Introducing Collage: DataI/OStream

The co::DataOStream and co::DataIStream form the core of the co::Object data distribution. They will gain even more importance in the next couple of weeks, when they will replace the current packet-based messaging (see 145). They will become the core of any communication between Collage nodes.

The data iostreams provide a std::iostream-like interface to send data over the network. They hide all the network connection details, allow overlapping of data serialization and sending through bucketization, do configurable compression and allow application-provided serializers for custom data types. We’re currently working on extending them to also do automatic endian conversion.

First of all they allow object serialization without a need of the application to know if the data has to be saved for later used (buffered objects), who will receive the data, whether or not to use multicast or how to compress it. The serialization in the application code is as simple as possible, here for example the eqPly::FrameData:

void FrameData::serialize( co::DataOStream& os, const uint64_t dirtyBits )       
    co::Serializable::serialize( os, dirtyBits );                                
    if( dirtyBits & DIRTY_CAMERA )                                               
        os << _position << _rotation << _modelRotation;                          
    if( dirtyBits & DIRTY_FLAGS )                                                
        os << _modelID << _renderMode << _colorMode << _quality << _ortho        
           << _statistics << _help << _wireframe << _pilotMode << _idle          
           << _compression;                                                      
    if( dirtyBits & DIRTY_VIEW )                                                 
        os << _currentViewID;                                                    
    if( dirtyBits & DIRTY_MESSAGE )                                              
        os << _message;                                                          

The deserialize method looks exactly the same, except using a DataIStream and the >> operator instead of <<. Applications can write free-standing serialization functions, similar to free-standing std::iostream operators.

Behind the scene Collage sets up the ostream with all connections to the nodes which have slave instance of the FrameData, preferring multicast connections. The output is bucketized, that is, whenever the accumulated data reaches a certain threshold (default ~64k), the current block is sent. This allows the OS to send data while the application prepares the next buffer.

Each DataOStream has a configurable compressor. The default algorithm uses heuristics to choose the best tradeoff between speed and compression ratio. Each outgoing packet is compressed before transmission. For buffered objects, the compressed data is retained to optimize memory usage.

Currently we are working on automatic endian conversion to form Collage networks between little and big endian hosts (see 146). Since most of the data types are known at deserialization time, a templated swap function will provide the endianness conversion. For void data we simply have to assume that it is already endian safe, or that the application doesn’t need endian safeness. In Collage and Equalizer we will make sure everything is endian-safe, e.g., by using portable boost serialization archives for the co::DataOStreamArchive.


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