mitogen/docs/ansible.rst

.. image:: images/ansible/cell_division.png
    :align: right

Mitogen for Ansible
===================


An extension to `Ansible`_ is included that implements connections over
Mitogen, replacing embedded shell invocations with pure-Python equivalents
invoked via highly efficient remote procedure calls to persistent interpreters
tunnelled over SSH. No changes are required to target hosts.

The extension is approaching stability and real-world testing is now
encouraged. `Bug reports`_ are welcome: Ansible is huge, and only wide testing
will ensure soundness.

.. _Ansible: https://www.ansible.com/

.. _Bug reports: https://goo.gl/yLKZiJ

Overview
--------

**Expect a 1.25x - 7x speedup** and a **CPU usage reduction of at least 2x**,
depending on network conditions, modules executed, and time already spent by
targets on useful work. Mitogen cannot improve a module once it is executing,
it can only ensure the module executes as quickly as possible.

* **One connection is used per target**, in addition to one sudo invocation per
  user account. This is much better than SSH multiplexing combined with
  pipelining, as significant state can be maintained in RAM between steps, and
  system logs aren't spammed with repeat authentication events.

* **A single network roundtrip is used** to execute a step whose code already
  exists in RAM on the target. Eliminating multiplexed SSH channel creation
  saves 5 ms runtime per 1 ms of network latency for every playbook step.

* **Processes are aggressively reused**, avoiding the cost of invoking Python
  and recompiling imports, saving 300-800 ms for every playbook step.

* Code is ephemerally cached in RAM, **reducing bandwidth usage by an order
  of magnitude** compared to SSH pipelining, with around 5x fewer frames
  traversing the network in a typical run.

* **No writes to the target's filesystem occur**, unless explicitly triggered
  by a playbook step. In all typical configurations, Ansible repeatedly
  rewrites and extracts ZIP files to multiple temporary directories on the
  target. Since no temporary files are used, security issues relating to those
  files in cross-account scenarios are entirely avoided.


Demo
~~~~

This demonstrates Ansible running a subset of the Mitogen integration tests
concurrent to an equivalent run using the extension.

.. raw:: html

    <video width="720" height="439" controls>
        <source src="http://k3.botanicus.net/tmp/ansible_mitogen.mp4" type="video/mp4">
    </video>


Testimonials
~~~~~~~~~~~~

* "With mitogen **my playbook runtime went from 45 minutes to just under 3
  minutes**. Awesome work!"

* "The runtime was reduced from **1.5 hours on 4 servers to just under 3
  minutes**. Thanks!"

* "Oh, performance improvement using Mitogen is *huge*. As mentioned before,
  running with Mitogen enables takes 7m36 (give or take a few seconds). Without
  Mitogen, the same run takes 19m49! **I'm not even deploying without Mitogen
  anymore** :)"

* "**Works like a charm**, thank you for your quick response"

* "I tried it out. **He is not kidding about the speed increase**."

* "I don't know what kind of dark magic @dmw_83 has done, but his Mitogen
  strategy took Clojars' Ansible runs from **14 minutes to 2 minutes**. I still
  can't quite believe it."


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

1. Thoroughly review the documented behavioural differences.
2. Verify Ansible 2.4/2.5 and Python 2.7 are listed in ``ansible --version``
   output.
3. Download and extract https://github.com/dw/mitogen/archive/master.zip
4. Modify ``ansible.cfg``:

   .. code-block:: dosini

        [defaults]
        strategy_plugins = /path/to/mitogen-master/ansible_mitogen/plugins/strategy
        strategy = mitogen_linear

   The ``strategy`` key is optional. If omitted, the
   ``ANSIBLE_STRATEGY=mitogen_linear`` environment variable can be set on a
   per-run basis. Like ``mitogen_linear``, the ``mitogen_free`` strategy exists
   to mimic the ``free`` strategy.


Noteworthy Differences
----------------------

* Ansible 2.4 and 2.5 are supported. File bugs to register interest in older
  releases.

* The ``sudo`` become method is available and ``su`` is planned. File bugs to
  register interest in additional methods.

* The ``ssh``, ``local`` and ``docker`` connection types are available, with
  more planned. File bugs to register interest.

* Local commands execute in a reuseable interpreter created identically to
  interpreters on targets. Presently one interpreter per ``become_user``
  exists, and so only one local action may execute simultaneously.

  Ansible usually permits up to ``forks`` simultaneous local actions. Any
  long-running local actions that execute for every target will experience
  artificial serialization, causing slowdown equivalent to `task_duration *
  num_targets`. This will be fixed soon.

* Asynchronous jobs presently exist only for the duration of a run, and time
  limits are not implemented.

* Due to use of :func:`select.select` the IO multiplexer breaks down around 100
  targets, expect performance degradation as this number is approached and
  errant behaviour as it is exceeded. A replacement will appear soon.

* The undocumented ability to extend :mod:`ansible.module_utils` by supplying a
  ``module_utils`` directory alongside a custom new-style module is not yet
  supported.

* "Module Replacer" style modules are not supported. These rarely appear in
  practice, and light web searches failed to reveal many examples of them.

* Ansible permits up to ``forks`` connections to be setup in parallel, whereas
  in Mitogen this is handled by a fixed-size thread pool. Up to 16 connections
  may be established in parallel by default, this can be modified by setting
  the ``MITOGEN_POOL_SIZE`` environment variable.

* Performance does not scale perfectly linearly with target count. This will
  improve over time.

* Timeouts normally apply to the combined runtime of the SSH and become steps
  of a task. As Mitogen treats SSH and sudo distincly, during a failure the
  effective timeout may appear to double.


New Features & Notes
--------------------


Connection Delegation
~~~~~~~~~~~~~~~~~~~~~

.. image:: images/jumpbox.png
    :align: right

Included is a preview of **Connection Delegation**, a Mitogen-specific
implementation of `stackable connection plug-ins`_. This enables multi-hop
connections via a bastion, or Docker connections delegated via their host
machine, where reaching the host may itself entail recursive delegation.

.. _Stackable connection plug-ins: https://github.com/ansible/proposals/issues/25

Unlike with SSH forwarding Ansible has complete visibility of the final
topology, declarative configuration via static/dynamic inventory is possible,
and data can be cached and re-served, and code executed on every intermediary.

For example when targeting Docker containers on a remote machine, each module
need only be uploaded once for the first task and container that requires it,
then cached and served from the SSH account for every future task in any
container.

.. raw:: html

    <div style="clear: both;"></div>


.. caution::

    Connection delegation is a work in progress, bug reports are welcome.

    * While imports are cached on intermediaries, module scripts are needlessly
      reuploaded for each target. Fixing this is equivalent to implementing
      **Topology-Aware File Synchronization**, so it may remain unfixed until
      that feature is started.

    * Delegated connection setup is single-threaded; only one connection can be
      constructed in parallel per intermediary.

    * Unbounded queue RAM growth may occur in an intermediary during large file
      transfers if the link between any two hops is slower than the link
      between the controller and the first hop.

    * Inferring the configuration of intermediaries may be buggy, manifesting
      as duplicate connections between hops, due to not perfectly replicating
      the configuration Ansible would normally use for the intermediary.

    * The extension does not understand the difference between a delegated
      connection and a ``become_user``. If interpreter recycling kicks in, a
      delegated connection could be prematurely recycled.

To enable connection delegation, set ``mitogen_via=<inventory name>`` on the
command line, or as host and group variables.

.. code-block:: ini

    # Docker container on web1.dc1 is reachable via web1.dc1.
    [app-containers.web1.dc1]
    app1.web1.dc1 ansible_host=app1 ansible_connection=docker mitogen_via=web1.dc1

    # Web servers in DC1 are reachable via bastion.dc1
    [dc1]
    web1.dc1
    web2.dc1
    web3.dc1

    [dc1:vars]
    mitogen_via = bastion.dc1

    # Web servers in DC2 are reachable via bastion.dc2
    [dc2]
    web1.dc2
    web2.dc2
    web3.dc2

    [dc2:vars]
    mitogen_via = bastion.dc2

    # Prod bastions are reachable via a magic account on a
    # corporate network gateway.
    [bastions]
    bastion.dc1 mitogen_via=prod-ssh-access@corp-gateway.internal
    bastion.dc2 mitogen_via=prod-ssh-access@corp-gateway.internal

    [corp-gateway]
    corp-gateway.internal


File Transfer
~~~~~~~~~~~~~

Normally a tool like ``scp`` is used to copy a file with the ``copy`` or
``template`` actions, or when uploading modules with pipelining disabled. With
Mitogen copies are implemented natively using the same interpreters, connection
tree, and routed message bus that carries RPCs.

This permits streaming directly between endpoints regardless of execution
environment, without necessitating temporary copies in intermediary accounts or
machines, for example when ``become`` is active, or in the presence of
connection delegation. It also neatly avoids the problem of securely sharing
temporary files between accounts and machines.

One roundtrip is required to initiate a transfer. For any tool that operates
via SSH multiplexing, 5 are required to configure the associated IO channel, in
addition to the time needed to start the local and remote processes. A complete
localhost invocation of ``scp`` requires around 15 ms.

As the implementation is self-contained, it is simple to make future
improvements like prioritizing transfers, supporting resume, or displaying
progress bars.


Interpreter Reuse
~~~~~~~~~~~~~~~~~

Python interpreters are aggressively reused to execute modules. While this
works well, it violates an unwritten assumption, and so it is possible an
earlier module execution could cause a subsequent module to fail, or for
unrelated modules to interact poorly due to bad hygiene, such as
monkey-patching that becomes stacked over repeat invocations.

Before reporting a bug relating to a misbehaving module, please re-run with
``-e mitogen_task_isolation=fork`` to see if the problem abates. This may be
set per-task, paying attention to the possibility an earlier task may be the
true cause of a failure.

.. code-block:: yaml

    - name: My task.
      broken_module:
        some_option: true
      vars:
        mitogen_task_isolation: fork

If forking solves your problem, **please report a bug regardless**, as an
internal list can be updated to prevent others bumping into the same problem.


Interpreter Recycling
~~~~~~~~~~~~~~~~~~~~~

There is a per-target limit on the number of interpreters. Once 20 exist, the
youngest is terminated before starting any new interpreter, preventing
situations like below from triggering memory exhaustion.

.. code-block:: yaml

    - hosts: corp_boxes
      vars:
        user_directory: [
          # 10,000 corporate user accounts
        ]
      tasks:
        - name: Create user bashrc
          become: true
          vars:
            ansible_become_user: "{{item}}"
          copy:
            src: bashrc
            dest: "~{{item}}/.bashrc"
          with_items: "{{user_directory}}"

The youngest is chosen to preserve useful accounts like ``root`` and
``postgresql`` that often appear early in a run, however it is simple to
construct a playbook that defeats this strategy. A future version will key
interpreters on the identity of their creating task, avoiding useful account
recycling in every scenario.

To modify the limit, set the ``MITOGEN_MAX_INTERPRETERS`` environment variable.


Standard IO
~~~~~~~~~~~

Ansible uses pseudo TTYs for most invocations to allow it to type interactive
passwords, however pseudo TTYs are disabled where standard input is required or
``sudo`` is not in use. Additionally when SSH multiplexing is enabled, a string
like ``Shared connection to localhost closed\r\n`` appears in ``stderr`` of
every invocation.

Mitogen does not naturally require either of these, as command output is always
embedded within framed messages, and it can simply call :py:func:`pty.openpty`
in any location an interactive password must be typed.

A major downside to Ansible's behaviour is that ``stdout`` and ``stderr`` are
merged together into a single ``stdout`` variable, with carriage returns
inserted in the output by the TTY layer. However ugly, the extension emulates
this precisely, to avoid breaking playbooks that expect text to appear in
specific variables with a particular linefeed style.


How Modules Execute
~~~~~~~~~~~~~~~~~~~

Ansible usually modifies, recompresses and reuploads modules every time they
run on a target, work that must be repeated by the controller for every
playbook step.

With the extension any modifications are done on the target, allowing pristine
copies of modules to be cached, reducing the necessity to re-transfer modules
for each invocation. Unmodified modules are uploaded once on first use and
cached in RAM for the remainder of the run.

**Binary**
    Native executables detected using a complex heuristic. Arguments are
    supplied as a JSON file whose path is the sole script parameter.

**Module Replacer**
    Python scripts detected by the presence of
    ``#<<INCLUDE_ANSIBLE_MODULE_COMMON>>`` appearing in their source. This type
    is not yet supported.

**New-Style**
    Python scripts detected by the presence of ``from ansible.module_utils.``
    appearing in their source. Arguments are supplied as JSON written to
    ``sys.stdin`` of the target interpreter.

**JSON_ARGS**
    Detected by the presence of ``INCLUDE_ANSIBLE_MODULE_JSON_ARGS`` appearing
    in the script source. The interpreter directive (``#!interpreter``) is
    adjusted to match the corresponding value of ``{{ansible_*_interpreter}}``
    if one is set. Arguments are supplied as JSON mixed into the script as a
    replacement for ``INCLUDE_ANSIBLE_MODULE_JSON_ARGS``.

**WANT_JSON**
    Detected by the presence of ``WANT_JSON`` appearing in the script source.
    The interpreter directive is adjusted as above. Arguments are supplied as a
    JSON file whose path is the sole script parameter.

**Old Style**
    Files not matching any of the above tests. The interpreter directive is
    adjusted as above. Arguments are supplied as a file whose path is the sole
    script parameter. The format of the file is ``"key=repr(value)[
    key2=repr(value2)[ ..]] "``.


Runtime Patches
~~~~~~~~~~~~~~~

Three small runtime patches are employed in ``strategy.py`` to hook into
desirable locations, in order to override uses of shell, the module executor,
and the mechanism for selecting a connection plug-in. While it is hoped the
patches can be avoided in future, for interesting versions of Ansible deployed
today this simply is not possible, and so they continue to be required.

The patches are concise and behave conservatively, including by disabling
themselves when non-Mitogen connections are in use. Additional third party
plug-ins are unlikely to attempt similar patches, so the risk to an established
configuration should be minimal.


Flag Emulation
~~~~~~~~~~~~~~

Mitogen re-parses ``sudo_flags``, ``become_flags``, and ``ssh_flags`` using
option parsers extracted from `sudo(1)` and `ssh(1)` in order to emulate their
equivalent semantics. This allows:

* robust support for common ``ansible.cfg`` tricks without reconfiguration,
  such as forwarding SSH agents across ``sudo`` invocations,
* reporting on conflicting flag combinations,
* reporting on unsupported flag combinations,
* internally special-casing certain behaviour (like recursive agent forwarding)
  without boring the user with the details,
* avoiding opening the extension up to untestable scenarios where users can
  insert arbitrary garbage between Mitogen and the components it integrates
  with,
* precise emulation by an alternative implementation, for example if Mitogen
  grew support for Paramiko.


Supported Variables
-------------------

Matching Ansible's model, variables are treated on a per-task basis, causing
establishment of additional reuseable interpreters as necessary to match the
configuration of each task.


SSH
~~~

This list will grow as more missing pieces are discovered.

* ``ansible_ssh_timeout``
* ``ansible_host``, ``ansible_ssh_host``
* ``ansible_user``, ``ansible_ssh_user``
* ``ansible_port``, ``ssh_port``
* ``ansible_ssh_executable``, ``ssh_executable``
* ``ansible_ssh_private_key_file``
* ``ansible_ssh_pass``, ``ansible_password`` (default: assume passwordless)
* ``ssh_args``, ``ssh_common_args``, ``ssh_extra_args``


Sudo
~~~~

* ``ansible_python_interpreter``
* ``ansible_sudo_exe``, ``ansible_become_exe``
* ``ansible_sudo_user``, ``ansible_become_user`` (default: ``root``)
* ``ansible_sudo_pass``, ``ansible_become_pass`` (default: assume passwordless)
* ``sudo_flags``, ``become_flags``
* ansible.cfg: ``timeout``


Docker
~~~~~~

Docker support has received relatively little testing, expect increased
probability of surprises for the time being.

* ``ansible_host``


Debugging
---------

Diagnostics and use of the :py:mod:`logging` package output on the target
machine are usually discarded. With Mitogen, all of this is captured and
returned to the controller, where it can be viewed as desired with ``-vvv``.
Basic high level logs are produced with ``-vvv``, with logging of all IO on the
controller with ``-vvvv`` or higher.

Although use of standard IO and the logging package on the target is forwarded
to the controller, it is not possible to receive IO activity logs, as the
processs of receiving those logs would would itself generate IO activity. To
receive a complete trace of every process on every machine, file-based logging
is necessary. File-based logging can be enabled by setting
``MITOGEN_ROUTER_DEBUG=1`` in your environment.

When file-based logging is enabled, one file per context will be created on the
local machine and every target machine, as ``/tmp/mitogen.<pid>.log``.


Getting Help
~~~~~~~~~~~~
Some users and developers hang out on the
`#mitogen <https://webchat.freenode.net/?channels=mitogen>`_ channel on the
FreeNode IRC network.


Sample Profiles
---------------

Local VM connection
~~~~~~~~~~~~~~~~~~~

This demonstrates Mitogen vs. connection pipelining to a local VM, executing
the 100 simple repeated steps of ``run_hostname_100_times.yml`` from the
examples directory. Mitogen requires **43x less bandwidth and 4.25x less
time**.

.. image:: images/ansible/run_hostname_100_times.png


Kathmandu to Paris
~~~~~~~~~~~~~~~~~~

This is a full Django application playbook over a ~180ms link between Kathmandu
and Paris. Aside from large pauses where the host performs useful work, the
high latency of this link means Mitogen only manages a 1.7x speedup.

Many early roundtrips are due to inefficiencies in Mitogen's importer that will
be fixed over time, however the majority, comprising at least 10 seconds, are
due to idling while the host's previous result and next command are in-flight
on the network.

The initial extension lays groundwork for exciting structural changes to the
execution model: a future version will tackle latency head-on by delegating
some control flow to the target host, melding the performance and scalability
benefits of pull-based operation with the management simplicity of push-based
operation.

.. image:: images/ansible/costapp.png