I need to do transactions (begin, commit or rollback), locks (select for update). How can I do it in a document model db?
Edit:
The case is this:
Can I solve this with CouchDB?
No. CouchDB uses an "optimistic concurrency" model. In the simplest terms, this just means that you send a document version along with your update, and CouchDB rejects the change if the current document version doesn't match what you've sent.
It's deceptively simple, really. You can reframe many normal transaction based scenarios for CouchDB. You do need to sort of throw out your RDBMS domain knowledge when learning CouchDB, though. It's helpful to approach problems from a higher level, rather than attempting to mold Couch to a SQL based world.
Keeping track of inventory
The problem you outlined is primarily an inventory issue. If you have a document describing an item, and it includes a field for "quantity available", you can handle concurrency issues like this:
_rev property that CouchDB sends along_rev property_rev matches the currently stored number, be done!_rev doesn't match), retrieve the newest document versionIn this instance, there are two possible failure scenarios to think about. If the most recent document version has a quantity of 0, you handle it just like you would in a RDBMS and alert the user that they can't actually buy what they wanted to purchase. If the most recent document version has a quantity greater than 0, you simply repeat the operation with the updated data, and start back at the beginning. This forces you to do a bit more work than an RDBMS would, and could get a little annoying if there are frequent, conflicting updates.
Now, the answer I just gave presupposes that you're going to do things in CouchDB in much the same way that you would in an RDBMS. I might approach this problem a bit differently:
I'd start with a "master product" document that includes all the descriptor data (name, picture, description, price, etc). Then I'd add an "inventory ticket" document for each specific instance, with fields for product_key and claimed_by. If you're selling a model of hammer, and have 20 of them to sell, you might have documents with keys like hammer-1, hammer-2, etc, to represent each available hammer.
Then, I'd create a view that gives me a list of available hammers, with a reduce function that lets me see a "total". These are completely off the cuff, but should give you an idea of what a working view would look like.
Map
function(doc)
{
if (doc.type == 'inventory_ticket' && doc.claimed_by == null ) {
emit(doc.product_key, { 'inventory_ticket' :doc.id, '_rev' : doc._rev });
}
}
This gives me a list of available "tickets", by product key. I could grab a group of these when someone wants to buy a hammer, then iterate through sending updates (using the id and _rev) until I successfully claim one (previously claimed tickets will result in an update error).
Reduce
function (keys, values, combine) {
return values.length;
}
This reduce function simply returns the total number of unclaimed inventory_ticket items, so you can tell how many "hammers" are available for purchase.
Caveats
This solution represents roughly 3.5 minutes of total thinking for the particular problem you've presented. There may be better ways of doing this! That said, it does substantially reduce conflicting updates, and cuts down on the need to respond to a conflict with a new update. Under this model, you won't have multiple users attempting to change data in primary product entry. At the very worst, you'll have multiple users attempting to claim a single ticket, and if you've grabbed several of those from your view, you simply move on to the next ticket and try again.
Expanding on MrKurt's answer. For lots of scenarios you don't need to have stock tickets redeemed in order. Instead of selecting the first ticket, you can select randomly from the remaining tickets. Given a large number tickets and a large number of concurrent requests, you will get much reduced contention on those tickets, versus everyone trying to get the first ticket.
Actually, you can in a way. Have a look at the HTTP Document API and scroll down to the heading "Modify Multiple Documents With a Single Request".
Basically you can create/update/delete a bunch of documents in a single post request to *URI /{dbname}/_bulk_docs* and they will either all succeed or all fail. The document does caution that this behaviour may change in the future, though.
EDIT: As predicted, from version 0.9 the bulk docs no longer works this way.
How do you do the classic "bank account" example of a database transaction? I.e. you want to atomically withdraw $100 from Alice's account and deposit it into Bob's. There are millions of accounts so you can't really expect Alice's and Bob's accounts to be the same document.
A design pattern for restfull transactions is to create a "tension" in the system. For the popular example use case of a bank account transaction you must ensure to update the total for both involved accounts:
The scanning for tension should be done in a backend process for all "tension documents" to keep the times of tension in the system short. In the above example there will be a short time anticipated inconsistence when the first account has been updated but the second is not updated yet. This must be taken into account the same way you'll deal with eventual consistency if your Couchdb is distributed.
Another possible implementation avoids the need for transactions completely: just store the tension documents and evaluate the state of your system by evaluating every involved tension document. In the example above this would mean that the total for a account is only determined as the sum values in the transaction documents where this account is involved. In Couchdb you can modles this very nicely as a map/reduce view.