Question

We’re looking into using CouchDB/CouchCocoa to replicate data to our mobile app.

Our system has a large number of users. Part of the database is private to each user — for example their tasks. These I’ve been able to replicate without problem using filtered replication.

Here’s the catch… The database also includes shared information only some of which pertains to a given user. How do I selectively replicate that shared information? For example a user’s task might reference specific shared documents. Is there a way to make sure those documents are included in the replication without including all the shared documents?

From the documentation it seems that adding doc_ids to the replication (or adding another replication with those doc ids) might be one solution. Has any one tried this? Are there other solutions?

EDIT: Given the number of users it seems impractical to tag each shared document with all the users sharing it but perhaps that’s the only way to do this?

Answer 1

Final solution mostly depends on your documents structure, but currently I see two use-cases:

1. As you keep everything within single database, probably you have some fields set to recognize, that document is shared or document is private, right? Example:

   owner: “Mike”

   participants: [] // if there is nobody mentioned, document looks like as private(?)

   So you just need some filter that would handle only private documents and only shared ones: by tags, number of participants, references or somehow.

   Also, if you need to replicate some documents only for specific user (e.g. only for Mike), than you need special view to handle all these documents and, yes, use replication by document ids, but this wouldn’t be an atomic request: you need some service script to handle these steps. If shared documents are defined by references to them, than the only solution is the same: some service script, view that generated document reference tree and replication by doc._id’s.

2. Review your architecture. Having per user database is normal use-case for CouchDB and follows way of data partitioning and isolation. So you may create per user database that would be private only for that user. For shared documents you may create additional databases playing with database members of security options. Each “shared” database will handle only certain number of participants by names or by groups, so there couldn’t be any data leaks unless that was not a CouchDB bug(:

   This approach looks too weird from first sight, but everything you’ve needed there is to create some management script that would handle database creation and publication, replications would be easy as possible and users data is in safe.

P.S. I’ve supposed that “sharing” operation makes document visible not for every one, but for some set of users. If I was wrong and “shared” state means “public” state than p2. will be more simpler: N users databases + 1 public one.