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1.2     ! takayama    1: % $OpenXM: OpenXM/doc/ascm2001p/session-management.tex,v 1.1 2001/06/19 07:32:59 noro Exp $
1.1       noro        2:
                      3: \section{Session Management}
                      4: \label{secsession}
                      5: %MEMO: key words:
                      6: %Security (ssh PAM), initial negotiation of byte order,
                      7: %mathcap, interruption, debugging window, etc.
                      8:
                      9: In this section we show our control integration in
                     10: OpenXM, which gives a dynamical and unified
                     11: method to start servers, to establish connections and to reset
                     12: computations.
                     13: %method to start servers and to establish connections.
                     14: %We assume that various clients and servers
                     15: %establish connections dynamically and communicate to each
                     16: %other. Therefore it is necessary to give a dynamical and unified
                     17: %method to start servers and to establish connections.
                     18: %In addition to that, interruption of execution and
                     19: %debugging facilities
                     20: %are necessary for interactive distributed computation.
                     21:
                     22: %\subsection{Interface of servers}
                     23: %
                     24: %A server has additional I/O streams for exchanging data between
                     25: %a client and itself other than ones for diagnostic
                     26: %messages. As the streams are for binary data,
                     27: %the byte order conversion is necessary when a
                     28: %client and a server have different byte orders. It is determined by
                     29: %exchanging the preferable byte order of each peer. If the preference
                     30: %does not coincide with each other,
                     31: %then the network byte order is used.
                     32: %This implies that all servers and clients should be able to
                     33: %handle the network byte
                     34: %order. Nevertheless it is necessary to negotiate the byte order to
                     35: %skip the byte order conversion because its cost is often dominant over
                     36: %fast networks.
                     37:
                     38: \subsection{Invocation of servers}
                     39: \label{launcher}
                     40:
                     41: Under OpenXM-RFC 100, servers are invoked as follows.
                     42: An application called {\it launcher} is invoked from a client.
                     43: The launcher sets up two communication channels: the data channel
1.2     ! takayama   44: and the control channel.
        !            45: Then it creates a server process which can communicate
1.1       noro       46: with the client via the data channel. Finally the launcher
                     47: acts as a control server and controls the server process.
                     48: As to the details of the control server see Section \ref{control}.
                     49: %As the data channel is used to exchange binary data,
                     50: %the byte order conversion is necessary when a
                     51: %client and a server have different byte orders. It is determined by
                     52: %exchanging the preferable byte order of each peer. If the preference
                     53: %does not coincide with each other,
                     54: %then the network byte order is used.
                     55: %This implies that all servers and clients should be able to
                     56: %handle the network byte
                     57: %order. Nevertheless it is necessary to negotiate the byte order to
                     58: %skip the byte order conversion because its cost is often dominant over
                     59: %fast networks.
1.2     ! takayama   60: OpenXM-RFC 101\cite{ox-rfc-101} gives a more secure and
1.1       noro       61: efficient way to start servers.
                     62: It uses ``ssh'' to launch a control server and
                     63: the control server can launch one or more than one
                     64: engines.
                     65:
                     66: \subsection{Control server}
                     67: \label{control}
                     68: %In OpenXM we adopted the following simple and robust method to
                     69: %control servers.
                     70: %
                     71: %An OpenXM server has logically two I/O channels: one for exchanging
                     72: %data for computations and the other for controlling computations. The
                     73: %control channel is used to send commands to control execution on the
                     74: %server. The launcher introduced in Section \ref{launcher}
                     75: %is used as a control process. We call such a process a {\it
                     76: %control server}. In contrast, we call a server for computation an {\it
                     77: %engine}. As the control server and the engine runs on the
                     78: %same machine, it is easy to send a signal from the control server.
                     79: %A control server is also an
                     80: %OpenXM stack machine and it accepts {\tt SM\_control\_*} commands
                     81: %to send signals to a server or to terminate a server.
                     82: %
                     83: %\subsection{Resetting an engine}
                     84:
                     85: {\tt SM\_control\_reset\_connection} sent to the control server
                     86: initiates a safe resetting of an engine.
                     87: After receiving the command, the control server sends {\tt SIGUSR1}
                     88: to the engine and the resetting procedure starts.
                     89: Figure \ref{reset} illustrates the flow of data under the OpenXM
                     90: reset protocol.
                     91: \begin{figure}[htbp]
                     92: \begin{center}
                     93: \epsfxsize=8cm
                     94: \epsffile{reset.eps}
                     95: \caption{OpenXM reset procedure}
                     96: \label{reset}
                     97: \end{center}
                     98: \end{figure}
                     99: {\tt OX\_SYNC\_BALL} is used to mark the end of data remaining in the
                    100: I/O streams. After reading it, it is assured that each stream is empty
                    101: and that the subsequent request from a client correctly
                    102: corresponds to the response from the engine.
                    103: %We note that we don't have to associate {\tt OX\_SYNC\_BALL} with
                    104: %any special action to be executed by the engine because it is
                    105: %assured that the engine is in the resetting state when it has received
                    106: %{\tt OX\_SYNC\_BALL}.
                    107:
                    108: %\subsection{Debugging facilities}
                    109: %Debugging is sometimes very hard for distributed computations.
                    110: %We provide two methods to help debugging on X window system:
                    111: %1. the diagnostic messages from the engine are displayed in a {\tt xterm}
                    112: %window;
                    113: %2. the engine can pop up a window to input debug commands.
                    114: %For example {\tt ox\_asir}, which is
                    115: %the OpenXM server of Risa/Asir, can pop up a window to input
                    116: %debug commands and the debugging similar to that on usual terminals is possible.
                    117: %One can also send {\tt SIGINT} by using {\tt SM\_control\_to\_debug\_mode}
                    118: %and it provides a similar functionality to entering the debugging
                    119: %mode from a keyboard interruption.