3rd party addon called Bucardo, which does multi-master replication with conflict resolution.

It's also possible you were thinking of Postgres-XC, but that project is intended to produce a synchronous, consistent, transparent multi-master cluster, so there'd be no conflict resolution in the first place.

There's also Rubyrep; I don't know enough about it to know if it'd fit your needs.

In the future PostgreSQL will support something akin to what you are describing, with logical replication / bi-directional replication, but it's pre-alpha quality for now, and is likely to land in PostgreSQL 9.5 at the soonest.