Pages/TcpRecovery.md

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\addtogroup tcp-recovery-facility

\brief Explains TCP Recovery facility.

\section tcp-recovery-concept Recovering from Data Loss

The Handler receives market data through the network multicast 
feeds. Due to a multicast nature, data may come in the wrong order 
or may be completely lost. The Handler does its best to fix all 
the network-related issues. However, packet gaps may still happen
while processing market data.

The CME MDP provides its clients with an ability to request 
and obtain lost data using a reliable TCP connection. This 
facility is called a TCP Recovery. If the Handler is configured 
to use a TCP Recovery facility, in case of data loss, it doesn't 
report an error immediately and does not fall into a book recovery. 
Instead, it suspends regular processing and requests a remote system 
to resend missed data. The Handler resumes regular processing without 
restarting its execution, once it successfully receives lost packets 
from the service.


If a request to recover missed data through a TCP Recovery 
feed fails, the Handler tries to recover lost packets for a
number of times defined by the 
OnixS::CME::MDH::TcpRecoverySettings::reconnectAttempts 
parameter. The Handler waits for  
OnixS::CME::MDH::TcpRecoverySettings::reconnectInterval
milliseconds before a new attempt to recover lost data is 
made. If the Handler was not able to recover lost packets for 
a predefined number of attempts, it spawns a recovery from 
snapshots or continues real-time processing as defined by 
session settings.

\section enabling-tcp-recovery Enabling TCP Recovery

It is necessary to contact CME support for TCP Recovery 
credentials to be able to use a TCP Recovery facility.

Also, the MDP does not allow multiple recovery requests served 
simultaneously using the same credentials. Only one TCP recovery 
session can be active at a time. For this reason, the SDK exposes 
a TCP recovery feature as a shared service which can be bound to 
multiple Handler instances. Implementation of this feature covers
all synchronization related aspects to avoid multiple requests at 
the same time.

The following example demonstrates how to use TCP Recovery with the Handler:

~~~{.cpp}

using namespace OnixS::CME::MDPH;

// Configuring TCP Recovery.

TcpRecoverySetting tcpRecoverySettings;

// Credentials are to be set as there's no defaults for
// these parameters. Other parameters have default values.

tcpRecoverySettings.username("CME");
tcpRecoverySettings.password("CME");

// Constructing instance of TCP Recovery
// service and binding to the Handler.

TcpRecoveryService tcpRecovery(tcpRecoverySettings);

// Configuring Handler.

HandlerSettings handlerSettings;

handlerSettings.channel(310);
handlerSettings.feeds().connectivityConfigurationFile("config.xml");

Handler handler(handlerSettings);

// Since this call, Handler will use
// TCP recovery in case of data loss.
handler.bindTcpRecovery(tcpRecovery);

~~~