CouchDB Implements a Fundamental Algorithm | Hacker News Reader

CouchDB Implements a Fundamental Algorithm

jchrisa.net

43 pointsropiku9 comments

I might be really retarded, but I have to admit, I didn't really understand a word in that article. Maybe there are a lot of CouchDB-specific words, maybe it's my lack of knowledge of map reduce, I don't know, I just didn't understand it. Sorry for a lame comment.

Transactions? Oh, nope, they removed those...

I'm not an expert in these things, and appreciate the opportunity to learn more.

It seems to me that a transactional database necessarily requires all sorts of locking and waits to ensure that everyone using the database sees the same, bang-up-to-date version of the data. This is probably essential in some applications.

The point of CouchDB seems to be that sometimes the system doesn't need the most up-to-date version of the data, and that data that's not-very-old will do, especially if it was valid at the start of the process. CouchDB then has a different set of invariants, a different collection of guarantees.

CouchDB seems to work on the idea that changes will propogate, and that the system will settle, and that processes will never get inconsistent data, although the data might sometimes be old/stale. These guarantees might then suffice, especially in places where locking and the delays/bottlenecks threatened by locaking would be unacceptable.

Is that a reasonable summary?

Read this: http://blog.woobling.org/2009/05/why-i-dont-use-couchdb.html

The system doesn't really settle. Imagine there are two versions of a record. Your process reads the old one, and generates a completely new record based on that data. Later on, "eventual consistency" removes that original "old" record; the "new" one won. Well, that affects that initial record, but it doesn't affect records that used data from that record to make new records. So the database gradually becomes filled with data that is completely inconsistent. There is no such thing as "eventual consistency". (Transactions handle this problem; if your new record is based off of old data, your transaction will abort. If you commit successfully, you know that your data is consistent.)

Now, if you never use your data to make new data, and none of your records are related to each other via links, and you never need to create more than one record at once, CouchDB is fine. But it seems to encourage people to throw away basic data integrity for some magical scalability reasons, which is probably a premature optimization.

(And I say this as someone who only uses relational database rarely. The relational model does not fit my problem space very often, but strict transactional semantics always do.)

I may be misunderstanding you, but is this loosely correct? It merges the distributed data like git, except in cases where there's a merge conflict, the newer one just wins?

It's certainly hard to come up with hueristics for automated merging in all corner cases, and punting and asking the (likely best-informed) user to merge, as git does, seems like the most practical solution. Unfortunately, automated replication doesn't seem to allow that option.

Am I really off base here?

If there is a conflict it is reported to the application using couchdb and the application can use whatever logic that it want to manage the conflict. Most recent is a simple method that an application can use.

Who needs 'em anyway?

I don't use them a lot too. But I suppose companies like Amazon, Barnes & Nobles probably require a lot of transaction support (otherwise their shopping carts could get really messy).

Many simple operations require transactions to be correct. If you've ever implemented a web request handler where you read session data at the beginning and write the whole session at the end; you lost data if you weren't using transactions and the user opened a new tab in the middle.

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