Hash-based sharding in MongoDB - Step-by-Step Execution

Concept Flow - Hash-based sharding

Start: Insert Document

↓

Extract Shard Key Value

↓

Apply Hash Function

↓

Compute Hash Value

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Modulo by Number of Shards

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Determine Target Shard

↓

Store Document in Target Shard

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End

Documents are assigned to shards by hashing the shard key value, then using the hash to pick the shard.

Execution Sample

MongoDB

shardKey = "user_id"
doc = {"user_id": 12345, "name": "Alice"}
hashValue = hash(doc[shardKey])
shardNumber = hashValue % 3
store(doc, shardNumber)

This code hashes the user_id to decide which of 3 shards stores the document.

Execution Table

Step	Action	Input	Hash Value	Modulo Result	Target Shard
1	Extract shard key value	user_id=12345
2	Apply hash function	12345	67890
3	Modulo by number of shards	67890 % 3	67890	0
4	Determine target shard	Modulo result=0			Shard 0
5	Store document	doc in Shard 0			Shard 0

💡 Document stored in Shard 0 based on hash modulo 3 result

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
shardKey	"user_id"	"user_id"	"user_id"	"user_id"	"user_id"	"user_id"
doc	{}	{"user_id":12345,"name":"Alice"}	{"user_id":12345,"name":"Alice"}	{"user_id":12345,"name":"Alice"}	{"user_id":12345,"name":"Alice"}	{"user_id":12345,"name":"Alice"}
hashValue			67890	67890	67890	67890
shardNumber				0	0	0

Key Moments - 3 Insights

Why do we use a hash function on the shard key value?

The hash function converts the shard key into a number that evenly distributes documents across shards, as shown in execution_table step 2.

What does the modulo operation do in hash-based sharding?

Modulo limits the hash value to the number of shards, deciding the exact shard to store the document, as seen in execution_table step 3.

Can two different shard key values end up in the same shard?

Yes, different keys can hash to values that modulo to the same shard number, so multiple keys can be stored in one shard (execution_table steps 3 and 4).

Visual Quiz - 3 Questions

Look at the execution_table, what is the hashValue after applying the hash function?

A 12345

B 67890

C 0

D 3

💡 Hint

Check the 'Hash Value' column at step 2 in the execution_table.

At which step is the target shard determined?

A Step 4

B Step 3

C Step 2

D Step 5

💡 Hint

Look for the step where 'Target Shard' column is filled in execution_table.

If the number of shards changes from 3 to 4, how does the modulo result change for the same hashValue?

A It becomes 1

B It remains 0

C It becomes 2

D It becomes 3

💡 Hint

Calculate 67890 % 4 and compare with the modulo result in execution_table step 3.

Concept Snapshot

Hash-based sharding assigns documents to shards by:
1. Extracting the shard key value.
2. Applying a hash function to it.
3. Using modulo by shard count to pick the shard.
This evenly distributes data and balances load.

Full Transcript

Hash-based sharding in MongoDB works by taking the shard key from a document, applying a hash function to convert it into a numeric hash value, then using modulo division by the number of shards to decide which shard stores the document. This method helps distribute data evenly across shards. The execution steps show extracting the shard key 'user_id' with value 12345, hashing it to 67890, then modulo 3 gives 0, so the document is stored in Shard 0. Variables like shardKey, doc, hashValue, and shardNumber change step by step as the document moves through the process. Key points include why hashing is used for distribution, how modulo picks the shard, and that different keys can map to the same shard. The quiz questions check understanding of hash values, step order, and effects of changing shard count.