--- a/CMakeLists.txt	Wed Sep 30 14:04:25 2015 +0200
+++ b/CMakeLists.txt	Tue Oct 06 13:31:40 2015 +0200
@@ -121,6 +121,7 @@
   Core/MultiThreading/ReaderWriterLock.cpp
   Core/MultiThreading/Semaphore.cpp
   Core/MultiThreading/SharedMessageQueue.cpp
+  Core/MultiThreading/RunnableWorkersPool.cpp
   Core/Images/Font.cpp
   Core/Images/FontRegistry.cpp
   Core/Images/ImageAccessor.cpp

--- a/Core/MultiThreading/IRunnableBySteps.h	Wed Sep 30 14:04:25 2015 +0200
+++ b/Core/MultiThreading/IRunnableBySteps.h	Tue Oct 06 13:31:40 2015 +0200
@@ -32,9 +32,11 @@
 
 #pragma once
 
+#include "../IDynamicObject.h"
+
 namespace Orthanc
 {
-  class IRunnableBySteps
+  class IRunnableBySteps : public IDynamicObject
   {
   public:
     virtual ~IRunnableBySteps()

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/Core/MultiThreading/RunnableWorkersPool.cpp	Tue Oct 06 13:31:40 2015 +0200
@@ -0,0 +1,149 @@
+/**
+ * Orthanc - A Lightweight, RESTful DICOM Store
+ * Copyright (C) 2012-2015 Sebastien Jodogne, Medical Physics
+ * Department, University Hospital of Liege, Belgium
+ *
+ * This program is free software: you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation, either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * In addition, as a special exception, the copyright holders of this
+ * program give permission to link the code of its release with the
+ * OpenSSL project's "OpenSSL" library (or with modified versions of it
+ * that use the same license as the "OpenSSL" library), and distribute
+ * the linked executables. You must obey the GNU General Public License
+ * in all respects for all of the code used other than "OpenSSL". If you
+ * modify file(s) with this exception, you may extend this exception to
+ * your version of the file(s), but you are not obligated to do so. If
+ * you do not wish to do so, delete this exception statement from your
+ * version. If you delete this exception statement from all source files
+ * in the program, then also delete it here.
+ * 
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.
+ **/
+
+
+#include "../PrecompiledHeaders.h"
+#include "RunnableWorkersPool.h"
+
+#include "SharedMessageQueue.h"
+#include "../OrthancException.h"
+#include "../Logging.h"
+
+namespace Orthanc
+{
+  struct RunnableWorkersPool::PImpl
+  {
+    class Worker
+    {
+    private:
+      const bool&           continue_;
+      SharedMessageQueue&   queue_;
+      boost::thread         thread_;
+ 
+      static void WorkerThread(Worker* that)
+      {
+        while (that->continue_)
+        {
+          std::auto_ptr<IDynamicObject>  obj(that->queue_.Dequeue(100));
+          if (obj.get() != NULL)
+          {
+            try
+            {
+              IRunnableBySteps& runnable = *dynamic_cast<IRunnableBySteps*>(obj.get());
+            
+              bool wishToContinue = runnable.Step();
+
+              if (wishToContinue)
+              {
+                // The runnable wishes to continue, reinsert it at the beginning of the queue
+                that->queue_.Enqueue(obj.release());
+              }
+            }
+            catch (OrthancException& e)
+            {
+              LOG(ERROR) << "Exception in a pool of working threads: " << e.What();
+            }
+          }
+        }
+      }
+
+    public:
+      Worker(const bool& globalContinue,
+             SharedMessageQueue& queue) : 
+        continue_(globalContinue),
+        queue_(queue)
+      {
+        thread_ = boost::thread(WorkerThread, this);
+      }
+
+      void Join()
+      {
+        if (thread_.joinable())
+        {
+          thread_.join();
+        }
+      }
+    };
+
+
+    bool                  continue_;
+    std::vector<Worker*>  workers_;
+    SharedMessageQueue    queue_;
+  };
+
+
+
+  RunnableWorkersPool::RunnableWorkersPool(size_t countWorkers) : pimpl_(new PImpl)
+  {
+    pimpl_->continue_ = true;
+
+    if (countWorkers <= 0)
+    {
+      throw OrthancException(ErrorCode_ParameterOutOfRange);
+    }
+
+    pimpl_->workers_.resize(countWorkers);
+
+    for (size_t i = 0; i < countWorkers; i++)
+    {
+      pimpl_->workers_[i] = new PImpl::Worker(pimpl_->continue_, pimpl_->queue_);
+    }
+  }
+
+
+  RunnableWorkersPool::~RunnableWorkersPool()
+  {
+    pimpl_->continue_ = false;
+
+    for (size_t i = 0; i < pimpl_->workers_.size(); i++)
+    {
+      PImpl::Worker* worker = pimpl_->workers_[i];
+
+      if (worker != NULL)
+      {
+        worker->Join();
+        delete worker;
+      }
+    }
+  }
+
+
+  void RunnableWorkersPool::Add(IRunnableBySteps* runnable)
+  {
+    pimpl_->queue_.Enqueue(runnable);
+  }
+
+
+  void RunnableWorkersPool::WaitDone()
+  {
+    pimpl_->queue_.WaitEmpty(0);
+  }
+}

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/Core/MultiThreading/RunnableWorkersPool.h	Tue Oct 06 13:31:40 2015 +0200
@@ -0,0 +1,56 @@
+/**
+ * Orthanc - A Lightweight, RESTful DICOM Store
+ * Copyright (C) 2012-2015 Sebastien Jodogne, Medical Physics
+ * Department, University Hospital of Liege, Belgium
+ *
+ * This program is free software: you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation, either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * In addition, as a special exception, the copyright holders of this
+ * program give permission to link the code of its release with the
+ * OpenSSL project's "OpenSSL" library (or with modified versions of it
+ * that use the same license as the "OpenSSL" library), and distribute
+ * the linked executables. You must obey the GNU General Public License
+ * in all respects for all of the code used other than "OpenSSL". If you
+ * modify file(s) with this exception, you may extend this exception to
+ * your version of the file(s), but you are not obligated to do so. If
+ * you do not wish to do so, delete this exception statement from your
+ * version. If you delete this exception statement from all source files
+ * in the program, then also delete it here.
+ * 
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.
+ **/
+
+
+#pragma once
+
+#include "IRunnableBySteps.h"
+
+#include <boost/shared_ptr.hpp>
+
+namespace Orthanc
+{
+  class RunnableWorkersPool : public boost::noncopyable
+  {
+  private:
+    struct PImpl;
+    boost::shared_ptr<PImpl> pimpl_;
+
+  public:
+    RunnableWorkersPool(size_t countWorkers);
+
+    ~RunnableWorkersPool();
+
+    void Add(IRunnableBySteps* runnable);  // Takes the ownership
+
+    void WaitDone();
+  };
+}

--- a/UnitTestsSources/MultiThreadingTests.cpp	Wed Sep 30 14:04:25 2015 +0200
+++ b/UnitTestsSources/MultiThreadingTests.cpp	Tue Oct 06 13:31:40 2015 +0200
@@ -39,6 +39,7 @@
 #include "../Core/MultiThreading/Locker.h"
 #include "../Core/MultiThreading/Mutex.h"
 #include "../Core/MultiThreading/ReaderWriterLock.h"
+#include "../Core/MultiThreading/RunnableWorkersPool.h"
 
 using namespace Orthanc;
 
@@ -183,7 +184,7 @@
       outputs.push_back(boost::lexical_cast<std::string>(b));
     }
 
-    Toolbox::USleep(100000);
+    Toolbox::USleep(30000);
 
     return true;
   }
@@ -202,7 +203,7 @@
     {
       printf(">> %s: %0.1f\n", it->c_str(), 100.0f * s->GetProgress(*it));
     }
-    Toolbox::USleep(10000);
+    Toolbox::USleep(3000);
   }
 }
 
@@ -257,3 +258,49 @@
     t.join();
   }
 }
+
+
+namespace
+{
+  class MyRunnable : public IRunnableBySteps
+  {
+  private:
+    unsigned int& output_;
+    unsigned int count_;
+
+  public:
+    MyRunnable(unsigned int& output) : output_(output), count_(0)
+    {
+    }
+
+    virtual bool Step()
+    {
+      count_ ++;
+      output_ ++;
+
+      boost::this_thread::sleep(boost::posix_time::milliseconds(3));
+
+      return (count_ < 7);
+    }
+  };
+}
+
+
+TEST(MultiThreading, RunnableWorkersPool)
+{
+  unsigned int output = 0;
+
+  {
+    RunnableWorkersPool pool(3);
+  
+    for (size_t i = 0; i < 11; i++)
+    {
+      pool.Add(new MyRunnable(output));
+    }
+
+    pool.WaitDone();
+  }
+
+
+  ASSERT_EQ(11 * 7, output);
+}