[ChicagoDave]

Yes and for crud systems relational is fine because you're unlikely to over-complicated your architecture. But when a system starts talking to other systems and its bounded contexts become complex, alternate solutions should be sought.

The problem with "schema change", and I did this for decades, is that it's always a massive blocker. In some companies the data architects had to approve and implement schema changes. You could wait days for that. NoSQL allows you to modify the document surface in mostly non-breaking change ways OR it's easier to version your APIs to handle different document versions.

Simple CRUD: Any data store is fine. Complex multiple bounded contexts: Choose the appropriate data store for each bounded context accordingly.

My point was no one should be reaching for a relational database or starting with an ERD to build a system. Document behaviors. Model the system. Let the system decide what data storage it requires.

[sgarland]

> Document behaviors. Model the system. Let the system decide what data storage it requires.

Counterpoint: force the system to use an RDBMS to store data in properly normalized schema, because it’s the only thing guaranteeing that the data will continue to exist as you expect.

[ChicagoDave]

This implies nosql data stores are not ACID capable. Mongo is fully capable and DynamoDB is mostly capable.

I would challenge you to look at event driven architectures, CQRS, event sourcing, and how to implement and leverage read models.

It will expand your architecture toolkit.

[sgarland]

Correct me if I'm wrong, but MongoDB appears to have a 100-msec gap for its journaling behavior [0] by default. This would be akin to setting innodb_flush_log_at_trx_commit [1] = 2, which is not its default.

I also note that in their FAQs [2], they erroneously state:

"MongoDB’s data modeling best practice suggests storing related data together in a single document using a variety of data types, including arrays and embedded documents. So, a lot of the time, ACID is not required as it is a single-document transaction."

Whether or not you're operating on a single document has nothing to do with its ability to meet durability guarantees (or consistency, for that matter).

NoSQL databases make tradeoffs for performance, and making the lives of devs easier in the short term. That's fine, if and only if you accept what you're losing, and document it for others who may not be aware. If at any point you can have your application get a write ack'd and subsequently lose the write, you do not have a durable data store, and you do not have ACID compliance. Whether that's the fault of the DBMS, the operating system (Postgres' fsyncgate), or hardware (drives lying to the OS about the write's durability without the benefit of PLP) is irrelevant – you have to understand the entire chain to make those guarantees, or at the very least, trust your upstream provider to have understood it for you and made the correct decisions.

0: https://www.mongodb.com/docs/manual/core/journaling/

1: https://dev.mysql.com/doc/refman/8.4/en/innodb-parameters.ht...

2: https://www.mongodb.com/resources/products/capabilities/acid...