From efc31a32800440bc3ea6b44fdbeee4bd0ecbfc3b Mon Sep 17 00:00:00 2001
From: Mahima Singh <105724608+smahima27@users.noreply.github.com>
Date: Fri, 26 Dec 2025 17:01:38 +0530
Subject: [PATCH] Add circuit breaker and adaptive timeout for provider
 resilience

---
 lib/vmpooler/adaptive_timeout.rb        | 130 ++++++++++++++++
 lib/vmpooler/circuit_breaker.rb         | 193 ++++++++++++++++++++++++
 lib/vmpooler/generic_connection_pool.rb |  28 ++++
 lib/vmpooler/pool_manager.rb            |  78 ++++++++++
 lib/vmpooler/providers/base.rb          |  89 +++++++++++
 vmpooler.yaml.example                   |  48 ++++++
 6 files changed, 566 insertions(+)
 create mode 100644 lib/vmpooler/adaptive_timeout.rb
 create mode 100644 lib/vmpooler/circuit_breaker.rb

diff --git a/lib/vmpooler/adaptive_timeout.rb b/lib/vmpooler/adaptive_timeout.rb
new file mode 100644
index 0000000..2bb47fb
--- /dev/null
+++ b/lib/vmpooler/adaptive_timeout.rb
@@ -0,0 +1,130 @@
+# frozen_string_literal: true
+
+module Vmpooler
+  # Adaptive timeout that adjusts based on observed connection performance
+  # to optimize between responsiveness and reliability.
+  #
+  # Tracks recent connection durations and adjusts timeout to p95 + buffer,
+  # reducing timeout on failures to fail faster during outages.
+  class AdaptiveTimeout
+    attr_reader :current_timeout
+
+    # Initialize adaptive timeout
+    #
+    # @param name [String] Name for logging (e.g., "vsphere_connections")
+    # @param logger [Object] Logger instance
+    # @param metrics [Object] Metrics instance
+    # @param min [Integer] Minimum timeout in seconds
+    # @param max [Integer] Maximum timeout in seconds
+    # @param initial [Integer] Initial timeout in seconds
+    # @param max_samples [Integer] Number of recent samples to track
+    def initialize(name:, logger:, metrics:, min: 5, max: 60, initial: 30, max_samples: 100)
+      @name = name
+      @logger = logger
+      @metrics = metrics
+      @min_timeout = min
+      @max_timeout = max
+      @current_timeout = initial
+      @recent_durations = []
+      @max_samples = max_samples
+      @mutex = Mutex.new
+    end
+
+    # Get current timeout value (thread-safe)
+    # @return [Integer] Current timeout in seconds
+    def timeout
+      @mutex.synchronize { @current_timeout }
+    end
+
+    # Record a successful operation duration
+    # @param duration [Float] Duration in seconds
+    def record_success(duration)
+      @mutex.synchronize do
+        @recent_durations << duration
+        @recent_durations.shift if @recent_durations.size > @max_samples
+
+        # Adjust timeout based on recent performance
+        adjust_timeout if @recent_durations.size >= 10
+      end
+    end
+
+    # Record a failure (timeout or error)
+    # Reduces current timeout to fail faster on subsequent attempts
+    def record_failure
+      @mutex.synchronize do
+        old_timeout = @current_timeout
+        # Reduce timeout by 20% on failure, but don't go below minimum
+        @current_timeout = [(@current_timeout * 0.8).round, @min_timeout].max
+
+        if old_timeout != @current_timeout
+          @logger.log('d', "[*] [adaptive_timeout] '#{@name}' reduced timeout #{old_timeout}s → #{@current_timeout}s after failure")
+          @metrics.gauge("adaptive_timeout.current.#{@name}", @current_timeout)
+        end
+      end
+    end
+
+    # Reset to initial timeout (useful after recovery)
+    def reset
+      @mutex.synchronize do
+        @recent_durations.clear
+        old_timeout = @current_timeout
+        @current_timeout = [@max_timeout, 30].min
+
+        @logger.log('d', "[*] [adaptive_timeout] '#{@name}' reset timeout #{old_timeout}s → #{@current_timeout}s")
+        @metrics.gauge("adaptive_timeout.current.#{@name}", @current_timeout)
+      end
+    end
+
+    # Get statistics about recent durations
+    # @return [Hash] Statistics including min, max, avg, p95
+    def stats
+      @mutex.synchronize do
+        return { samples: 0 } if @recent_durations.empty?
+
+        sorted = @recent_durations.sort
+        {
+          samples: sorted.size,
+          min: sorted.first.round(2),
+          max: sorted.last.round(2),
+          avg: (sorted.sum / sorted.size.to_f).round(2),
+          p50: percentile(sorted, 0.50).round(2),
+          p95: percentile(sorted, 0.95).round(2),
+          p99: percentile(sorted, 0.99).round(2),
+          current_timeout: @current_timeout
+        }
+      end
+    end
+
+    private
+
+    def adjust_timeout
+      return if @recent_durations.empty?
+
+      sorted = @recent_durations.sort
+      p95_duration = percentile(sorted, 0.95)
+
+      # Set timeout to p95 + 50% buffer, bounded by min/max
+      new_timeout = (p95_duration * 1.5).round
+      new_timeout = [[new_timeout, @min_timeout].max, @max_timeout].min
+
+      # Only adjust if change is significant (> 5 seconds)
+      if (new_timeout - @current_timeout).abs > 5
+        old_timeout = @current_timeout
+        @current_timeout = new_timeout
+
+        @logger.log('d', "[*] [adaptive_timeout] '#{@name}' adjusted timeout #{old_timeout}s → #{@current_timeout}s (p95: #{p95_duration.round(2)}s)")
+        @metrics.gauge("adaptive_timeout.current.#{@name}", @current_timeout)
+        @metrics.gauge("adaptive_timeout.p95.#{@name}", p95_duration)
+      end
+    end
+
+    def percentile(sorted_array, percentile)
+      return 0 if sorted_array.empty?
+
+      index = (sorted_array.size * percentile).ceil - 1
+      index = [index, 0].max
+      index = [index, sorted_array.size - 1].min
+      sorted_array[index]
+    end
+  end
+end
diff --git a/lib/vmpooler/circuit_breaker.rb b/lib/vmpooler/circuit_breaker.rb
new file mode 100644
index 0000000..48407c9
--- /dev/null
+++ b/lib/vmpooler/circuit_breaker.rb
@@ -0,0 +1,193 @@
+# frozen_string_literal: true
+
+module Vmpooler
+  # Circuit breaker pattern implementation to prevent cascading failures
+  # when a provider becomes unresponsive or experiences repeated failures.
+  #
+  # States:
+  # - CLOSED: Normal operation, requests flow through
+  # - OPEN: Provider is failing, reject requests immediately (fail fast)
+  # - HALF_OPEN: Testing if provider has recovered with limited requests
+  class CircuitBreaker
+    STATES = [:closed, :open, :half_open].freeze
+
+    class CircuitOpenError < StandardError; end
+
+    attr_reader :state, :failure_count, :success_count
+
+    # Initialize a new circuit breaker
+    #
+    # @param name [String] Name for logging/metrics (e.g., "vsphere_provider")
+    # @param logger [Object] Logger instance
+    # @param metrics [Object] Metrics instance
+    # @param failure_threshold [Integer] Number of failures before opening circuit
+    # @param timeout [Integer] Seconds to wait in open state before testing (half-open)
+    # @param half_open_attempts [Integer] Number of successful test requests needed to close
+    def initialize(name:, logger:, metrics:, failure_threshold: 5, timeout: 30, half_open_attempts: 3)
+      @name = name
+      @logger = logger
+      @metrics = metrics
+      @failure_threshold = failure_threshold
+      @timeout = timeout
+      @half_open_attempts = half_open_attempts
+
+      @state = :closed
+      @failure_count = 0
+      @success_count = 0
+      @last_failure_time = nil
+      @mutex = Mutex.new
+    end
+
+    # Execute a block with circuit breaker protection
+    #
+    # @yield Block to execute if circuit allows
+    # @return Result of the block
+    # @raise CircuitOpenError if circuit is open and timeout hasn't elapsed
+    def call
+      check_state
+
+      begin
+        result = yield
+        on_success
+        result
+      rescue StandardError => e
+        on_failure(e)
+        raise
+      end
+    end
+
+    # Check if circuit allows requests
+    # @return [Boolean] true if circuit is closed or half-open
+    def allow_request?
+      @mutex.synchronize do
+        case @state
+        when :closed
+          true
+        when :half_open
+          true
+        when :open
+          if should_attempt_reset?
+            true
+          else
+            false
+          end
+        end
+      end
+    end
+
+    # Get current circuit breaker status
+    # @return [Hash] Status information
+    def status
+      @mutex.synchronize do
+        {
+          name: @name,
+          state: @state,
+          failure_count: @failure_count,
+          success_count: @success_count,
+          last_failure_time: @last_failure_time,
+          next_retry_time: next_retry_time
+        }
+      end
+    end
+
+    private
+
+    def check_state
+      @mutex.synchronize do
+        case @state
+        when :open
+          if should_attempt_reset?
+            transition_to_half_open
+          else
+            time_remaining = (@timeout - (Time.now - @last_failure_time)).round(1)
+            raise CircuitOpenError, "Circuit breaker '#{@name}' is open (#{@failure_count} failures, retry in #{time_remaining}s)"
+          end
+        when :half_open
+          # Allow limited requests through for testing
+        when :closed
+          # Normal operation
+        end
+      end
+    end
+
+    def should_attempt_reset?
+      return false unless @last_failure_time
+
+      Time.now - @last_failure_time >= @timeout
+    end
+
+    def next_retry_time
+      return nil unless @last_failure_time && @state == :open
+
+      @last_failure_time + @timeout
+    end
+
+    def on_success
+      @mutex.synchronize do
+        case @state
+        when :closed
+          # Reset failure count on success in closed state
+          @failure_count = 0 if @failure_count > 0
+        when :half_open
+          @success_count += 1
+          @failure_count = 0
+          @logger.log('d', "[+] [circuit_breaker] '#{@name}' successful test request (#{@success_count}/#{@half_open_attempts})")
+
+          if @success_count >= @half_open_attempts
+            transition_to_closed
+          end
+        when :open
+          # Should not happen, but reset if we somehow get a success
+          transition_to_closed
+        end
+      end
+    end
+
+    def on_failure(error)
+      @mutex.synchronize do
+        @failure_count += 1
+        @last_failure_time = Time.now
+
+        case @state
+        when :closed
+          @logger.log('d', "[!] [circuit_breaker] '#{@name}' failure #{@failure_count}/#{@failure_threshold}: #{error.class}")
+          if @failure_count >= @failure_threshold
+            transition_to_open
+          end
+        when :half_open
+          @logger.log('d', "[!] [circuit_breaker] '#{@name}' failed during half-open test")
+          transition_to_open
+        when :open
+          # Already open, just log
+          @logger.log('d', "[!] [circuit_breaker] '#{@name}' additional failure while open")
+        end
+      end
+    end
+
+    def transition_to_open
+      @state = :open
+      @success_count = 0
+      @logger.log('s', "[!] [circuit_breaker] '#{@name}' OPENED after #{@failure_count} failures (will retry in #{@timeout}s)")
+      @metrics.increment("circuit_breaker.opened.#{@name}")
+      @metrics.gauge("circuit_breaker.state.#{@name}", 1) # 1 = open
+    end
+
+    def transition_to_half_open
+      @state = :half_open
+      @success_count = 0
+      @failure_count = 0
+      @logger.log('s', "[*] [circuit_breaker] '#{@name}' HALF-OPEN, testing provider health")
+      @metrics.increment("circuit_breaker.half_open.#{@name}")
+      @metrics.gauge("circuit_breaker.state.#{@name}", 0.5) # 0.5 = half-open
+    end
+
+    def transition_to_closed
+      @state = :closed
+      @failure_count = 0
+      @success_count = 0
+      @logger.log('s', "[+] [circuit_breaker] '#{@name}' CLOSED, provider recovered")
+      @metrics.increment("circuit_breaker.closed.#{@name}")
+      @metrics.gauge("circuit_breaker.state.#{@name}", 0) # 0 = closed
+    end
+  end
+end
diff --git a/lib/vmpooler/generic_connection_pool.rb b/lib/vmpooler/generic_connection_pool.rb
index 5299b23..56a0b1f 100644
--- a/lib/vmpooler/generic_connection_pool.rb
+++ b/lib/vmpooler/generic_connection_pool.rb
@@ -34,6 +34,34 @@ module Vmpooler
           end
         end
       end
+
+      # Get connection pool health status
+      # @return [Hash] Health status including utilization and queue depth
+      def health_status
+        {
+          size: @size,
+          available: @available.length,
+          in_use: @size - @available.length,
+          utilization: ((@size - @available.length).to_f / @size * 100).round(2),
+          waiting_threads: (@queue.respond_to?(:length) ? @queue.length : 0),
+          state: determine_health_state
+        }
+      end
+
+      private
+
+      def determine_health_state
+        utilization = ((@size - @available.length).to_f / @size * 100)
+        waiting = @queue.respond_to?(:length) ? @queue.length : 0
+
+        if utilization >= 90 || waiting > 5
+          :critical  # Pool exhausted or many waiting threads
+        elsif utilization >= 70 || waiting > 2
+          :warning   # Pool under stress
+        else
+          :healthy   # Normal operation
+        end
+      end
     end
   end
 end
diff --git a/lib/vmpooler/pool_manager.rb b/lib/vmpooler/pool_manager.rb
index 9c6def6..fedc697 100644
--- a/lib/vmpooler/pool_manager.rb
+++ b/lib/vmpooler/pool_manager.rb
@@ -1287,6 +1287,63 @@ module Vmpooler
       $metrics.gauge('vmpooler_health.status', status_value)
     end
 
+    # Monitor connection pool health across all providers
+    def monitor_connection_pools
+      return unless $providers
+
+      $providers.each do |provider_name, provider|
+        begin
+          next unless provider.respond_to?(:connection_pool)
+
+          pool = provider.connection_pool
+          next unless pool.respond_to?(:health_status)
+
+          health = pool.health_status
+
+          # Push metrics
+          $metrics.gauge("connection_pool.#{provider_name}.available", health[:available])
+          $metrics.gauge("connection_pool.#{provider_name}.in_use", health[:in_use])
+          $metrics.gauge("connection_pool.#{provider_name}.utilization", health[:utilization])
+          $metrics.gauge("connection_pool.#{provider_name}.waiting", health[:waiting_threads])
+
+          # Log warnings for unhealthy states
+          if health[:state] == :critical
+            $logger.log('s', "[!] [connection_pool] '#{provider_name}' CRITICAL: #{health[:utilization]}% used, #{health[:waiting_threads]} waiting")
+            $metrics.increment("connection_pool.#{provider_name}.critical")
+          elsif health[:state] == :warning
+            $logger.log('d', "[*] [connection_pool] '#{provider_name}' WARNING: #{health[:utilization]}% used, #{health[:waiting_threads]} waiting")
+            $metrics.increment("connection_pool.#{provider_name}.warning")
+          end
+
+          # Check circuit breaker status
+          if provider.respond_to?(:circuit_breaker) && provider.circuit_breaker
+            cb_status = provider.circuit_breaker.status
+            state_value = { closed: 0, half_open: 0.5, open: 1 }[cb_status[:state]] || 1
+            $metrics.gauge("circuit_breaker.state.#{provider_name}", state_value)
+            $metrics.gauge("circuit_breaker.failures.#{provider_name}", cb_status[:failure_count])
+          end
+
+          # Log adaptive timeout stats
+          if provider.respond_to?(:adaptive_timeout) && provider.adaptive_timeout
+            timeout_stats = provider.adaptive_timeout.stats
+            if timeout_stats[:samples] > 0
+              $metrics.gauge("adaptive_timeout.current.#{provider_name}", timeout_stats[:current_timeout])
+              $metrics.gauge("adaptive_timeout.p95.#{provider_name}", timeout_stats[:p95])
+            end
+          end
+        rescue StandardError => e
+          $logger.log('d', "[!] [connection_pool_monitor] Failed to monitor '#{provider_name}': #{e}")
+        end
+      end
+    end
+
+    def connection_pool_monitor_enabled?
+      global_config = $config[:config] || {}
+      enabled = global_config['connection_pool_monitor_enabled']
+      enabled = true if enabled.nil? # Default to enabled
+      enabled
+    end
+
     def create_vm_disk(pool_name, vm, disk_size, provider)
       Thread.new do
         begin
@@ -2525,6 +2582,27 @@ module Vmpooler
           end
         end
 
+        # Connection pool monitoring thread
+        if connection_pool_monitor_enabled?
+          monitor_interval = ($config[:config] && $config[:config]['connection_pool_monitor_interval']) || 10 # default 10 seconds
+          if !$threads['connection_pool_monitor']
+            $threads['connection_pool_monitor'] = Thread.new do
+              loop do
+                monitor_connection_pools
+                sleep(monitor_interval)
+              end
+            end
+          elsif !$threads['connection_pool_monitor'].alive?
+            $logger.log('d', '[!] [connection_pool_monitor] worker thread died, restarting')
+            $threads['connection_pool_monitor'] = Thread.new do
+              loop do
+                monitor_connection_pools
+                sleep(monitor_interval)
+              end
+            end
+          end
+        end
+
         sleep(loop_delay)
 
         unless maxloop == 0
diff --git a/lib/vmpooler/providers/base.rb b/lib/vmpooler/providers/base.rb
index e5d458e..10b89a3 100644
--- a/lib/vmpooler/providers/base.rb
+++ b/lib/vmpooler/providers/base.rb
@@ -13,6 +13,10 @@ module Vmpooler
         attr_reader :metrics
         # Provider options passed in during initialization
         attr_reader :provider_options
+        # Circuit breaker for provider resilience
+        attr_reader :circuit_breaker
+        # Adaptive timeout for connections
+        attr_reader :adaptive_timeout
 
         def initialize(config, logger, metrics, redis_connection_pool, name, options)
           @config = config
@@ -30,6 +34,11 @@ module Vmpooler
 
           @provider_options = options
           logger.log('s', "[!] Creating provider '#{name}'")
+
+          # Initialize circuit breaker if enabled
+          initialize_circuit_breaker if circuit_breaker_enabled?
+          # Initialize adaptive timeout if enabled
+          initialize_adaptive_timeout if adaptive_timeout_enabled?
         end
 
         # Helper Methods
@@ -271,6 +280,86 @@ module Vmpooler
           logger.log('s', '[!] purge_unconfigured_folders was renamed to purge_unconfigured_resources, please update your provider implementation')
           purge_unconfigured_resources(allowlist)
         end
+
+        private
+
+        # Circuit breaker configuration and initialization
+
+        def circuit_breaker_enabled?
+          global_config = @config[:config] || {}
+          provider_cfg = provider_config || {}
+
+          # Check provider-specific setting first, then global
+          enabled = provider_cfg['circuit_breaker_enabled']
+          enabled = global_config['circuit_breaker_enabled'] if enabled.nil?
+          enabled = true if enabled.nil? # Default to enabled
+
+          enabled
+        end
+
+        def initialize_circuit_breaker
+          require 'vmpooler/circuit_breaker'
+
+          global_config = @config[:config] || {}
+          provider_cfg = provider_config || {}
+
+          # Get circuit breaker settings (provider-specific overrides global)
+          failure_threshold = provider_cfg['circuit_breaker_failure_threshold'] ||
+                              global_config['circuit_breaker_failure_threshold'] || 5
+          timeout = provider_cfg['circuit_breaker_timeout'] ||
+                    global_config['circuit_breaker_timeout'] || 30
+          half_open_attempts = provider_cfg['circuit_breaker_half_open_attempts'] ||
+                               global_config['circuit_breaker_half_open_attempts'] || 3
+
+          @circuit_breaker = Vmpooler::CircuitBreaker.new(
+            name: @provider_name,
+            logger: @logger,
+            metrics: @metrics,
+            failure_threshold: failure_threshold.to_i,
+            timeout: timeout.to_i,
+            half_open_attempts: half_open_attempts.to_i
+          )
+
+          @logger.log('d', "[*] [#{@provider_name}] Circuit breaker initialized (threshold: #{failure_threshold}, timeout: #{timeout}s)")
+        end
+
+        def adaptive_timeout_enabled?
+          global_config = @config[:config] || {}
+          provider_cfg = provider_config || {}
+
+          # Check provider-specific setting first, then global
+          enabled = provider_cfg['adaptive_timeout_enabled']
+          enabled = global_config['adaptive_timeout_enabled'] if enabled.nil?
+          enabled = true if enabled.nil? # Default to enabled
+
+          enabled
+        end
+
+        def initialize_adaptive_timeout
+          require 'vmpooler/adaptive_timeout'
+
+          global_config = @config[:config] || {}
+          provider_cfg = provider_config || {}
+
+          # Get adaptive timeout settings (provider-specific overrides global)
+          min = provider_cfg['connection_pool_timeout_min'] ||
+                global_config['connection_pool_timeout_min'] || 5
+          max = provider_cfg['connection_pool_timeout_max'] ||
+                global_config['connection_pool_timeout_max'] || 60
+          initial = provider_cfg['connection_pool_timeout_initial'] ||
+                    global_config['connection_pool_timeout_initial'] || 30
+
+          @adaptive_timeout = Vmpooler::AdaptiveTimeout.new(
+            name: "#{@provider_name}_connections",
+            logger: @logger,
+            metrics: @metrics,
+            min: min.to_i,
+            max: max.to_i,
+            initial: initial.to_i
+          )
+
+          @logger.log('d', "[*] [#{@provider_name}] Adaptive timeout initialized (min: #{min}s, max: #{max}s, initial: #{initial}s)")
+        end
       end
     end
   end
diff --git a/vmpooler.yaml.example b/vmpooler.yaml.example
index f05ded2..fd3fd22 100644
--- a/vmpooler.yaml.example
+++ b/vmpooler.yaml.example
@@ -522,6 +522,54 @@
 #         for example, the first time is 2 seconds, then 4, 8, 16 etc. until it reaches check_loop_delay_max.
 #         This value must be greater than 1.0.
 #
+#   - circuit_breaker_enabled (optional; default: true)
+#     Enable circuit breaker pattern for provider connections to prevent cascading failures.
+#     When a provider experiences repeated failures, the circuit breaker will "open" and reject
+#     requests immediately (fail-fast) rather than waiting for timeouts, allowing the provider
+#     to recover while protecting the system from thread exhaustion.
+#     (optional; default: true)
+#
+#   - circuit_breaker_failure_threshold (optional; default: 5)
+#     Number of consecutive failures before opening the circuit breaker.
+#     Lower values make the circuit breaker more sensitive to failures.
+#     (optional; default: 5)
+#
+#   - circuit_breaker_timeout (optional; default: 30) seconds
+#     How long to keep the circuit breaker open before attempting to test recovery.
+#     After this timeout, the circuit enters "half-open" state to test if the provider has recovered.
+#     (optional; default: 30)
+#
+#   - circuit_breaker_half_open_attempts (optional; default: 3)
+#     Number of successful test requests required in half-open state before closing the circuit.
+#     (optional; default: 3)
+#
+#   - adaptive_timeout_enabled (optional; default: true)
+#     Enable adaptive timeout that adjusts connection timeouts based on observed performance.
+#     The timeout will adapt to p95 latency + 50% buffer, bounded by min/max values.
+#     On failures, timeout is reduced to fail faster on subsequent attempts.
+#     (optional; default: true)
+#
+#   - connection_pool_timeout_min (optional; default: 5) seconds
+#     Minimum connection timeout for adaptive timeout mechanism.
+#     (optional; default: 5)
+#
+#   - connection_pool_timeout_max (optional; default: 60) seconds
+#     Maximum connection timeout for adaptive timeout mechanism.
+#     (optional; default: 60)
+#
+#   - connection_pool_timeout_initial (optional; default: 30) seconds
+#     Initial connection timeout before adaptation begins.
+#     (optional; default: 30)
+#
+#   - connection_pool_monitor_enabled (optional; default: true)
+#     Enable monitoring of connection pool health across all providers.
+#     Emits metrics for pool utilization, waiting threads, and circuit breaker status.
+#     (optional; default: true)
+#
+#   - connection_pool_monitor_interval (optional; default: 10) seconds
+#     How often to check connection pool health and emit metrics.
+#     (optional; default: 10)
+#
 #   - manage_host_selection (Only affects vSphere Provider)
 #     Allow host selection to be determined by vmpooler
 #     Hosts are selected based on current CPU utilization and cycled between when there are multiple targets