Day 14: Practice System Design Problems (e.g., design a URL shortener)
System Design Overview
System design involves architecting large-scale systems with a focus on scalability, availability, reliability, and performance. In interviews, you’re expected to create a system from scratch, taking into consideration various trade-offs and choices based on requirements.
System design problems often require you to:
Clarify requirements: Functional and non-functional requirements.
Define high-level architecture: Components and interactions.
Address scalability, availability, and performance: Use caching, load balancing, databases, etc.
Plan for trade-offs and bottlenecks: Handle potential issues like high traffic or data consistency.
Discuss data storage, redundancy, and fault tolerance.
System Design Problem: Design a URL Shortener
A URL shortener like Bit.ly converts long URLs into short ones, making it easier to share links. Let’s break down how to approach the design step-by-step.
1. Requirements Gathering
Before diving into the design, clarify both functional and non-functional requirements.
Functional Requirements:
Shorten a long URL and generate a unique short URL.
Retrieve the original URL when given the short URL.
Handle edge cases like invalid URLs or expired URLs.
Support custom short URLs (optional).
Non-Functional Requirements:
The system should be highly available (since users rely on links working).
The system should be scalable to handle billions of URL requests.
Low latency: The redirection process should be fast.
The system should be fault-tolerant to avoid losing data.
The short URLs should not clash or duplicate, ensuring uniqueness.
2. High-Level Design
A high-level architecture of a URL shortener typically consists of the following components:
2.1 Components
API Layer:
URL Shortening API: Accepts long URLs, generates a unique short URL.
Redirection API: Accepts short URLs and redirects users to the original URL.
Database:
Stores mappings between long URLs and their corresponding short URLs.
Stores metadata such as timestamp, expiration date, etc.
Caching:
A cache can store frequently accessed URL mappings to reduce database lookups and increase performance.
Load Balancer:
Ensures traffic is evenly distributed across the application servers.
Distributed System Considerations:
Ensure the design can scale horizontally by adding more servers as the system grows.
3. URL Generation Strategy
The main challenge in URL shorteners is to generate unique, short URLs. There are several methods to achieve this:
3.1 Hash-based Approach
Description: Use a hash function (e.g., MD5, SHA256) to convert a long URL into a fixed-length string (hash).
Pros: Easy to implement, can generate a unique identifier.
Cons: Hash collisions can occur (i.e., two different URLs generate the same hash), and this needs to be handled with collision resolution strategies (e.g., append a counter or timestamp).
3.2 Base62 Encoding
Description: Use a numeric identifier for each URL, then encode the number in a Base62 format (using digits 0-9, lowercase letters a-z, and uppercase letters A-Z).
Example: URL ID
125in Base62 might be encoded ascB.
Pros: Base62 provides a compact and human-readable encoding.
Cons: The system needs to manage unique numeric IDs for each URL, typically using an auto-incrementing field in the database.
3.3 Random String Generation
Description: Generate a random string (e.g., 6-8 characters) for each URL using characters from a predefined set (Base62 is common).
Pros: Simple, ensures short URLs.
Cons: Needs to ensure uniqueness by checking the database for existing URLs (i.e., collisions may occur).
4. Database Design
The URL shortener needs a robust data storage strategy to store mappings between short URLs and long URLs.
4.1 Table Schema
URL Table:
id(Primary Key, Auto Increment)original_url(Text)short_url(String)creation_date(Timestamp)expiration_date(Optional, if you want URLs to expire after a period)
4.2 Database Choices
Relational Database (e.g., MySQL, PostgreSQL):
Works well for storing URL mappings with atomic transactions.
Indexed lookups are fast for read-heavy operations.
NoSQL Database (e.g., DynamoDB, Cassandra):
Useful for high throughput and distributed environments.
Can store key-value pairs where the key is the short URL and the value is the original URL.
Sharding for Scalability:
As the number of URLs grows, horizontal partitioning (sharding) may be necessary.
One way to shard is based on the short URL prefix (e.g., URLs starting with ‘a’ are stored on one shard, ‘b’ on another).
5. Caching
To improve performance and reduce the load on the database, frequently accessed URLs should be cached.
5.1 Caching Strategy
Use a distributed cache like Redis or Memcached.
Cache mappings of short URLs to long URLs for fast redirection.
Set an appropriate time-to-live (TTL) on cached entries to avoid stale data.
5.2 Cache Invalidation
If a URL expires or is deleted, the cache entry must be invalidated to prevent serving outdated data.
6. Handling High Availability and Fault Tolerance
6.1 Replication and Redundancy
Ensure that the database is replicated across multiple servers to avoid a single point of failure.
Master-slave replication can be used for load distribution.
6.2 Load Balancing
Use a load balancer (e.g., AWS ELB, NGINX) to distribute incoming traffic across multiple application servers.
6.3 Failover Strategy
In case of server failure, the system should automatically switch to backup servers (replicated databases, backup caches, etc.).
7. URL Expiry and Deletion
Some URL shorteners provide an expiration date for short URLs. After a certain period, the URLs become invalid and are removed.
Implementation:
Add an
expiration_datefield in the database.Periodically run a background job to delete expired URLs or mark them as inactive.
8. Security and Considerations
8.1 Prevent Abuse
URL shorteners can be misused for phishing or redirecting to malicious websites. To mitigate this:
Blacklist known malicious domains.
Rate limiting to prevent users from creating too many short URLs in a short period.
8.2 Redirection Security
Ensure that HTTP requests for short URLs are redirected properly using HTTP 301 (Permanent Redirect) or 302 (Temporary Redirect) to avoid issues with search engine indexing.
8.3 Custom Short URLs
Users may want to create custom short URLs (e.g.,
http://short.ly/my-link).Ensure that custom URLs are unique and not already taken.
Allow users to reserve specific URLs if needed.
9. Traffic Estimation and Scaling
9.1 Read and Write Traffic
Assume the service is popular and handles billions of URLs.
Write operations (URL shortening) are less frequent than read operations (redirections), so the system needs to optimize for reads.
9.2 Scaling with High Traffic
Horizontally scale application servers to handle millions of concurrent requests.
Use CDNs to cache responses globally and reduce latency for users far from the origin servers.
10. Example API Design
10.1 URL Shortening API
10.2 URL Redirection API
11. Conclusion: Trade-offs and Considerations
Key Trade-offs:
Consistency vs Availability: Choose an eventual consistency model (AP system) to prioritize availability over strict consistency. If the short URL lookup is slightly delayed, it’s acceptable as long as the system remains available.
Scalability: The system must handle billions of URL mappings. Horizontal scalability is essential to maintain performance as traffic grows.
Fault Tolerance: Design for fault tolerance by using replication and load balancing to avoid a single point of failure.
By understanding these steps and reasoning through the choices made during the system design process, you'll be well-prepared to tackle system design questions like URL shortener during interviews.
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