Horizontal Scaling with Sharding

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Distribute data across multiple shards for horizontal scaling using MongoDB sharded clusters with config servers and mongos routers.

Prerequisites

-MongoDB 6+ installed
-Config server replica set running
-At least 2 shard replica sets

Steps

Start the config server replica set

Launch config server instances that store cluster metadata and routing information.

$ mongod --configsvr --replSet configRS --port 27019 --dbpath /data/configsvr --bind_ip localhost --fork --logpath /var/log/mongodb/configsvr.log

In production, run 3 config server members for redundancy. A single member works for testing.

Initialize config server replica set

Connect to the config server and initiate the replica set.

$ mongosh --port 27019 --eval 'rs.initiate({_id: "configRS", configsvr: true, members: [{_id: 0, host: "localhost:27019"}]})'

Start a mongos router

Launch the query router that applications connect to for routing queries to the correct shards.

$ mongos --configdb configRS/localhost:27019 --port 27017 --bind_ip localhost --fork --logpath /var/log/mongodb/mongos.log

Add shards to the cluster

$ mongosh --port 27017 --eval 'sh.addShard("shard0RS/localhost:27020"); sh.addShard("shard1RS/localhost:27021");'

Enable sharding on a database and shard a collection

Choose a shard key and enable sharding for a specific collection.

$ mongosh --port 27017 --eval 'sh.enableSharding("mydb"); sh.shardCollection("mydb.orders", {customerId: "hashed"});'

The shard key cannot be changed after sharding. Choose carefully based on your query patterns and data distribution.

Check sharding status

View the overall sharding topology, chunk distribution, and balancer status.

$ mongosh --port 27017 --eval 'sh.status()'

Full Script

full-script.sh

#!/bin/bash
# MongoDB Sharding Status and Health Check
set -euo pipefail

MONGOS_PORT=27017

echo "=== Sharding Status ==="
mongosh --port "$MONGOS_PORT" --quiet --eval "sh.status()"

echo ""
echo "=== Shard Distribution ==="
mongosh --port "$MONGOS_PORT" --quiet --eval "
const dbs = db.adminCommand({listDatabases: 1}).databases;
dbs.forEach(d => {
  if (d.name !== 'admin' && d.name !== 'config' && d.name !== 'local') {
    print('--- ' + d.name + ' (' + Math.round(d.sizeOnDisk / 1024 / 1024) + 'MB) ---');
    const colls = db.getSiblingDB(d.name).getCollectionNames();
    colls.forEach(c => {
      const stats = db.getSiblingDB(d.name)[c].stats();
      if (stats.sharded) {
        print('  ' + c + ' (sharded): ' + stats.count + ' docs');
        Object.keys(stats.shards || {}).forEach(s => {
          print('    ' + s + ': ' + stats.shards[s].count + ' docs, ' +
            Math.round(stats.shards[s].size / 1024 / 1024) + 'MB');
        });
      }
    });
  }
});
"

echo ""
echo "=== Chunk Distribution ==="
mongosh --port "$MONGOS_PORT" --quiet --eval "
db.getSiblingDB('config').chunks.aggregate([
  {$group: {_id: {ns: '$ns', shard: '$shard'}, chunks: {$sum: 1}}},
  {$sort: {'_id.ns': 1, '_id.shard': 1}}
]).forEach(doc => {
  print(doc._id.ns + ' -> ' + doc._id.shard + ': ' + doc.chunks + ' chunks');
});
"

echo ""
echo "=== Balancer Status ==="
mongosh --port "$MONGOS_PORT" --quiet --eval "
const status = sh.getBalancerState();
print('Balancer enabled: ' + status);
const running = sh.isBalancerRunning();
print('Balancer running: ' + running);
"

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#!/bin/bash # MongoDB Sharding Status and Health Check set -euo pipefail MONGOS_PORT=27017 echo "=== Sharding Status ===" mongosh --port "$MONGOS_PORT" --quiet --eval "sh.status()" echo "" echo "=== Shard Distribution ===" mongosh --port "$MONGOS_PORT" --quiet --eval " const dbs = db.adminCommand({listDatabases: 1}).databases; dbs.forEach(d => { if (d.name !== 'admin' && d.name !== 'config' && d.name !== 'local') { print('--- ' + d.name + ' (' + Math.round(d.sizeOnDisk / 1024 / 1024) + 'MB) ---'); const colls = db.getSiblingDB(d.name).getCollectionNames(); colls.forEach(c => { const stats = db.getSiblingDB(d.name)[c].stats(); if (stats.sharded) { print(' ' + c + ' (sharded): ' + stats.count + ' docs'); Object.keys(stats.shards || {}).forEach(s => { print(' ' + s + ': ' + stats.shards[s].count + ' docs, ' + Math.round(stats.shards[s].size / 1024 / 1024) + 'MB'); }); } }); } }); " echo "" echo "=== Chunk Distribution ===" mongosh --port "$MONGOS_PORT" --quiet --eval " db.getSiblingDB('config').chunks.aggregate([ {$group: {_id: {ns: '$ns', shard: '$shard'}, chunks: {$sum: 1}}}, {$sort: {'_id.ns': 1, '_id.shard': 1}} ]).forEach(doc => { print(doc._id.ns + ' -> ' + doc._id.shard + ': ' + doc.chunks + ' chunks'); }); " echo "" echo "=== Balancer Status ===" mongosh --port "$MONGOS_PORT" --quiet --eval " const status = sh.getBalancerState(); print('Balancer enabled: ' + status); const running = sh.isBalancerRunning(); print('Balancer running: ' + running); "