Fixed Window vs. Sliding Window Rate Limiting in .NET

1. Fixed Window Rate Limiting (in .NET)
What it is
The Fixed Window algorithm counts requests within a strict time window (like a box of time). When the window resets, the count resets too.

Example

• Limit: 10 requests per minute.
• Every minute (e.g., from 10:00:00 to 10:01:00), a new window opens.
• If a client makes 10 requests at 10:00:05, they are allowed.
• The next request (11th) before 10:01:00 will get blocked (HTTP 429 error).
• At 10:01:00, the window resets and they can make requests again.

In .NET

options.AddSlidingWindowLimiter("sliding", opt =>
{
    opt.Window = TimeSpan.FromMinutes(1);   // 1-minute rolling window
    opt.PermitLimit = 10;                  // 10 requests allowed in the last minute
    opt.SegmentsPerWindow = 4;             // divide into 4 parts (15 seconds each)
    opt.QueueProcessingOrder = QueueProcessingOrder.OldestFirst;
    opt.QueueLimit = 2;
});

Pros

• Very simple and fast to implement.
• Good for low-traffic or non-critical APIs.

Cons

• Allows “burst traffic” at the start of the window (all 10 requests may come instantly).
• Not very "smooth" — users may hit the limit early and wait for the whole window.

2. Sliding Window Rate Limiting (in .NET)
What it is

The Sliding Window algorithm spreads the limit over a rolling period, not tied to a fixed block of time. It calculates how many requests were made in the last X minutes — no matter when the requests came.

Example

• Limit: 10 requests per minute (rolling).
• If 10 requests were made between 10:00:30 and 10:01:30, the limit applies based on that rolling window — not the wall clock.
• This avoids a sudden "burst" problem and gives a smoother, fairer limit.

In .NET

options.AddSlidingWindowLimiter("sliding", opt =>
{
    opt.Window = TimeSpan.FromMinutes(1);   // 1-minute rolling window
    opt.PermitLimit = 10;                  // 10 requests allowed in the last minute
    opt.SegmentsPerWindow = 4;             // divide into 4 parts (15 seconds each)
    opt.QueueProcessingOrder = QueueProcessingOrder.OldestFirst;
    opt.QueueLimit = 2;
});

SegmentsPerWindow: The window is divided into smaller parts (helps tracking requests better).

Pros

• Fairer and smoother — no sudden resets or spikes.
• Prevents "burst" requests at the start of each minute.

Cons

• Slightly more memory and CPU usage (to track rolling windows).
• Complex to implement manually (but .NET does this for you).

When to Use Which?

Scenario	Fixed Window	Sliding Window
Simple internal APIs	✅ Good	❌ Overkill
Public APIs used by 1000s of users	❌ Not Recommended	✅ Better
Need smooth traffic flow (e.g., payment gateway)	❌ Bad for bursts	✅ Ideal
Want easy implementation without extra load	✅ Very Easy	❌ Slightly heavy

Interview Perspective

Q: What is the key difference between Fixed and Sliding window algorithms in API Rate Limiting?

• Fixed Window resets the counter after each period (e.g., every minute).
• Sliding Window checks requests across a rolling period, making limits smoother and avoiding bursts.
• Sliding is better for high-traffic public APIs, while Fixed is good for simple, low-risk endpoints.

Real Example: Login API in .NET

Fixed Window

// Allow 5 login attempts every 60 seconds
options.AddFixedWindowLimiter("loginFixed", opt =>
{
    opt.Window = TimeSpan.FromSeconds(60);
    opt.PermitLimit = 5;
});

C#

Sliding Window

// Allow 5 login attempts in any rolling 60 seconds
options.AddSlidingWindowLimiter("loginSliding", opt =>
{
    opt.Window = TimeSpan.FromSeconds(60);
    opt.PermitLimit = 5;
    opt.SegmentsPerWindow = 6;  // 10 seconds each
});

Summary

Fixed Window	Sliding Window
Simple, less resource-hungry	Smooth, prevents burst requests
Allows initial bursts	Spreads requests over time
Good for small/private APIs	Best for public APIs, payment, login
Reset happens strictly by time	Reset happens by rolling calculation

If you want, I can

• Build a full sample .NET app demonstrating both types.
• Help you prepare exact interview answers.
• Show how to apply rate limiting per user/IP dynamically.

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