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REP: 109 Title: Unary Stacks Author: Ken Conley <[email protected]> Status: Final Type: Standards Track Content-Type: text/x-rst Created: 25-May-2011 Post-History: 25-May-2011

Abstract

The definition of ROS stacks and packages is changed to enable a directory to be both a stack and a package. We call this combined entity a unary stack to indicate that it is a stack with a single package entity.

Motivation

ROS enables developers to organize directories into both packages and stacks. Packages are the basic unit of the ROS build system and they typically contain libraries, nodes, and configuration files. Stacks are the basic unit of the ROS release system

Packages and stacks are usually organized at different levels of granularity. Packages are focused on build products and thus follow a 'Goldilocks' principle: enough functionality to be useful, not too much to be heavyweight. A package might provide an implementation of a specific algorithm or data structure.

Stacks are focused on release products, so they usually organize packages together that provide an aggregate functionality, like a navigation system. Stacks are also organized by software development concerns: they often collect code that is co-developed and has interdependent APIs. Releasing this code together enables shared internal APIs to be changed together within a single release.

While most stacks contain multiple packages, there are several instances of stacks only containing a single package. This type of organization is common with external libraries that are integrated into ROS and also common with packages that are developed in isolation, such as a ROS client library. For example, the slam_gmapping stack just contains the gmapping package. This demonstrates the awkward naming prefix that is often used to differentiate the stack from the package.

There are also cases where stacks collect packages that either have similar functionality or were co-developed, but are better provided separately. For example, the various driver stacks in ROS collect similar drivers, like camera_drivers, and laser_drivers. This collection requires users to install many drivers, instead of drivers just for the hardware they have.

One final interesting case is external libraries that wish to provide ROS integration. While these libraries can add a ROS package manifest to their source tree to achieve integration with the build system, they must create an additional level of directory hierarchy to be release-able.

Unary stacks enable a filesystem directory to be treated as both a package and stack. This will alleviate pain felt by developers when the package and stack organizational levels overlap and also make it easier for external library maintainers to release code for use in a ROS system.

Specification

Unary stacks are defined to be directories that contain both a stack manifest [1] and package manifest [2]. With respect to ROS command-line tools, the directory is both a stack and a package. The stack contains a single package, which shares the same name.

Stack version

The stack version number is currently contained the CMakeLists.txt file. For example, this indicate a stack at version 1.2.5:

rosbuild_make_distribution(1.2.5)

For backwards compatibility, we will continue to support the rosbuild_make_distribution() macro as a version number source.

In order to support external libraries that cannot use this CMake macro, this REP introduces a <version> tag in the stack manifest:

<stack>
   <version>1.2.5</version>
   ...
</stack>

The <version> tag has a single text element, which is interpreted to be the version number of the stack. Surrounding whitespace in this text element is stripped.

The <version> tag has precedence over the rosbuild_make_distribution() macro.

Affected tools and libraries

The following tools are expected to be directly impacted by this change and will require testing:

  • rospack
  • rosstack
  • rosdep
  • roscreate-stack
  • rosmake
  • rosdoc

The following Python libraries will be updated to follow the definition of unary stacks:

  • roslib.packages
  • roslib.stacks
  • roslib.manifest
  • roslib.stack_manifest

The following ROS wiki documentation macros will be updated to represent unary stacks:

  • StackHeader()
  • PackageHeader()

For ease of deployment, StackHeader() and PackageHeader() will treat unary stacks the same and represent both stack and package metadata.

Releasing a unary stack

The largest required change is to the ROS release system [3]. Previously, stacks and packages each have a CMakeLists.txt file that handles the required release behavior. In order to simplify combining these two entities, stacks will no longer be built by CMake. Instead, the release scripts will perform the necessary release packaging.

Additional rosmake modifications

The behavior of rosmake <stack-name> will be modify to mean, "rosmake all the packages in this stack, as well as all of the stacks this stack depends on." Previously, the behavior was, "rosmake all of the packages in this stack."

Rationale

Unary stacks vs. bare packages

One of the main alternatives to unary stacks is to release bare packages. This creates issues relating to separation of concerns. Stacks contain information about how a body of code is released, whereas packages contain information about how code is built. Keeping this information semantically separate simplifies and clarifies the toolchain. For similar reasons, the specified implementation is much more straightforward as the existing toolchain can maintain the existing semantics of stacks and packages and not have to introduce a new hybrid semantic.

rosmake changes

The changes to the behavior in rosmake are necessitated by the desire to release empty stacks. Empty stacks are a convenient mechanism for migrating to unary stacks and are also a useful mechanism for declaring 'meta stacks' that simply aggregate multiple stacks. For the purposes of migrating to unary stacks, empty stacks are used to declare a dependency on the new unary stack. For example, the simulator_stage stack will still exist in ROS Electric, but it will be empty and declare a dependency on the stage stack.

Deployment

Unary stacks are targeted at the ROS Electric Emys release. We anticipate migrating a small number of stacks to use the unary stack system, but our priority is the stability of the distribution release.

Backwards Compatibility

Unary stacks are not backwards compatible with previous ROS distributions and will not be released into ROS Diamondback.

In order to minimize the impact to stack dependencies, we will release shell stacks for any stacks that are migrated to use the unary stack system. For example, when camera_drivers are separated into individual unary stacks for each driver, we will release a new camera_drivers stack that depends on each of these stacks. Support for these shell stacks will be maintained for a single ROS distribution release, then removed.

References

[1]Stack Manifests (http://ros.org/wiki/Stack Manifest)
[2]Package Manifests (http://ros.org/wiki/Manifest)
[3]ROS release system (http://www.ros.org/wiki/release)

Copyright

This document has been placed in the public domain.