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Interviews / Databases / What are bitmaps and hyperloglogs and what are they used for in Redis?

Questions

What are the different data structures in Redis and when would you use them?
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What are Redis Streams and what are they used for?
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What are bitmaps and hyperloglogs and what are they used for in Redis?
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How do you do persistence in Redis?
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How do you do replication in Redis?
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How do you do sharding in Redis?
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What are transactions and how do you use them in Redis?
Q C
What is Lua scripting and how do you use it in Redis?
Q C
What is Redis Sentinel and how do you use it for high availability?
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What is Redis Cluster and how do you use it for scalability?
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What is the difference between `DEL` and `UNLINK` in Redis?
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How do you use Redis as a cache?
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What are bitmaps and hyperloglogs and what are they used for in Redis?

conceptual Data Structures Interactive Quiz Code Examples

The Scenario

You are a backend engineer at a social media company. You are building a new service that needs to track a large number of events, such as which users have liked a post or which users have seen an ad.

You need to find a way to store this data in a way that is efficient and that does not use a lot of memory.

The Challenge

Explain what bitmaps and hyperloglogs are in Redis and how you would use them to solve this problem. What are the key benefits of using these data structures?

Wrong Approach

A junior engineer might try to solve this problem by using a set. This would work, but it would use a lot of memory for a large number of events. They might not be aware of bitmaps and hyperloglogs, which are much more memory-efficient.

Right Approach

A senior engineer would know that bitmaps and hyperloglogs are the perfect tools for this job. They would be able to explain what these data structures are and how to use them to track a large number of events in a memory-efficient way.

Step 1: Understand What Bitmaps and HyperLogLogs Are

Data StructureDescriptionUse Cases
BitmapsA bitmap is a data structure that allows you to store a boolean value for each element in a set.Tracking user activity, such as which users have liked a post or which users have seen an ad.
HyperLogLogsA HyperLogLog is a probabilistic data structure that is used to estimate the cardinality of a set.Counting the number of unique visitors to a website or the number of unique searches for a keyword.

Step 2: Choose the Right Tool for the Job

  • If you need to know whether a specific user has performed an action, you should use a bitmap.
  • If you only need to know the number of unique users who have performed an action, you should use a HyperLogLog.

Step 3: Code Examples

Here are some code examples that show how to use these data structures:

Bitmaps:

# Mark user 123 as having seen the ad
SETBIT ad:1:seen 123 1

# Check if user 123 has seen the ad
GETBIT ad:1:seen 123

# Count the number of users who have seen the ad
BITCOUNT ad:1:seen

HyperLogLogs:

# Add a user to the set of unique visitors
PFADD unique_visitors "user:123"

# Get the number of unique visitors
PFCOUNT unique_visitors

Practice Question

You need to count the number of unique visitors to your website. Which of the following would be the most appropriate?