URL Shortner System Design

https://media.geeksforgeeks.org/wp-content/uploads/20251006182804290214/url_shortner.webpThe need for an efficient and concise URL management system has become significant concern in the digital age. URL shortening services, such as bit.ly, TinyURL, and ZipZy.in, play a massive role in transforming lengthy web addresses into shorter, shareable links.

Lets understand this Activity Diagram of URL Shortner:

Requirements for URL Shortner Service System Design

1. Functional requirements

• Given a long URL, the service should generate a shorter and unique alias for it.
• When the user hits a short link, the service should redirect to the original link.
• Links will expire after a standard default time span.

2. Non-Functional requirements

• The system should be highly available. This is really important to consider because if the service goes down, all the URL redirection will start failing.
• URL redirection should happen in real-time with minimal latency.
• Shortened links should not be predictable.

Capacity Estimation

Let's assume our service has 30M new URL shortenings per month. Let’s assume we store every URL shortening request (and associated shortened link) for 5 years. For this period the service will generate about 1.8 B records.

30 million * 5 years * 12 months = 1.8B

Note: Let's consider we are using 7 characters to generate a short URL. These characters are a combination of 62 characters [A-Z, a-z, 0-9] something like https://zpzy.in/redirecting?code=abXdef2.

Data Capacity Modeling

Discuss the data capacity model to estimate the storage of the system. We need to understand how much data we might have to insert into our system. Think about the different columns or attributes that will be stored in our database and calculate the storage of data for five years. Let's make the assumption given below for different attributes.

• Consider the average long URL size of 2KB ie for 2048 characters.
• Short URL size: 17 Bytes for 17 characters
• created_at- 7 bytes
• expiration_length_in_minutes -7 bytes

The above calculation will give a total of 2.031KB per shortened URL entry in the database.
If we calculate the total storage then for 30 M active users
total size = 30000000 * 2.031 = 60780000 KB = 60.78 GB per month. In a Year of 0.7284 TB and in 5 years 3.642 TB of data.

Note: We need to think about the reads and writes that will happen on our system for this amount of data. This will decide what kind of database (RDBMS or NoSQL) we need to use.

Low-Level Design

URL Encoding Techniques to create Shortened URL

To convert a long URL into a unique short URL we can use some hashing techniques like Base62 or MD5. We will discuss both approaches.

1. Base62 Encoding

• Base62 encoder allows us to use the combination of characters and numbers which contains A-Z, a-z, 0–9 total( 26 + 26 + 10 = 62).
• So for 7 characters short URL, we can serve 62^7 ~= 3500 billion URLs which is quite enough in comparison to base10 (base10 only contains numbers 0-9 so you will get only 10M combinations).
• We can generate a random number for the given long URL and convert it to base62 and use the hash as a short URL id.

If we use base62 making the assumption that the service is generating 1000 tiny URLs/sec then it will take 110 years to exhaust this 3500 billion combination.

#include <string>
#include <vector>

std::string to_base_62(int deci) {
    std::string s = "012345689abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
    std::string hash_str = "";
    while (deci > 0) {
        hash_str = s[deci % 62] + hash_str;
        deci /= 62;
    }
    return hash_str;
}

public String toBase62(int deci) {
    String s = "012345689abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
    String hashStr = "";
    while (deci > 0) {
        hashStr = s.charAt(deci % 62) + hashStr;
        deci /= 62;
    }
    return hashStr;
}

def to_base_62(deci):
    s = '012345689abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ'
    hash_str = ''
    while deci > 0:
        hash_str = s[deci % 62] + hash_str
        deci /= 62
    return hash_str


print to_base_62(999)

function toBase62(deci) {
    const s = '012345689abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
    let hashStr = '';
    while (deci > 0) {
        hashStr = s[deci % 62] + hashStr;
        deci = Math.floor(deci / 62);
    }
    return hashStr;
}

2. MD5 Encoding

MD5 also gives base62 output but the MD5 hash gives a lengthy output which is more than 7 characters.

• MD5 hash generates 128-bit long output so out of 128 bits we will take 43 bits to generate a tiny URL of 7 characters.
• MD5 can create a lot of collisions. For two or many different long URL inputs we may get the same unique id for a short URL and that could cause data corruption.
• So we need to perform some checks to ensure that this unique id doesn't exist in the database already.

Efficient Database Storage & Retrieval of TinyURL

Let's discuss the mapping of a long URL into a short URL in our database:

1. Using Base62 Encoding

Assume we generate the Tiny URL using base62 encoding then we need to perform the steps given below:

• The tiny URL should be unique so firstly check the existence of this tiny URL in the database (doing get(tiny) on DB). If it's already present there for some other long URL then generate a new short URL.
• If the short URL isn’t present in DB then put the long URL and TinyURL in DB (put(TinyURL, long URL)).

This technique works with one server very well but if there will be multiple servers then this technique will create a race condition .

• When multiple servers will work together, there will be a possibility that they all can generate the same unique id or the same tiny URL for different long URLs.
• Even after checking the database, they will be allowed to insert the same tiny URLs simultaneously in the database and this may end up corrupting the data.

2. Using MD5 Approach

• Encode the long URL using the MD5 approach and take only the first 7 chars to generate TinyURL.
• The first 7 characters could be the same for different long URLs so check the DB (as we have discussed in Technique 1) to verify that TinyURL is not used already.

This approach saves some space in the database but how?

• If two users want to generate a tiny URL for the same long URL then the first technique will generate two random numbers and it requires two rows in the database.
• In the second technique, both the longer URL will have the same MD5 so it will have the same first 43 bits.
• This means we will get some deduping and we will end up with saving some space since we only need to store one row instead of two rows in the database.

3. Using Counter Approach

Using a counter is a good decision for a scalable solution because counters always get incremented so we can get a new value for every new request.

Single server approach:

• A single host or server (say database) will be responsible for maintaining the counter.
• When the worker host receives a request it talks to the counter host, which returns a unique number and increments the counter. When the next request comes the counter host again returns the unique number and this goes on.
• Every worker host gets a unique number which is used to generate TinyURL.

High-level Design of a URL-Shortening Service

• User Interface/Clients
  Users enter a long URL via web form or API and receive a shortened link.
• Application Server
  Generates unique short keys, stores mappings in the database, handles redirects, and tracks clicks.
• Load Balancer
  Distributes traffic across servers at three points:
• Clients - Application Servers
• Application Servers - Database
• Application Servers - Cache
• Database
  Stores URL-key mappings, ensures uniqueness, and scales with NoSQL solutions like MongoDB or Cassandra.
• Caching
  Speeds up reads with in-memory caches (Redis/Memcached) for frequently accessed URLs.
• Cleanup Service
  Removes expired or unused data from storage.
• Redirection
  Short key lookup retrieves the original URL and redirects using HTTP 301.
• Analytics
  Tracks clicks, referrers, browsers, and devices for user insights.
• Security
  Prevents abuse (phishing/malware links, DDoS, brute force) using firewalls, rate-limiting, and authentication.

Database Design

Let us explore some of the choices for System Design of Databases of URL Shortner:

• We can use RDBMS which uses ACID properties but you will be facing the scalability issue with relational databases.
• Now if you think you can use sharding and resolve the scalability issue in RDBMS then that will increase the complexity of the system.
• There are 30M active users so there will be conversions and a lot of Short URL resolution and redirections.
• Read and write will be heavy for these 30M users so scaling the RDBMS using shard will increase the complexity of the design.

You may have to use consistent hashing to balance the traffic and DB queries in the case of RDBMS and which is a complicated process. So to handle this amount of huge traffic on our system relational databases are not fit and also it won't be a good decision to scale the RDBMS.

So let's take a look at NoSQL Database:

• The only problem with using the NoSQL database is its eventual consistency.
• We write something and it takes some time to replicate to a different node but our system needs high availability and NoSQL fits this requirement.
• NoSQL can easily handle the 30M of active users and it is easy to scale. We just need to keep adding the nodes when we want to expand the storage.

Caching and Load Balancing in URL Shortening service

In a URL shortening service, caching and load balancing are essential for managing high demand and optimizing response times. The service could greatly benefit from read-through caching or write-through caching mechanisms.

• A read-through cache automatically loads data into the cache when a miss occurs
• While a write-through cache updates the cache whenever the database is updated.
• Caching
  Use a read-through cache (Redis/Memcached) to quickly serve frequently accessed shortened URLs.
• Load Balancing
• Round Robin: Evenly spreads traffic, simple to scale.
• Least Connections: Best when request processing times vary, balances load more efficiently.

Design a tiny URL generator - System Design

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