What is the best Mongodb Sharding key for my schema?

Question

I am design a Mongodb collection which can save the statistic for daily volume

Here is my DB schema

mongos> db.arq.findOne()
{
    "_id" : ObjectId("553b78637e6962c36d67c728"),
    "ip" : NumberLong(635860665),
    "ts" : ISODate("2015-04-25T00:00:00Z"),
    "values" : {
        "07" : 2,
        "12" : 1
    },
    "daily_ct" : 5
}
mongos>

And Here is my Indexes

mongos> db.arq.getIndexes()
[
    {
        "v" : 1,
        "key" : {
            "_id" : 1
        },
        "name" : "_id_",
        "ns" : "Query_Volume.test"
    },
    {
        "v" : 1,
        "key" : {
            "ip" : 1
        },
        "name" : "ip_1",
        "ns" : "Query_Volume.test"
    },
    {
        "v" : 1,
        "key" : {
            "ts" : 1
        },
        "name" : "ts_1",
        "expireAfterSeconds" : 15552000,
        "ns" : "Query_Volume.test"
    }
]
mongos>

Note: I have a time stamp index since I need to use TTL mechanism.

But the Sharding Key has any suggestion?

Answer 1

You have multiple options:

1. {ts: 1} Your timestamp. The data of certain ranges will be located together, but the key is monotonically increasing, and I'm not sure, whether the TTL index will clean up shard chunks. Means: The write load switches from shard to shard, and you have a shard with high write load whereas the other shards will get no writes for the data. This pattern works nicely if you query contiguous time ranges but has downsides in writing.
2. {ts: "hashed"} Hash-based sharding. The data will be sharded more or less evenly across the shards. Hash-based sharding distributes the write load but involves all shards (more or less) when querying for data.

You will need to test, what fits the best for your reads and writes. The sharding key depends on the data structure and the read/write patterns of your application.