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10.4  Remote Peer Queues

As discussed in section 10.2, peer queues can be formed interactively during an attachment between the two sides. The protocol provides for the creation and closing of these queues on both sides.

The handling of data on these queues are performed by data handlers, routines which either provide data to the queue or consume data from the queue. They are triggered by the appropriate control messages, so that a data consumer handler would consume data arriving on a queue, and a data provider handler would send data onto a queue (which would be consumed on the other side). These data handlers can be user defined.

The programmer for the remote side needs to provide the remote side of the interface to the peer queue.

10.4.1  Synchronous peer queues

The implementation of a synchronous peer queue on the ECLiPSe side is shown in figure 10.2. There is an in-memory buffer, which is

Figure 10.2: ECLiPSe implementation of remote synchronous peer queue

implemented as a memory queue in ECLiPSe (i.e. queue("") option for open/3). Associated with the memory queue is the actual socket stream to the remote side. The memory queue is the queue that the user sees on the ECLiPSe side, with name Name and a unique integer id Queue, which is the stream id for the memory queue. The id is used by both sides to identify the queue, as it is unique (a symbolic name can always be reassigned), and is used in the control messages. To conform to normal ECLiPSe streams, these queues appear uni-directional to the user, i.e. data can be either written to or read from the queue. The direction is fromec if the direction of the data is from ECLiPSe to the remote side (i.e. ECLiPSe can output to the queue), and toec if the direction is from the remote side to ECLiPSe (i.e. ECLiPSe can read from the queue). The user perform normal I/O operations on the memory queue, and the protocol ensures that the data is transferred correctly to the remote side without blocking.

The socket stream is largely hidden from the user on the ECLiPSe side, with data automatically transferred between it and the buffer. The buffer is needed to ensure that when data is transferred between the two sides, the I/O operation is synchronised (the data is being read on one end of the socket while it is being written at the other), so no blocking will occur. When a side needs to initiate an I/O operation with the other side (either to write data to or read data from the other side via the socket stream), it must have control (otherwise it would not be in a position to initiate an action). Before performing the I/O operation, a control message is sent to the other side to allow it to prepare to read the data before the I/O operation is actually initiated. The memory queue thus serves to buffer the data before it is transferred to the socket stream.

The socket stream must be connected to the remote side before it could be used. Control messages are used also to synchronise the connection of the socket. The protocol does not specify if a buffer is needed on the remote side, this depends on the facilities available in the language used for the remote side.

For a particular synchronous queue, a data handler can only be defined on one side of the queue, but not both.

10.4.2  Asynchronous peer queues

I/O operations on these queues do not need to be controlled by the remote protocol, but their creation and closing is controlled by the remote protocol, so that the remote interface can keep track of these queues. These are implemented as raw socket streams on the ECLiPSe side, with I/O operations performed directly on the socket. The Name and Queue id for the stream is thus those for the ECLiPSe socket stream. It is the responsibility of the user to ensure that I/O operations do not block on these queues. They are provided to allow asynchronous transfer of data (e.g. from ECLiPSe side to the remote side without handing over control), and also they allow more efficient transfer of data.

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