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\addtogroup new-features

\brief Summaries major features and improvements of given release. 

\section new-messaging-api The New Messaging Subsystem

The messaging subsystem has been redesigned in the SDK. Previously, 
a message was representing a FIX container that provided access to 
the field values using tags as keys. Also, the processing machinery 
used generic SBE (Simple Binary Encoding) decoder to extract data from 
the SBE-encoded content transmitted by CME MDP and to build a FIX message. 

The given approach of manipulating market data provided certain benefit 
as it allowed to work with all market data through a unified FIX container. 
The drawback of the given approach was latency. SBE-encoded content had to 
decoded and copied to the FIX container on a processing layer. 

The messaging subsystem has been redesigned to provide customers 
with maximal performance, and now the API exposes classes for each 
type of SBE message defined by SBE coding templates. 

The given approach allows the customers to be much closer to the 
original data transmitted by CME, as the SBE to FIX mapping is rather 
semantical and thus losses certain important characteristics which 
the customers may require. 

Also, the performance is dramatically improved due to eliminating SBE 
to FIX transformation subsystem, and now data is directly accessed without 
any intermediate copying. 

\section new-listeners New Events And Listeners

Events and listeners, reflecting miscellaneous aspects 
of the Handler behavior, have been redesigned, and now 
are more consistent compare to the previous design. 

The table below represents a quick overview of major 
listeners and key improvements.

<table>
  <tr>
    <th>Listener</th>
    <th>Description</th>
  </tr>
  <tr>
    <td>OnixS::CME::MDH::HandlerListener</td>
    <td>
      Encapsulates Handler events. 
      
      Previously, the Handler behavior-related events were spread 
     in a few listeners like a listener for the Handler state and 
     issues (warning, error). Now, everything is gathered into a 
     single listener. The states have been substituted with the 
     correspondent callbacks and enhanced. 

      For example, now the callback identifying the end of recovery 
     exposes the status of recovery completion indicating whether 
     the recovery is succeeded or failed with a detailed description 
     of the last case. 
    </td>
  </tr>
  <tr>
    <td>OnixS::CME::MDH::MarketDataListener</td>
    <td>
      Encapsulates market data events. 
      
      The given listener is a close analog to the 
     OnixS::CME::MDP3::MessageProcessingListener exposed 
     by the previous generations of the SDK. However, it was 
     enhanced with packet-related events exposing original 
     data received by Handler instances. 
    </td>
  </tr>
  <tr>
    <td>OnixS::CME::MDH::SecurityListener</td>
    <td>
      Encapsulates security-related events. 
      
      Previously, security-related events were raised through 
     a number of listeners like the OnixS::CME::MDP3::TradeListener 
     and the OnixS::CME::MDP3::DirectChangeBookLister. 
    </td>
  </tr>
</table>

\section new-session-settings Market Data Processing (Session) Settings

Previously, the way how Handler processed market data was defined by which 
overload of the OnixS::CME::MDH::Handler::start member was used to spawning  
processing. 

Starting with version 4 of the SDK, the OnixS::CME::MDH::SessionSettings 
class was invented to define market data processing for the session. Settings 
are supplied to the OnixS::CME::MDH::Handler::start member as a parameter. 
Currently, settings allow defining whether market data recovery must be done 
at the beginning of market data processing or in case of data loss or other 
processing error and so on. Also, settings allow defining which type of market 
(by price or by order or both) is to be recovered. New parameters offer a wider 
variety of processing ways compare to previous versions. 

\section tcp-recovery-with-natural-refresh Using TCP Recovery With Natural Refresh

Previously, it was not possible to use TCP recovery service while maintaining 
order books in natural refresh mode. Starting with version 4 of the SDK a 
TCP recovery facility can be used while processing market data in any selected
way. 

\section new-book-management Book Management Improvements

The users must explicitly configure the Handler concerning which types 
of order books are to be maintained starting with the given major release. 
Previously, maintenance was defined by either the Handler's settings or 
by the fact of associating correspondent book listener with a Handler 
instance. See \ref book-handling for more information. 

\section miscellaneous-new-features Miscellaneous

\ref what-is-new section enlists other features 
and improvements as individual topics. 

\see \ref migration-guide