Question

I’m having trouble finding documentation on the request object argument used in replication filters (‘req’ in the sample below):

function(doc, req) {
  // what is inside req???
  return false;
}

This old CouchBase blog post has a little code snippet that shows the userCtx variable being a part of the request object:

What is this userCtx? When you make an authenticated request against
CouchDB, either using HTTP basic auth, secure cookie auth or OAuth,
CouchDB will verify the user’s credentials. If they match a CouchDB
user, it populates the req.userCtx object with information about the
user.

This userCtx object is extremely useful for restricting replication of documents to the owner of the document. Check out this example:

function(doc, req) {
  // require a valid request user that owns the current doc
  if (!req.userCtx.name) {
    throw("Unauthorized!");
  }
  if(req.userCtx.name == doc.owner) {
    return true;
  }
  return false;
}

But the problem now is that CouchDB requires the filter method to be explicitly chosen by the initiator of the replication (in this case, the initiator is a mobile user of my web app):

curl -X POST http://127.0.0.1:5984/_replicate \
-d '{"source":"database", \
     "target":"http://example.com:5984/database", \
     "filter":"example/filtername"
     }'

The Question

Is there a way to enforce a specific filter by default so that users are restricted to replicating only their own data? I’m thinking the best way to do this is to use a front end to CouchDB, like Nginx, and restrict all replication requests to ones that include that filter. Thoughts? Would love a way to do this without another layer in front of CouchDB.

Answer 1

Data replication stands right with user ability to read data. Since if your users shares data within single database all of them has right to replicate all of them to their local couches. So you couldn’t apply any documents read restriction unless you’ve split single shared database into several personal ones – this is common use case for such situations.

There is no any way to enforce apply changes feed filter or other parameters like views has. However, you can use rewrites to wraps requests to some resources with predefined query parameters or even with dynamic ones. This is a little not solution that you’d expected, but still better that nginx and some logic at his side: probably, you’d to allow users to specify custom filters with custom query parameters and enforce you’re own only if nothing specified, right?

P.S. Inside req object is very useful about current request. Partially it was described at wiki, but it’s a little out of date. However, it’s easily to view it with simple show function:

function(doc, req){
    return {json: req}
}