Design a Search Autocomplete System

Functional Requirements:

• Prefix-based Suggestions: Given a query prefix, return autocomplete suggestions that start with the prefix.
• Top-K Results: For each prefix, return the top K (e.g., 5) most popular or relevant suggestions.
• Case Insensitivity: Suggestions should be case-insensitive ("App" and "app" are treated the same).
• Alphabetic Characters Only: Ignore special characters and numbers in suggestions.
• No Spell Correction: The system does not handle typos or fuzzy matching.

Non-Functional Requirements:

• Low Latency: Suggestions should be returned quickly (within ~100ms per request).
• Scalability: The system should support a large number of users and queries per second.
• High Availability: The system should be resilient to failures and remain available.
• Data Growth: Should handle increasing numbers of queries and growing suggestion lists gracefully.
• Graceful Degradation: In case of failures, the system should serve stale data rather than fail completely.