• Work performed as part of on-going VMS consultancy to Australia Post.
  
  
• AP has a large distributed network of VAXes, both clusters asnd standalone.
  
  
• Interprocessor communication often required to perform tasks eg. remote queue control, log name translation, data transfer, etc.
  
  
• Traditional solution:
  - network object - shared file (cluster) - lock value block (cluster)
  
  
• Trend of the 90's: remote Procedure Calls

• Provides a program with the ability to invoke a procedure which executes on a remote node.
  
  
• Arguments and function return status
  
  
• Arguments may be modified, just like any program function call.

Normal program

sub1(x)

call sub1(x)

RPC Client Runtime call sub1(x)

RPC Server Runtime sub1(x)

• High degree of transparency, protocol handled by lower layer of software.
  
  
• Simplifies development of distributed applications.
  
  
• Access to remote data and services.
  
  
• Load sharing through application design.
  
  
• Multi-vendor support for HP-Apollo's NCS.
  
  
• DECrpc conforms to NCS 1.5, bundled with UCX (requires TCP/IP transport).
  
  
• OSF DCE support

• D-I-Y Message Exchange (MX).
  
  
• Transport is DECnet instead of TCP/IP.
  
  
• Written in Fortran and Macro, uses non-transparent task-to-task communications.
  
  
• Totally non-standard protocol, intended for in-house use to support system management utilities and programs(APSTD).
  
  
• Application program usage not supported currently although can be extended.
  
  
• Supports 2 types of calls:
  
  - Long calls
  
  - Short calls

• Function directly executed by MXserver process at remote node.
  
  
• Fast but limited to serving 1 call at a time. Must not tie up processor eg. no AST, HIB, CPU or IO intensive.
  
  
• Typically used for logical name translation.

• Program opens a session with remote node by activating MXLserver object.
  
  
• Issues several RPCs.
  
  
• Close the session.
  
  
• Process creation overhead.
  
  
• Allows multiple function invocation without delaying other callers.

• MXserver: detached process, permanently known object 133, UIC [APSTD].
  
  
• Processes short calls from client nodes.
  
  
• MXLserver: DECnet object 132, uses APSTD_SERVER account.
  
  
• Processes long calls from client nodes.
  
  
• MXSHR: shareable image containing MX runtime and user routines. Programs using MX must link against this iamge.
  
  
• MXTEST_PR: echo program for self-test.

• Client program echoes HV3's node name
  
  

        character node*6
        
        status=MX_$SHORT('HV3','MX_$ECHO',%ref(node),len(node))
  

  
  
  
• MX_$ECHO routine in MXSHR.EXE called by MXserver on HV3
  
  

        integer function MX_$ECHO(node)
        structure /node_struct/
          character s*6
        end structure
        record /node_struct/ node
        
        MX_$ECHO=sys$getsyi(...)
        node.s=...
        return
  

• Client MX runtime:
  
  - establish session with MX server on remote node.
  
  - calculate total arg size and tells MX server to allocate buffer.
  
  - send function name and args to MX server.
  
  
• MX server processing:
  
  - receives function name and args from client.
  
  - LIB$FIND_IMAGE_SYMBOL of function in MXSHR.EXE.
  
  - allocate buffer, build argument list.
  
  - LIB$CALLG arg_list.
  
  - returns status and arguments to client.
  
  
• Client MX runtime:
  
  - unpacks buffer, copy modified args and status.
  
  - close network connection.

• translate logical name and get queue information from HV8.

        ok=MX_$OPEN('HV8',chan)
        
        ok=MX_$LONG(chan,'MX_$TRNLNM',%ref('APQ'),len('APQ'),%ref('lnm$system'),len('lnm$system'),
          %ref(buf),sizeof(buf),buflen,4)
        
        if (ok) then
          ok=MX_$LONG(chan,'MX_$GETQUE',%ref(buf),blen,%ref(qrec),sizeof(qrec))
        endif
        
        call MX_$CLOSE(chan)
  

      SHORT CALL:

      status=MX_$SHORT(node,function,arg1,len1,...)

      LONG CALL:
      
      status=MX_$OPEN(node,chan)
      status=MX_$LONG(chan,function,arg1,len1,...)
      status=MX_$CLOSE(chan)
      

• Client-server authentication.
  
  
• If node name is blank or same as client node, function is executed directly by client node's runtime.
  
  
• variable length argument list (max 32).
  
  
• Must specify argument size in bytes for each argument.

• Routine must be INTEGER FUNCTION and return VMS condition code.
  
  
• All arguments passed by reference.
  
  
• Argument must be writable if modified by server node.
  
  
• Max of 32 arguments per function call.
  
  
• Server runs under APSTD_SERVER username at remote node.
  
  
• Executes with same current privileges as calling process.

• MX_$ECHO(node_name)
  
  
• MX_$GETUEA(pid,user,env,appl)
  
  
• MX_$GETJOB(queue,user,entry,buf,ctx)
  
  
• MX_$ALTJOB(entry,reque,user,action)
  
  
• MX_$GETQUE(queue,buf)
  
  
• MX_$ALTQUEUE(queue,mntform,action)
  
  
• MX_$TRNLNM(lognam,table,resnam,rlen)
  
  
• MX_$BRKTHRU(term,termlen,msg,msglen)