Design a Sequencer Class - System Design and Coding Interview

Question

## Problem

Design a `Sequencer` class that generates unique, monotonically increasing IDs. It must be safe for concurrent use and support the following interface:

```python
class Sequencer:
    def __init__(self, start: int = 1, step: int = 1): ...
    def next_id(self) -> int: ...         # returns next unique ID
    def reserve(self, count: int) -> range: ...  # atomically reserve a block of `count` IDs
    def reset(self, start: int) -> None: ...     # reset counter (only valid at startup)
    def peek(self) -> int: ...            # returns next ID without consuming it
```

## Round 2 - System Design

Now extend to a distributed setting: multiple application servers share a single logical sequencer. IDs must still be globally unique and roughly monotonically increasing.

- Describe your architecture (e.g., centralized counter in Redis vs. range pre-allocation vs. Snowflake-style bit-packing).
- How do you handle the case where a server crashes after reserving a block but before using all IDs?
- What guarantees can you provide: strict monotonicity, best-effort monotonicity, or no ordering at all?

## Follow-ups
- What is the maximum throughput of a centralized Redis-based sequencer? What are the bottlenecks?
- How does Snowflake-style ID encoding handle clock skew?

Design a Sequencer Class - System Design and Coding Interview

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