Question

I have a Rails 3.1 app, running on Ruby 1.9.2, Mongo 2.0.2, using Mongoid as a ODM, however, this should be bypassing most of Mongoid and just using the MongoDB Ruby Driver. I am running a MapReduce query a few times on one page load. This will query will run acceptably for a while in development, say like half a day, then fail with a

(assertion: 'value has to be set'; 
 assertionCode: '10210'; 
 errmsg: 'db assertion failure'; 
 ok: '0.0')

On a refresh of the page, everything continues to work wonderfully for some time. The log from execution (with some trimmed out)…

(R) MONGODB development['$cmd'].find({"mapreduce"=>"values", "map"=><BSON::Code:70098680478720 @data="function() {
  var value = { a: 0, b: 0 };
  var count = { a: 0, b: 0 };
  var min = { a: 100, b: 100 };
  var max = { a: -100, b: -100 };

  if (this.c == "a" || this.c == "b") {
    value[this.c] = parseInt(this.value);
    count[this.c]++;
    min[this.c] = parseInt(this.value);
    max[this.c] = parseInt(this.value);

    emit(this.item_id,
      {
        count: count,
        values: value,
        min: min,
        max: max,
        avg: { a: null, b: null }
      }
    );
  }
}" @scope="{}">, "reduce"=><BSON::Code:70098680478680 @data="function(key, data) {
  var newDatum = data.pop();

  data.forEach(function(item) {
    newDatum.values.a += item.values.a;
    newDatum.count.a += item.count.a;
    if (item.min.a < newDatum.min.a)
      newDatum.min.a = item.min.a;
    if (item.max.a > newDatum.max.a)
      newDatum.max.a = item.max.a;
    // same for b
  });

  newDatum.avg.a = newDatum.values.actual / newDatum.count.a;
  newDatum.avg.b = newDatum.values.ideal / newDatum.count.b;

  return newDatum;
}" @scope="{}">, :out=>{:inline=>1}, :query=>{:about_id=>BSON::ObjectId('4f1d84de69c238941d000012'), :by_id=>{"$ne"=>BSON::ObjectId('4f1d84de69c238941d000012')}, :comp_id=>{"$in"=>[BSON::ObjectId('4f1d84de69c238941d000016'), BSON::ObjectId('4f1d84de69c238941d000019')]}}}).limit(-1)

Now the query part for one that fails in the same request… (but again, will significantly more often than not succeed)

}" @scope="{}">, :out=>{:inline=>1}, :query=>{:about_id=>BSON::ObjectId('4f1d84de69c238941d000012'), :by_id=>BSON::ObjectId('4f1d84de69c238941d000012'), :comp_id=>BSON::ObjectId('4f1d84de69c238941d000019')}}).limit(-1)
Completed 500 Internal Server Error in 516ms

Mongo::OperationFailure (Database command 'mapreduce' failed: (assertion: 'value has to be set'; assertionCode: '10210'; errmsg: 'db assertion failure'; ok: '0.0').):

If it’s of any value, this is happening on Mac OS X Lion with Mongo installed via homebrew.

$ mongod --version
db version v2.0.2, pdfile version 4.5
Wed Feb  8 16:33:11 git version: 514b122d308928517f5841888ceaa4246a7f18e3

And what I can see in the log file during this event. The one line about mr failed seems to match to the event.

Wed Feb  8 15:08:36 [clientcursormon] mem (MB) res:15 virt:2630 mapped:80
Wed Feb  8 15:11:37 [clientcursormon] mem (MB) res:17 virt:2630 mapped:80
Wed Feb  8 15:16:37 [clientcursormon] mem (MB) res:17 virt:2630 mapped:80
Wed Feb  8 15:21:37 [clientcursormon] mem (MB) res:17 virt:2630 mapped:80
Wed Feb  8 15:26:37 [clientcursormon] mem (MB) res:17 virt:2630 mapped:80
Wed Feb  8 15:31:37 [clientcursormon] mem (MB) res:17 virt:2630 mapped:80
Wed Feb  8 15:32:55 [conn3] mr failed, removing collection :: caused
by :: 10210 value has to be set
Wed Feb  8 15:36:37 [clientcursormon] mem (MB) res:18 virt:2630 mapped:80
Wed Feb  8 15:39:26 [initandlisten] connection accepted from 127.0.0.1:57001 #4

So my question is, what can I do to help track this down? I am not noticing any additional issues with our use of Mongo. I’m not sure how to dig into this and figure out what’s going on.

Answer 1

So it appears the issue was JavaScript stored in the db itself. db.system.js.find() listed a an entry without a value. Per a post on the mongodb mailing list, removing that js seemed to correct the error. Hopefully if someone else runs into this, they’ll learn to check system.js.