Not able to shard collection

Question

I have enabled sharding using the below commands.

mongos> use news
switched to db news
mongos> sh.enableSharding("news");
{
    "ok" : 1,
    "$clusterTime" : {
        "clusterTime" : Timestamp(1633500027, 1),
        "signature" : {
            "hash" : BinData(0,"AAAAAAAAAAAAAAAAAAAAAAAAAAA="),
            "keyId" : NumberLong(0)
        }
    },
    "operationTime" : Timestamp(1633500026, 1)
}
mongos> db.createCollection("articles");
{
    "ok" : 1,
    "$clusterTime" : {
        "clusterTime" : Timestamp(1633500039, 2),
        "signature" : {
            "hash" : BinData(0,"AAAAAAAAAAAAAAAAAAAAAAAAAAA="),
            "keyId" : NumberLong(0)
        }
    },
    "operationTime" : Timestamp(1633500039, 2)
}
mongos> sh.shardCollection("news.articles", {"dc": 1});
{
    "collectionsharded" : "news.articles",
    "ok" : 1,
    "$clusterTime" : {
        "clusterTime" : Timestamp(1633500077, 25),
        "signature" : {
            "hash" : BinData(0,"AAAAAAAAAAAAAAAAAAAAAAAAAAA="),
            "keyId" : NumberLong(0)
        }
    },
    "operationTime" : Timestamp(1633500077, 21)
}
mongos> sh.status();
--- Sharding Status ---
  sharding version: {
    "_id" : 1,
    "minCompatibleVersion" : 5,
    "currentVersion" : 6,
    "clusterId" : ObjectId("615d26c277fa05dc0bf74e23")
  }
  shards:
        {  "_id" : "shard1rs",  "host" : "shard1rs/192.168.226.180:50001,192.168.226.180:50002,192.168.226.180:50003",  "state" : 1,  "topologyTime" : Timestamp(1633495862, 1),  "tags" : [ "bos" ] }
        {  "_id" : "shard2rs",  "host" : "shard2rs/192.168.226.180:50004,192.168.226.180:50005,192.168.226.180:50006",  "state" : 1,  "topologyTime" : Timestamp(1633496033, 2),  "tags" : [ "dfw" ] }
  active mongoses:
        "5.0.3" : 1
  autosplit:
        Currently enabled: yes
  balancer:
        Currently enabled:  yes
        Currently running:  no
        Failed balancer rounds in last 5 attempts:  0
        Migration Results for the last 24 hours:
                512 : Success
  databases:
        {  "_id" : "config",  "primary" : "config",  "partitioned" : true }
                config.system.sessions
                        shard key: { "_id" : 1 }
                        unique: false
                        balancing: true
                        chunks:
        {  "_id" : "news",  "primary" : "shard2rs",  "partitioned" : true,  "version" : {  "uuid" : UUID("dde4cff3-a2e4-4c81-87c1-4fd306370986"),  "timestamp" : Timestamp(1633500025, 2),  "lastMod" : 1 } }
                news.articles
                        shard key: { "dc" : 1 }
                        unique: false
                        balancing: true
                        chunks:

Even though, I have Sharded the collection using the above commands but getShardDistribution() is returning collection is not sharded.

mongos> db.articles.getShardDistribution();
Collection news.articles is not sharded.

Please let me know if I am missing something here.

Answer 1

You need to add data to the collection. Only then the system can tell "how" data is shared. Before that, the collection is not sharded.

Answer 2

As workaround you can run this query:

db.getSiblingDB("config").chunks.aggregate([
   { $match: { ns: "news.articles" } },
   { $group: { _id: { shard: "$shard", ns: "$ns" }, chunks: { $sum: 1 } } },
   { $group: { _id: "$_id.ns", shards: { $push: { shard: "$_id.shard", chunks: "$chunks" } } } },
   { $unwind: "$shards" },
   { $group: { _id: "$_id", shards: { $push: { k: "$shards.shard", v: "$shards.chunks" } } } },
   { $replaceRoot: { newRoot: { $mergeObjects: [{ns:"$_id"}, { $arrayToObject: "$shards" }] } } },
   { $sort: { ns: 1 } }
])