Scaling CouchDB

Conflict Resolution

When replicating, you will inevitably run into document update conflicts. This can happen when a document with the same ID has been updated independently on two or more CouchDB nodes. To handle document update conflicts, you will need to create a view to find conflicts. Here is the Map function that will let us find conflicts:

function(doc) {
   if (doc._conflicts) {
       for (var i in doc._conflicts) {
           emit(doc._conflicts[i]);
       }
   }
}

Let’s save this to a design document:

curl -X PUT http://localhost:5984/catalog-a/_design/default -d \
'{
   "_id": "_design/default",
   "language": "javascript",
   "views": {
       "conflicts": {
           "map":
"function(doc) {
   if (doc._conflicts) {
       for (var i in doc._conflicts) {
           emit(doc._conflicts[i]);
       }
   }
}"       }
   }
}'

The response:

{
   "ok":true,
   "id":"_design/default",
   "rev":"1-05df67108309b2783233f27ffff31c59"
}

Now let’s create a conflict. First, create the following document within the catalog-a database:

curl -X PUT http://localhost:5984/catalog-a/978-0-596-80579-1 \
-H "Content-Type: application/json" \
-d '{
   "title":"Building iPhone Apps with HTML, CSS, and JavaScript",
}'

The response:

{
   "ok":true,
   "id":"978-0-596-80579-1",
   "rev":"1-8e68b2b2f14a81190889dab9d04481d2"
}

Next, create the same document within the catalog-b database, but with a slightly different title (the comma is missing after “CSS”):

curl -X PUT http://localhost:5984/catalog-b/978-0-596-80579-1 \ -H "Content-Type: application/json" \ -d '{ "title":"Building iPhone Apps with HTML, CSS and JavaScript", ...