aeron-client/src/main/java/io/aeron/logbuffer/TermAppender.java
public int appendFragmentedMessage(
        final HeaderWriter header,
        final DirectBuffer bufferOne,
        final int offsetOne,
        final int lengthOne,
        final DirectBuffer bufferTwo,
        final int offsetTwo,
        final int lengthTwo,
        final int maxPayloadLength,
        final ReservedValueSupplier reservedValueSupplier,
        final int activeTermId)
    {
        final int length = lengthOne + lengthTwo;
        final int numMaxPayloads = length / maxPayloadLength;
        final int remainingPayload = length % maxPayloadLength;
        final int lastFrameLength = remainingPayload > 0 ? align(remainingPayload + HEADER_LENGTH, FRAME_ALIGNMENT) : 0;
        final int requiredLength = (numMaxPayloads * (maxPayloadLength + HEADER_LENGTH)) + lastFrameLength;
        final UnsafeBuffer termBuffer = this.termBuffer;
        final int termLength = termBuffer.capacity();

        final long rawTail = getAndAddRawTail(requiredLength);
        final int termId = termId(rawTail);
        final long termOffset = rawTail & 0xFFFF_FFFFL;

        checkTerm(activeTermId, termId);

        long resultingOffset = termOffset + requiredLength;
        if (resultingOffset > termLength)
        {
            resultingOffset = handleEndOfLogCondition(termBuffer, termOffset, header, termLength, termId);
        }
        else
        {
            int frameOffset = (int)termOffset;
            byte flags = BEGIN_FRAG_FLAG;
            int remaining = length;
            int positionOne = 0;
            int positionTwo = 0;

            do
            {
                final int bytesToWrite = Math.min(remaining, maxPayloadLength);
                final int frameLength = bytesToWrite + HEADER_LENGTH;
                final int alignedLength = align(frameLength, FRAME_ALIGNMENT);

                header.write(termBuffer, frameOffset, frameLength, termId);

                int bytesWritten = 0;
                int payloadOffset = frameOffset + HEADER_LENGTH;
                do
                {
                    final int remainingOne = lengthOne - positionOne;
                    if (remainingOne > 0)
                    {
                        final int numBytes = Math.min(bytesToWrite - bytesWritten, remainingOne);
                        termBuffer.putBytes(payloadOffset, bufferOne, offsetOne + positionOne, numBytes);

                        bytesWritten += numBytes;
                        payloadOffset += numBytes;
                        positionOne += numBytes;
                    }
                    else
                    {
                        final int numBytes = Math.min(bytesToWrite - bytesWritten, lengthTwo - positionTwo);
                        termBuffer.putBytes(payloadOffset, bufferTwo, offsetTwo + positionTwo, numBytes);

                        bytesWritten += numBytes;
                        payloadOffset += numBytes;
                        positionTwo += numBytes;
                    }
                }
                while (bytesWritten < bytesToWrite);

                if (remaining <= maxPayloadLength)
                {
                    flags |= END_FRAG_FLAG;
                }

                frameFlags(termBuffer, frameOffset, flags);

                if (null != reservedValueSupplier)
                {
                    final long reservedValue = reservedValueSupplier.get(termBuffer, frameOffset, frameLength);
                    termBuffer.putLong(frameOffset + RESERVED_VALUE_OFFSET, reservedValue, LITTLE_ENDIAN);
                }

                frameLengthOrdered(termBuffer, frameOffset, frameLength);

                flags = 0;
                frameOffset += alignedLength;
                remaining -= bytesToWrite;
            }
            while (remaining > 0);
        }

        return (int)resultingOffset;
    }
public int appendFragmentedMessage(
        final HeaderWriter header,
        final DirectBuffer bufferOne,
        final int offsetOne,
        final int lengthOne,
        final DirectBuffer bufferTwo,
        final int offsetTwo,
        final int lengthTwo,
        final int maxPayloadLength,
        final ReservedValueSupplier reservedValueSupplier,
        final int activeTermId)
    {
        final int length = lengthOne + lengthTwo;
        final int numMaxPayloads = length / maxPayloadLength;
        final int remainingPayload = length % maxPayloadLength;
        final int lastFrameLength = remainingPayload > 0 ? align(remainingPayload + HEADER_LENGTH, FRAME_ALIGNMENT) : 0;
        final int requiredLength = (numMaxPayloads * (maxPayloadLength + HEADER_LENGTH)) + lastFrameLength;
        final UnsafeBuffer termBuffer = this.termBuffer;
        final int termLength = termBuffer.capacity();

        final long rawTail = getAndAddRawTail(requiredLength);
        final int termId = termId(rawTail);
        final long termOffset = rawTail & 0xFFFF_FFFFL;

        checkTerm(activeTermId, termId);

        long resultingOffset = termOffset + requiredLength;
        if (resultingOffset > termLength)
        {
            resultingOffset = handleEndOfLogCondition(termBuffer, termOffset, header, termLength, termId);
        }
        else
        {
            int frameOffset = (int)termOffset;
            byte flags = BEGIN_FRAG_FLAG;
            int remaining = length;
            int positionOne = 0;
            int positionTwo = 0;

            do
            {
                final int bytesToWrite = Math.min(remaining, maxPayloadLength);
                final int frameLength = bytesToWrite + HEADER_LENGTH;
                final int alignedLength = align(frameLength, FRAME_ALIGNMENT);

                header.write(termBuffer, frameOffset, frameLength, termId);

                int bytesWritten = 0;
                int payloadOffset = frameOffset + HEADER_LENGTH;
                do
                {
                    final int remainingOne = lengthOne - positionOne;
                    if (remainingOne > 0)
                    {
                        final int numBytes = Math.min(bytesToWrite - bytesWritten, remainingOne);
                        termBuffer.putBytes(payloadOffset, bufferOne, offsetOne + positionOne, numBytes);

                        bytesWritten += numBytes;
                        payloadOffset += numBytes;
                        positionOne += numBytes;
                    }
                    else
                    {
                        final int numBytes = Math.min(bytesToWrite - bytesWritten, lengthTwo - positionTwo);
                        termBuffer.putBytes(payloadOffset, bufferTwo, offsetTwo + positionTwo, numBytes);

                        bytesWritten += numBytes;
                        payloadOffset += numBytes;
                        positionTwo += numBytes;
                    }
                }
                while (bytesWritten < bytesToWrite);

                if (remaining <= maxPayloadLength)
                {
                    flags |= END_FRAG_FLAG;
                }

                frameFlags(termBuffer, frameOffset, flags);

                if (null != reservedValueSupplier)
                {
                    final long reservedValue = reservedValueSupplier.get(termBuffer, frameOffset, frameLength);
                    termBuffer.putLong(frameOffset + RESERVED_VALUE_OFFSET, reservedValue, LITTLE_ENDIAN);
                }

                frameLengthOrdered(termBuffer, frameOffset, frameLength);

                flags = 0;
                frameOffset += alignedLength;
                remaining -= bytesToWrite;
            }
            while (remaining > 0);
        }

        return (int)resultingOffset;
    }
Append a fragmented message to the the term buffer.
The message will be split up into fragments of MTU length minus header.

@param header                for writing the default header.
@param bufferOne             containing the first part of the message.
@param offsetOne             at which the first part of the message begins.
@param lengthOne             of the first part of the message.
@param bufferTwo             containing the second part of the message.
@param offsetTwo             at which the second part of the message begins.
@param lengthTwo             of the second part of the message.
@param maxPayloadLength      that the message will be fragmented into.
@param reservedValueSupplier {@link ReservedValueSupplier} for the frame.
@param activeTermId          used for flow control.
@return the resulting offset of the term after the append on success otherwise  {@link #FAILED}.