hash - how do hashed sharding keys work in mongodb? -

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i wondered internals hashed sharding keys in mongodb. constant mongodb chooses hash field if timestamp instance ?
guess key timestamp % n ; n ?
if design, guess n cannot change on time. how can sure if have lot of data there not many collisions in 1 shard if n small ?
thanks

a hashed key uses data in document, , standard hash function (md5) generate hash value document. example, suppose index on {name: 'hashed'} , have these documents:

{name: 'john', height: 73} {name: 'zardosht', height: 68}

mongodb run 'john' , 'zardosht' through md5 hash function , random-looking byte sequences. then, it'll use byte sequences actual b-tree key use in index. when want query {name: 'john'}, md5 hash again, gets same byte sequence have gotten before, , index lookup value.

you can play mongo shell , see these values yourself:

 % mongodb-linux-x86_64-2.2.5/bin/mongo                  mongodb shell version: 2.2.5 connecting to: test > db.runcommand({_hashbsonelement: {name: "john"}}) {     "key" : {         "name" : "john"     },     "seed" : 0,     "out" : numberlong("5553133884637223031"),     "ok" : 1 } > db.runcommand({_hashbsonelement: {name: "zardosht"}}) {     "key" : {         "name" : "zardosht"     },     "seed" : 0,     "out" : numberlong("8641066842148283865"),     "ok" : 1 }

note there isn't n in i've described, don't have worry limits that.


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