The following is a list (and re-explanation) of term definitions used elsewhere in the Ansible documentation.
Consult the documentation home page for the full documentation and to see the terms in context, but this should be a good resource to check your knowledge of Ansible’s components and understand how they fit together. It’s something you might wish to read for review or when a term comes up on the mailing list.
An action is a part of a task that specifies which of the modules to run and which arguments to pass to that module. Each task can have only one action, but it may also have other parameters.
Refers to running Ansible to perform some quick command, using /usr/bin/ansible, rather than the orchestration language, which is /usr/bin/ansible-playbook. An example of an ad hoc command might be rebooting 50 machines in your infrastructure. Anything you can do ad hoc can be accomplished by writing a playbook and playbooks can also glue lots of other operations together.
Refers to a task that is configured to run in the background rather than waiting for completion. If you have a long process that would run longer than the SSH timeout, it would make sense to launch that task in async mode. Async modes can poll for completion every so many seconds or can be configured to “fire and forget”, in which case Ansible will not even check on the task again; it will just kick it off and proceed to future steps. Async modes work with both /usr/bin/ansible and /usr/bin/ansible-playbook.
Refers to some user-written code that can intercept results from Ansible and do something with them. Some supplied examples in the GitHub project perform custom logging, send email, or even play sound effects.
Refers to running Ansible with the --check
option, which does not
make any changes on the remote systems, but only outputs the changes
that might occur if the command ran without this flag. This is
analogous to so-called “dry run” modes in other systems, though the
user should be warned that this does not take into account unexpected
command failures or cascade effects (which is true of similar modes in
other systems). Use this to get an idea of what might happen, but do
not substitute it for a good staging environment.
By default, Ansible talks to remote machines through pluggable
libraries. Ansible supports native OpenSSH (SSH (Native)) or
a Python implementation called paramiko. OpenSSH is preferred
if you are using a recent version, and also enables some features like
Kerberos and jump hosts. This is covered in the getting
started section. There are also
other connection types like accelerate
mode, which must be
bootstrapped over one of the SSH-based connection types but is very
fast, and local mode, which acts on the local system. Users can also
write their own connection plugins.
A conditional is an expression that evaluates to true or false that decides whether a given task is executed on a given machine or not. Ansible’s conditionals are powered by the ‘when’ statement, which are discussed in the Working With Playbooks.
An approach to achieving a task that uses a description of the final state rather than a description of the sequence of steps necessary to achieve that state. For a real world example, a declarative specification of a task would be: “put me in California”. Depending on your current location, the sequence of steps to get you to California may vary, and if you are already in California, nothing at all needs to be done. Ansible’s Resources are declarative; it figures out the steps needed to achieve the final state. It also lets you know whether or not any steps needed to be taken to get to the final state.
A --diff
flag can be passed to Ansible to show what changed on
modules that support it. You can combine it with --check
to get a
good ‘dry run’. File diffs are normally in unified diff format.
A core software component of Ansible that is the power behind /usr/bin/ansible directly – and corresponds to the invocation of each task in a playbook. The Executor is something Ansible developers may talk about, but it’s not really user land vocabulary.
Facts are simply things that are discovered about remote nodes. While
they can be used in playbooks and templates just like
variables, facts are things that are inferred, rather than set. Facts
are automatically discovered by Ansible when running plays by
executing the internal setup module on the remote nodes. You
never have to call the setup module explicitly, it just runs, but it
can be disabled to save time if it is not needed or you can tell
ansible to collect only a subset of the full facts via the
gather_subset:
option. For the convenience of users who are
switching from other configuration management systems, the fact module
will also pull in facts from the ohai and facter
tools if they are installed. These are fact libraries from Chef and
Puppet, respectively. (These may also be disabled via
gather_subset:
)
A filter plugin is something that most users will never need to understand. These allow for the creation of new Jinja2 filters, which are more or less only of use to people who know what Jinja2 filters are. If you need them, you can learn how to write them in the API docs section.
Ansible talks to remote nodes in parallel and the level of parallelism
can be set either by passing --forks
or editing the default in
a configuration file. The default is a very conservative five (5)
forks, though if you have a lot of RAM, you can easily set this to
a value like 50 for increased parallelism.
Facts are mentioned above. Sometimes when running a multi-play
playbook, it is desirable to have some plays that
don’t bother with fact computation if they aren’t going to need to
utilize any of these values. Setting gather_facts: False
on
a playbook allows this implicit fact gathering to be skipped.
Globbing is a way to select lots of hosts based on wildcards, rather
than the name of the host specifically, or the name of the group they
are in. For instance, it is possible to select ww*
to match all
hosts starting with www
. This concept is pulled directly from
Func, one of Michael DeHaan’s (an Ansible Founder) earlier
projects. In addition to basic globbing, various set operations are
also possible, such as ‘hosts in this group and not in another group’,
and so on.
A group consists of several hosts assigned to a pool that can be conveniently targeted together, as well as given variables that they share in common.
The group_vars/
files are files that live in a directory
alongside an inventory file, with an optional filename named after
each group. This is a convenient place to put variables that are
provided to a given group, especially complex data structures, so that
these variables do not have to be embedded in the inventory
file or playbook.
Handlers are just like regular tasks in an Ansible
playbook (see Tasks) but are only run if
the Task contains a notify
directive and also indicates that it
changed something. For example, if a config file is changed, then the
task referencing the config file templating operation may notify
a service restart handler. This means services can be bounced only if
they need to be restarted. Handlers can be used for things other than
service restarts, but service restarts are the most common usage.
A host is simply a remote machine that Ansible manages. They can have individual variables assigned to them, and can also be organized in groups. All hosts have a name they can be reached at (which is either an IP address or a domain name) and, optionally, a port number, if they are not to be accessed on the default SSH port.
Each Play in Ansible maps a series of tasks (which define the role, purpose, or orders of a system) to a set of systems.
This hosts:
directive in each play is often called the hosts specifier.
It may select one system, many systems, one or more groups, or even some hosts that are in one group and explicitly not in another.
Just like Group Vars, a directory alongside the inventory file named
host_vars/
can contain a file named after each hostname in the
inventory file, in YAML format. This provides a convenient place to
assign variables to the host without having to embed them in the
inventory file. The Host Vars file can also be used to define complex
data structures that can’t be represented in the inventory file.
An operation is idempotent if the result of performing it once is exactly the same as the result of performing it repeatedly without any intervening actions.
The idea that playbook files (which are nothing
more than lists of plays) can include other lists of plays,
and task lists can externalize lists of tasks in other files,
and similarly with handlers. Includes can be parameterized,
which means that the loaded file can pass variables. For instance, an
included play for setting up a WordPress blog may take a parameter
called user
and that play could be included more than once to
create a blog for both alice
and bob
.
A file (by default, Ansible uses a simple INI format) that describes Hosts and Groups in Ansible. Inventory can also be provided via an Inventory Script (sometimes called an “External Inventory Script”).
A very simple program (or a complicated one) that looks up hosts, group membership for hosts, and variable information from an external resource – whether that be a SQL database, a CMDB solution, or something like LDAP. This concept was adapted from Puppet (where it is called an “External Nodes Classifier”) and works more or less exactly the same way.
Jinja2 is the preferred templating language of Ansible’s template module. It is a very simple Python template language that is generally readable and easy to write.
Ansible uses JSON for return data from remote modules. This allows modules to be written in any language, not just Python.
In general, Ansible evaluates any variables in playbook content at the last possible second, which means that if you define a data structure that data structure itself can define variable values within it, and everything “just works” as you would expect. This also means variable strings can include other variables inside of those strings.
A collection of modules made available to /usr/bin/ansible or an Ansible playbook.
By passing --limit somegroup
to ansible or
ansible-playbook, the commands can be limited to a subset
of hosts. For instance, this can be used to run
a playbook that normally targets an entire set of
servers to one particular server.
A local_action directive in a playbook targeting
remote machines means that the given step will actually occur on the
local machine, but that the variable {{ ansible_hostname }}
can be
passed in to reference the remote hostname being referred to in that
step. This can be used to trigger, for example, an rsync operation.
By using connection: local
in a playbook, or
passing -c local
to /usr/bin/ansible, this indicates
that we are managing the local host and not a remote machine.
A lookup plugin is a way to get data into Ansible from the outside world.
Lookup plugins are an extension of Jinja2 and can be accessed in templates, e.g.,
{{ lookup('file','/path/to/file') }}
.
These are how such things as with_items
, are implemented.
There are also lookup plugins like file
which loads data from
a file and ones for querying environment variables, DNS text records,
or key value stores.
Generally, Ansible is not a programming language. It prefers to be
more declarative, though various constructs like loop
allow
a particular task to be repeated for multiple items in a list.
Certain modules, like yum and apt, actually take
lists directly, and can install all packages given in those lists
within a single transaction, dramatically speeding up total time to
configuration, so they can be used without loops.
Modules are the units of work that Ansible ships out to remote machines. Modules are kicked off by either /usr/bin/ansible or /usr/bin/ansible-playbook (where multiple tasks use lots of different modules in conjunction). Modules can be implemented in any language, including Perl, Bash, or Ruby – but can leverage some useful communal library code if written in Python. Modules just have to return JSON. Once modules are executed on remote machines, they are removed, so no long running daemons are used. Ansible refers to the collection of available modules as a library.
The concept that IT systems are not managed one system at a time, but by interactions between multiple systems and groups of systems in well defined orders. For instance, a web server may need to be updated before a database server and pieces on the web server may need to be updated after THAT database server and various load balancers and monitoring servers may need to be contacted. Ansible models entire IT topologies and workflows rather than looking at configuration from a “one system at a time” perspective.
The act of a task registering a change event and informing a handler task that another action needs to be run at the end of the play. If a handler is notified by multiple tasks, it will still be run only once. Handlers are run in the order they are listed, not in the order that they are notified.
Many software automation systems use this word to mean different things. Ansible uses it as a conductor would conduct an orchestra. A datacenter or cloud architecture is full of many systems, playing many parts – web servers, database servers, maybe load balancers, monitoring systems, continuous integration systems, etc. In performing any process, it is necessary to touch systems in particular orders, often to simulate rolling updates or to deploy software correctly. Some system may perform some steps, then others, then previous systems already processed may need to perform more steps. Along the way, emails may need to be sent or web services contacted. Ansible orchestration is all about modeling that kind of process.
By default, Ansible manages machines over SSH. The library that
Ansible uses by default to do this is a Python-powered library called
paramiko. The paramiko library is generally fast and easy to manage,
though users desiring Kerberos or Jump Host support may wish to switch
to a native SSH binary such as OpenSSH by specifying the connection
type in their playbooks, or using the -c ssh
flag.
Playbooks are the language by which Ansible orchestrates, configures, administers, or deploys systems. They are called playbooks partially because it’s a sports analogy, and it’s supposed to be fun using them. They aren’t workbooks :)
A playbook is a list of plays. A play is minimally a mapping between a set of hosts selected by a host specifier (usually chosen by groups but sometimes by hostname globs) and the tasks which run on those hosts to define the role that those systems will perform. There can be one or many plays in a playbook.
By default, Ansible runs in push mode, which allows it very fine-grained control over when it talks to each system. Pull mode is provided for when you would rather have nodes check in every N minutes on a particular schedule. It uses a program called ansible-pull and can also be set up (or reconfigured) using a push-mode playbook. Most Ansible users use push mode, but pull mode is included for variety and the sake of having choices.
ansible-pull works by checking configuration orders out of git on a crontab and then managing the machine locally, using the local connection plugin.
Push mode is the default mode of Ansible. In fact, it’s not really a mode at all – it’s just how Ansible works when you aren’t thinking about it. Push mode allows Ansible to be fine-grained and conduct nodes through complex orchestration processes without waiting for them to check in.
The result of running any task in Ansible can be
stored in a variable for use in a template or a conditional statement.
The keyword used to define the variable is called register
, taking
its name from the idea of registers in assembly programming (though
Ansible will never feel like assembly programming). There are an
infinite number of variable names you can use for registration.
Ansible modules work in terms of resources. For instance, the
file module will select a particular file and ensure
that the attributes of that resource match a particular model. As an
example, we might wish to change the owner of /etc/motd
to
root
if it is not already set to root
, or set its mode to
0644
if it is not already set to 0644
. The resource models
are idempotent meaning change commands are not
run unless needed, and Ansible will bring the system back to a desired
state regardless of the actual state – rather than you having to tell
it how to get to the state.
Roles are units of organization in Ansible. Assigning a role to a group of hosts (or a set of groups, or host patterns, etc.) implies that they should implement a specific behavior. A role may include applying certain variable values, certain tasks, and certain handlers – or just one or more of these things. Because of the file structure associated with a role, roles become redistributable units that allow you to share behavior among playbooks – or even with other users.
The act of addressing a number of nodes in a group N at a time to
avoid updating them all at once and bringing the system offline. For
instance, in a web topology of 500 nodes handling very large volume,
it may be reasonable to update 10 or 20 machines at a time, moving on
to the next 10 or 20 when done. The serial:
keyword in an Ansible
playbooks control the size of the rolling update pool. The
default is to address the batch size all at once, so this is something
that you must opt-in to. OS configuration (such as making sure config
files are correct) does not typically have to use the rolling update
model, but can do so if desired.
See also
Ansible does not require root logins, and since it’s daemonless, definitely does not require root level daemons (which can be a security concern in sensitive environments). Ansible can log in and perform many operations wrapped in a sudo command, and can work with both password-less and password-based sudo. Some operations that don’t normally work with sudo (like scp file transfer) can be achieved with Ansible’s copy, template, and fetch modules while running in sudo mode.
Native OpenSSH as an Ansible transport is specified with -c ssh
(or a config file, or a directive in the playbook)
and can be useful if wanting to login via Kerberized SSH or using SSH
jump hosts, etc. In 1.2.1, ssh
will be used by default if the
OpenSSH binary on the control machine is sufficiently new.
Previously, Ansible selected paramiko
as a default. Using
a client that supports ControlMaster
and ControlPersist
is
recommended for maximum performance – if you don’t have that and
don’t need Kerberos, jump hosts, or other features, paramiko
is
a good choice. Ansible will warn you if it doesn’t detect
ControlMaster/ControlPersist capability.
Ansible allows tagging resources in a playbook
with arbitrary keywords, and then running only the parts of the
playbook that correspond to those keywords. For instance, it is
possible to have an entire OS configuration, and have certain steps
labeled ntp
, and then run just the ntp
steps to reconfigure
the time server information on a remote host.
Playbooks exist to run tasks. Tasks combine an action (a module and its arguments) with a name and optionally some other keywords (like looping directives). Handlers are also tasks, but they are a special kind of task that do not run unless they are notified by name when a task reports an underlying change on a remote system.
A list of Task.
Ansible can easily transfer files to remote systems but often it is desirable to substitute variables in other files. Variables may come from the inventory file, Host Vars, Group Vars, or Facts. Templates use the Jinja2 template engine and can also include logical constructs like loops and if statements.
Ansible uses :term:Connection Plugins
to define types of available
transports. These are simply how Ansible will reach out to managed
systems. Transports included are paramiko,
ssh (using OpenSSH), and
local.
An optional conditional statement attached to a task that is used to
determine if the task should run or not. If the expression following
the when:
keyword evaluates to false, the task will be ignored.
As opposed to Facts, variables are names of values (they can be simple scalar values – integers, booleans, strings) or complex ones (dictionaries/hashes, lists) that can be used in templates and playbooks. They are declared things, not things that are inferred from the remote system’s current state or nature (which is what Facts are).
Ansible does not want to force people to write programming language code to automate infrastructure, so Ansible uses YAML to define playbook configuration languages and also variable files. YAML is nice because it has a minimum of syntax and is very clean and easy for people to skim. It is a good data format for configuration files and humans, but also machine readable. Ansible’s usage of YAML stemmed from Michael DeHaan’s first use of it inside of Cobbler around 2006. YAML is fairly popular in the dynamic language community and the format has libraries available for serialization in many languages (Python, Perl, Ruby, etc.).
See also
Frequently asked questions
An introduction to playbooks
Best practices advice
Have a question? Stop by the google group!
#ansible IRC chat channel