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The Apache module for gSOAP

updated Mon Feb 22 2016
 
The Apache module for gSOAP

Table of Contents

By Christian Aberger, Mick Wall, Robert van Engelen, David Viner, Ryan Troll, and La Cam Chung.

Overview

SOAP/XML and REST Web services can be easily created and deployed as gSOAP standalone services or installed as (Fast)CGI applications. In addition, the mod_gsoap Apache module offers the ability to run gSOAP services directly inside the Apache HTTP server. The mod_gsoap Apache module supports the deployment of multiple gSOAP services that can run together with the usual services on Apache. This approach offers a production-quality Web services deployment scenario.

The mod_gsoap Apache module is designed to keep things simple so that existing gSOAP services can be recompiled for Apache HTTP server deployment without modification of the source code. The Apache apxs command compiles your gSOAP service code and installs it with mod_gsoap. Add your new service to Apache httpd.conf and presto!

The original Apache module for gSOAP home page is at http://mod-gsoap.sourceforge.net.

Apache modules for gSOAP are provided for both Apache 1.3 and 2.x. In the following we will discuss the Apache module for Apache 2.x.

Installation

First download and install Apache httpd in a new directory, say apachegsoap:

mkdir apachegsoap
cd apachegsoap
tar -xjf httpd-2.4.16.tar.bz2
cd httpd-2.2.31
./configure --prefix=`pwd`/.. --with-mpm=worker --enable-mods-shared=most
make -j4
make install

To use the Apache extension mechanism, your platform has to support the DSO feature and your Apache httpd binary has to be built with the mod_so module. The apxs tool automatically complains if this is not the case. You can check this yourself by manually running the httpd -l command (here installed in apachegsoap/bin with our local installation):

cd apachegsoap
bin/httpd -l

The module mod_so should be on the displayed list of modules.

Next, we will build and install mod_gsoap for Apache 2.x and up. The source code files are located under gsoap/mod_gsoap/mod_gsoap-0.9/apache_20 and include apache_gsoap.h, mod_gsoap.c, and a Visual Studio project file mod_gsoap.vcproj.

To compile mod_gsoap, execute:

cd /path/to/gsoap/installation/gsoap/mod_gsoap/mod_gsoap-0.9/apache_20
ln -s /path/to/gsoap/installation/gsoap/stdsoap2.h .
sudo apxs -a -i -c mod_gsoap.c

Root permissions are required, so we used sudo apxs here.

The apxs command should be on your path or located in apachegsoap/bin (which is the case with our local installation).

If a specific C compiler is required, say cc, then try apxs -S CC=cc ....

Deploying C services with the Apache module

After building mod_gsoap we are ready to deploy gSOAP services written in C with the Apache module.

The gSOAP package contains a calculator example. We will use this example to walk you through the creation and deployment of an Apache module gSOAP service.

First, copy the calculator example:

cd apachegsoap
cp /path/to/gsoap/installation/gsoap/samples/calc/* .
cp /path/to/gsoap/installation/gsoap/stdsoap2.* .

Next, edit calcserver.c by removing main() and replace it with IMPLEMENT_GSOAP_SERVER() as follows:

#include "apache_gsoap.h"
IMPLEMENT_GSOAP_SERVER() /* replaces main() { ... } */

Then compile and build the service:

soapcpp2 -c -SL -wx calc.h
apxs -a -c calcserver.c soapC.c soapServer.c stdsoap2.c
chmod 755 .libs/calcserver.so

Again, the apxs command should be on your path or located in apachegsoap/bin (which is the case with our local installation).

This creates .libs/calcserver.so service module that is universally readable. Also make sure that .libs/calcserver.so is readable through the entire path, that is through /home/testuser/apachegsoap/.libs.

If you have installed source files in other directories, then you will need to add appropriate -I and -L options with the apxs command shown above.

To deploy the service, we will need to add our module with its properties to httpd.conf (for example we can add it at the end):

<IfModule mod_gsoap.c>
 <Location /soap>
  SetHandler gsoap_handler
  SOAPLibrary /home/testuser/apachegsoap/.libs/calcserver.so
  Order allow,deny
  Allow from all
 </Location>
</IfModule>

The httpd.conf file is usually found under /private/etc/apache2/httpd.conf. However, since we use a locally installed Apache httpd server that searches the /home/testuser/apachegsoap path, you will find httpd.conf in /home/testuser/apachegsoap/conf.

The Location property sets part of the URL of the service, which in this case will be http://localhost/soap or if you set a port that is different than the standard HTTP port 80, say 9080, the full URL is http://localhost:9080/soap. To change the port from 80 to 9080, edit httpd.conf and change Listen:

Listen 9080

To start the service:

cd apachegsoap
bin/apacectl start

Point your browser to http://localhost:9080 and the page should show the message "It works!".

To stop the server:

bin/apacectl stop

To use the service by client applications, direct the endpoint URL of clients to "http://localhost:9080/soap", for example in the calcclient.c code that came with the gSOAP example:

const char server[] = "http://localhost:9080/soap";

Then we build the client:

soapcpp2 -c -CL -wx calc.h
cc -o calcclient calcclient.c soapC.c soapClient.c stdsoap2.c

and run it:

./calcclient add 2 3
result = 5

To let clients access the WSDL of a service, you can use the query ?wsdl as part of the URL such as http://localhost:9080/soap?wsdl to pull the file calc.wsdl from the current location of the service. To do so, copy the calc.wsdl file there to make it available to the Apache server.

Dynamic libraries

If you are using dynamic libraries to deploy services with mod_gsoap, then those should be closed propertly to avoid memory leaks. To do so add:

dlclose(pConfig->m_pLibraries->m_pSOAPLibrary->m_hLibrary);

at the end of the gsoap_handler() function in mod_gsoap.c.

Building C++ services from service classes

The Apache server is written in C. Building Apache modules in C++ is tricky and cannot be fully guaranteed due to compiler differences. Several online resources exist that can help to implement C++ modules for Apache 2.0. If this fails, the best alternative is to use FastCGI (see gSOAP user guide on "FastCGI Support").

When using C++ gSOAP service classes generated by soapcpp2 options -i or -j we need to implement the C function soap_serve() that dispatches the services.

We will walk through the implementation of a service using the same calculator example demonstrated above.

First, run soapcpp2 with option -j to generate a service class:

soapcpp2 -j -SL -wx calc.h

Create a new calcerver.cpp file with the following code:

#include "soapcalcService.h"
#include "calc.nsmap"
#include "apache_gsoap.h"
extern "C" int soap_serve(struct soap *soap)
{
calcService service(soap);
int err = service.serve();
service.destroy();
return err;
}
int calcService::add(double a, double b, double *result)
{
*result = a + b;
return SOAP_OK;
}
int calcService::sub(double a, double b, double *result)
{
*result = a - b;
return SOAP_OK;
}
int calcService::mul(double a, double b, double *result)
{
*result = a * b;
return SOAP_OK;
}
int calcService::div(double a, double b, double *result)
{
*result = b != 0 ? a / b : 0.0;
return SOAP_OK;
}
int calcService::pow(double a, double b, double *result)
{
*result = ::pow(a, b);
return SOAP_OK;
}

Here, calcService is the service class defined in the generated soapcalcService.h and soapcalcService.cpp files.

The apxs command is used to compile as follows, with the -S CC=c++ option:

apxs -a -c -S CC=c++ calcserver.cpp soapC.cpp soapcalcService.cpp stdsoap2.cpp
chmod 755 .lib/calcserver.so

This creates .libs/calcserver.so service module that is universally readable. Also make sure that .libs/calcserver.so is readable through the entire path, that is through /home/testuser/apachegsoap/.libs.

The module should also be added to httpd.conf, as was explained above.

When multiple service classes are defined, when soapcpp2 -j is applied to multiple .h files, then you have two options:

  1. create an .so library for each service and add each module to httpd.conf with a new Location property.
  2. create one .so library with the services combined, all listening to the same service URL. Only one module is added to httpd.conf since the Location property is the same. Note that the ?wsdl query is not as useful in this case, since we cannot publicize the service WSDLs combined.

The second option requires C++ namespaces as explained in section "How to Chain C++ Server Classes to Accept Messages on the Same Port" in the gSOAP user guide. Basically, you should chain the services as follows:

#include "apache_gsoap.h"
extern "C" int soap_serve(struct soap *soap)
{
int err;
if ((err = soap_begin_serve(soap)) == SOAP_OK)
{
X::Service service_x(soap);
if ((err = service_x.dispatch()) == SOAP_NO_METHOD)
{
Y::Service service_y(soap);
if ((err = service_y.dispatch()) == SOAP_NO_METHOD)
{
Z::Service service_z(soap);
err = service_z.dispatch();
}
}
}
soap_destroy(soap);
soap_end(soap);
return err;
}

Troubleshooting

It is recommended to test the service first as a stand-alone server over a port using soap_bind() and soap_accept(). Debugging as much easier this way.

In case you need to debug the service as it is deployed while is httpd running, we first stop the service and start it up in single threaded mode so we can attach to it to a debugger such as gdb or lldb.

cd apachegsoap
bin/apacectl stop
bin/httpd -X -f /home/testuser/apachegsoap/conf/httpd.conf &

You will get a process ID, say 12345, which we will attach to a debugger:

gdb /home/testuser/apachegsoap/bin/httpd 12345

You will see a load of symbols loading, including mod_gsoap.so.

Next, we set a breakpoint on the soap_serve call (the main entry point), let gdb know that we accept the pending breakpoint, and then continue the process:

(gdb) b soap_serve
Function "soap_serve" not defined.
Make breakpoint pending on future shared library load? (y or [n]) y
Breakpoint 1 (soap_serve) pending.
(gdb) c
Continuing.

We execute our calcclient from another window, which triggers the breakpoint.

[Switching to Thread 0x7f3fbd209950 (LWP 25493)]
Breakpoint 1, 0x00007f3fb059e398 in soap_serve () from /home/testuser/apachegsoap/gsoap-2.8/gsoap/samples/calc/.libs/calcserver.so
Current language: auto; currently asm
(gdb) n
Single stepping until exit from function soap_serve,
which has no line number information.
0x00007f3fb05a5eec in apache_default_soap_serve () from /home/testuser/apachegsoap/gsoap-2.8/gsoap/samples/calc/.libs/calcserver.so
(gdb)

Note that when Apache httpd runs as user wwwrun (or another user) then you will have to run the debugger as that same user. To do so, execute:

sudo -u wwwrun gdb /usr/sbin/httpd2-worker 25487

Thanks to Jon Scobie for suggestions.

License

The Apache modules for gSOAP are released under the gSOAP open source public license (compatible with commercial licensing) and GPLv2.

Further reading

All things `mod_gsoap` by Jon Scobie

Apache HTTP server project.