Custom encoders allow using SBE Encoder in the situations where default encoding scheme doesn�t work properly. For example, when a non-standard wire-level format used, or it is needed to use specific handling of normal wire-level data.

Such encoders should implement specific interface and must be created in response to specific callbacks of the OnixS::FIX::SBE::ISbeCustomCoderLibrary interface.

Where to use the customized SBE encoding	Interface to be implemented	Method of the library
A specific field that is not a FIX group.	OnixS::FIX::SBE::ISbeFieldEncoder	OnixS::FIX::SBE::ISbeCustomCoderLibrary::queryEncoder(ONIXS_FIXENGINE_UNUSED CoderOwnerId owner, ONIXS_FIXENGINE_UNUSED SbeFieldMetaData field)
Dimensions for a FIX group.	OnixS::FIX::SBE::ISbeGroupDimensionEncoder	OnixS::FIX::SBE::ISbeCustomCoderLibrary::queryGroupDimensionEncoder(ONIXS_FIXENGINE_UNUSED CoderOwnerId owner, ONIXS_FIXENGINE_UNUSED SbeFieldSetMetaData group)
A composite that is not included into FIX message (message header).	OnixS::FIX::SBE::ISbeCompositeEncoder	OnixS::FIX::SBE::ISbeCustomCoderLibrary::queryEncoder(ONIXS_FIXENGINE_UNUSED CoderOwnerId owner, ONIXS_FIXENGINE_UNUSED SbeTypeMetaData composite)

Each decoder is created for particular owner that is denoted by the owner parameter of the appropriate callback. Once created, the custom encoders must remain valid until the OnixS::FIX::SBE::ISbeCustomCoderLibrary::detach will be issued with the owner parameter equal to the one used for creating of the custom encoder.

All the methods that create custom encoders have parameter that provides metadata of the object for which the encoder is requested. It is possible to keep copy of the metadata inside of the custom encoder but it will be valid only while the owning SBE Encoder is valid also. The metadata object becomes invalid once the SBE decoder is deleted.

There are no specific requirements regarding uniqueness of the custom encoder, so it is possible to use the same custom encoder for different FIX fields or even to share it between different SBE coders. Ability of such sharing or reusing depends on the implementation.

Custom encoding of FIX field

To encode a single FIX field that is not a FIX group it is needed to implement OnixS::FIX::SBE::ISbeFieldEncoder interface and return appropriate pointer from the OnixS::FIX::SBE::ISbeCustomCoderLibrary::queryEncoder((ONIXS_FIXENGINE_UNUSED CoderOwnerId owner, ONIXS_FIXENGINE_UNUSED SbeFieldMetaData field) method.

The custom encoder will be requested by the main Encoder with the following order:

First of all the Encoder determines amount of binary data used by the custom encoder. – If the field is the fixed-size field (that is determined by its place in the SBE template: it must be the <field> node), the Encoder calls OnixS::FIX::SBE::ISbeFieldEncoder::queryFixedSize method. This method should return size of the encoded field in bytes. Early determining of the field size is required to compose a proper message or group header. Since the <data> or <group> fields are not used for such determining, this method is never called in the cases; – If the field is **<data>** field, the OnixS::FIX::SBE::ISbeFieldEncoder::queryFixedSize is not called;
The Encoder calls OnixS::FIX::SBE::ISbeFieldEncoder::encode method to perform encoding.

The field encoder should use OnixS::FIX::SBE::SbeBinaryOutputStream object to put the encoded data which values is obtained from the object of the OnixS::FIX::SBE::SbeConstFieldAccess type. Errors of the process (if issued) must be reported via exceptions derived from the std::domain_error.

The version parameter repeats data retrieved from the message header. The custom encoder should adjust its behaviour on the base of the version of the message encoded (if applicable).

Custom encoding of FIX group

To encode a FIX group dimensions it is needed to implement OnixS::FIX::SBE::ISbeGroupDimensionEncoder interface and return appropriate pointer from the OnixS::FIX::SBE::ISbeCustomCoderLibrary::queryGroupDimensionEncoder method.

Main Encoder calls only OnixS::FIX::SBE::ISbeGroupDimensionEncoder::encode method to encode group dimension data.

The group dimension encoder is applied only for the composite that encodes dimension of the particular repeating group (length of the root block, number of items, etc.). This encoder does not allow to encode the entire group, i.e. all its fields.

It is possible to use the SBE v2 RC2 extensions to the dimension encoding: the numGroups and numVarDataFields values. The need to use them depends on the specific SBE template.

Custom encoding of composite

To encode a composite that is not a part of FIX message it is needed to implement OnixS::FIX::SBE::ISbeCompositeEncoder interface and return appropriate pointer from the OnixS::FIX::SBE::ISbeCustomCoderLibrary::queryEncoder(ONIXS_FIXENGINE_UNUSED CoderOwnerId owner, ONIXS_FIXENGINE_UNUSED SbeTypeMetaData composite) method.

Such decoder is applied only for composites that are never included into the FIX message itself and SBE Encoder 'knows' how to use them. For now there is a single only such composite: the one that encodes the message header. Name of the type is mentioned in the 'headerType' attribute of the SBE template XML.

This encoder used by the OnixS::FIX::SBE::Encoder::encodeWithHeader method before encoding of the FIX message and there is particular dependency between input parameters of this method and values that arrive to the decoder:

The version input parameter and the version value that is received from OnixS::FIX::SBE::SbeSpecialCompositeInput::getMessageHeaderData are equal and repeat input value of the encodeWithHeader method;
The templateId value that is received from OnixS::FIX::SBE::SbeSpecialCompositeInput::getMessageHeaderData is equal to the input parameter of the encodeWithHeader method.