**Design a URL Shortener**\nYou are tasked with designing a URL shortener service similar to bit.ly. The service should take a long URL as input and generate a short URL that redirects to the long URL when visited. Consider aspects such as scalability, fault tolerance, and performance. Explain how you would structure the database and APIs to handle associated tasks.\n\n**Functionality should include:** \n1. Generating a short URL from a long URL. \n2. Redirecting from a short URL back to the original URL. \n3. Tracking usage statistics for each short URL.

**LeetCode #159 - Longest Substring with At Most Two Distinct Characters**\nGiven a string s, return the length of the longest substring that contains at most two distinct characters. This problem requires an understanding of the sliding window technique to efficiently find the maximum length of the substring under the specified constraint.\n**Function Signature:** `def length_of_longest_substring_two_distinct(s: str) -> int:`\n**Example 1:**\nInput: `s = "eceba"`\nOutput: `3`\nExplanation: The substring is "ece" which contains two distinct characters.\n**Example 2:**\nInput: `s = "aa"`\nOutput: `2`\nExplanation: The substring is "aa" which contains one distinct character.\n**Constraints:**\n- `1 <= s.length <= 10^5`\n- `s` consists of English letters, digits, symbols, and spaces.

**Design a URL Shortener Service**\nURL shorteners are widely used for concisely hiding long URLs to make links easily shareable. Your task is to design a URL shortener service like bit.ly. Consider the requirements of URL storage, generating short keys, managing redirection, handling collisions, and providing analytics. Describe the major components, databases, and technologies you would use to implement this service at scale.\n**Functionality Requirements:** 1. Shorten a URL 2. Redirect to the original URL 3. Track statistics (clicks, etc.)\n**Non-Functional Requirements:** 1. High availability 2. Low latency 3. Secure access\n**Considerations:** Scalability and performance in handling high volume traffic.