Problem Statement
What is zone sharding in MongoDB and when would you use it?
Explanation
Zone sharding, also called tag-aware sharding, allows you to control which shards store specific ranges of data based on shard key values. You create zones, associate shards with zones, and define shard key ranges for each zone. The balancer then ensures chunks falling within zone ranges are migrated to shards associated with those zones.
Zone sharding is useful for several scenarios. First, geographic data distribution where you want to keep data close to users. For example, you can create US zone and EU zone, assign shards in US data centers to US zone and shards in EU data centers to EU zone, then define ranges like user IDs starting with 1 for US and 2 for EU.
Second, data tiering based on access patterns. You might have hot data that is frequently accessed and cold data that is rarely accessed. Create hot zone on fast SSD-backed shards and cold zone on cheaper HDD-backed shards, then assign recent data to hot zone and old data to cold zone based on date ranges.
Third, multi-tenancy where you want to isolate different customers on different hardware. Create zones for different customer tiers, like premium zone on powerful hardware and standard zone on regular hardware, then assign customers to appropriate zones.
To implement zone sharding, first add shards to zones using sh.addShardToZone. Then define the ranges of shard key values for each zone using sh.updateZoneKeyRange. The balancer automatically migrates chunks to respect zone boundaries. This happens gradually and transparently.
Zone sharding adds complexity because you must manage zone definitions and shard assignments. Use it only when you have clear requirements for data placement. For most applications, automatic chunk distribution without zones is sufficient.
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