MapReduce fails without error message

[Dror Cohen]

Hi everyone,

I'm performing an incremental map reduce on a 2.6 mongod instance and everything worked fine and dandy until recently.

db.runCommand({ 
mapreduce: "somePreAggregatedCollection", 
query: {
    "start" : {
        $gt: previousRun,
        }
    },
map : function Map() {
    delete this.start;
    delete this.end;
    delete this._id;
    var key = this.user,
        value = this;
    delete value.user;
    emit(key, value);
},
reduce : function Reduce(user, aggregationData) {
    var result = {};
    aggregationData.forEach(function(timespan){
        Object.keys(timespan).forEach(function(field){
            if (!result[field] || (field.indexOf('Last', field.length - 5) != -1)) {
                result[field] = timespan[field];
            } else if ((field.indexOf('Count', field.length - 5) != -1) || (field.indexOf('Sum', field.length - 5) != -1)) {
                result[field] += timespan[field];
            } else if (field.indexOf('Min', field.length - 5) != -1) {
                result[field] = (result[field] > timespan[field]) ? timespan[field] : result[field];
            } else if (field.indexOf('Max', field.length - 5) != -1) {
                result[field] = (result[field] < timespan[field]) ? timespan[field] : result[field];
            }
        });
    });
    return result;
},
sort : { "user" : 1, "start" : 1 },
out : { reduce: "lifetime_agg"  },
jsMode: true

I'm pretty sure that I'm not breaking any requirements and nothing is nowhere near the limits, but the things is that if I don't use the query to make chunks small enough the command does nothing at all, it simply responds with:

counts: {
    input: 0,
    emit: 0,
    reduce: 0,
    output: number of records already in lifetime_agg
}

I would have expected some sort of an error message, and the whole thing still works if I force it to run with a smaller query result.

P.S. jsMode is set to true because I was tweaking with it and the number of records is nowhere near 500,000. It behaves the same way with jsMode set to false.

[Asya Kamsky]

Your mapReduce is doing a query to select a subset of records to run over:

query: {
"start" : {
$gt: previousRun,
}
}

Is it possible that either "previousRun" value is suddenly not being
populated properly or "start" field is not being set properly in new
records?

Asya

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