NSCL DAQ Software Documentation
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Chapter 34. The RingMaster server

The NSCL data acquisition system uses single producer multi-consumer ring buffers at the core of its data flow system. These ring buffers provide a single put 'pointer' for the single producer and an array of get 'pointers' for consumers. Processes that attach to ring buffers as producers or consumers, allocate their own pointers without the benefit of a coordinating server.

These pointers are allocated by the creation of a CRingBuffer object and deallocated by the destruction of such an object. This means that if a program that is a client exits without destroying all of its CRingBuffer objects, the corresponding pointers would be lost.

The RingMaster server is a persistent server that eliminates this problem. CRingBuffer communicates with the RingMaster in such a way that the RingMaster knows which clients exist, which ring pointers they are using and when they exit, normally or abnormally.

If a client exits abnormally, the RingMaster forces the pointers left dangling to be free again.

34.1. The RingMaster Protocol

This section describes the protocol supported by the RingMaster. The RingMaster server uses the NSCL PortManager to allocate its listen port. The application name it uses is RingMaster. To connect to he RingMaster an application must first locate it using the programmatic interface to the PortManager. The starting point for PortManager is the Service Port Manager chapter.

The first time an application creates a producer or consumer client for any ring buffer, CRingBuffer connects to the RingMaster. This connection is held open for the lifetime of the client application. It is the loss of this connection that informs the RingMaster of the exit of a client. The loss of a connection triggers the RingMaster to release all ring resources held by the application that formed the link.

Construction of a CRingBuffer object registers the put/get pointer allocated with the RingMaster destruction of an object informs the RingMaster which pointer is no longer allocated.

Registration involves sending a message of the form:

Example 34-1. The CONNECT message format


CONNECT ringname connecttype pid comment

The command should be a valid Tcl list, using quoting where needed (e.g. surround the comment with {}'s if it has spaces.

ringname is the name of the ring to which the client is rattaching. connecttype identifies the type of connection being formed. This can be either the text producer for a producer connection or conumer.nnn. Where nnn is the index of the consumer pointer allocated for consumer connections.

On success, the server replies "OK\r\n. The client must be local else the server will drop the connection.

Destruction sends a DISCONNECT message The form of this messge is:

Example 34-2. Format of the DISCONNECT message


 DISCONNECT   ringname connecttype pid

The ringname is the name of the ring buffer, connecttype is the connection type passed in the CONNECT command that is being 'undone'. The pid argument is similarly the pid that was used in the CONNECT commnd being undone.

On success, the server replies with "OK\r\n". If the client is not from localhost, the connection is immediatel closed.

Clients can also probe ring buffer usage. This is done via a command of the form

Example 34-3. Format of the LIST command


LIST

This message is acceptable from both local and remote hosts. It is used to inquire about rings and ring usage. Note that only rings that have producer or consumer clients will be reported. The reply from this will be a line that reads OK followed by a properly formatte Tcl list of ring information.

Each element of the list reports on the usage of a single ring. Each list element is a pair. The first element of the pair is the ring name. The second element, the ring usage as returned from the Tcl ring package's ringbuffer usage command.

This is returned as a Tcl list that has the following elements:

size

The number of data bytes in the ring buffer.

putAvailable

The number of bytes that can be put by the producer without blocking. Note that in order to be able to distinguish between the full and empty ring buffer cases, this can never be larger than size - 1

maxConsumers

Returns the maximum number of clients that can connect to the ring buffer at any given time.

producer

The PID of the producer process. This is -1 if there is no producer.

maxGet

The amount of data that can be gotten without blocking by the consumer that is most caught up. (maximum that can be gotten by any consumer). If no consumers are connected, this returns 0.

minGet

Same as for maxGet but gives the amount of data that can be gotten by the consumer that is least caught up.

consumers

This is a list of consumers attached to the ring buffer. Each consumer is represented by a pair of values. The first, the consumer's pocess ID. The second, the amount of data that consumer could get without blocking.

The RingMaster process keeps track of all rings on the host system. When it starts, it enumerates the rings that have persisted since the last time it ran. When new rings are created, a REGISTER message informs the RingMaster of that fact. This message is transparently sent by the CRingBuffer::create call.

Example 34-4. Format of the REGISTER command


REGISTER newRingName

When a ring is destroyed, the CRingBuffer::remove member function transparently contacts the RingMaster process and sends it a UNREGISTER message so that the server forgets about the existence of the ring.

Example 34-5. Format of the UNREGISTER message


UNREGISTER destroyedRingName

The RingMaster allows applications on remote systems to get data from the ring bufffers it manages. The CRingAccess class is the preferred method to gain access to remote data. It ensures that data are not repetitively sent if several consumers from the same remote host want data from a ring buffer. It does this by creating a proxy ring to which all consumers attach and arranging for data from the ring to be copied to the proxy ring.

The REMOTE command requests data from a ring. If successful, the RingMaster replies: OK BINARY FOLLOWS. After that message, the client process should no longer use that socket to send data to the ring master. Instead, the RingMaster will arrange for all data submitted to the target ring to be transmitted to the client on that socket.

Example 34-6. Format of the REMOTE message


REMOTE sourceRingName

Once the socket to the ring master has become a data transfer socket:

  1. No further commands can be sent to the RingMaster.

  2. The ring master will have treated a request for REMOTE data to be identical to a disconnection of the socket, releasing any resources that were allocated via that socket.

I recommend that if you are requesting remote data, you open a separate socket to the ring master and only use it for that purpose. I further recommend, that you use the CRingAccess class to do this rather than directly interacting with the ring master, or rather than interacting via the CRingMaster class.

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