Resilience4J - SlidingWindow Protocal with CircuitBreaker

 In previous articel discussed about the BulkHead Pattern, Now we are discussing on Sliding Window.

Good 👍 this is core internal logic of CircuitBreaker in Resilience4j.

Most developers use @CircuitBreaker but don’t understand how sliding window actually calculates failure rate.

Let’s break it clearly.

🔥 What is Sliding Window in Resilience4j?

Sliding window is the statistical window used by CircuitBreaker to decide:

Should we OPEN the circuit or keep it CLOSED?

It calculates:

• Failure rate %

• Slow call rate %

• Total calls count

Based on last N calls or last N seconds.

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📌 Two Types of Sliding Windows

1️⃣ COUNT_BASED Sliding Window

Based on number of calls.

Example:

CircuitBreakerConfig config = CircuitBreakerConfig.custom()
    .slidingWindowType(SlidingWindowType.COUNT_BASED)
    .slidingWindowSize(10)
    .failureRateThreshold(50)
    .build();

Meaning:

• Observe last 10 calls

• If more than 50% fail

• Circuit goes OPEN

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Example Scenario:

Last 10 calls:

S F S F F S F F S F

Failures = 6
Failure rate = 60%

If threshold = 50% → Circuit OPEN

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2️⃣ TIME_BASED Sliding Window

Based on time duration.

Example:

.slidingWindowType(SlidingWindowType.TIME_BASED)
.slidingWindowSize(10)

Meaning:

• Observe calls in last 10 seconds

• Calculate failure rate

• If threshold crossed → OPEN

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🧠 How Sliding Window Internally Works

Internally it maintains:

• Circular array (ring buffer)

• Buckets for time-based

• Atomic counters

Every new call:

1. Old data expires

2. New result added

3. Failure rate recalculated

4. Decision made

This is O(1) time complexity per update.

Very efficient.

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🎯 Important Configurations (Architect Level)

🔹 Minimum Number of Calls

.minimumNumberOfCalls(5)

Circuit will not evaluate failure rate unless at least 5 calls happen.

This avoids false positives in low traffic systems.

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🔹 Failure Rate Threshold

.failureRateThreshold(50)

If failure % > threshold → OPEN

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🔹 Slow Call Rate Threshold

.slowCallRateThreshold(60)
.slowCallDurationThreshold(Duration.ofSeconds(2))

If 60% calls take > 2 seconds → OPEN

This protects against latency spikes.

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🏦 Real Banking Example (APS Context)

Let’s say:

Loan SOR:

• Sliding window size = 20 calls

• Failure threshold = 40%

• Minimum calls = 10

If last 20 calls:

• 8 failures

• Failure rate = 40%

Circuit remains CLOSED.

But if 9 failures:

• 45%

• Circuit OPEN

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🔄 Difference Between Count vs Time Based

Feature	COUNT_BASED	TIME_BASED
Best For	Stable traffic	Variable traffic
Banking Core APIs	✅ Good	⚠️ Depends
High burst systems	❌ Risky	✅ Better
Predictability	High	Medium