OnixS C++ FIX Engine 4.13.0
API Documentation
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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 shareable storage. If one server with the FIX Engine fails and another reserve server stands up, the FIX Engine can restore session states from the same shareable storage. Such storage should implement the OnixS::FIX::ISessionStorage interface.

Implementing Pluggable Storage

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

  • The user should manage the lifecycle of the storage

    The Engine core treats the instance of Pluggable Storage as a "foreign" object and, therefore, never attempts to delete it. On the other hand, there is a strong requirement to keep the 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 directly affects 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 are thrown from Pluggable Storage callbacks, always immediately break a session. So you usually need to use an 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 an 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 Pluggable Storage methods 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 during the session startup, initial login, and session shutdown of 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

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 the counterparty in dependency on the value of the OnixS::FIX::EngineSettings::sendLogoutOnException setting.

Outgoing message handling

Note
If a message is sent from the OnixS::FIX::ISessionListener::onInboundApplicationMsg() callback, the session checks it and calls the OnixS::FIX::ISessionStorage::storeInbound() first before calling the OnixS::FIX::ISessionStorage::storeOutbound(). The session does it to keep the logging order. Otherwise, the OnixS::FIX::ISessionStorage::storeOutbound() will be called before the OnixS::FIX::ISessionStorage::storeInbound(), and the outbound message will appear in the log before the inbound message.

Resend Request handling

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 OnixS::FIX::ISessionStorage
{
public:
void close(bool /*keepSequenceNumbers*/, bool /*doBackup*/) ONIXS_FIXENGINE_OVERRIDE {}
void getOutbound(SequenceNumber /*beginSequenceNumber*/, SequenceNumber /*endSequenceNumber*/, ISessionStorageListener * /*listener*/) ONIXS_FIXENGINE_OVERRIDE {}
void inSeqNum(SequenceNumber /*messageSequenceNumber*/) ONIXS_FIXENGINE_OVERRIDE {}
void storeInbound(const Message &, SequenceNumber /*messageSequenceNumber*/, const RawMessagePointer & /*pointer*/, bool /*logMessage*/) ONIXS_FIXENGINE_OVERRIDE {}
void storeInbound(const FlatMessage &, SequenceNumber /*messageSequenceNumber*/, const RawMessagePointer& /*pointer*/, bool /*logMessage*/) ONIXS_FIXENGINE_OVERRIDE {}
void storeOutbound(const Message &, const RawMessagePointer & /*pointer*/, bool /*logMessage*/) ONIXS_FIXENGINE_OVERRIDE {}
void storeOutbound(const FlatMessage &, SequenceNumber /*sequenceNumber*/, const RawMessagePointer & /*pointer*/, bool /*logMessage*/) ONIXS_FIXENGINE_OVERRIDE {}
void outSeqNum(SequenceNumber /*messageSequenceNumber*/) ONIXS_FIXENGINE_OVERRIDE {}
Timestamp sessionCreationTime() ONIXS_FIXENGINE_OVERRIDE { return Timestamp(); }
size_t resendingQueueSize() const ONIXS_FIXENGINE_OVERRIDE { return 1000; }
};
// Declare the instance of the storage.
MyPluggableStorage myStorage;
// Create the session which uses the storage.
Session initiator("SenderCompId", "TargetCompId", ProtocolVersion::FIX_42, &listener, SessionStorageType::Pluggable, &myStorage);

Example

Please, check the Pluggable Storage Sample included in the distribution package of the FIX Engine.