Chapter 39. Networked ring buffer access

The NSCL data acquisition system allows data inserted in local ring buffers to be acquired by remote systems. This is done in such a way as to minimize data transfer. The first application to request data from a remote ring contacts the remote system's RingMaster server. The RingMaster runs an instance of ringtostdout on the requested ring, with stdout redirected across the request socket. The local system sets up a proxy ring and starts up an instance of stdintoring with stdin connected to the data transmission socket. Finally the application is connected as a consumer to the proxy ring. Subsequent applications that request data from the same remote ring are simply connected to the proxy ring.

The networked ring buffer access class CRingAccess provides member functions that make this operation completely transparent to application code. Ring buffers are specified using a URI (Universal Resource Identifier), that specifies the host and remote ring name.

tcp://spdaq22.nscl.msu.edu/mydaq
        

The example above requests data from the ring mydaq on the host spdaq22.nscl.msu.edu. If the host specified is localhost the CRingAccess will connect the application directly to the specified local ring without creating a proxy ring. When attaching to a ring you should construct URIs for rings as shown below:

#include <unistd.h>
#include <string>
using namespace std;
...

string ringUri(string host, string ring)
{
    char localHost[100];
    char domain[100];
    gethostname(localhost, sizeof(localhost));   
    getdomainname(domain, sizeof(domain));       
    
    string fqdn(localHost);
    if (host == fqdn) {
       host = "localhost";
    }
    fqdn += domain;                              
    if (host == fqdn) {
       host = "localhost";
    }
    string uri("tcp://");
    uri   += host;
    uri   += "/";                              
    uri   += ring;
    return uri;
    
}
        

To use the remote access software, you must incorporate the class definitions into your software, tell the compiler how to locate the class header and link your application to the data flow library. The examples below show how to do each of these. The compiler and linker lines assume that you have defined DAQROOT to point to the top level directory of the installed NSCLDAQ.

#include <CRemoteAccess.h>
...
        

g++ -c -I$DAQROOT/include mycode.cpp
        

g++ -o myapp myapp.cpp -L$DAQROOT/lib -lDataFlow -Wl,"-rpath=$DAQROOT/lib"
        

Remote Ring Access provides reference information about this class library.