aeron-client/src/main/java/io/aeron/Subscription.java
public int controlledPoll(final ControlledFragmentHandler fragmentHandler, final int fragmentLimit)
    {
        final Image[] images = this.images;
        final int length = images.length;
        int fragmentsRead = 0;

        int startingIndex = roundRobinIndex++;
        if (startingIndex >= length)
        {
            roundRobinIndex = startingIndex = 0;
        }

        for (int i = startingIndex; i < length && fragmentsRead < fragmentLimit; i++)
        {
            fragmentsRead += images[i].controlledPoll(fragmentHandler, fragmentLimit - fragmentsRead);
        }

        for (int i = 0; i < startingIndex && fragmentsRead < fragmentLimit; i++)
        {
            fragmentsRead += images[i].controlledPoll(fragmentHandler, fragmentLimit - fragmentsRead);
        }

        return fragmentsRead;
    }
public int controlledPoll(final ControlledFragmentHandler fragmentHandler, final int fragmentLimit)
    {
        final Image[] images = this.images;
        final int length = images.length;
        int fragmentsRead = 0;

        int startingIndex = roundRobinIndex++;
        if (startingIndex >= length)
        {
            roundRobinIndex = startingIndex = 0;
        }

        for (int i = startingIndex; i < length && fragmentsRead < fragmentLimit; i++)
        {
            fragmentsRead += images[i].controlledPoll(fragmentHandler, fragmentLimit - fragmentsRead);
        }

        for (int i = 0; i < startingIndex && fragmentsRead < fragmentLimit; i++)
        {
            fragmentsRead += images[i].controlledPoll(fragmentHandler, fragmentLimit - fragmentsRead);
        }

        return fragmentsRead;
    }
Poll in a controlled manner the {@link Image}s under the subscription for available message fragments.
Control is applied to fragments in the stream. If more fragments can be read on another stream
they will even if BREAK or ABORT is returned from the fragment handler.
<p>
Each fragment read will be a whole message if it is under MTU length. If larger than MTU then it will come
as a series of fragments ordered within a session.
<p>
To assemble messages that span multiple fragments then use {@link ControlledFragmentAssembler}.

@param fragmentHandler callback for handling each message fragment as it is read.
@param fragmentLimit   number of message fragments to limit when polling across multiple {@link Image}s.
@return the number of fragments received
@see ControlledFragmentHandler