RUBY-3798 Backpressue examples

spec/integration/backpressure_examples_spec.rb

@@ -0,0 +1,127 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe 'backpressure examples' do
+  RETRYABLE_ERROR_LABEL = 'RetryableError'
+  SYSTEM_OVERLOADED_ERROR = 'SystemOverloadedError'
+  BASE_BACKOFF_MS = 100
+  MAX_BACKOFF_MS = 10_000
+
+  def system_overloaded_error?(error)
+    error.respond_to?(:label?) && error.label?(SYSTEM_OVERLOADED_ERROR)
+  end
+
+  def calculate_exponential_backoff(attempt)
+    rand * [ MAX_BACKOFF_MS, BASE_BACKOFF_MS * (2**(attempt - 1)) ].min
+  end
+
+  def with_retries(max_attempts: 2)
+    max_attempts.times do |attempt|
+      is_retry = attempt > 0
+      if is_retry
+        delay = calculate_exponential_backoff(attempt)
+        sleep(delay / 1000.0)
+      end
+      begin
+        return yield
+      rescue StandardError => e
+        is_retryable_overload_error = system_overloaded_error?(e) && e.label?(RETRYABLE_ERROR_LABEL)
+        can_retry = is_retryable_overload_error && attempt + 1 < max_attempts
+        raise unless can_retry
+      end
+    end
+  end
+  describe '#system_overloaded_error?' do
+    it 'returns true for an error with the SystemOverloadedError label' do
+      error = Mongo::Error.new('overloaded')
+      error.add_label(SYSTEM_OVERLOADED_ERROR)
+      expect(system_overloaded_error?(error)).to be true
+    end
+
+    it 'returns false for an error without the SystemOverloadedError label' do
+      error = Mongo::Error.new('other')
+      expect(system_overloaded_error?(error)).to be_falsey
+    end
+
+    it 'returns false for a plain StandardError' do
+      error = StandardError.new('plain')
+      expect(system_overloaded_error?(error)).to be_falsey
+    end
+  end
+
+  describe '#calculate_exponential_backoff' do
+    it 'returns a value between 0 and BASE_BACKOFF_MS for the first retry' do
+      results = Array.new(100) { calculate_exponential_backoff(1) }
+      expect(results).to all(be >= 0)
+      expect(results).to all(be <= BASE_BACKOFF_MS)
+    end
+
+    it 'caps at MAX_BACKOFF_MS for high attempt numbers' do
+      results = Array.new(100) { calculate_exponential_backoff(100) }
+      expect(results).to all(be >= 0)
+      expect(results).to all(be <= MAX_BACKOFF_MS)
+    end
+  end
+
+  describe '#with_retries' do
+    it 'returns the result of the block on success' do
+      result = with_retries { 42 }
+      expect(result).to eq(42)
+    end
+
+    it 'raises non-retryable errors immediately' do
+      attempts = 0
+      expect do
+        with_retries(max_attempts: 3) do
+          attempts += 1
+          raise StandardError, 'fatal'
+        end
+      end.to raise_error(StandardError, 'fatal')
+      expect(attempts).to eq(1)
+    end
+
+    it 'raises overload errors that lack the RetryableError label' do
+      attempts = 0
+      expect do
+        with_retries(max_attempts: 3) do
+          attempts += 1
+          error = Mongo::Error.new('overloaded')
+          error.add_label(SYSTEM_OVERLOADED_ERROR)
+          raise error
+        end
+      end.to raise_error(Mongo::Error, 'overloaded')
+      expect(attempts).to eq(1)
+    end
+
+    it 'retries retryable overload errors up to max_attempts' do
+      attempts = 0
+      expect do
+        with_retries(max_attempts: 3) do
+          attempts += 1
+          error = Mongo::Error.new('overloaded')
+          error.add_label(SYSTEM_OVERLOADED_ERROR)
+          error.add_label(RETRYABLE_ERROR_LABEL)
+          raise error
+        end
+      end.to raise_error(Mongo::Error, 'overloaded')
+      expect(attempts).to eq(3)
+    end
+
+    it 'succeeds on retry after a retryable overload error' do
+      attempts = 0
+      result = with_retries(max_attempts: 3) do
+        attempts += 1
+        if attempts < 2
+          error = Mongo::Error.new('overloaded')
+          error.add_label(SYSTEM_OVERLOADED_ERROR)
+          error.add_label(RETRYABLE_ERROR_LABEL)
+          raise error
+        end
+        'ok'
+      end
+      expect(result).to eq('ok')
+      expect(attempts).to eq(2)
+    end
+  end
+end