How do they know your username is already taken so fast?
Tags: bloomfilters, redis, systemdesign, scalablearchitecture
Published: 2023-03-01 13:10:17

---
title: How do they know your username is already taken so fast?
published: true
description: Understanding Bloom Filters
tags: bloomfilters, redis, systemdesign, scalablearchitecture
cover_image: https://miro.medium.com/v2/resize:fit:1400/0*maEX3dATKcctix-a
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# published_at: 2023-03-01 12:50 +0000
---
How do some websites tell you immediately that your username is already in use? How can they search your username from disks so fast? The magic trick is an efficient data structure called Bloom Filters.

Bloom filters are a space-efficient probabilistic data structure used to test whether an element is a member of a set. It is often used to reduce the number of disk I/O operations required to check if an element is in a set by avoiding disk access altogether. In this blog, we’ll explore the inner workings of bloom filters, how they are used, the advantages and limitations of this powerful data structure.


![Bloom Filter Pattern | Redis](https://miro.medium.com/v2/resize:fit:1034/format:webp/1*iWvvmfPRzRUc2ds2abMX_Q.jpeg)
 

**Implementation Details:**

* Bloom filters are implemented as a bit array with a set of hash functions that map elements to the indices in the bit array.
* When an element is added to the set, the hash functions are applied to the element, and the corresponding bits in the bit array are set to 1.
* When testing for membership of an element, the hash functions are applied to the element, and the corresponding bits in the bit array are checked.
* If all the bits are set to 1, then the element is probably in the set. Otherwise, it is definitely not in the set.

The main advantage of Bloom filters is that they allow for quick and efficient membership tests without the need for disk I/O operations. This is especially important for applications that require large sets and need to test for membership quickly and frequently. Another advantage of Bloom filters is that they can be implemented with a low false positive rate, which is the rate at which the Bloom filter returns that an element is in the set when it is actually not.

**Applications:**

One of the main applications of Bloom filters is in the field of distributed systems, where they are used to reduce the number of network requests between nodes. For example, if a node needs to check if an element is in a set, it can first check its local Bloom filter before sending a request to another node. This can significantly reduce the number of network requests and improve overall system performance.

Another application of Bloom filters is in network security, where they are used to detect malicious traffic. For example, a Bloom filter can be used to maintain a list of known malicious IP addresses and to test incoming traffic against the list to detect malicious traffic. This approach is much more efficient than checking every incoming IP address against a list of known malicious IP addresses, as the number of false positive results is kept to a minimum.

Bloom filters can also be used in other areas, such as spell-checking, database indexing, username availability tests and text indexing.

**Limitations:**

One of the main limitations of Bloom filters is their limited capacity. As the number of elements in the set grows, the false positive rate increases, eventually reaching a point where the Bloom filter becomes less helpful. However, this can be mitigated by increasing the size of the bit array or by using more hash functions, although this will increase the amount of memory required by the Bloom filter.

You can take advantage of this in redis as well. To know more please checkout this [link](https://redis.com/blog/bloom-filter/). Originally published at [medium](https://medium.com/@himankbh/what-are-bloom-filters-50790a39add8)