Ansible Extension ================= .. image:: images/ansible/cell_division.png :align: right An experimental extension to `Ansible`_ is included that implements host connections over Mitogen, replacing embedded shell invocations with pure-Python equivalents invoked via highly efficient remote procedure calls tunnelled over SSH. No changes are required to the target hosts. The extension isn't nearly in a generally dependable state yet, however it already works well enough for testing against real-world playbooks. `Bug reports`_ in this area are very welcome – Ansible is a huge beast, and only significant testing will prove the extension's soundness. Divergence from Ansible's normal behaviour is considered a bug, so please report anything you notice, regardless of how inconsequential it may seem. .. _Ansible: https://www.ansible.com/ .. _Bug reports: https://goo.gl/yLKZiJ Overview -------- You should **expect a 1.25x - 7x speedup** and a **CPU usage reduction of at least 2x**, depending on network conditions, the specific modules executed, and time spent by the target host already doing useful work. Mitogen cannot speed up a module once it is executing, it can only ensure the module executes as quickly as possible. * **A single SSH connection is used for each target host**, in addition to one sudo invocation per distinct user account. Subsequent playbook steps always reuse the same connection. This is much better than SSH multiplexing combined with pipelining, as significant state can be maintained in RAM between steps, and the system logs aren't filled with spam from repeat SSH and sudo invocations. * **A single Python interpreter is used** per host and sudo account combination for the duration of the run, avoiding the repeat cost of invoking multiple interpreters and recompiling imports, saving 300-800 ms for every playbook step. * Remote interpreters reuse Mitogen's module import mechanism, caching uploaded dependencies between steps at the host and user account level. As a consequence, **bandwidth usage is consistently an order of magnitude lower** compared to SSH pipelining, and around 5x fewer frames are required to traverse the wire for a run to complete successfully. * **No writes to the target host'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 host. 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 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 ------------ .. caution:: Thoroughly review the list of limitations before use, and **do not test the prototype in a live environment until this notice is removed**. 1. Verify Ansible 2.4 and Python 2.7 are listed in the output of ``ansible --version`` 2. Download and extract https://github.com/dw/mitogen/archive/master.zip 3. 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, you can set the ``ANSIBLE_STRATEGY=mitogen_linear`` environment variable on a per-run basis. Like ``mitogen_linear``, the ``mitogen_free`` strategy also exists to mimic the built-in ``free`` strategy. 4. Cross your fingers and try it. Limitations ----------- This is a proof of concept: issues below are exclusively due to code immaturity. High Risk ~~~~~~~~~ * Transfer of large files using certain Ansible-internal APIs, such as triggered via the ``copy`` module, will cause corresponding memory and CPU spikes on both host and target machine, due to delivering the file as a single message. If many machines are targetted, the controller could easily exhaust available RAM. This will be fixed soon as it's likely to be tickled by common playbooks. * No mechanism exists to bound the number of interpreters created during a run. For some playbooks that parameterize ``become_user`` over many accounts, resource exhaustion may be triggered on the target machine. * Only Ansible 2.4 is being used for development, with occasional tests under 2.5, 2.3 and 2.2. It should be more than possible to fully support at least 2.3, if not also 2.2. Low Risk ~~~~~~~~ * Only UNIX machines running Python 2.x are supported, Windows will come later. * Only the ``sudo`` become method is available, however adding new methods is straightforward, and eventually at least ``su`` will be included. * The extension's performance benefits do not scale perfectly linearly with the number of targets. This is a subject of ongoing investigation and improvements will appear in time. * "Module Replacer" style modules are not yet supported. These rarely appear in practice, and light Github code searches failed to reveal many examples of them. Behavioural Differences ----------------------- * Ansible permits up to ``forks`` SSH connections to be setup simultaneously, whereas in Mitogen this is handled by a thread pool. Eventually this pool will become per-CPU, but meanwhile, a maximum of 16 SSH connections may be established simultaneously by default. This can be increased or decreased setting the ``MITOGEN_POOL_SIZE`` environment variable. * Mitogen treats connection timeouts for the SSH and become steps of a task invocation separately, meaning that in some circumstances the configured timeout may appear to be doubled. This is since Mitogen internally treats the creation of an SSH account context separately to the creation of a sudo account context proxied via that SSH account. A future revision may detect a sudo account context created immediately following its parent SSH account, and try to emulate Ansible's existing timeout semantics. * Local commands are executed in a reuseable Python interpreter created identically to interpreters used on remote hosts. At present only one such interpreter per ``become_user`` exists, and so only one local action may be executed simultaneously per local user account. Ansible usually permits up to ``ansible.cfg: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 exist only for the duration of a run, and cannot be queried by subsequent ansible-playbook invocations. Since the ability to query job IDs across runs relied on an implementation detail, it is not expected this will break any real-world playbooks. 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 ``#<>`` appearing in their source. * 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)[ ..]] "``. 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 SSH Variables ------------- Matching Ansible's existing model, these variables are treated on a per-task basis, causing establishment of additional reuseable interpreters as necessary to match the configuration of each task. 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`` * ``mitogen_ssh_discriminator``: if present, a string mixed into the key used to deduplicate connections. This permits intentional duplicate Mitogen connections to a single host, which is probably only useful for testing. Sudo Variables -------------- * ``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 Variables ---------------- Note: Docker support is only intended for developer testing, it might disappear entirely prior to a stable release. * ansible_host Chat on IRC ----------- Some users and developers hang out on the `#mitogen `_ channel on the FreeNode IRC network. Debugging --------- Normally with Ansible, diagnostics and use of the :py:mod:`logging` package output on the target machine are discarded. With Mitogen, all of this is captured and returned to the host machine, 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..log``. Implementation Notes -------------------- Interpreter Reuse ~~~~~~~~~~~~~~~~~ The extension aggressively reuses the single target Python interpreter to execute every module. While this works well, it violates an unwritten assumption regarding Ansible modules, and so it is possible a buggy module could cause a run to fail, or for unrelated modules to interact with each other due to bad hygiene. Mitigations (such as forking) will be added as necessary if problems of this sort ever actually manfest. 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. Standard IO ~~~~~~~~~~~ Ansible uses pseudo TTYs for most invocations, to allow it to handle typing passwords interactively, however it disables pseudo TTYs for certain commands 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 embedded within the SSH stream, and it can simply call :py:func:`pty.openpty` in every 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 all of this behaviour precisely, to avoid breaking playbooks that expect certain text to appear in certain variables with certain linefeed characters. See `Ansible#14377`_ for related discussion. .. _Ansible#14377: https://github.com/ansible/ansible/issues/14377 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.