DistAlgo

DistAlgo is a very high-level language for programming distributed algorithms. This project implements a DistAlgo compiler with Python as the target language. In the following text, the name 'DistAlgo' refers to the compiler and not the language.

1. Requirements

Python

DistAlgo requires Python version 3.5, 3.6, or 3.7, which can be obtained from http://www.python.org. This document assumes that your installation uses the default name python for the Python executable.

NOTE: If your system has both Python 2.x and Python 3.x installed, your Python executable is likely Python 2. In that case, you should replace python with python3 (or pythonX.Y where 'X.Y' is the exact Python version you want to use) in all following command line examples. To find out which version of Python is installed on your system, type:

  python --version

Operating system

DistAlgo has been tested on GNU/Linux and Microsoft Windows. The command line instructions given in this document use GNU Bash syntax. If you are using a different shell (e.g., Windows 'cmd.exe' shell), please adjust the commands accordingly.

2. Installation

Installation of DistAlgo is entirely optional. The installation process consists of copying or extracting the DistAlgo files to a path in the local filesystem (designated as <DAROOT> in the following texts), then adding <DAROOT> to PYTHONPATH so that Python can load the da module. You can accomplish this through either one of the following options:

Option 1: Using pip to install DistAlgo

pip is a command line utility for installing Python packages from the Python Package Index(PyPI). pip is the recommended method of installing DistAlgo. Using pip, you do not need to manually download the DistAlgo distribution package or setup environment variables, as pip will manage all of that for you. The name of the DistAlgo package on PyPI is 'pyDistAlgo'.

To install DistAlgo as a system-wide package:

 pip install pyDistAlgo

This command will likely require administrator privileges.

To install DistAlgo for the current user only:

 pip install --user pyDistAlgo

If you have installed DistAlgo for both system and the current user, the user installation will take precedence.

To upgrade an existing DistAlgo installation to a newer version:

 pip install --upgrade [--user] pyDistAlgo

Installing pre-release versions using pip

By default, pip only installs "stable" versions from the PyPI repository. If you would like to install or upgrade to the latest version, pass the --pre flag to pip install (this is needed now if you use Python 3.7):

 pip install --pre pyDistAlgo
 pip install --upgrade --pre pyDistAlgo

Option 2: Using setup.py

If you have already downloaded a DistAlgo distribution package, you can install it using the included setup.py file. To see full usage description, type:

  cd <DAROOT>; python setup.py --help

The following command installs DistAlgo as system-wide package:

  cd <DAROOT>; python setup.py install

The following command installs DistAlgo for the current user:

  cd <DAROOT>; python setup.py install --user

If you have installed DistAlgo for both system and user, the user installation takes precedence.

Option 3: Manually adding the DistAlgo root directory to PYTHONPATH

If you have downloaded and extracted the DistAlgo files to <DAROOT>, you can simply add the DistAlgo root directory to your PYTHONPATH environment variable by running the following command in your shell:

  export PYTHONPATH=<DAROOT>:${PYTHONPATH}

Afterwards, the da module will be available in all python instances launched from this shell. You can add the above command to the initialization scripts for your shell to avoid typing this command in each new shell instance.

The <DAROOT> directory installed using this method takes precedence over any DistAlgo packages installed by pip or setup.py.

Option 4: Running DistAlgo without installation

Alternatively, if you do not wish to install the DistAlgo package or modify the PYTHONPATH environment variable, you can simply run DistAlgo using the scripts provided under the directory <DAROOT>/bin. This directory contains two Python scripts, dac and dar, that runs the DistAlgo compiler and runtime, respectively. These scripts will automatically detect <DAROOT> and add it to the Python variable sys.path so no installation is required.

To avoid typing <DAROOT>/bin in running the scripts, add it to your PATH environment variable:

  export PATH=<DAROOT>/bin:${PATH}

NOTE: The scripts assume your Python executable is installed to /usr/bin/python3; if that is not the case, you must modify the first line in the scripts to point to your Python executable.

For Windows only: The Windows program loader does not recognize the "Shebang" (#!) sequence, so scripts dac and dar will not work under the 'cmd.exe' shell. To work around this limitation, the '/bin' directory also contains dac.bat and dar.bat, which are simple batch script wrappers for dac and dar. To use these batch scripts under Windows, substitute dac and dar with dac.bat and dar.bat, respectively, for all following examples. The batch scripts assume your Python executable is python, if that is not the case then you must modify the batch files with the full path to your Python executable.

3. Running DistAlgo

The DistAlgo system consists of a compiler and a runtime. Under normal circumstances, you do not need to invoke the compiler directly, because the runtime will invoke the compiler if necessary.

For both the compiler and runtime, use command line argument -h to see a full description of available options.

Invoking the compiler

You only need to run the compiler if you wish to see the generated Python code for a DistAlgo source file. Note that the generated Python file is for informational purposes only, it is not used by the runtime for the purpose of running a DistAlgo module -- the runtime always compiles and loads the code directly from the '.da' source file.

If you have installed DistAlgo, run module da.compiler, passing a DistAlgo source file <SOURCE> as argument:

  python -m da.compiler <SOURCE>

Otherwise, call the dac script (adding prefix <DAROOT>/bin/ if you did not add <DAROOT>/bin to your PATH variable):

  dac <SOURCE>

Invoking the runtime

To run DistAlgo programs, run the da module, passing a DistAlgo source file as argument:

  python -m da <SOURCE>

or call the dar script:

  dar <SOURCE>

The --version(or -v) command line option will print out the version number of the installed DistAlgo system. The --help(or -h) command line option will print out a list of all command line options along with a brief description of each option.

Running a DistAlgo module as a script

Instead of passing a path to a DistAlgo source file on the command line, you can use the '-m' option to run a DistAlgo module as though it were a script:

 python -m da -m <MODULE>

The DistAlgo command line option '-m' mimics Python's own '-m' option. <MODULE> must be a DistAlgo module in dotted form and without the '.da' suffix. The source file for the module is located by the same rules that govern Python's own module loading process.

Passing command line arguments to the DistAlgo program

Command line arguments before the <SOURCE> argument are passed to the DistAlgo runtime; arguments after the <SOURCE> argument are passed to the DistAlgo source program in the global sys.argv list. Alternatively, if you are using the '-m' option to run a DistAlgo module, command line arguments before the -m <MODULE> argument are passed to the DistAlgo runtime; arguments after the -m <MODULE> argument are passed to the DistAlgo source program in the global sys.argv list.

For example, the following command passes argument -i to the DistAlgo runtime, and passes arguments a and 1 to source program mutex.da:

   dar -i mutex.da a 1

Quitting

If you wish to quit your program before it terminates, press Ctrl-C. Depending on the timing of this interrupt, you may see some exceptions being thrown. This is expected behavior; simply press Ctrl-C again to fully terminate the program.

For Cygwin with native Python only: If you are running native Windows Python under a Cygwin terminal, Ctrl-C will not propagate to the child processes: only the parent process will be killed, and the children will continue to run in the background. You need to manually terminate the child processes from the Windows task manager.

Examples

This section assumes you have installed DistAlgo; otherwise, please replace python -m da with dar or with <DAROOT>/bin/dar.

The following command runs the Lamport mutual exclusion example:

   python -m da -m da.examples.lamutex.orig

The following command runs the same program, but passes 20 to orig.da, causing the program to create 20 processes:

   python -m da -m da.examples.lamutex.orig 20

The following command runs the same program, but passes -f to the runtime, causing a log file to be created for this run:

   python -m da -f -m da.examples.lamutex.orig

After running the above command, you should find a file orig.da.log under the current directory.

The following command runs the same program, but passes -L debug to the runtime, causing debugging output to be printed to the console:

   python -m da -L debug -m da.examples.lamutex.orig

Running multiple nodes

When you start a DistAlgo program, a special DistAlgo process known as the "node process" is created. The node process is responsible for running the main method of the program.

By default, the node process is unnamed, and as such will not be able to talk to other node processes, and any DistAlgo process running on an unnamed node will not be able to communicate with DistAlgo processes running on other nodes. In order to have multiple node processes that communicate with each other, you must give each one a unique name. A node name can be any string that does not include the characters '@', '#', and ':'. A node process can be named by using the -n <NAME> command line argument. For example, the command

  python -m da -n Node1 -m da.examples.lamutex.orig

will start a node named 'Node1', which then runs the mutual exclusion example. Notice that the formatted process ids in the output of this command now include a "#Node1" suffix, to indicate that the processes are running on the 'Node1' node.

Node names can be used as the at argument when calling the new function, which instructs the system to create the new processes on the named node(s) instead of locally. The following program, 'pingpong.da', creates a Pong process on the node named 'PongNode', then creates a Ping process on the local node, and finally starts both:

  class Ping(process):
      def setup(pong): pass
      def run():
          send(('Ping',), to=pong)
          await(received(('Pong',)))
          output("Ponged.")
  
  class Pong(process):
      def setup(): pass
      def run():
          await(some(received(('Ping',), from_=ping)))
          output("Pinged.")
          send(('Pong',), to=ping)
  
  def main():
      pong = new(Pong, args=(), at='PongNode')
      ping = new(Ping, args=(pong,))
      start(pong)
      start(ping)

To run this example program, first start a node named anything other than 'PongNode', for example 'PingNode', and tell it to run 'pingpong.da':

 python -m da -n PingNode pingpong.da

'PingNode' will run the main method and attempt to create a Pong process on a node named 'PongNode'. But since it does not yet know which node is 'PongNode', it will block at the first new statement waiting to resolve the name 'PongNode'.

In order for the program execution to continue, you must start another node and name it 'PongNode':

 python -m da -n PongNode -D pingpong.da

The same 'pingpong.da' file has to be specified on the command line, because the 'PongNode' needs to have access to the Pong class in order to create Pong processes. The command line parameter '-D'(or equivalently, '--idle') tells the system to create an "idle" node. Idle nodes do not execute their main method, and their only responsibility is to create DistAlgo processes on behalf of other nodes. If you omit '-D', then 'PongNode' will also run the main method, creating an additional Ping and Pong process each, which may or may not be your desired outcome.

At this point, 'PingNode' will be able to resolve the name 'PongNode', and the execution of main can continue as usual. After 'PingNode' terminates, you should be able to observe the line

 pingpong.Ping<Ping:eb002#PingNode>:OUTPUT: Ponged.

on the terminal running the 'PingNode', and the line

 pingpong.Pong<Pong:54802#PongNode>:OUTPUT: Pinged.

on the terminal running the 'PongNode' (the 5 hex-digit process id values may differ).

To specify nodes running on remote hosts, add the remote hostname as a suffix to the node name using the @ separator. For example PongNode@PongHost specifies a node named 'PongNode' that is running on the host named 'PongHost'.

Cookies

In a DistAlgo system involving multiple nodes, a pre-shared secret key, known as a "cookie", can be used to authenticate processes and prevent unauthorized processes from sending messages to DistAlgo processes. Cookies can be set when starting a node process, and any DistAlgo processes started on that node will automatically inherit its cookie value. You can set the cookie for a node by using the '--cookie' command line option:

 python -m da --cookie SECRET -n PongNode pingpong.da

In this case, any process that does not have a matching cookie will not be able to send messages to 'PongNode' or any DistAlgo process running on 'PongNode'.

Alternatively, you can store the cookie value in a file named '.da.cookie' under you home directory:

 echo -n "SECRET" > ${HOME}/.da.cookie
 chmod 600 ${HOME}/.da.cookie

This way, all DistAlgo nodes will automatically use the contents of '${HOME}/.da.cookie' as their cookie, unless you explicitly specify one on the command line using '--cookie'.

As a special case, when an unnamed node is started, it sets its cookie to a random value, thus preventing this node and any DistAlgo processes created by this node from accidentally communicating with other nodes and processes.

SECURITY WARNING: Any remote or local process that knows your cookie and can send UDP packets to the UDP port or make TCP connections to the TCP port used by any DistAlgo process, will be able to trick the DistAlgo system into executing arbitrary code on your system. Never share your cookie with untrusted parties!

4. Further References

For a full description of the DistAlgo language, see <DAROOT>/doc/language.pdf. For a quick reference of all DistAlgo built-in functions, run the following command:

  python -m da -B

For DistAlgo examples, see <DAROOT>/da/examples/.