This documentation best read in doxygen. You can view the doxygen docs online here.

What is this

This repository provides an implementation of the GHS (for Gallager, Humblet and Spira) algorithm, which allows creation of a distributed minimum spanning tree on a message passing system

It was designed and implemented originally at the NASA Jet Propulsion Laboratory, under NASA and US Government Sponsorship and is now maintained here as open-source software. (NTR 52251, titled "Library for Leader Election and Spanning Tree Distributed Construction", submitted by Joshua Vander Hook 12/14/2021)

It also provides an exectuable that can be run on a set of linux machines so that those machines can demonstrate the execution of GHS.

What does the repo provide

This rep provides a few things, in C/C++:

A C++ GHS implementation library libghs.so
A C++ statically-sized messager queue that is based on a circular buffer libghs_seque.so
An optional set of extensions that help to build algorithms around it libghs_ext.so (configure with -DBUILD_EXT=On)
An optional set of tools to help with studies of performance on various graphs, and to output meaningful visuals using the dot linux utility to-dot, random-graph, ghs-score (configure with -DBUILD_TOOLS=On)
An optional doctest executable, that will unit-test the currently built library ghs-doctest (configure with -DBUILD_DOCTEST=On)
An optional demo executable, that is suitable for integration-type testing on multi-machine, multi-process environments ghs-demo (configure with -DBUILD_DEMO=On)
An experimental integration of miniz to do message compressing in ghs-demo (configure with -DENABLE_COMPRESSION=On)

Dependencies

The union of all dependencies can be obtained by running get_deps.sh on an ubuntu-like system.

libghs.so and libghs_seque.so: C++11
libghs_ext.so: as above, and std::
The tools: all of previous
ghs-doctest: all of previous, and libdoctest-dev or a similar doctest installation
ghs-demo: all of previous, and libnng-dev, libinih-dev and miniz checked out in ext/miniz if compression is enabled

There's also ROS support, which brings with it a lot of dependencies not handled by this repo or get_deps.sh

Configuring

I use cmake. apt install cmake

To configure,

mkdir build
pushd build
cmake .. -D<option>=On -D<option 2>=On 
popd  

See CMakeLists.txt for details, but the set of <options> are:

BUILD_DOCS (default=On): build doxygen docs
BUILD_EXT (default=Off): build libghs_ext.so
BUILD_DOCTEST (default=Off): build ghs-doctest. Implies BUILD_EXT
BUILD_DEMO (default=Off): build ghs-demo. Implies BUILD_EXT
ENABLE_ROS (default=Off): Add some CMake sugar to play well with catkin and ROS
ENABLE_COMPRESSION (default=Off): Try out experimental (i.e., not really working) libz compression
BUILD_TOOLS (default=Off): build the utilities ghs-score, to-dot, and random-graph for testing and visualization.

Code coverage checks and performance testing are not implemented.

Trying it out using ghs-demo

You can try it out on various machines. You'll have to set up a config that describes the network, then run ghs-demo on each machine. you can run them all locally, just set the agent endpoints to something like tcp://localhost:<a port per agent> or ipc:///tmp/agent0, ip:///tmp/agent1 etc

This should work fine for a the le_config.ini file:

[agents]
; valid endpoints are sockets, so
; tcp://hostname:port 
; ipc://<file>
; are both valid
; Agents must have consecutive ids starting at 0
0=tcp://localhost:5000
1=tcp://localhost:5001
2=tcp://localhost:5002
3=tcp://localhost:5003
 
; Or locally:
;0=ipc:///tmp/agent0
;1=ipc:///tmp/agent1
;2=ipc:///tmp/agent2
;3=ipc:///tmp/agent3
 
; Or for real fun, on diff machines
;0=tcp://192.168.1.100:5000
;0=tcp://192.168.1.101:5000
;0=tcp://192.168.1.102:5000
;0=tcp://192.168.1.103:5000
 
[runtime]
retry_connections=false

Then, in four terminals, execute:

<le_config.ini ghs-demo --start -i <ID> -w <S>

(usually ghs-demo is in build/src/demo/)

where:

<ID> is the id of the current node (so we know where to listen)
<S> is an optional wait-time in seconds. The node will wait before starting the algorithm this many seconds, to give you a chance to start all nodes. All nodes should be started before any wait timer expires!!

If it works, you'll see Converged!! in all windows, and after a few seconds it should shut down. You can also look at the step-by-step edges used by each node in the output stream, if you have the stomach to look through it.

Installing

There is no install target configured at this time.

Implementation

You don't need to read any of the following, but you may be interested.

Algorithm

The algorithm implemented is best understood by reading chapter 15.5 of Distributed Algorithms by Lynch. In short, each agent begins isolated as leaders of their own partition. Each partition finds an outgoing edge to another partition which is of minimum weight, and the two partitions join up. This continues log(n) rounds until all are in the same partition.

Each round, the leader initiates a search for a new MWOE by all nodes in its partition, and compares returned edges for the minimum weight. The leader broadcasts JOIN messages, which are carefully handled by all nodes to ensure that the resulting MST is consistent and correct. The subtleties of this process are elegantly handled, and it's worth reading the chapter to understand what's gong on. (though I read it many times and may not fully grok all the corner cases yet)

Implementation

A working implementation is provided by ghs-demo, which can be built by configuring with -DBUILD_DEMO=On.

See the documentation of ghs-demo.h, in particular demo::GhsDemoExec for full implementation details.

Style

File organization

h (header) and cpp (src files) are in the same folder: src/
Implementation headers hpp are, too
This is not always well observed under src/ghs-demo

Naming conventions

Types (Structs and Classes) are Capitalized.
Enums are used when all possible values are known apriori.
methods are snake_case
class members are snake_case
files are flying-snake-case

Copyright

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.
Neither the name of Caltech nor its operating division, the Jet Propulsion Laboratory, nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

License

Apache-2.0

See LICENSE.md