You need to think of how you approach the application in an Document oriented way. If you simply try and replicate how you would model the problem in an RDBMS you will fail. There are also different trade-offs that you might want to make. Remember as well that CouchDB's design is assuming that you will have an active active cluster of many nodes that could fail at any time. How is your app going to handle one of the database nodes dissapearing from under it?
One way to think about it is to imagine you didn't have any computers, just paper documents. How would you create an efficient business process using bits of paper being passed around? How can you avoid bottlenecks? What if something goes wrong?
Another angle you should think about is eventual consistency, where you will get into a consistent state eventually, but you may be inconsistent for some period of time. This is an anathema in RMDBS land, but is extremely common in the real world. The canonical transaction example is of transferring money from bank accounts. How does this actually happen in the real world? Through a single atomic transactions or through different banks issuing credit and debit notices to each other? What happens when you write a cheque?
So lets look at your examples:
- CRUD of entities with some fields with unique index on it.
If I understand this correctly in CouchDB terms, you want to have a collection of documents where some that some named value is guaranteed to be unique across all those documents? That case isn't generally supportable because documents may be created on different replicas.
So we need to look at the real world problem and see if we can model that. Do you really need them to be unique? Can your application handle multiple docs with the same value? Do you need to assign a unique identifier? Can you do that deterministically? A common scenario where this is required is where you need a unique sequential identifier. This is tough to solve in a replicated environment. In fact if the unique id is need to be strictly sequential with respect to time created it's impossible if you need the id straight away. You need to relax at least one of those constraints.
- ecommerce web app like ebay
I'm not sure what to add here as the last comment you made on that post was to say "very useful! thanks". Was there something missing from the approach outlined there that is still causing you a problem? I thought MrKurt's answer was pretty full and I added a little enhancement that would reduce contention.