How to Develop Web Services in WebASPart 2:
Design your own Web services
with ABAP HTTP Extensions
By Axel Angeli, logosworld.com
Editor's Note: To be honest, I didn't really understand the Web Application Server as well as I
wanted to—until I read Axel's SAPtips white paper on the subject. Axel has a knack for making
sense of SAP®'s solutions and helping others to understand their value. He believes that SAP's
technological evolution is part of the overall trend towards open platforms and universal
standards. What some SAP users may not realize is that the “future is now”—SAP has already
reached the point where it can provide support for basic, integrated Web services initiatives. But
stepping into SAP Web services does require mastery of new buzzwords like SOAP and WDSL.
And, some key decisions about development environments must also be made. These choices
may depend on which version of SAP is in use, which messaging systems are running, and many
other factors. In this excellent how-to article, Axel digs right into all these issues. Building on his
last piece, Axel starts with a basic introduction to Web services technologies, and then he takes a
closer look at how basic Web services can be designed using WebAS, either with Business
Server Pages or with HTTP extensions. Because this technology is still emerging, there are some
judgment calls that must be made, such as whether to use traditional RFC/SAP HTTP Protocol,
or WebAS's new ABAP Objects/HTTP classes. If this kind of techie talk peaks your interest,
you're going to love Axel's opinionated, bird's eye view of SAP's Web services technologies.
Chock full of actual coding examples, this article gives the real scoop on the technical capabilities
SAP has to offer—capabilities that anyone who is looking to upgrade to the latest versions of SAP
will want (and need) to know about.
Technical Overview: The first part of this white paper1 uncovered the technical landscape of
SAP’s WebAS and outlined how to develop classical server pages through the Business Server
Pages (BSP). Web services are the recognized technology that provides automated services via
the Internet. Given that fact, and given that a majority of international companies have SAP as
their ERP system, it is a natural approach to make use of SAP itself to provide Web services. In
this article, we’ll start by guiding you through an introduction to Web services and emerging
protocols like SOAP and WDSL. Then, we'll discuss how Web services can be implemented
using WebAS, either by means of Business Server Pages, or through SAP’s adaptation of the
Java Servlet technology by means of the HTTP extensions super class.
A note on public XML services: The examples for Web services in this document make use
of the public services provided by Amazon.com. The developer key in the examples has
been invalidated. The examples will still work, but we kindly ask you to register for your
own developer token at http://amazon.com/xml, in order to comply with their code of
conduct and as a gesture to proper “netiquette.”
1
Part one of this series is available for SAPtips subscribers in the SAPtips document library. The
article is titled "How to Develop Web Services in WebAS – Part 1," by Axel Angeli.
Copyright © 2003 by Klee Associates, Inc.
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How to Develop Web Services in WebASPart 2:
Design your own Web services
with ABAP HTTP Extensions
Consulting Web Services from SAP and Anywhere
Accessing SAP from Any Web Server
Nowadays, the SAP ERP suite is the omnipresent software solution for enterprise computing.
Many companies rely on SAP to manage their important finance and asset management issues,
as well as their sales and distribution process, just to name a couple of the functional areas that
SAP encompasses. A common—and definitely not very new—desire of users of information
technology is to access the data stored in a centralized ERP system from anywhere in the world;
especially now, since we have a feature-rich Internet browser client stored on every computer,
including small pervasive devices like the palm-top computers. Under this premise, it appears as
a natural choice to access your centralized SAP system via HTTP, the protocol your browser
uses. Once you start talking about managing business processes remotely via the Internet, you're
talking about the emerging technology that we call “Web services.”
Accessing SAP ERP via HTTP
HTTP Client Application
(can be another R/3 system, too)
HTTP
Somewhere in the Internet/Intranet
SAP WebAS ICM
Internet Communication Manager
COM
Any HTTP
Server
HTTP Communication Interface
Understands both HTTP and RFC
RFC
RFC
RFC Function Modules
BAPI
SAP R/3
Figure 1: Accessing an ERP System HTTP
Web services are designed to be accessed from another program, although they can generally be
consulted from a Web browser as well. The calling program can be any program that understands
HTTP, including SAP R/3 starting with release 3.0E. Before we go into details, let’s first learn
more about Web services by example.
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How to Develop Web Services in WebASPart 2:
Design your own Web services
with ABAP HTTP Extensions
Example of a Web Service
Let's look at what a typical Web service might look like. The principle of our example is very
simple:
•
You send an HTTP request to the Web service provider.
•
You receive an XML document with the result.
To visualize this, we shall have a look at Amazon.com, which allows XML-based inquiries to their
bookstore through automated Web services. You can test this easily by entering the HTTP
request shown in Figure 2 in your Web browser:
http://xml.amazon.com/onca/xml2?t=webservices-20&tag=logosworldcom&devt=D2H30000000000&AsinSearch=3528057297&type=lite&f=xml.
Figure 2: Calling the Amazon.com Web Service
To make the request string a little better readable, we insert some line breaks after each part of
the URL (Figure 3). However, the URL above must be entered as one string without any spaces
as shown in Figure 2.
http://xml.amazon.com/onca/xml2
?t=webservices-20
&tag=logosworldcom
&dev-t=D2H30000000000
&AsinSearch=3528057297
&type=lite&f=xml.
Figure 3: The Amazon.com Web Service Request URL Formatted for Better Readability
This request (which asks Amazon.com to list the books authored by someone in my company,
logosworld.com), returns a plain XML document with the requested information included. See
Figure 4 below.
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How to Develop Web Services in WebASPart 2:
Design your own Web services
with ABAP HTTP Extensions
<?xml version="1.0" encoding="UTF-8"?>
<ProductInfo
xmlns:xsi=http://www.w3.org/2001/XMLSchema-instance
xsi:noNamespaceSchemaLocation
="http://xml.amazon.com/schemas2/dev-lite.xsd">
<Details>
<Asin>3528057297</Asin>
<ProductName>The SAP R/3 Guide to EDI and
Interfaces</ProductName>
<Authors>
<Author>Axel Angeli</Author>
<Author>Ulrich Streit</Author>
<Author>Robi Gonfalonieri</Author>
</Authors>
<Manufacturer>Friedrick Vieweg & Son</Manufacturer>
<OurPrice>$50.95</OurPrice>
</Details>
</ProductInfo>
Tree representation of above XML
ProductInfo
└─ Details
├─ Asin
│ └─ 3528057297
├─ ProductName
│
└─ The SAP R/3 Guide to EDI and Interfaces
├─ Authors
│ └─ Author
│
├─ Axel Angeli
│
├─ Ulrich Streit
│
└─ Robi Gonfalonieri
└─ OurPrice
└─ $50.95
Figure 4: XML-Response from Amazon.com
About URLs, URIs and Points of Content
In the theory of electronic communication, the partners at either end of a communication line are
called end points. End points are also known as terminals, but this is a very ambiguous
expression, so we better avoid it. Typically, a communication consists of two partners. At one end
is a client, or requester, and at the other end is a point of content located on a server. In simple
cases, the point of content is a file.
In order to locate a point of content, the requester needs to specify sufficient information about
the requested content, so that the Internet framework can route the request to a listener that runs
on a server and is able to retrieve the content from the point of content.
Typically, the requester specifies three items:
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How to Develop Web Services in WebASPart 2:
Design your own Web services
with ABAP HTTP Extensions
•
The public name of the server and the domain; alternately, it can specify the server’s unique
IP address.
•
The name of the resource in a format that it is understood by the listening port on the server
•
Optionally, parameters to pass further details about the request are provided.
The assembly of these three information items is called a URI – Uniform Resource Indicator. In
cases of Web pages, they are also known as URL – Uniform Resource Locator, where a URL is
generally recognized as a subset or special variant of a URI. So we can say that a reference to a
general point of content within the Internet is called a URI, a Uniform Resource Identifier, while
URL is reserved to address a Web page. Figure 5 shows how a URI is constructed.
Traditionally, a URL references a file on a specific server. Nowadays a URL may point to any kind
of electronic resource that can be reached via HTTP. In these cases, the Web server captures
the URL, and instead of returning a simple file, it calls a program to return a computed result.
Parts of a URI
Resource type
Resource address
Resource parameters
r
ete
me
me
a
m
a
n
ol
ara
in
rN
file
toc erve
I-P
ma
th&
o
G
o
r
a
C
p
D
P
S
http: //www.
logosworld.de /asp/testdrive.htm ?user=micky&pass=mausi
Resource address
This is an identifying name of the requested ressource. Usually, this is simply the
filename of the requested document, but it can be any string that is understood
by the server. Usually the server tells from the suffix (e.g., .htm, .txt, .asp, .php,
.cgi).
Ressource type
Tells the server which protocol to use. There are a wide variety of protocols. The
most important protocols are the following ones:
http:
HyperText Transfer Protocol
Tells the server that the requester (i.e., the browser) wants to receive
general Unicode-coded documents (e.g., HTML).
ftp:
File Transfer Protocol
Protocol specially for file transfers.
smtp: Simple Mail Transfer Protocol
Most important protocol for eMail distribution.
Figure 5: Elements of a URI
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How to Develop Web Services in WebASPart 2:
Design your own Web services
with ABAP HTTP Extensions
Sending Parameters via HTTP
In the previous example (Figure 2), we sent all the information that is needed by the server to
build the proper result in the browser line. We did this by adding the parameters behind the actual
URL. We could also have sent the parameters as an attachment to the HTTP request within the
HTTP document body. The request for a search at Amazon.com is a relatively simply one. That is
why we could pass all the search parameters as part of the requesting URI string. Issuing an
HTTP request without sending any additional data is known as an “HTTP Get request”, and that
is what we did. The form we chose to pass the parameters, i.e., by separating parameters with
ampersands (&), is called a canonical URL. The history of this parameter-passing convention
goes back to the long established CGI protocol2.
A Detailed View on Web Services
There is no precise, or even an official definition of “Web service.” Roughly speaking, Web
services are programs that can be accessed via an Internet connection. Web services are
typically comprised of self-service applications and automated workflows. Consider this typical
self-service application: picture an order tracking service where a company provides an interface
through the Internet that lets a client query the actual status of an order. A typical process flow
may involve the following steps:
1. order receipt
2. production start and end
3. packing
4. shipping
Such tracking systems are already established by many shipping companies. In this case,
imagine that deliveries are transported from Sweden down to the Frankfurt airport, then flown
“over the pond” to Chicago, and transported down to Kansas on a train. This obviously takes
time, and in the course of order fulfillment, the commodities are passed through several hands.
Given that there are so many uncertainties during the order flow, both the customer and vendor
have a vested interest in regularly inspecting the status and location of each shipment. Many
shipping companies offer these kinds of services already through the Internet, where the
authorized parties can check the status and location of an order in process.
As a natural extension, this “online tracking” can be formalized as a Web service by
presenting the data in a computer-readable format that can be easily interpreted by a
client program. Furthermore, the individual “legs”, or processing points, can report their
status to the same Web service interface, so that the data no longer needs to be passed
via paper notes, faxes, and manual re-entry. Via a basic Web service, this crucial
information is now available in “quasi real-time”.
Of course, you cannot simply load a program on your Web server and wait for anyone to call it up
from anywhere within the reach of your network. Because Web services are designed to smoothly
cooperate with other programs, they must abide by some guidelines:
1. Web services need to be easily accessible without any restrictive documentation.
2
If readers are looking for more explanation of the technical terms involved in Web services, we
recommend taking a look at http://whatis.com, delivered by Techtarget.com
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How to Develop Web Services in WebASPart 2:
Design your own Web services
with ABAP HTTP Extensions
One great feature of the “human readable XML format” is that it usually allows an easy discovery
of its contents and the document structure without the need for external documentation. As a
point of comparison, many EDI projects failed primarily because the chosen formats were
proprietary and had been disclosed only to a pre-selected, exclusive audience (E.G. EDIFACT
documentation is only accessible by paying dues to the organization).
2. Web services should be easily adaptable to a range of clients and operating systems.
The “Alpha and Omega” of Web services is “compatibility through convention”. No matter the
content, Web services look largely the same. With the use of XML, it is possible to parse
documents for necessary information, instead of mapping to a strict, inflexible format. Given this
flexibility, a handler program can react smoothly on changes and additions in the sent documents.
Characteristics of a Web Service
From what is offered as Web services today, we can generally characterize Web services as
having the following attributes:
•
Web services are invoked via an HTTP request.
HTTP and the sister protocols—like the secure HTTPS option—are the undisputed standard as
Internet protocol, and the only Internet protocol that is understood by all modern computers.
•
Web services deliver their results as an XML data stream.
XML has become the de facto standard for Internet data exchange. XML documents have
become so ubiquitous that it is an easy bet to say that XML will be the exclusive data exchange
format for any kind of online, automated document-distribution system. Many document
processing applications are transitioning from proprietary formats to storing data as XML data, or
adding XML interfaces for external programs, e.g., the next version of Microsoft Office (Word,
PowerPoint, etc.) will be XML-enabled, as will Adobe Framemaker. It is already obvious that the
EDI world will soon be dominated by XML. Sooner or later all those acknowledged EDI standards
like EDIFACT/UN or ANSI X.12 will go the way of the dinosaurs.
•
Web services results are structured data—data that can be easily interpreted by another
program.
This is an important characteristic that defines the essential difference between a classic Web
page and a Web service. Web services are designed to be consulted primarily by other programs.
Due to the text-based nature of XML documents, the results are naturally readable as well—
another helpful XML feature. However, the response data must provide sufficient information so
that a program can formally derive the requested information with as little effort as possible.
•
Web service results are either called by:
•
passing the requested data to an agent as an XML document, or
•
passing parameters as part of the URI, using the canonical URL as introduced by the CGI.
(We'll elaborate on canonical URLs on the next page.)
Web services via the SOAP-XML Standard
As the request itself becomes more complicated, the URI(URL) used to issue the request would
become lengthy. Therefore, the request can also be handed over to HTTP as a data block; this is
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How to Develop Web Services in WebASPart 2:
Design your own Web services
with ABAP HTTP Extensions
then called an “HTTP Post request”. There are many uses of HTTP Post requests, e.g., an HTTP
Post request is sent to a server if you fill out an online HTML form in your browser and submit the
form data. Amazon.com accepts requests from an HTTP Post as well and, in this case, it uses
the SOAP protocol standard.
SOAP is the acronym for the Simple Object Access Protocol and is itself an XML document. In
addition to having the same features as a plain XML document, SOAP defines restrictions on
how the document must be structured and how the XML tags have to be named. SOAP was
designed out of the simple need to provide a standard format for the multiple ways you could
structure a response with respect to naming your tags and laying out your response tree. SOAP
has been designed to be used out of programs; therefore it is not that easy to test this variant in
your browser, but in Figure 6, we show what a SOAP request might look like.
<SOAP-ENV:Envelope>
<SOAP-ENV:Body>
<AsinSearchRequest>
<AsinSearchRequest>
<asin>3528057297</asin>
<tag>logosworldcom</tag>
<type>lite</type>
<dev-tag>12345678901234</dev-tag>
</AsinSearchRequest>
</typens:AsinSearchRequest>
</SOAP-ENV:Body>
</SOAP-ENV:Envelope>
Tree representation of above XML
AsinSearchRequest
├─ asin
│ └─ 3528057297
├─ tag
│
└─ logosworldcom
├─ type
│
└─ lite
└─ dev-tag
└─ 12345678901234
Figure 6: SOAP Request for Amazon.com
SAP as Web Client
So far, we have seen how a Web service works in principle. The typical client of a Web service,
however, is not an individual user, but another program, e.g., a program within SAP R/3. Let's
take a closer look at how Web services function within various SAP releases.
While the WebAS (SAP 4.7 and higher) can act generically as an HTTP client, the older releases
make use of an SAP-provided utility called SAPHTTP, which is found both in the “/bin/exe”
directory of the SAP application server (“/usr/sap/SID/SYS/bin/exe”) and as “SAPHTTP.EXE” as
part of the SAPGUI installation package. Before you can use SAPHTTP, a valid RFC destination
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How to Develop Web Services in WebASPart 2:
Design your own Web services
with ABAP HTTP Extensions
must be defined with transaction SM59 in SAP (Figure 8), pointing to SAPHTTP either on the
server or on the PC that has SAPGUI on. In the latter case, SAPHTTP will be called via the
SAPGUI on the workstation.
Using SAPHTTP, you call the following function modules:
•
function HTTP_GET, and
•
function HTTP_POST
with the destination of SAPHTTP. See Figure 7 for an example of accessing Web services in
SAP, using our Amazon.com book query example.
DATA: ABSOLUTE_URI(128) type c.
data: response_headers(80) occurs 0 with header line.
data: RESPONSE_ENTITY_BODY(120) occurs 0 with header line.
ABSOLUTE_URI =
'http://xml.amazon.com/onca/xml2?t=webservices-20' &
'&tag=logosworldcom&dev-t=D2H30000000000' &
'&AsinSearch=3528057297&type=lite&f=xml'.
CALL FUNCTION 'HTTP_GET'
EXPORTING
ABSOLUTE_URI
= ABSOLUTE_URI
RFC_DESTINATION
= 'SAPHTTPA'
PROXY
= '192.168.69.64:8080'
TABLES
* REQUEST_ENTITY_BODY
=
RESPONSE_ENTITY_BODY = RESPONSE_ENTITY_BODY
RESPONSE_HEADERS
= RESPONSE_HEADERS
* REQUEST_HEADERS =
EXCEPTIONS
OTHERS
= 8.
LOOP AT response_entity_body.
WRITE: / response_entity_body.
ENDLOOP.
Figure 7: SAP as Web Service Client with ABAP via SAPHTTP Utility
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How to Develop Web Services in WebASPart 2:
Design your own Web services
with ABAP HTTP Extensions
Figure 8: Definition of RFC Destination SAPHTTP in Transaction SM59
Calling a Web Services with the SAP WebAS HTTP Extension Class
WebAS, with its generic HTTP interface, proposes to use the IF_HTTP_CLIENT interface class
instead. Standard SAP WebAS distribution includes the report SXSLT_TEST, whose form
LOAD_HTTP gives an example of how to request an HTTP page. In Figure 9 we constructed
another example to demonstrate the essential.
Intermezzo (and key SAP RFC tip):
I recommend continuing to use the traditional method via the RFC destination to SAPHTTP
whenever possible. Not only will it keep your development backward-compatible with older SAP
releases, but the new access method, via the ABAP Object syntax, is still in a state of evolution,
and cannot currently satisfy the needs of a modern developer. Unfortunately, ABAP Workbench,
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How to Develop Web Services in WebASPart 2:
Design your own Web services
with ABAP HTTP Extensions
the traditional SAP development environment, only has very basic support for ABAP/Objects. The
core tool of every modern development workbench, the debugger, does not allow the display of
essential information about ABAP/Objects and the current setting of attributes. The ABAP editor
does also not support ABAP/Objects in a convenient way. Editing aids, like Intellisense, which
displays methods and attributes for an object while you type, are either completely missing or
awkward to use. With respect to the HTTP classes that ship with WebAS, it must be said that a
large part of the coding is realized on the kernel level, which makes it nearly impossible to debug
there. I am aware that a large community of developers, who dedicate their time, ambition, and
professional obsession to object oriented development will heavily object, but from an
experienced software architect’s and software auditor’s point of view, this way in which HTTP
services are currently implemented in WebAS cannot be endorsed at this time.
The only advantage of the HTTP classes over the traditional SAPHTTP is the direct
implementation of all HTTP features, as opposed to SAPHTTP, which basically does HTTP
forwarding to your proxy. But when you take into account that real life implementations of Web
clients from SAP will forcibly access the Internet via a proxy anyway, this is not much of an
advantage. Therefore, I decidedly state that, as of this writing, the best practice is to ignore
the HTTP classes of WebAS, while continuing to use SAPHTTP for outbound Internet
access and BSP for inbound HTTP calls. I will keep an eye on the evolving ABAP/Objects
and HTTP classes of WebAS, and at the point where these new technologies surpass the
traditional SAP HTTP protocols, I will author an update to SAPtips readers. For a detailed
look at the WebAS HTTP client classes, see Figure 9 below, which submits a query to
Amazon.com.
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How to Develop Web Services in WebASPart 2:
Design your own Web services
with ABAP HTTP Extensions
REPORT ZAXXTEST_CLHTTP.
.
CLASS cl_ixml
DEFINITION LOAD.
DATA: response TYPE string.
DATA: g_ixml TYPE REF TO if_ixml.
DATA: g_stream_factory TYPE REF TO if_ixml_stream_factory.
DATA: headfields TYPE tihttpnvp.
DATA: client TYPE REF TO if_http_client,
host TYPE string,
port TYPE string,
proxy_host TYPE string, proxy_port TYPE string,
path TYPE string,
scheme TYPE i.
During program load we create factory (helper) classes for XML handling
LOAD-OF-PROGRAM.
g_ixml = cl_ixml=>create( ).
g_stream_factory = g_ixml->create_stream_factory( ).
START-OF-SELECTION.
Setting the parameters needed by the object
host = 'http://xml.amazon.com'.
port = '80'.
path =
'/onca/xml2?t=webservices-20' &
'&tag=logosworldcom&dev-t=D2H30000000000' &
'&AsinSearch=3528057297&type=lite&f=xml'.
proxy_host = '192.168.69.64'.
proxy_port = '8080'.
scheme = 1. "1=HTTP 2=HTTPS
* Create HTTP client object
CALL METHOD cl_http_client=>create
EXPORTING
host
= host
service
= port
proxy_host
= proxy_host
proxy_service = proxy_port
scheme
= scheme
IMPORTING
client
= client.
* Create HTTP client object, send & receive
client->request->set_header_field( name =
value =
client->request->set_header_field( name =
value =
'~request_uri'
path ).
'~request_method'
'GET' ).
* send & receive
client->send( ).
client->receive( ).
* Capture the result data
response = client->response->get_cdata( ).
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How to Develop Web Services in WebASPart 2:
Design your own Web services
with ABAP HTTP Extensions
* Capture the header fields from the result
CALL METHOD client->response->get_header_fields
CHANGING
fields = headfields.
WRITE: / response COLOR COL_NEGATIVE.
Figure 9: WebAS as HTTP Client
Discovering a Web Service with WSDL and UDDI
While the previous examples are certainly clear enough to understand the idea behind sending
Web service requests and interpreting the responses, it is not clear how to find out how the SOAP
requests need to be formatted. To do this, we need some sort of documentation. The required
documentation for the form of a Web service can in turn be easily represented as a formal XML
document.
These formal descriptions of SOAP requests are done in a language called WSDL, the Web
Services Description Language. The WSDL tells you how the tags of the request and response
have to be named and what tree layout the XML documents will have. The WSDL format for
Amazon.com is illustrated in Figure 10.
If your Web service is to be accessed by many foreign sources, the next step is to publish the
WSDL in a public directory, just as you publish a telephone number in the yellow pages. One of
the popular Web service directories is the UDDI (Universal Description, Discovery and
Integration), which is upheld by a consortium mainly driven by IBM. However, like the yellow
pages, these directories are of interest only for a certain niche group of clients, as the directories
do not indicate anything about the quality of a service. In practice, the use of a Web service is not
typically established on an ad hoc basis. Rather, in advance, you would know exactly what kind of
service you wanted to use, and you would already know the information you needed to access it,
including the WSDL, without having to consult the UDDI.
This key point should not go unmentioned: if you search for WSDL in SAP’s Interface Repository
(http://ifr.sap.com), you may not find what you expect. SAP has not yet committed itself to any
standards like WSDL. WSDL is mainly driven by IBM and Microsoft and it already has the status
of a recommendation by the W3C (World Wide Web Consortium). Instead of WDSL, SAP has
affiliated itself with a competing standard called the Web Service Choreography Interface (WSCI),
an initiative mainly driven by SUN Microsystems and BEA. There appear to be some political
issues in play here, because while WSCI may not be worse than WSDL, it isn't better either.
Needless to say, competing Web services standards only slow down the corporate acceptance of
Web services, so we can only hope that the political issues will be set aside in the interests of
universal interoperability.
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How to Develop Web Services in WebASPart 2:
Design your own Web services
with ABAP HTTP Extensions
<definitions targetNamespace=
"urn:PI/DevCentral/SoapService" name="AmazonSearch">
<types>
<xsd:schema>
<xsd:complexType name="AsinRequest">
<xsd:all>
<xsd:element name="asin" />
<xsd:element name="tag" />
<xsd:element name="type" />
<xsd:element name="devtag" />
<xsd:element name="offer" />
<xsd:element name="offerpage" minOccurs="0"/>
</xsd:all>
</xsd:complexType>
</xsd:schema>
</types>
<message name="AsinSearchRequest">
<part name="AsinSearchRequest"
type="typens:AsinRequest"/>
</message>
<message name="AsinSearchResponse">
<part name="return" type="typens:ProductInfo"/>
</message>
</definitions>
Figure 10: WSDL-Definition of an Amazon Search by Article Number (ASIN)
Implementing Your Own Web Services
Now that we have seen how we can make use of other people’s Web services, what we really
want to see is how we can implement our own services. Let us explore this by showing how it is
done in Java, WebAS, and Microsoft.NET.
There are basically two different approaches to outline here. In both cases, our goal will be to
create an XML result page. One option is to create the page by defining a template page that
outlines the structure of the response document and by generating some code that evaluates the
response data.
The alternative is to set up a program that is called directly by the URL through an interface shim
and returns the resulting data stream. Such a program is known as a servlet. The way that
servlets are called and parameters are passed to them is derived from the old CGI standard,
which allowed for the calling of executable programs from a Web server. While the concept of
server pages, with its predefined templates, is nearly identical for all Web servers, the servlet
concept differs in some essential ways, which we'll explore shortly.
Web Services Through a BSP
Let us start by taking the example of a Business Server Page (BSP) from the previous article I
wrote on WebAS, and manipulate it to return an XML document instead of an HTML page. In the
previous article, we defined a BSP that calls the BAPI to return the actual currency exchange
rates as they are stored in the SAP system. The result was inserted within HTML tags to display
an HTML table.
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We’ll now vary this example by inserting some useful XML tags to represent a proper request
response (see Figure 11). The format of the resulting XML document is taken from the template
defined in the interface repository http://ifr.sap.com. As another variation, we could also enhance
the template from the IFR and easily construct a SOAP document, which differs only in some
named tags. We'll do this later in this paper.
<%@page language="abap"%>
<%
DATA: exch_rate_list type bapi1093_0.
DATA:
DATA:
DATA:
DATA:
tfrom_curr_range TYPE STANDARD TABLE OF bapi1093_3.
tto_currncy_range TYPE STANDARD TABLE OF bapi1093_4.
texch_rate_list TYPE STANDARD TABLE OF bapi1093_0.
treturn TYPE STANDARD TABLE OF bapiret1.
CALL FUNCTION 'BAPI_EXCHRATE_GETCURRENTRATES'
DESTINATION 'NONE'
EXPORTING
date
= sy-datum
TABLES
from_curr_range
= tfrom_curr_range
to_currncy_range
= tto_currncy_range
exch_rate_list
= texch_rate_list
return
= treturn.
%>
<doc:ExchangeRate.GetCurrentRates.Response xmlns:doc="urn:sapcom:document:sap:business">

<ExchRateList>

<% LOOP AT texch_rate_list into exch_rate_list. %>
<item>

<FROM_CURR>
<%= exch_rate_list-from_curr %>
</FROM_CURR>
<TO_CURRNCY>
<%= exch_rate_list-to_currncy %>
</TO_CURRNCY>
<VALID_FROM>
<%= exch_rate_list-valid_from %>
</VALID_FROM>
<EXCH_RATE>
<%= exch_rate_list-exch_rate %>
</EXCH_RATE>
</item>
<% ENDLOOP. %>
</ExchRateList>
</doc:ExchangeRate.GetCurrentRates.Response>
Figure 11: XML Service Defined Through a BSP
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Passing Parameters to the Web Service
The previous example shown in Figure 11 is missing an important feature, the passing of
parameters to the Web service. While ASP.NET or Java servlets resolve the parameters
immediately into the request object, they are accessed through header variables in WebAS.
Because this feature is poorly documented in the current WebAS release, we demonstrate their
use in Figure 12.
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<%@page language="abap"%>
<html>
<head>
<link rel="stylesheet"
href="../../sap/public/bc/bsp/styles/sapbsp.css">
<title>Simulation of AMAZON Web Services through BSP</title>
<%
DATA: BEGIN OF parm,
name TYPE string, value TYPE string, rc TYPE i,
END OF parm.
DATA: querystring TYPE string.
DATA: request_uri TYPE string.
DATA: query TYPE REF TO cl_swlwp_uri.
DATA: xml TYPE string.
DATA: tfields TYPE tihttpnvp, field TYPE LINE OF tihttpnvp.
**** Here we read all information pairs of the HTTP request ********
CALL METHOD request->get_header_fields
CHANGING fields = tfields.
**** This one reads the query_string explicitely ********
CALL METHOD request->get_header_field
EXPORTING name = '~query_string'
RECEIVING value = querystring.
*********** URI Helper Class **************************************
* Initialize the helper class to better deal with query strings URI
* need a dummy prefix to make the class work properly
CONCATENATE 'dummy?' querystring INTO querystring.
CALL METHOD cl_swlwp_uri=>create_from_string
EXPORTING im_string = querystring
IMPORTING ex_uri
= query.
***********
parm-name =
CALL METHOD
EXPORTING
IMPORTING
Extract a single parameter ****************
'ASIN'.
query->get_query_parameter
im_name = parm-name
ex_value = parm-value.
**** Here we better call a function module to do our service.
* CALL FUNCTION 'Z_AXX_XML_BOOKSTORE'
*
EXPORTING querystring = querystring
*
IMPORTING XML
= XML.
*******************************************************
%>
</head>
<body class="bspBody1">
<H1>Simulation of AMAZON Web Services through BSP</H1>
<HR>QueryString: <% = querystring %>
<br>PARAM: Name=<% = parm-name %> Value=<% = parm-value %> RC= <% =
parm-rc %>
<HR><table>
<% LOOP AT tfields INTO field. %>
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<tr><td><% = field-name %></td><td><% = field-value %></td></tr>
<% ENDLOOP. %>
</table>
</body></html>
Figure 12: Catching the Query String Parameters to Simulate the Amazon Service
In Figure 13 we show the way to simulate the Amazon.com article number search. The example
returns an XML data stream in the way it is done by Amazon.com. Since there is no bookstore
available in the WebAS basic version, we simply echo the received parameters in the result
string, assuming that this is sufficient guidance to add the necessary code to return the desired
result data in the resulting XML stream.
<%@page language="abap"%>
<%
...
Same as in previous example
%>
<?xml version="1.0" encoding="UTF-8"?>
<ProductInfo xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<QueryString> <% = querystring %> </QueryString>
<PARAM><NAME><% = parm-name %> </NAME><VALUE><% = parm-value
%></VALUE></PARAM>
<return>
<% LOOP AT tfields INTO field. %>
<Details>
<Name><% = field-name %></Name>
<Value><% = field-value %></Value>
</Details>
<% ENDLOOP. %>
</return>
</ProductInfo>
Figure 13: Returning the Query String Data as XML Stream in the Amazon Fashion
WebAS Servlets Through HTTP Extensions
The previous WebAS examples returned the both HTML and XML result through a Business
Server Page. Another way to produce the result string from the WebAS is through the use of the
HTTP extensions, which we will call WebAS servlets, as they use an approach similar to the Java
servlets. While Java servlets extend the base class javax.servlet.*, the WebAS
extends3 the ABAP class IF_HTTP_EXTENSION (the prefix IF stands for interface). Extending
the class is also known as inheriting the class. In the case of Web services, the newly developed
class inherits the IF_HTTP_EXTENSION and overwrites one distinct method:
HANDLE_REQUEST (). The request handler itself can access the HTTP objects through a
3
Extending a class is also known as inheriting a class, depending on which terminology of object
oriented programming is used
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series of helper classes, e.g., IF_HTTP_RESPONSE and IF_HTTP_REQUEST, which are
inherited via the parent class IF_HTTP_EXTENSION.
As a simple first example, for an HTTP service we shall inspect the predefined class
CL_HTTP_EXT_PING, which can be reviewed with the class builder (SE24). The class has
been mapped with transaction SICF to the service name pingservice. Hence, with an up and
running WebAS, you could call the service by entering a URL like the one below in your browser,
where ”linux:8080” is the HTTP address of the WebAS.
http://linux:8080/pingservice
Figure 14: Sample URL to Call a Web Service of the WebAS
Request Handling with Method IF_HTTP_EXTENSION~HANDLE_REQUEST
When the HTTP service request is received by the WebAS, it calls the method
HANDLE_REQUEST() of the named class. In our example, pingservice is the public name
that has been assigned to the implementing class in transaction SICF. The implementing class
should be an extension of the default handler class IF_HTTP_EXTENSION, and it should
implement a method with the fixed name HANDLE_REQUEST().
The IF_HTTP_EXTENSION class implements all important methods and attributes that are
needed for handling the response and request parts of an HTTP communication. See Figure 15.
method IF_HTTP_EXTENSION~HANDLE_REQUEST.
data: result type string.
server->response->set_header_field(
name = 'Content-Type'
value = 'text/html' ).
result = '<html><body>'(001) &
'Connection to server successful'(101) &
'SystemID: &SYSID&'(102) &
'</body></html'(999).
replace '&SYSID&' with sy-sysid into result.
server->response->set_cdata( data = result ).
endmethod.
Figure 15: Sample Implementation of a Pingservice Class
Within the HANDLE_REQUEST method, an arbitrary ABAP Object code can be executed.
Finally, the response object must be furnished with the response data, according to the
requirements of the requesting HTTP client. In the pingservice example, we assume that the
request expects a simple standard HTML response. So the response header is set with the
method
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How to Develop Web Services in WebASPart 2:
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server->response->set_header_field
accordingly to text/html and the result data block is set to the response data with method
server->response->set_cdata( data = ) .
In addition to returning the data, we need to set the HTTP header information, which is
sometimes mandatory and sometimes only useful. Typically, the header would at least provide
information about the content of the document data that is sent in the document body. In our
case, we tell the receiver of the document that the body contains HTML data. The valid content
descriptors are defined by the W3C MIME recommendation.
server->response->set_header_field(
name = 'Content-Type'
value = 'text/html' ).
As a next step, we build the result string, which actually can be an arbitrary text. As a best
practice rule, you would insert a call to a function module that returns the final result data string.
This is “best practice” because it reduces the amount of code within an interface handler to a
minimum. Doing so, the request handler is used as a wrapper for the application and as a
jumping board to call the actual processing only:
result = '<html><body>'(001) &
'Connection to server successful'(101) &
'SystemID: &SYSID&'(102) &
'</body></html'(999).
replace '&SYSID&' with sy-sysid into result.
Finally, the result is assigned to the HTTP body with the set_cdata method.
server->response->set_cdata( data = result ).
Web Services Through Java Servlets
With the upcoming hype for Java J2EE support as part of the WebAS starting with release 6.2,
you might be interested to see how the same code looks when it is written in Java. In Java, a
program that can be called via HTTP is called a servlet. When a servlet is written, it is deployed in
a formal structure called the servlet container.
Basically, the program in Java looks the same as in ABAP. Again, there is a formal base class
that carries the HTTP extensions. This class is inherited by any servlet class that you develop
and deploy. Other than in Java, the servlet does not work on top of a general request handler, but
the servlet container explicitly calls some methods that match the name of the HTTP request. For
every HTTP command, such as GET or POST, there has to be a method implemented whose
name is built by prefixing the HTTP command with “do” as follows:
•
doXxx(), where Xxx is to be replaced by the name of the handled HTTP command, e.g.,
doGet for HTTP Get and doPost for HTTP Post.
A calling Web client may expect that a servlet handles at least the basic HTTP commands Get
and Post, so consequently a servlet must support and implement the standard methods:
•
doGet()
•
doPost()
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Every method will be called exclusively from the matching context, and the response of each
method is a string that is returned in the body of HTTP response. Before we can use the servlet,
the servlet class must be deployed in the Web server’s servlet container. When this is
successfully done, it can be called via an HTTP Get or HTTP Post request by specifying nothing
more than the class name in the URL. If the class requires parameter information, it can be
appended to the URL separated by a question mark (?) to build a canonical URI, which we are
already familiar with. See Figure 16.
public class HelloJSP {
public static void doGet(String[] args) {
System.out.println("<html><body>Hello World from
JSP</body></html>");
}
public static void main(String[] args) {
System.out.println("<html><body>Welcome to Hello World for
JSP</body></html>");
}
}
Figure 16: Implementation of Servlet Class HelloWorldExample in Java
SAP XML and SOAP Utilities
SAP also provides for a number of helper classes to allow for easier (I don't dare say easy)
creation of the resulting XML documents, e.g., creating a proper SOAP response document. In
Figure 17, there is an example that is also found in a similar fashion in the demo section of the
WebAS, and is called the “info service.” The example may look cryptic at the first glance. The key
thing to understand is that the method soapdoc->add_parameter converts the given
ABAP data structure into a SOAP-compliant XML sub tree.
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method if_http_extension~handle_request.
* parameters:
*
[in] if_http_server& server
* Call through browser: http://linux:8080/sap/public/info
data:
data:
data:
data:
data:
data:
data:
data:
data:
l_rfcsi type rfcsi.
soapdoc type ref to CSoapDocument.
isoapdoc type ref to ISoapSerialize.
exUsage type ref to CSoapExceptionUsage.
exResrc type ref to CSoapExceptionResource.
exIntrn type ref to CSoapExceptionInternal.
dref
type ref to data.
xdata
type xstring.
data type string.
call function 'RFC_SYSTEM_INFO'
importing
rfcsi_export = l_rfcsi.
clear xdata.
create object soapdoc.
if not soapdoc is initial.
try.
call method soapdoc->set_method
exporting name = 'RFC_SYSTEM_INFO.Response'
nsprefix = 'rfc'
nsvalue = CSoapConstants=>sc_rfc_function_ns.
get reference of l_rfcsi into dref.
call method soapdoc->add_parameter
exporting name = 'RFCSI'
value = dref
direction = CSoapConstants=>ic_param_in.
isoapdoc = soapdoc.
call method isoapdoc->serialize
changing document = xdata.
* -- set header field
call method server->response->set_header_field(
name = 'Content-Type'
"#EC NOTEXT
value = 'text/xml' ).
catch CSoapExceptionUsage into exUsage.
"-- don't care
clear xdata.
catch CSoapExceptionResource into exResrc.
clear xdata.
catch CSoapExceptionInternal into exIntrn.
clear xdata.
endtry.
endif.
if xdata is initial.
call method server->response->set_header_field(
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name = 'Content-Type'
"#EC NOTEXT
value = 'text/html' ).
* Bild an HTML framework to properly display it in a browser
concatenate '<html>' '<body>' l_rfcsi '</body>' '</html>'
into data.
call method server->response->set_cdata( data = data ).
else.
call method server->response->set_data( data = xdata ).
endif.
endmethod.
Figure 17: Producing a SOAP Document Out of a Function Call Result
In my system, the resulting SOAP document looks as follows in Figure 18. If you look closer, you
will find that the SOAP document is nearly identical with the template provided by the SAP
interface repository, but with one small enhancement: the template is embraced by the SOAPENV envelope tags.
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Figure 18: SOAP Document Produced with Aid of the SoapdocÆadd_parameter Method
Do You Need WebAS or SAP Enterprise for Web Services?
Calling SAP via RFC or via HTTP
From what we have seen so far, it is certainly clear that there is no need to upgrade to WebAS or
SAP Enterprise (4.7) for the sole purpose of integrating your SAP system with the Web. If you
want to give your HTTP-based SAP clients real-time access to all the wealth of data inside of
SAP, you can use any modern Web server, and access your SAP ERP system via RFC calls
from Java or Windows DCOM calls.
Realistically, if you designed a solution that delivered information from your ERP system to an
Internet requester, you would not even think of plugging in your Internet connection directly to
your SAP box, for the simple reason of security. Exposing your ERP system directly to the
Internet would be like depositing the safe with the Queen’s crown jewels in the garden of
Buckingham palace.
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However, there are other arguments for upgrading your SAP ERP system to SAP Enterprise 4.7:
•
SAP Enterprise will allow you to easily design browser access to SAP for Intranet solutions.
These solutions are becoming more and more popular, because they don't involve deploying
SAPGUI on every workstation.
•
Secondly, having SAP function like any HTTP server, as it does in SAP Enterprise, allows a
smoother communication with other internal TCP/IP applications, including your own Internet
Web server.
Smooth HTTP interaction can provoke a nominal return of investment when you have a complex
heterogeneous server landscape. Whereas having RFC access means deploying the necessary
drivers on the client computer, calling dedicated libraries, and being dependent on SAP-provided
utilities and documentation during development, the use of WebAS allows you to access your
SAP ERP system in the same manner and use the same utilities as you would to access any
other online location or Web server.
Although programming RFC access is relatively easy, and it's available for most common
operating platforms (including Windows, Java on Windows, Linux, and UNIX), deploying an RFC
application requires that the client computer has the necessary RFC drivers installed.
Installing drivers on a productive machine is always critical. You have to consider:
•
deployment and registering of the drivers
•
rebooting the box after installation
•
allowing for the proper security mechanisms
•
hat testing your application may interfere with existing applications and even bring them
down.
When you decide to activate HTTP communication with SAP, you will benefit from the alreadyinstalled TCP/IP tools that are universally installed on every modern computer from mainframes
down to pervasive palm-top clients. This allows access to SAP from every computer within reach
of your network, without any special preparation of the client.
Another benefit of SAP HTTP: your development and deployment team will also avoid endless
discussions with the Basis and security administration team. Every proprietary standard used in
your server landscape will likely explode your training and planning costs. What may look like an
easy five-minute task to your development team suddenly turns out to become an elaborate
ceremony requiring weeks of planning. Once your SAP talks HTTP, it is only up to whoever is
handling the firewall to open or deny the access to your SAP ERP system. This takes the
discussion away from technical complexities and allows for a more strategic focus.
So, based on the strengths of the WebAS and Enterprise 4.7, what is my final assessment in
terms of the best approach to Web services? My current recommendation, as of this writing, is
that WebAS and Enterprise 4.7 is the ideal platform for SAP Web services, for all the reasons I've
outlined in this paper. But, keep in mind, that specifically when submitting a SOAP request to the
SAP system, I recommend continuing to use SAP HTTP and RFC, not the HTTP classes of
WebAS, until the HTTP classes and ABAP/Objects technologies mature. Even if you're running
on WebAS, you can still continue to use SAP HTTP and traditional RFC methods. And, best of
all, you do not need to be running WebAS or 4.7 to start using Web services in your application
environment.
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SAP As the Universal Development Platform
It is already pretty obvious that SAP, with the addition of HTTP service to its kernel, has matured
into one of the leading competitors in the enterprise application server market. From now on, we
will have to clearly distinguish between the SAP ERP suite—which is a class of its own anyway—
and the SAP kernel technology, currently marketed as the NetWeaver WebAS.
Thanks to the strong transaction management, the database integration, the fully integrated
development workbench, and sophisticated change and deployment management, SAP WebAS
has already become a serious contender to the established virtual machine frameworks for
enterprise application servers. In the next white paper, I shall assess the strengths and
weaknesses of the three major development and runtime environments, namely WebSphere
J2EE, Microsoft.NET, and SAP WebAS, and explain why the “Clash of the Frameworks” will
end in favor of the newcomer SAP WebAS and ABAP—at least for SAP users.
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Axel Angeli is a senior SAP and EAI advisor and principal of logosworld.com, a Germanbased enterprise specializing in coaching SAP and EAI project teams and advising IT
management on implementation issues. Axel has been in the IT business since 1984, and
throughout his career, he has always worked with cutting edge technologies. Axel's SAP
experience stems back from the good old R/2 days and he is an expert on SAP’s NetWeaver
technology and any kind of ABAP development. A speaker of several languages, Axel
specializes in coaching and leading large multi-national teams on complex projects with
heterogeneous platforms and international rollouts. Known for his intensive and successful
trouble-shooting experience, Axel has been nicknamed by his colleagues as the “Red Adair”
of SAP projects. He is the author of the best-selling tutorial “The SAP R/3 Guide to EDI, IDocs,
ALE and Interfaces.”
The information in our publications and on our Website is the copyrighted work of Klee Associates, Inc. and is owned by
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NO WARRANTY: This documentation is delivered as is, and Klee Associates, Inc. makes no warranty as to its accuracy
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