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by TekMol 583 days ago
Should replication really be a concern of the DB layer?

Replication means writing queries which alter the data to multiple machines, right?

Shouldn't that be done by a software one level up? Which takes in the queries via some network protocol and then sends them to all machines.

That would sound more logical to me.
1 comments
emseetech 583 days ago
Historically, yes. Databases were software that were concerned with both storage and networking.

It's fine to want to separate those out, but it's not easy to do so and there are reasons they've been coupled for decades.

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TekMol 583 days ago
What makes it hard?

Having a single DB that takes write queries via a proxy which spreads them out to multiple read-only-DBs sounds easy at first.

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abtinf 583 days ago
When do you consider the write/transaction to be completed?

What do you do about out-of-sync read replicas?

ACID gets real hard real fast when introducing replication.

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TekMol 583 days ago
> When do you consider the write/transaction to be completed?

Sending a UPDATE/INSERT/DELETE statement to SQLite is not blocking? I would think it is, because in my code I can read the number of affected rows right after I sent the query.

> What do you do about out-of-sync read replicas?

Delete them and replace them by uploading a checkpoint and replaying a log of the queries since then.

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Tostino 583 days ago
If you are doing statement level replication, you better make sure every query is run in the exact same order (and finishes in the same order).

Without that you will have drift from your master database.

With that, you have a whole new host of synchronization issues you need to deal with.

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