Question

I am trying to run group command via mongo shell

db.contract.group({
   key:{id: 1}, 
   initial: {v: []}, 
   reduce: function(obj, prev){ 
     prev.v.push(obj.name)
   } 
});

and see the following error on client

Thu Nov 17 12:12:49 uncaught exception: group command failed: {
    "errmsg" : "exception: JS_NewObject failed: toJSObject2",
    "code" : 13072,
    "ok" : 0
}

and mongod logs says the following

Thu Nov 17 12:12:17 [initandlisten] connection accepted from 127.0.0.1:58509 #1
Thu Nov 17 12:12:49 [conn1] JS Error: out of memory
Thu Nov 17 12:12:49 [conn1] Assertion: 13072:JS_NewObject failed: toJSObject2
0x10008de9b 0x1001565bd 0x100156c9e 0x10037011e 0x10037204c 0x10034c4d6 0x10034d877 0x100180cc4 0x100184649 0x1002b9e89 0x1002c3f18 0x100433888 0x100446d74 0x7fff86e00fd6 0x7fff86e00e89 
 0   mongod                              0x000000010008de9b _ZN5mongo11msgassertedEiPKc + 315
 1   mongod                              0x00000001001565bd _ZN5mongo9Convertor10toJSObjectEPKNS_7BSONObjEb + 1229
 2   mongod                              0x0000000100156c9e _ZN5mongo7SMScope9setObjectEPKcRKNS_7BSONObjEb + 78
 3   mongod                              0x000000010037011e _ZN5mongo12GroupCommand5groupESsRKSsRKNS_7BSONObjES3_SsSsPKcS3_SsRSsRNS_14BSONObjBuilderE + 2110
 4   mongod                              0x000000010037204c _ZN5mongo12GroupCommand3runERKSsRNS_7BSONObjERSsRNS_14BSONObjBuilderEb + 3676
 5   mongod                              0x000000010034c4d6 _ZN5mongo11execCommandEPNS_7CommandERNS_6ClientEiPKcRNS_7BSONObjERNS_14BSONObjBuilderEb + 1350
 6   mongod                              0x000000010034d877 _ZN5mongo12_runCommandsEPKcRNS_7BSONObjERNS_10BufBuilderERNS_14BSONObjBuilderEbi + 2151
 7   mongod                              0x0000000100180cc4 _ZN5mongo11runCommandsEPKcRNS_7BSONObjERNS_5CurOpERNS_10BufBuilderERNS_14BSONObjBuilderEbi + 52
 8   mongod                              0x0000000100184649 _ZN5mongo8runQueryERNS_7MessageERNS_12QueryMessageERNS_5CurOpES1_ + 10585
 9   mongod                              0x00000001002b9e89 _ZN5mongo13receivedQueryERNS_6ClientERNS_10DbResponseERNS_7MessageE + 569
 10  mongod                              0x00000001002c3f18 _ZN5mongo16assembleResponseERNS_7MessageERNS_10DbResponseERKNS_8SockAddrE + 1528
 11  mongod                              0x0000000100433888 _ZN5mongo10connThreadEPNS_13MessagingPortE + 616
 12  mongod                              0x0000000100446d74 thread_proxy + 132
 13  libSystem.B.dylib                   0x00007fff86e00fd6 _pthread_start + 331
 14  libSystem.B.dylib                   0x00007fff86e00e89 thread_start + 13
Thu Nov 17 12:12:49 [conn1] query staging.$cmd ntoreturn:1 command: { group: { key: { asset_id: 1.0 }, initial: { v: {} }, ns: "contract", $reduce: function (obj, prev) {
    prev.v.push(obj.name);
} } } reslen:119 21013ms

I tried checking if the virtual memory is unlimited and it is

bash-3.2$ ulimit -a | egrep virtual\|open
open files                      (-n) 256
virtual memory          (kbytes, -v) unlimited

so I am not sure how to fix this problem

Thank you

Answer 1

Mongo is grouping in memory, so if you have a large database with many different values for id things might get too big.

Also you are piling up all names for a given id in one array. There is also a limit for the maximum document size in Mongo which might be the reason for the out of memory error if you have many docs for a given id.

I guess the solution is to use map/reduce instead.