Chapter 37. 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.

Example 37-1. A sample ring specification in URI form


The example above requests data from the ring mydaq on the host 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:

Example 37-2. Substituting local host for the hostname in URI's

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

string ringUri(string host, string ring)
    char localHost[100];
    char domain[100];
    gethostname(localhost, sizeof(localhost));   (1)
    getdomainname(domain, sizeof(domain));       (2)
    string fqdn(localHost);
    if (host == fqdn) {
       host = "localhost";
    fqdn += domain;                              (3)
    if (host == fqdn) {
       host = "localhost";
    string uri("tcp://");
    uri   += host;
    uri   += "/";                              (4)
    uri   += ring;
    return uri;
This section of code determines the name of the host. For e.g., this will fill localhost with localhost
This section of code determines DNS domain name of the system. For domain gets filled with the string
This section of code considers the case that host is either the unqualified host name (e.g. spdaq20), or the fully qualified host name (e.g., and if so substitutes localhost to prevent the formation of a proxy ring, when the URI is used to connect to the ring
Finally this section of code constructs and returns a string containing the URI that will specify ring in the system host.

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.

Example 37-3. Including the header

#include <CRemoteAccess.h>

Example 37-4. Compiling code that uses CRingAccess

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

Example 37-5. Linking code that uses CRingAccess

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

Remote Ring Access provides reference information about this class library.