ch01.htm

这个是sap开发语言abap的教育文档

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<H2><A NAME="UnderstandingBasis"><FONT SIZE=5 COLOR=#FF0000>
Understanding Basis</FONT></A></H2>
<P>
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<P>
<I>Basis</I> is like an operating system for R/3. It sits between
the ABAP/4 code and the computer's operating system. SAP likes
to call it <I>middleware</I> because it sits in the middle, between
ABAP/4 and the operating system.<P>
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<TR VALIGN=TOP><TD WIDTH=600><B>NOTE</B></TD></TR>
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<BLOCKQUOTE>
The predecessor to R/3 is R/2. R/2 is mainframe-based, and SAP ported it to the client/server environment. To do this, SAP created Basis. Creating Basis enabled the existing ABAP/4 code to run on other platforms.</BLOCKQUOTE>

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<P>
If you turn back to Figure 1.1, you can see Basis sitting between
ABAP/4 and the operating system. ABAP/4 cannot run directly on
an operating system. It requires a set of programs (collectively
called Basis) to load, interpret, and buffer its input and output.
<P>
Basis, in some respects, is like the Windows environment. Windows
starts up, and while running it provides an environment in which
Windows programs can run. Without Windows, programs written for
the Windows environment cannot run.
<P>
Basis is to ABAP/4 programs as Windows is to Windows programs.
Basis provides the runtime environment for ABAP/4 programs. Without
Basis, ABAP/4 programs cannot run. When the operator starts up
R/3, you can think of him as starting up Basis. Basis is a collection
of R/3 system programs that present you with an interface. Using
this interface the user can start ABAP/4 programs.
<P>
To install Basis, an installer runs the program <TT>r3inst</TT>
at the command-prompt level of the operating system. Like most
installs, this creates a directory structure and copies a set
of executables into it. These executables taken together as a
unit form Basis. 
<P>
To start up the R/3 system, the operator enters the <TT>startsap</TT>
command. The Basis executables start up and stay running, accepting
requests from the user to run ABAP/4 programs.
<P>
ABAP/4 programs run within the protective Basis environment; they
are not executables that run on the operating system. Instead,
Basis reads ABAP/4 code and interprets it into operating system
instructions.
<P>
ABAP/4 programs do not access operating system functions directly.
Instead, they use Basis functions to perform file I/O and display
data in windows. This level of isolation from the operating system
enables ABAP/4 programs to be ported <I>without modification</I>
to any system that supports R/3. This buffering is built right
into the ABAP/4 language itself and is actually totally transparent
to the programmer.
<P>
Basis makes ABAP/4 programs portable. The platforms that R/3 can
run on are shown in Table 1.1.<BR>
<P>
<CENTER><B>Table 1.1&nbsp;&nbsp;Platforms and Databases Supported
by R/3</B></CENTER><CENTER>
<TABLE BORDER=1>
<TR VALIGN=TOP><TD WIDTH=144><CENTER><B>Operating Systems</B></CENTER></TD>
<TD WIDTH=144><CENTER><B>Supported Hardware</B></CENTER></TD>
<TD WIDTH=144><CENTER><B>Supported Front-Ends</B></CENTER></TD>
<TD WIDTH=144><CENTER><B>Supported Databases</B></CENTER></TD>
</TR>
<TR VALIGN=TOP><TD WIDTH=144>AIX SINIX </TD><TD WIDTH=144>IBM SNI SUN</TD>
<TD WIDTH=144>Win 3.1/95/NT</TD><TD WIDTH=144>DB2 for AIX</TD>
</TR>
<TR VALIGN=TOP><TD WIDTH=144>SOLARIS HP-UX</TD><TD WIDTH=144>Digital HP</TD>
<TD WIDTH=144>OSF/Motif </TD><TD WIDTH=144>Informix-Online</TD>
</TR>
<TR VALIGN=TOP><TD WIDTH=144>Digital-UNIX</TD><TD WIDTH=144>Bull </TD><TD WIDTH=144>OS/2 
</TD><TD WIDTH=144>Oracle 7.1</TD></TR>
<TR VALIGN=TOP><TD WIDTH=144>&nbsp;</TD><TD WIDTH=144>&nbsp;</TD><TD WIDTH=144>Macintosh
</TD><TD WIDTH=144>ADABAS D</TD></TR>
<TR VALIGN=TOP><TD WIDTH=144>Windows NT</TD><TD WIDTH=144>AT&amp;T Compaq
</TD><TD WIDTH=144>Win 3.1/95/NT</TD><TD WIDTH=144>Oracle 7.1
</TD></TR>
<TR VALIGN=TOP><TD WIDTH=144>&nbsp;</TD><TD WIDTH=144>Bull/Zenith </TD><TD WIDTH=144>OSF/Motif 
</TD><TD WIDTH=144>SQL Server 6.0</TD></TR>
<TR VALIGN=TOP><TD WIDTH=144>&nbsp;</TD><TD WIDTH=144>HP (Intel) SNI</TD>
<TD WIDTH=144>OS/2</TD><TD WIDTH=144>ADABAS D</TD></TR>
<TR VALIGN=TOP><TD WIDTH=144>&nbsp;</TD><TD WIDTH=144>IBM (Intel)</TD><TD WIDTH=144>Macintosh
</TD><TD WIDTH=144>&nbsp;</TD></TR>
<TR VALIGN=TOP><TD WIDTH=144>&nbsp;</TD><TD WIDTH=144>Digital (Intel)</TD>
<TD WIDTH=144>&nbsp;</TD><TD WIDTH=144>&nbsp;</TD></TR>
<TR VALIGN=TOP><TD WIDTH=144>&nbsp;</TD><TD WIDTH=144>Data-General</TD><TD WIDTH=144>&nbsp;
</TD><TD WIDTH=144>&nbsp;</TD></TR>
<TR VALIGN=TOP><TD WIDTH=144>OS/400</TD><TD WIDTH=144>AS/400</TD><TD WIDTH=144>Win95 OS/2
</TD><TD WIDTH=144>DB2/400</TD></TR>
</TABLE>
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<P>
For example, if you write an ABAP/4 program on Digital UNIX with
an Informix database and an OSF/Motif interface, that same program
should run <I>without modification </I>on a Windows NT machine
with an Oracle database and a Windows 95 interface. Or, it could
run on an AS/400 with a DB2 database using OS/2 as the front-end.
<P>
SAP also provides a suite of tools for administering the Basis
system. These tools perform tasks such as system performance monitoring,
configuration, and system maintenance. To access the Basis administration
tools from the main menu, choose the path Tools-&gt;Administration.
<P>
Here are some examples of Basis administration tools:
<UL>
<LI>To see a list of the servers currently running in your R/3
system, choose the menu path Tools-&gt;Administration, Monitoring-&gt;System
Monitoring-&gt;Servers.
<LI>To view the current system log, choose Tools-&gt;Administration,
Monitoring-&gt;System Log.
<LI>To see system performance statistics, run transaction st03
(enter <B>/nst03</B> in the Command Field), choose This Application
Server, Last Minute Load, and analyze the last 15 minutes. Press
the Dialog button at the bottom of the screen, and you will see
the average user response time for the last 15 minutes (look at
Av. Response Time).
</UL>
<P>
Basis is designed to run in a client/server configuration.
<H2><A NAME="UnderstandingClientServer"><FONT SIZE=5 COLOR=#FF0000>
Understanding Client/Server</FONT></A></H2>
<P>
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<P>
<I>Client/server</I> is two programs talking to each other (see
Figure 1.15).
<P>
<A HREF="javascript:popUp('f1-15.gif')"><B>Figure 1.15: </B><I>The essence of client/server</I>.</A>
<P>
Here we see Program 1 asking Program 2 for some information. Program
1 is the <I>client</I> and Program 2 is the <I>server</I>. Program
2 <I>serves</I> Program 1 with the information it requested. This
is different than a main program calling a subroutine and returning.
A program that calls a subroutine transfers control to the subroutine
and cannot perform any processing until the subroutine returns
control. 
<P>
With client/server, the client and server programs are independent
processes. If the client sends a request to the server, it is
free to perform other work while waiting for the response.
<P>
Figure 1.16 shows the three standard client/server configurations.
R/3 can be tailored to run in any of these configurations.
<P>
<A HREF="javascript:popUp('f1-16.gif')"><B>Figure 1.16: </B><I>One-, two-, and three-tiered client/server
configurations</I>.</A>
<P>
When the client and server programs both run on the same computer,
the configuration is referred to as <I>single-tier</I> client/server.
(A <I>tier</I> is the boundary between two computers.) When they
run on different computers, the configuration is referred to as
<I>two-tier</I> client/server.
<P>
A program can function as both a client <I>and</I> a server if
it both requests information and replies to requests. When you
have three programs in communication, such as is shown in Figure
1.16, the configuration is called three-tier client/server. 
<P>
The client/server configuration enables the R/3 system to spread
its load across multiple computers. This provides the customer
with the ability to scale the processing power of the system up
or down by simply adding another computer to an existing configuration,
instead of replacing a single computer that performs all of the
processing, such as that which occurs in the mainframe world.
<H2><A NAME="RSystemArchitecture"><FONT SIZE=5 COLOR=#FF0000>
R/3 System Architecture</FONT></A></H2>
<P>
SAP based the architecture of R/3 on a three-tier client/server
model. The R/3 system architecture appears in Figure 1.17.
<P>
<A HREF="javascript:popUp('f1-17.gif')"><B>Figure 1.17: </B><I>The R/3 system architecture</I>.</A>
<H3><A NAME="PresentationServer">
Presentation Server</A></H3>
<P>
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<P>
The <I>presentation server </I>is actually a program named <TT>sapgui.exe</TT>.
It is usually installed on a user's workstation. To start it,
the user double-clicks on an icon on the desktop or chooses a
menu path. When started, the presentation server displays the
R/3 menus within a window. This window is commonly known as the
SAPGUI, or the user interface (or simply, <I>the interface</I>).
The interface accepts input from the user in the form of keystrokes,
mouse-clicks, and function keys, and sends these requests to the
application server to be processed. The application server sends
the results back to the SAPGUI which then formats the output for
display to the user.
<H3><A NAME="ApplicationServer">
Application Server</A></H3>
<P>
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<P>
An <I>application server </I>is a set of executables that collectively
interpret the ABAP/4 programs and manage the input and output
for them. When an application server is started, these executables
all start at the same time. When an application server is stopped,
they all shut down together. The number of processes that start
up when you bring up the application server is defined in a single
configuration file called the <I>application server profile</I>.
<P>
Each application server has a profile that specifies its characteristics
when it starts up and while it is running. For example, an application
sever profile specifies:
<UL>
<LI>Number of processes and their types
<LI>Amount of memory each process may use
<LI>Length of time a user is inactive before being automatically
logged off
</UL>
<P>
The application server exists to interpret ABAP/4 programs, and
they only run there-the programs do not run on the presentation
server. An ABAP/4 program can start an executable on the presentation
server, but an ABAP/4 program cannot execute there.
<P>
If your ABAP/4 program requests information from the database,
the application server will format the request and send it to
the database server.
<H3><A NAME="DiscoveringtheDatabaseServer">
Discovering the Database Server</A></H3>
<P>
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<P>
The <I>database server</I> is a set of executables that accept
database requests from the application server. These requests
are passed on to the RDBMS (Relation Database Management System).
The RDBMS sends the data back to the database server, which then
passes the information back to the application server. The application
server in turn passes that information to your ABAP/4 program.
<P>
There is usually a separate computer dedicated to house the database
server, and the RDBMS may run on that computer also, or may be
installed on its own computer.
<H3><A NAME="ConfiguringtheServers">
Configuring the Servers</A></H3>
<P>
During installation, the servers can be configured in four ways
(see Figure 1.18).
<P>
<A HREF="javascript:popUp('f1-18.gif')"><B>Figure 1.18: </B><I>Possible R/3 system configurations</I>.</A>
<P>
In a three-tier client/server configuration, the presentation
servers, applications servers, and database server all run on
separate machines. This is the most common configuration for large
systems, and is common in production.
<P>
In the distribution presentation configuration, the application
and database servers are combined on one computer and the presentation
servers run separately. This is used for smaller systems, and
is often seen on a development system. 
<P>
In the two-tier client/server configuration, the presentation
and application servers are combined and the database server is
separate. This configuration is used in conjunction with other
application servers. It is used for a batch server when the batch
is segregated from the online servers. A SAPGUI is installed on