lynk 0.3.1
Client for using AWS DynamoDB as a distributed lock.
License: Apache License 2.0

Lynk
====

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.. intro-begin

Lynk is a Distributed Lock Manager (DLM) that uses DynamoDB to track the state
of its locks. Lynk is a cooperative locking scheme where each client assumes
that all others in the system are obeying a set of rules in order to assure
the integrity of the locks.

.. intro-end


Documentation
=============

The docs are hosted at `readthedocs <https://lynk.readthedocs.io/en/latest/>`_


Quickstart
==========

The quickstart guide looks better at
`readthedocs <https://lynk.readthedocs.io/en/latest/quickstart.html>`_

.. quick-start-begin


Installation
------------

Lynk is available on PyPi as ``lynk`` and can be installed in the usual way
with pip::

  $ pip install lynk


AWS Credentials
---------------

Lynk uses boto3 in order to make all calls to AWS, which means it uses the
boto3 standard credential chain. Make sure your machine has AWS credentials
configured in the way `boto3 expects <https://boto3.amazonaws.com/v1/documentation/api/latest/guide/configuration.html>`_.


Creating a table
----------------

In order to store the locks we need to create a DynamoDB table. For ease of
getting started there is a command line tool installed along with the package
to help manage lynk tables.

To create a table called ``quickstart`` run the ``lynk create-table`` command::

  $ lynk create-table lynk-quickstart
  Creating table lynk-quickstart
  Created

With the ``lynk list-tables`` command line tool you can check a list of tables
created this way by lynk::

  $ lynk list-tables
  lynk-quickstart


Creating a lock
---------------

Locks are shared through a DynamoDB table, in our case we will be using the
``lynk-quickstart`` we created earlier table. Locks are distinguished by a
lock name, within their table. To get create a lock, we first need to create a
:class:`lynk.session.Session` that is bound to our table. The session can
then be used to create multiple locks that will be backed by that table.

The easiet way to make a :class:`lynk.session.Session` is by using the
:func:`lynk.get_session` function. This function only takes one argument
which is the name of the table it is bound to. Once a session has been created
it can be used to create lock objects using the
:meth:`lynk.session.Session.create_lock` method.

.. code-block:: python

   import lynk

   session = lynk.get_session('lynk-quickstart')
   lock = session.create_lock('my lock')

``lock`` is an instance of :class:`lynk.lock.Lock` which is bound to both our
table ``lynk-quickstart``, and the logical lock name ``my lock``. If we create
another lock object bound to the same table, with the same lock name, only one
will be acquireable at a time, with the second having to wait for the first one
to release before being able to acquire it. This is a little bit awkard to
show in a single code segment since it requires muiltiple threads. Below is a
minimal but complete example of using two threads to contend for the same lock.

.. code-block:: python

   import time
   import logging
   import threading

   import lynk


   LOG = logging.getLogger(__file__)


   def configure_logging():
       LOG.setLevel(logging.DEBUG)
       formatter = logging.Formatter('%(threadName)s - %(message)s')
       ch = logging.StreamHandler()
       ch.setFormatter(formatter)
       LOG.addHandler(ch)


   def thread(session):
       LOG.debug('Starting')
       lock = session.create_lock('my lock')
       lock.acquire()
       LOG.debug('Lock acquired')
       time.sleep(10)
       lock.release()
       LOG.debug('Lock released')


   def main():
       configure_logging()
       session = lynk.get_session('lynk-quickstart')
       t1 = threading.Thread(target=thread, args=(session,))
       t2 = threading.Thread(target=thread, args=(session,))

       t1.start()
       t2.start()
       t1.join()
       t2.join()


   if __name__ == "__main__":
       main()


First, we can ignore the ``configure_logging`` function, it just sets up
logging to show which thread is emitting the logs. This makes it easier to track
the flow of our program.

Looking at the ``main`` function, the first real thing that happens  we create
a session that can create locks bound to our table ``lynk-quickstart``.

.. code-block:: python

   session = lynk.get_session('lynk-quickstart')

We then create two thread objects, and pass our ``session`` object into each
as a shared variable. Once started each thread will execute the ``thread``
function.

.. code-block:: python

   t1 = threading.Thread(target=thread, args=(session,))
   t2 = threading.Thread(target=thread, args=(session,))


The last thing the ``main`` function does is start both threads, then join on
them, which will wait for them to complete before exiting.

.. code-block:: python

   t1.start()
   t2.start()
   t1.join()
   t2.join()


Now we have two threads executing the ``thread`` function. Following along each
thread, disregarding the log statements, the first thing it does is create a
lock object.

.. code-block:: python

   lock = session.create_lock('my lock')

This means each thread will have its own unique lock object linked logically to
the name ``my lock``. The threads share a session, which is bound to the table
``lynk-quickstart``. Simply creating the lock does not interact with the
DynamoDB Tables in any way.

Next each thread tries to acquire the lock.

.. code-block:: python

   lock.acquire()

This simple statement is what makes the call to write an entry in our DynamoDB
Table. Once an entry is written, this indicates that the lock is in-use and
we are safe to operate on whatever resource this lock was responsible for
protecting. In this example case we simply sleep for 10 seconds and then
release the lock.

.. code-block:: python

   time.sleep(10)
   lock.release()

The ``time.sleep(10)`` call would be replaced with real work in an actual
application. Once the protected resource is done being operated on, and has
been safely written and is ready for another agent to use, we release the
lock. The :meth:`lynk.lock.Lock.release` call deletes the entry from the table
freeing the lock name up to be used by another agent.


The output of our little sample application is shown below. You can see one
thread gets the lock (in this case ``Thread-2``) and does it work while the
other thread waits for it to be released. Once released, the other thread
repeats the same process::

   Thread-1 - Starting
   Thread-2 - Starting
   Thread-2 - Lock acquired
   Thread-2 - Lock released
   Thread-1 - Lock acquired
   Thread-1 - Lock released


More complex but similar examples can be seen in the
`examples <https://github.com/stealthycoin/lynk/tree/master/examples>`_
directory of the source repo.


Lock entry details
------------------

If you have the AWS CLI installed you can run the following command while the
example script above is running (shouldn't be too difficult since the script
takes around 30 seconds to complete)::

  $ aws dynamodb scan --table-name lynk-quickstart --query Items
  [
      {
	  "lockKey": {
	      "S": "my lock"
	  },
	  "leaseDuration": {
	      "N": "20"
	  },
	  "versionNumber": {
	      "S": "dabbbfde-93cb-47f8-a249-fbae84c4a5e3"
	  },
	  "hostIdentifier": {
	      "S": "Johns-MacBook-Pro.local"
	  }
      }
  ]

While the lock is held by a thread, we can see the entry that marks it as in
use. It has four components, the ``lockKey`` which is clearly the lock name
that we selected when creating our lock object. A ``leaseDuration``, this is
the amount of time we have a lease on this lock. Any other agent that wants
to acquire this lock must wait at least that long before trying again. Our
example code will refresh this lock automatically, even if we had slept longer
than 20 seconds.
The ``versionNumber`` is used as a fencing token, each write to this entry
changes this value. You can read more about how the ``leaseDuration`` and
``versionNumber`` are used to ensure the lock integrity in the documentation
for the :class:`lynk.techniques.VersionLeaseTechinque`. Finally there is a
``hostIdentifier`` which is just there to show the host that created the lock.
This can be used for debugging a distributed multi-agent system all using one
lock table.

More examples can be found in the
`examples <https://github.com/stealthycoin/lynk/tree/master/examples>`_
directory in the source repo.


Context manager
---------------

In the above example we manually call ``acquire()`` and ``release()``. This depends on no
exceptions ocurring, and would generally be safer in a ``try: finally:`` block. For
convenience the :class:`lynk.lock.Lock` object can be called and used as a context manager.
The following code:

.. code-block:: python

   lock.acquire()
   time.sleep(10)
   lock.release()

Can be re-written more safely, and conveinently, as:

.. code-block:: python

   with lock():
       time.sleep(10)

This ensures the releasing in the lock in the case of an unexpected exception.


Teardown
--------

To tear down the resources created during the quickstart tutorial run the
``lynk delete-table`` command::

  $ lynk delete-table lynk-quickstart
  Deleting table lynk-quickstart
  Deleted

Verify that there are no left over tables checking that the following has no
output::

  $ lynk list-tables


.. quick-start-end