LearnViz CompSci
Home / Learn / Databases / Sharding

Advanced Topic 38 of 51 Databases

Sharding

Distributing data horizontally across nodes.

How it works

Sharding splits data horizontally across multiple database servers, each holding a subset called a shard, chosen by a shard key using range, hash or directory-based strategies. Queries that include the shard key go to one shard, while others must fan out. It scales storage and write throughput beyond a single machine, unlike replication, which copies the same data.

Step-by-step walkthrough

  1. Data is divided across servers by a shard key.
  2. A request for user 1.5M reaches the router.
  3. The router sends it to the correct shard.
  4. Each shard holds only part of the data.
  5. Capacity grows by adding shards, but cross-shard queries and rebalancing are hard.

Where it is used

Very large social networks, e-commerce platforms, multi-tenant services and systems like MongoDB, Vitess and Citus use sharding to grow.

Common pitfall

A bad shard key causes hot spots, such as sequential IDs sending all writes to one shard. Cross-shard joins and transactions are slow or unsupported, and resharding is operationally difficult. Application logic becomes more complex. Delay sharding until needed, try vertical scaling, caching and read replicas first, and choose keys matching dominant access patterns.

Where it fits in the course

Sharding is topic 38 of 51 in Databases, rated Advanced. It follows Replication. The next topic is NoSQL Databases.

Practice and explore

How to study this topic

  1. Read the explanation above and restate the definition in your own words.
  2. Open the interactive lesson and answer its mini quiz.
  3. Read the common pitfall, then write a short example that avoids it.
  4. Check your understanding in the Databases quiz, and revisit the topic if you miss a question.

Frequently asked questions

What is Sharding?
Distributing data horizontally across nodes. It belongs to Databases and is an advanced-level topic.
Where do people use Sharding?
Very large social networks, e-commerce platforms, multi-tenant services and systems like MongoDB, Vitess and Citus use sharding to grow.
What is the most common mistake with Sharding?
A bad shard key causes hot spots, such as sequential IDs sending all writes to one shard.
What should I learn before this topic?
Start with Replication, the topic just before it in Databases.
How can I practice it?
Answer the mini quiz in the interactive lesson, then take the Databases quiz.
Is Sharding hard to learn?
It is an Advanced topic. Expect to read the explanation more than once and to connect it with earlier topics. Working through a small example by hand helps most.
How long does it take to learn Sharding?
Most learners can read the explanation and finish the interactive lesson in under an hour. Fluency comes from applying the idea, so plan a few short practice sessions with the quiz.
Which topics are related to Sharding?
Replication, NoSQL Databases, Database Partitioning are closely connected. The full list is in the related topics section of this page.
What should I study after Sharding?
Continue with NoSQL Databases, the next topic in Databases.

Related topics