Using Some Maven Ideas And Best Practices With SCons

Table of Contents

Abstract

Up to now we used make to build most of our projects. However, in our new mid-size project we decided to use scons instead. Some requirements which had to be fulfilled are (as you will see, we tried to integrate some of the ideas introduced by the build tool maven):

Multimodule Projects

The project has a hierarchical directory structure containing multiple subprojects, which in turn may have mutliple dependent modules. The build tool should be able to recursively resolve dependencies between projects and modules in order to build a particular subproject at any level of the hierarchy.

Short Setup Time

Especially at early stages of the project numerous subprojects must be created. The development members, located at different subsidiaries, must be able to independently create new subprojects when needed. Thus the build tool should allow subproject creation with little setup effort without specifying the build process in an imperative or declarative way each time.

Common Directory Structure

A high number of subprojects requires a common directory structure based on best practices as introduced in maven. The build tool should support implementation of best practices by configuration of common source code, target, test code, resources ... directories.

Model-Based Approach

The directory structures of subprojects and modules are self similar. Thus the same logic can be used for builds at any hierarchical level. Consequently the build tool should enable to separate the build logic from the so called [p]roject [o]bject [m]odel (maven naming convention). The pom includes the name and version of the subproject, its dependencies and artifacts to build.

Build Phases

The pom fully specifies a piece of code. Given the pom various build phases like compile, test, install, site can be applied to the source code each resulting in a particular artifact. The build tool should be able to interpret and handle a pom to process the source code appropriately.

Approach

We integrated the build logic in a SConstruct, which is wrapped by a shell script. Thus for each subproject the developers only have to provide its name, version, type and dependencies and then call sconswrapper compile. It is very convenient, and it works fine for us. Next I would like to explain, how we use the script and what are the benefits of using it by means of an example.

Workflow

The wrapper script mentioned ...

• checks, whether the current directory contains a SConscript (we use the name pom.scons, see below),
• copies SConstruc from a dedicated directory to the current directory,
• executes scons,

• and finally deletes SConstruct from the current directory.

  We use it like in the following workflow example:

  Step 1

  Create new subproject helloworld including the common directory structure.

project
|--helloworld
    |--src
       |--main
       |  |--c++ <-- source code goes here 

Step 2

Create the corresponding pom for helloworld

project
|--helloworld
    |--pom.scons <-- project object model
    |--src
       |--main
          |--c++

Step 3

The pom.scons file is actually a SConscript. The following example defines the Program helloworld at version 0.9. The ext key of the dependency pthread is set to yes, thus before building the artifact, the existance of the external library phtread is checked. The resulting artifact is copied (exported) to the common bin location.

# file: helloworld/pom.scons
# This SConscript contains no build logic but the project object model only. 
import os
import os.path
Import(['env', 'build'])
env = env.Copy()
env['pom']['group'] = ''
env['pom']['name'] = 'helloworld'
env['pom']['version'] = ' 0.9'
env['pom']['artifacts'] = 'Program'
env['pom']['deps'] = [
    {'group':'', 'name':'pthread', 'version':'', 'scopes':['compile'], 'ext':'yes'}] 
env['pom']['exports'] = [
    {'source':'main/helloworld-0.9', 'target':'helloworld', 'type':'bin'}]
targets = build(env)
Return('targets')

Step 4

Build and install the program helloworld. By default helloworld will be copied to $HOME/local/bin. If the artifact would be a SharedLibrary, it would be copied to $HOME/local/lib by default and so on.

# cd project/helloworld
# sconswrapper compile install

Step 5

Now assume, helloworld depends on an internal library called libhelloworld. Create new subproject libhelloworld including the common directory structure.

project
|--helloworld
|   |--pom.scons
|   |--src
|      |--main
|         |--c++
|--libhelloworld
    |--pom.scons
    |--src
       |--main
          |--c++ 

Step 6

Create the corresponding pom for libhelloworld.

# file: libhelloworld/pom.scons
# This SConscript contains no build logic but the project object model only.
import os
import os.path
Import(['env', 'build']) 
env = env.Copy()
env['pom']['group'] = ''
env['pom']['name'] = 'libhelloworld'
env['pom']['version'] = '0.9'
env['pom']['artifacts'] = 'SharedLibrary' 
env['pom']['deps'] = [
    {'group':'', 'name':'pthread', 'version':'', 'scopes':['compile'], 'ext':'yes'}]
env['pom']['exports'] = [
    {'source':'main/libhelloworld- 0.9.so', 'target':'libhelloworld.so', 'type':'lib'}]
targets = build(env)
Return('targets')

Step 7

Modify the project object model for helloworld by just adding new dependency helloworld, which is automatically expanded to libhelloworld.so by scons. The ext key is set to no, thus scons does not check the existence of the library before building.

# file: helloworld/pom.scons
# This SConscript contains no build logic but the project object model only.
import os
import os.path
Import(['env', 'build'])
env = env.Copy()
env['pom']['group'] = '' 
env['pom']['name'] = 'helloworld'
env['pom']['version'] = '0.9'
env['pom']['artifacts'] = 'Program'
env['pom']['deps'] = [
    {'group':'', 'name':'pthread', 'version':'', 'scopes':['compile'], 'ext':'yes'},
    {'group':'', 'name':'helloworld', 'version':'', 'scopes':['compile'], 'ext':'no'}] # new dependency
env['pom']['exports'] = [
    {'source':'main/helloworld-0.9', 'target':'helloworld', 'type':'bin'}] 
targets = build(env)
Return('targets')

Step 8

Now assume, you don't want to build each artifact (helloworld, libhelloworld) manually, but let scons resolve the dependencies and build them in the right order. Thus if you call sconswrapper at project level, libhelloworld.so should be built first, helloworld should be built next. For that to work simply add another pom for project. The artifact type Meta means: recursively search for subprojects and build them in the right order.

project
|--pom.scons <-- pom for 'project'
|--helloworld
|   |--pom.scons
|   |--src
|      |--main
|         |--c++
|--libhelloworld
    |--pom.scons
    |--src
       |--main
          |--c++

# file: pom.scons
# This SConscript contains no build logic but the project object model only.
import os
import os.path
Import(['env', 'build']) 
env = env.Copy()
env['pom']['group'] = ''
env['pom']['name'] = 'greeting'
env['pom']['version'] = '0.9'
env['pom']['artifacts'] = 'Meta' # artifact type 'Meta' 
env['pom']['deps'] = []
env['pom']['exports'] = []
targets = build(env)
Return('targets')

Step 9

Build and install the top level project. As a result libhelloworld is built and copied to $HOME/local/lib. Next helloworld is built and copied to $HOME/local/bin.

# cd project
# sconswrapper compile install

Step 10

Create documentation. For this to work doxygen must be in the path.

# cd project 
# sconswrapper site

Step 11

Clean up. This deletes all created target directories. To remove artifacts, created by the current phase only, use sconswrapper <phase> -c.

# cd project
# sconswrapper clean

Sources

The sources used in this Wiki:

sconswrapper

This is the shell wrapper script calling scons with SConstruct.

SConstruct

This is the scons script containing all build logic to interpret subproject specific project object models.

pom.scons.template

This is a commented template file for project object models.

Conclusion

Although it works fine for us, the SConstruct is by far not perfect. I am still new to python and scons and therefore would like to know your opinion on how to do things right. So if it sounds interesting to you or you have any questions or suggestions, please let me know. Any comments would be very much appreciated.

-- JakobScheck 2007-03-07 10:31:48