diff --git a/src/db/pool.cr b/src/db/pool.cr
index 575c512..5acffaa 100644
--- a/src/db/pool.cr
+++ b/src/db/pool.cr
@@ -2,47 +2,97 @@ require "weak_ref"
 
 module DB
   class Pool(T)
+    # Pool configuration
+
+    # initial number of connections in the pool
     @initial_pool_size : Int32
-    # maximum amount of objects in the pool. Either available or in use.
+    # maximum amount of connections in the pool (Idle + InUse)
     @max_pool_size : Int32
-    @available = Set(T).new
-    @total = [] of T
+    # maximum amount of idle connections in the pool
+    @max_idle_pool_size : Int32
+    # seconds to wait before timeout while doing a checkout
     @checkout_timeout : Float64
-    # maximum amount of retry attempts to reconnect to the db. See `Pool#retry`.
+    # maximum amount of retry attempts to reconnect to the db. See `Pool#retry`
     @retry_attempts : Int32
+    # seconds to wait before a retry attempt
     @retry_delay : Float64
 
+    # Pool state
+
+    # total of open connections managed by this pool
+    @total = [] of T
+    # connections available for checkout
+    @idle = Set(T).new
+    # connections waiting to be stablished (they are not in *@idle* nor in *@total*)
+    @inflight : Int32
+
+    # Sync state
+
+    # communicate that a connection is available for checkout
+    @availability_channel : Channel(Nil)
+    # signal how many existing connections are waited for
+    @waiting_resource : Int32
+    # global pool mutex
+    @mutex : Mutex
+
     def initialize(@initial_pool_size = 1, @max_pool_size = 0, @max_idle_pool_size = 1, @checkout_timeout = 5.0,
                    @retry_attempts = 1, @retry_delay = 0.2, &@factory : -> T)
-      @initial_pool_size.times { build_resource }
-
       @availability_channel = Channel(Nil).new
       @waiting_resource = 0
+      @inflight = 0
       @mutex = Mutex.new
+
+      @initial_pool_size.times { build_resource }
     end
 
     # close all resources in the pool
     def close : Nil
       @total.each &.close
       @total.clear
-      @available.clear
+      @idle.clear
+    end
+
+    record Stats, open_connections : Int32
+
+    # Returns stats of the pool
+    def stats
+      Stats.new(
+        open_connections: @total.size
+      )
     end
 
     def checkout : T
-      resource = if @available.empty?
-                   if can_increase_pool
-                     build_resource
-                   else
-                     wait_for_available
-                     pick_available
-                   end
-                 else
-                   pick_available
-                 end
+      res = sync do
+        resource = nil
 
-      @available.delete resource
-      resource.before_checkout
-      resource
+        until resource
+          resource = if @idle.empty?
+                       if can_increase_pool?
+                         @inflight += 1
+                         r = unsync { build_resource }
+                         @inflight -= 1
+                         r
+                       else
+                         unsync { wait_for_available }
+                         # The wait for available can unlock
+                         # multiple fibers waiting for a resource.
+                         # Although only one will pick it due to the lock
+                         # in the end of the unsync, the pick_available
+                         # will return nil
+                         pick_available
+                       end
+                     else
+                       pick_available
+                     end
+        end
+
+        @idle.delete resource
+
+        resource
+      end
+
+      res.before_checkout
+      res
     end
 
     # ```
@@ -51,14 +101,14 @@ module DB
     # `selected` be a resource from the `candidates` list and `is_candidate` == `true`
     # or `selected` will be a new resource and `is_candidate` == `false`
     def checkout_some(candidates : Enumerable(WeakRef(T))) : {T, Bool}
-      # TODO honor candidates while waiting for availables
-      # this will allow us to remove `candidates.includes?(resource)`
-      candidates.each do |ref|
-        resource = ref.value
-        if resource && is_available?(resource)
-          @available.delete resource
-          resource.before_checkout
-          return {resource, true}
+      sync do
+        candidates.each do |ref|
+          resource = ref.value
+          if resource && is_available?(resource)
+            @idle.delete resource
+            resource.before_checkout
+            return {resource, true}
+          end
         end
       end
 
@@ -67,13 +117,21 @@ module DB
     end
 
     def release(resource : T) : Nil
-      if can_increase_idle_pool
-        @available << resource
-        resource.after_release
-        @availability_channel.send nil if are_waiting_for_resource?
-      else
-        resource.close
-        @total.delete(resource)
+      idle_pushed = false
+
+      sync do
+        if can_increase_idle_pool
+          @idle << resource
+          resource.after_release
+          idle_pushed = true
+        else
+          resource.close
+          @total.delete(resource)
+        end
+      end
+
+      if idle_pushed && are_waiting_for_resource?
+        @availability_channel.send nil
       end
     end
 
@@ -82,10 +140,14 @@ module DB
     # It will try to reuse all of the available connection right away,
     # but if a new connection is needed there is a `retry_delay` seconds delay.
     def retry
-      current_available = @available.size
-      # if the pool hasn't reach the max size, allow 1 attempt
-      # to make a new connection if needed without sleeping
-      current_available += 1 if can_increase_pool
+      current_available = 0
+
+      sync do
+        current_available = @idle.size
+        # if the pool hasn't reach the max size, allow 1 attempt
+        # to make a new connection if needed without sleeping
+        current_available += 1 if can_increase_pool?
+      end
 
       (current_available + @retry_attempts).times do |i|
         begin
@@ -94,7 +156,7 @@ module DB
         rescue e : ConnectionLost
           # if the connection is lost close it to release resources
           # and remove it from the known pool.
-          delete(e.connection)
+          sync { delete(e.connection) }
           e.connection.close
         rescue e : ConnectionRefused
           # a ConnectionRefused means a new connection
@@ -107,44 +169,46 @@ module DB
 
     # :nodoc:
     def each_resource
-      @available.each do |resource|
-        yield resource
+      sync do
+        @idle.each do |resource|
+          yield resource
+        end
       end
     end
 
     # :nodoc:
     def is_available?(resource : T)
-      @available.includes?(resource)
+      @idle.includes?(resource)
     end
 
     # :nodoc:
     def delete(resource : T)
       @total.delete(resource)
-      @available.delete(resource)
+      @idle.delete(resource)
     end
 
     private def build_resource : T
       resource = @factory.call
       @total << resource
-      @available << resource
+      @idle << resource
       resource
     end
 
-    private def can_increase_pool
-      @max_pool_size == 0 || @total.size < @max_pool_size
+    private def can_increase_pool?
+      @max_pool_size == 0 || @total.size + @inflight < @max_pool_size
     end
 
     private def can_increase_idle_pool
-      @available.size < @max_idle_pool_size
+      @idle.size < @max_idle_pool_size
     end
 
     private def pick_available
-      @available.first
+      @idle.first?
     end
 
     private def wait_for_available
       timeout = TimeoutHelper.new(@checkout_timeout.to_f64)
-      inc_waiting_resource
+      sync_inc_waiting_resource
 
       timeout.start
 
@@ -153,30 +217,42 @@ module DB
       case index
       when 0
         timeout.cancel
-        dec_waiting_resource
+        sync_dec_waiting_resource
       when 1
-        dec_waiting_resource
+        sync_dec_waiting_resource
         raise DB::PoolTimeout.new
       else
         raise DB::Error.new
       end
     end
 
-    private def inc_waiting_resource
-      @mutex.synchronize do
-        @waiting_resource += 1
-      end
+    private def sync_inc_waiting_resource
+      sync { @waiting_resource += 1 }
     end
 
-    private def dec_waiting_resource
-      @mutex.synchronize do
-        @waiting_resource -= 1
-      end
+    private def sync_dec_waiting_resource
+      sync { @waiting_resource -= 1 }
     end
 
     private def are_waiting_for_resource?
-      @mutex.synchronize do
-        @waiting_resource > 0
+      @waiting_resource > 0
+    end
+
+    private def sync
+      @mutex.lock
+      begin
+        yield
+      ensure
+        @mutex.unlock
+      end
+    end
+
+    private def unsync
+      @mutex.unlock
+      begin
+        yield
+      ensure
+        @mutex.lock
       end
     end