Pluggable Session Storage

Pluggable session storage is the storage which implemented by the user. For example, it could be used to implement High-Availability (HA) Solutions, one can implement a Pluggable Session Storage which stores data to a shareable storage and if one server, with the FIX Engine, fails and an another reserve server stands up then the FIX Engine can restore session states from the same shareable storage. Such a storage should implement the OnixS::FIX::ISessionStorage interface.

Implementing Pluggable Storage

There are few basic rules that should be applied to an implementation of Pluggable Storage.

• Lifecycle of the storage should be managed by the user

  The Engine core treats instance of Pluggable Storage as a "foreign" object and, therefore, never attempts to delete it. On the other hand, there is strong requirement to keep storage instance valid, until the OnixS::FIX::Session instance is destroyed.

• Be as fast as possible

  Since Pluggable Storage is incorporated into the message send/receive route, it has a direct affection on sending/receiving latency, and overall throughput of the system. So, to keep high performance, it is needed to handle callbacks as fast as possible.

• Prevent against exceptions

  Exceptions thrown from Pluggable Storage callbacks, always immediately break a session. So you usually need to use exception-free code for callbacks.

• Don't make assumptions about parameters lifecycle

  Messages and pointers, passed to Pluggable Storage callbacks, should be completely handled, before these callbacks return control to Engine core. Whenever you need to keep such data for separate usage, you should make copies of messages or memory blocks, and store them in appropriate place, but don't keep raw pointers, passed to callbacks.

• No specific synchronization is needed

  The Engine core manages all synchronization issues by itself, so all methods of Pluggable Storage are called sequentially. Therefore, you don't need to implement separate synchronization to ensure, that ISessionStorage::logInbound and ISessionStorage::logOutbound methods are not called simultaneously. Of course, it is still possible to use synchronizations for other needs.

• Track message numbers, when the message is logged

  Message sequence numbers should be tracked while messages are logged, i.e. during OnixS::FIX::ISessionStorage::storeInbound or OnixS::FIX::ISessionStorage::storeOutbound methods are handled. These numbers should be kept and returned to Engine through OnixS::FIX::ISessionStorage::inSeqNum() and OnixS::FIX::ISessionStorage::outSeqNum() calls for incoming and outgoing messages respectively.

  OnixS::FIX::ISessionStorage::inSeqNum(int) and OnixS::FIX::ISessionStorage::outSeqNum(int) are called only duringsession startup, the initial login and session shutdown the processes.

Call Order of Pluggable Storage Methods

This section explains how different methods of Pluggable Storage are called in different situations.

Incoming message handling

• Reading message from wire. On this stage, verification of the message content and the message classification are performed.
• When there is a session-level message
  • OnixS::FIX::ISessionStorage::storeInbound.
  • OnixS::FIX::ISessionListener methods, if the listener is present.
• When there is an application-level message, call order to listeners methods is reversed against the previous case
  • OnixS::FIX::ISessionListener methods, if the listener is present.
  • OnixS::FIX::ISessionStorage::storeInbound.

If an exception is thrown by the user's code on any stage (during ISessionStorage or ISessionListener handling), the session becomes disconnected, and subsequent stages are omitted. Logout message will be sent to counterparty in dependency on the value of the OnixS::FIX::EngineSettings::sendLogoutOnException setting.

Outgoing message handling

• OnixS::FIX::ISessionListener methods. If the method throws exception, the handling is stopped.
• OnixS::FIX::ISessionStorage::storeInbound. If the method throws exception, the handling is stopped.
• Passing message to the internal queue for subsequent sending .

Resend Request handling

• OnixS::FIX::ISessionStorage::storeInbound with incoming ResendRequest<2> message.
• OnixS::FIX::ISessionStorage::getOutbound method to restore all or a part of the requested messages. These messages are kept in an internal buffer, until the OnixS::FIX::ISessionStorage::getOutbound method returns control to Engine.
• The following loop is repeated for each one restored message:
  • OnixS::FIX::ISessionListener::onOutboundApplicationMsg or OnixS::FIX::ISessionListener::onOutboundSessionMsg.
  • OnixS::FIX::ISessionStorage::storeOutbound method.

Attaching Pluggable Storage

There is a quite schematic illustration for the pluggable storage usage.

// Declare the own class and implement ISessionStorage interface callbacks.
class MyPluggableStorage : public ISessionStorage { ... };

// Declare the instance of the storage.
MyPluggableStorage myStorage;

// Create the session which uses the storage.
Session initiator(senderCompId, targetCompId, dictionary, &listener, SessionStorageType::Pluggable, &myStorage);

Example

Please, check the PluggableStorage sample, included into distribution package of FIX Engine.