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1.10    ! noro        1: % $OpenXM: OpenXM/doc/issac2000/session-management.tex,v 1.9 2000/01/16 06:39:39 takayama Exp $
1.2       takayama    2:
1.3       noro        3: \section{Session Management}
1.4       noro        4: \label{secsession}
1.3       noro        5: %MEMO: key words:
                      6: %Security (ssh PAM), initial negotiation of byte order,
                      7: %mathcap, interruption, debugging window, etc.
                      8:
1.8       noro        9: In this section we explain our control integration in
                     10: OpenXM.  We assume that various clients and servers
1.3       noro       11: establish connections dynamically and communicate to each
1.8       noro       12: other. Therefore it is necessary to give a dynamical and unified
                     13: method to start servers and to establish connections.
1.10    ! noro       14: In addition to that, interruption of executions and
        !            15: debugging facilities
        !            16: are necessary for interactive distributed computation.
1.8       noro       17:
                     18: %\subsection{Interface of servers}
                     19: %
                     20: %A server has additional I/O streams for exchanging data between
                     21: %a client and itself other than ones for diagnostic
                     22: %messages. As the streams are for binary data,
                     23: %the byte order conversion is necessary when a
                     24: %client and a server have different byte orders. It is determined by
                     25: %exchanging the preferable byte order of each peer. If the preference
                     26: %does not coincide with each other,
                     27: %then the network byte order is used.
                     28: %This implies that all servers and clients should be able to
                     29: %handle the network byte
                     30: %order. Nevertheless it is necessary to negotiate the byte order to
                     31: %skip the byte order conversion because its cost is often dominant over
                     32: %fast networks.
1.3       noro       33:
                     34: \subsection{Invocation of servers}
                     35: \label{launcher}
                     36:
1.8       noro       37: An application called {\it launcher} is provided to start servers
                     38: and to establish connections as follows.
1.3       noro       39:
                     40: \begin{enumerate}
1.8       noro       41: \item A launcher is invoked from a client.
                     42: When the launcher is invoked, the client
                     43: informs the launcher of a port number for TCP/IP connection
                     44: and the name of a server.
1.4       noro       45: \item The launcher and the client establish a connection with the
1.10    ! noro       46: specified port number. One time password may be used to prevent
        !            47: launcher spoofing.
1.8       noro       48: \item The launcher creates a process and executes the server after
                     49: setting the data channel appropriately.
1.3       noro       50: \end{enumerate}
                     51:
1.7       noro       52: After finishing the above task as a launcher, the launcher process
1.3       noro       53: acts as a control server and controls the server process created by
1.10    ! noro       54: itself. As to the control server see Section \ref{control}.
1.3       noro       55:
1.8       noro       56: As the data channel is used to exchange binary data,
                     57: the byte order conversion is necessary when a
                     58: client and a server have different byte orders. It is determined by
                     59: exchanging the preferable byte order of each peer. If the preference
                     60: does not coincide with each other,
                     61: then the network byte order is used.
                     62: This implies that all servers and clients should be able to
                     63: handle the network byte
                     64: order. Nevertheless it is necessary to negotiate the byte order to
                     65: skip the byte order conversion because its cost is often dominant over
                     66: fast networks.
                     67:
                     68:
1.3       noro       69: \subsection{Control server}
                     70: \label{control}
1.8       noro       71: In OpenXM we adopted the following simple and robust method to
                     72: control servers.
1.3       noro       73:
                     74: An OpenXM server has logically two I/O channels: one for exchanging
1.4       noro       75: data for computations and the other for controlling computations. The
1.3       noro       76: control channel is used to send commands to control execution on the
1.6       takayama   77: server. The launcher introduced in Section \ref{launcher}
1.8       noro       78: is used as a control process. We call such a process a {\it
                     79: control server}. In contrast, we call a server for computation an {\it
                     80: engine}. As the control server and the engine runs on the
                     81: same machine, it is easy to manipulate the engine, especially to
1.3       noro       82: send a signal from the control server. A control server is also an
1.8       noro       83: OpenXM stack machine and it accepts {\tt SM\_control\_*} commands
1.6       takayama   84: to send signals to a server or to terminate a server.
1.3       noro       85:
1.8       noro       86: \subsection{Resetting a server}
1.3       noro       87:
1.8       noro       88: A client can send a signal to an engine by using the control channel
                     89: at any time. However, I/O operations are usually buffered,
1.10    ! noro       90: which may cause troubles.
        !            91: To reset a server safely the following are required.
1.3       noro       92:
                     93: \begin{enumerate}
1.10    ! noro       94: \item Any OX message must be a synchronized object in the sense of Java.
1.3       noro       95:
                     96: As an {\tt OX} message is sent as a combination of several {\tt CMO}
1.10    ! noro       97: data, a global exit without sending all may generate broken data.
1.3       noro       98:
                     99: \item After restarting a server, a request from a client
                    100: must correctly corresponds to the response from the server.
                    101:
                    102: An incorrect correspondence occurs if some data remain on the stream
                    103: after restarting a server.
                    104: \end{enumerate}
                    105:
                    106: {\tt SM\_control\_reset\_connection} is an {\tt SM} command to
1.10    ! noro      107: initiate a safe resetting of a server.
        !           108: Under the OpenXM reset protocol a server and a client act as follows.
1.3       noro      109:
1.6       takayama  110: \centerline{\fbox{client}}
1.3       noro      111:
                    112: \begin{enumerate}
                    113: \item The client sends {\tt SM\_control\_reset\_connection} to the
1.7       noro      114: control server. The control server sends {\tt SIGUSR1} to the engine.
1.8       noro      115: \item The client enters the resetting state. It skips all {\tt
1.4       noro      116: OX} messages from the engine until it receives {\tt OX\_SYNC\_BALL}.
1.3       noro      117: \item After receiving {\tt OX\_SYNC\_BALL} the client sends
                    118: {\tt OX\_SYNC\_BALL} to the engine and returns to the usual state.
                    119: \end{enumerate}
                    120:
1.6       takayama  121: \centerline{\fbox{engine}}
1.3       noro      122:
                    123: \begin{enumerate}
                    124: \item After receiving {\tt SIGUSR1} from the control server,
                    125: the engine enters the resetting state.
1.8       noro      126: \item The engine sends {\tt OX\_SYNC\_BALL} to the client.
                    127: We note that the operation does not block because
1.3       noro      128: the client reads and skips {\tt OX} messages soon after sending
                    129: {\tt SM\_control\_reset\_connection}.
                    130: \item The engine skips all {\tt OX} messages from the engine until it
1.4       noro      131: receives {\tt OX\_SYNC\_BALL}.
1.3       noro      132: \item After receiving {\tt OX\_SYNC\_BALL} the engine returns to the
                    133: usual state.
                    134: \end{enumerate}
                    135:
1.8       noro      136: {\tt OX\_SYNC\_BALL} is used to mark the end of data remaining in the
                    137: I/O streams. After reading it, it is assured that each stream is empty
1.7       noro      138: and that the subsequent request from a client correctly
                    139: corresponds to the response from the server.
                    140: We note that we don't have to associate {\tt OX\_SYNC\_BALL} with
1.6       takayama  141: any special action to be executed by the server because it is
1.8       noro      142: assured that the peer is in the resetting state when one has received
1.6       takayama  143: {\tt OX\_SYNC\_BALL}.
1.3       noro      144:
1.10    ! noro      145: \subsection{Debugging facilities}
1.8       noro      146: Debugging is not easy for distributed computations.
1.10    ! noro      147: We provide two methods to help debugging.
        !           148: If servers are executed on X window system,
        !           149: the diagnostic messages from the engine are displayed in a {\tt xterm}
        !           150: window.
1.6       takayama  151: Furthermore, if the engine provides an interface to input commands,
1.10    ! noro      152: then each engine can pop up a debug window.
        !           153: For example {\tt ox\_asir}, which is
1.6       takayama  154: the OpenXM server of {\tt Risa/Asir}, can pop up a window to input
                    155: debug commands and the debugging similar to that on usual terminals is possible.
1.7       noro      156: One can also send {\tt SIGINT} by using {\tt SM\_control\_to\_debug\_mode}
1.4       noro      157: and it provides a similar functionality to entering the debugging
1.3       noro      158: mode from a keyboard interruption.