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Question

I am scraping an 90K record database using JSON-RPC and I am trying to put in some basic error checking. I want to start by scraping the database twice using two different settings and adding a prefix to the second scrape. This way I can check to ensure that the two settings are not producing different records (due to dropped updates, etc). I wanted to implement the comparison using a view which compares each document from the first scrape with it's twin produced by the second scrape and then emit the names of records with a difference between them.

However, I cannot quite figure out how to pull in another doc in the view, everything I have read only discusses external docs using the emit() function, which is too late to permit me to compare it. In the example below, the lookup() function would grab the referenced document.

Is this just not possible?

function(doc) {
  if(doc._id.slice(0,1)!=='$' && doc._id.slice(0,1)!== "_"){
    var otherDoc = lookup('$test" + doc._id);
    if(otherDoc){
    var keys = doc.value.keys();
    var same = true;
    keys.forEach(function(key) {
      if ((key.slice(0,1) !== '_') && (key.slice(0,1) !=='$') && (key!=='expires')) {
        if (!Object.equal(otherDoc[key], doc[key])) {
          same = false;
        }
      }
    });
      if(!same){
        emit(doc._id, 1);
      }
    }
  }
}

Answer 1

Context

You are correct that this is not possible in CouchDB. The whole point of the map function is that it must be idempotent, otherwise you lose all the other nice benefits of a pre-calculated index.

This is why you cannot access external resources in the map function, whether they be other records or the clock. Any time you run a map you must always get the same result if you put the same record into it. Since there are no relationships between records in CouchDB, you cannot promise that this is possible.

Solution

However, you can still achieve your end goal, just be different means. Some possibilities...

• Assuming there is some meaningful numeric value in each doc, you could use a view to take the sum of all those values and group them by which import you did ( {key: <batch id>, value: <meaningful number>} ). Then compare the two numbers in your client or the browser to see if they match.

• A brute force approach would be to use a view to pair the docs that should match. Each doc is on a different row, but they're grouped by a common field. Then iterate through the entire index comparing the pairs. This would certainly be the quickest to code and doesn't depend on your application or data.

• Implement a validation function to enforce a schema on your data. Just be warned that this will reduce your write throughput since each written record will be piped out of Erlang and into the JS engine. Also, this is only applicable if you're worried about properly formed records instead of their precise content, which might not be the case.

• Instead of your different batch jobs creating different docs, have them place them into the same doc. The structure might look like this: { "_id": "something meaningful", "batch_one": { ..data.. }, "batch_two": { ..data.. } } Then your validation function could compare them or you could create a view that indexes all the docs that don't match. All depends on where in your pipeline you want to do the error checking and correction.

Personally I like the last option better, but only if you don't plan to use the database as is in production. Ie., you wouldn't want to carry around all that extra data in each record.

Hope that helps.

Cheers.