Any one of these database types can have a SQL interface (in various dialects and standards), ACID, different consistency models and scale-out options. It's just up to the specific database product in what it actually offers.

Perhaps it should be said that relational is DB that's for general purpose, while specialized (key/value, graph, etc) requires you to understand your data so you can make proper trade-offs when picking specialized db. You are not getting those features advertised by other databases for free.

Relational is specialized too - in dealing with relational data, foreign keys, normalization, joins, etc.

ACID is just a feature of a product, not tied to relational as there are relational databases that don't offer it or can have it disabled. Tradeoffs and understanding your data apply to every single type of database.

HBase is strongly consistent and given that it a core part of the Hadoop stack and used by Facebook for Messages would indicate it is hardly has systemic data loss issues. Cassandra likewise is used for PSN, Steam, EA Online, Spotify and is quite capable of running in strongly consistent mode. That's just two of the many NoSQL databases that are strongly consistent and being used for major systems where data loss would be unacceptable.

Also your comparisons between PostgreSQL and MongoDB are meaningless. The complete overhaul of the default engine in MongoDB 3 (WiredTiger) has increased performance massively with a notable reduction in disk space usage. It's basically a whole new database.

https://www.mongodb.com/blog/post/performance-testing-mongod...

Regarding the Mongo, the second link I provided was comparing WiredTiger. In link you provided they made an improvement, but 2.x was already slow. Initially MongoDB gained its popularity because it supposed to be fast. And it was when it did not worry about making sure the written data ends up on disk. Once people started losing data when system crashed they changed defaults, but then became much slower. And worst of all it still was losing data[1]. There's no good reason to use it, even if you would want to use it as a cache, there are far better solutions.

[1] https://aphyr.com/posts/322-call-me-maybe-mongodb-stale-read...

You're arguing the word "consistent" is used wrong, and then you go on to mixing ACID and CAP — two acronyms which use the word "consistent" with completely different meanings.

The "C" in ACID stands for data consistency, which is that you shouldn't be allowed to store any data which isn't internally consistent with the schema's consistency rules. For example, you're not allowed to store a "null" value in a "not null" column, and you can't delete a row that is pointed to by a different table's foreign key constraint.

The "C" in Brewer's original CAP theorem really represents atomic consistency. It's the idea that if you submit data, you should get the exact same data back, even if you ask a different node. It sort of represents both the "A" (atomicity, in the sense that either everything or nothing is updated) in ACID, as well as the "I" (isolation, in the sense that nodes don't see partial writes) and "D" (durability, in the sense that data written stays written).

In CAP terms, HBase is strongly consistent, I believe, as long as you query the primary region server and not a replica. But HBase is not transactional, and therefore it's pretty meaningless to talk about ACID.

By the ACID definition a database can either be consistent or not. It can't sometimes be one or the other (unless there is some bug. I was referring to the CAP definition which I assume is what you are referring to. And by that definition HBase is absolutely strong consistent.

And that second link was pretty bad (with all respect to the great guys over at Datastax). It is benchmarking a release candidate build of MongoDB (2.8.0-rc4) where quite a number of important fixes went in before GM. They should have just waited until 3.0 was released.

Also it would be good for that Call Me Maybe test to be redone considering that stale-read bug has been fixed:

https://jira.mongodb.org/browse/SERVER-18022

Not entirely accurate. Relational database schemas are typically normalized into many tables and make heavy use of joins; it's fair to say that the concept of joins are integral the relational model. But joins, by definition, require fast access to data; it's extremely inefficient to join across network boundaries.

So-called NoSQL databases aren't just about ACID or CAP, but also about denormalization. In fact, I would go so far as to say that the single founding tenet of all of "NoSQL" was denormalization across multiple nodes. Google's Bigtable (on which HBase is based, and which arguably started the public's interest in "NoSQL") depends on extreme denormalization in its data model‚ which does away with tables and instead treats everything as columns, and also discards referential integrity (the "C" in ACID, which isn't the same as in CAP) and isolation (the "I"). Sharding flows naturally out of denormalization. Eventual consistency came later with Amazon's Dynamo paper; Bigtable was strongly consistent.

A consequence of denormalization is that you can hit any database node and get consistent performance. No query will ever get bogged down in complicated, long-running nested loops and hash joins because the mechanisms don't exist. It creates an even playing field for queries because they're mostly individual key lookups; the result is (in theory) predictable, stable performance across the cluster. The complexity of join logic that was in the relational database is moved into the client, which gets the burden of untangling data. Which is why Google also invented early solutions like MapReduce and Sawzall, because once you have a huge, denormalized, distributed cluster of data in this poor man's relational model, your individual clients can no longer access the data efficiently. So denormalization also leads to parallel computation, and especially precomputation, rather than the ad-hoc queries that relational databases are good at.

It's all about scale, and fitting the data model to work in your favour at that scale.

So yeah, I would definitely say that relational databases are designed in a way that make them hard to distribute. The above is just the tip of the iceberg. How do you enforce referential integrity (foreign keys, etc.) across nodes? How do you assign unique primary keys? You talk about making tradeoffs, but would you really want a distributed relational database without distributed transactions?

Can you implement a performant distributed database with joins and other relational features? Yes. VoltDB and Vertica come to mind. They are designed from the ground up to do this, and have complicated logic for partitioning queries. They make other tradeoffs: for example, VoltDB is in-memory and to perform well, all transactional access code has to be written as a "stored procedure", a Java JAR file that you upload to the server, and transactions are expected to be very short-lived. But they will, pretty much by definition, not perform at scale the way that "NoSQL" databases are designed to.