gutter

by disqus

disqus / gutter

Fully featured Python feature switch toolkit

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Gutter

NOTE: This repo is the client for Gargoyle 2, known as "Gutter". It does not work with the existing

Gargoyle 1 codebase 
_.

Gutter is feature switch management library. It allows users to create feature switches and setup conditions those switches will be enabled for. Once configured, switches can then be checked against inputs (requests, user objects, etc) to see if the switches are active.

For a UI to configure Gutter with see the

gutter-django project 
_

Table of Contents

  • Configuration_
  • Setup_
  • Arguments_
  • Switches
    _
  • Conditions
    _
  • Checking Switches as Active
    _
  • Signals_
  • Namespaces_
  • Decorators_
  • Testing Utilities
    _

Configuration

Gutter requires a small bit of configuration before usage.

Choosing Storage ~~~~~~~~~~~~~~~~

Switches are persisted in a

storage
object, which is a
dict
or any object which provides the
types.MappingType
interface (
__setitem__
and
__getitem__
methods). By default,
gutter
uses an instance of
MemoryDict
from the
durabledict library 
_. This engine does not persist data once the process ends so a more persistent data store should be used.

Autocreate ~~~~~~~~~~

gutter
can also "autocreate" switches. If
autocreate
is enabled, and
gutter
is asked if the switch is active but the switch has not been created yet,
gutter
will create the switch automatically. When autocreated, a switch's state is set to "disabled."

This behavior is off by default, but can be enabled through a setting. More on "settings" below.

Configuring Settings ~~~~~~~~~~~~~~~~~~~~

To change the

storage
and/or
autocreate
settings, simply import the settings module and set the appropriate variables:

.. code:: python

from gutter.client.settings import manager as manager_settings
from durabledict.dict import RedisDict
from redis import RedisClient

manager_settings.storage_engine = RedisDict('gutter', RedisClient())) manager_settings.autocreate = True

In this case, we are changing the engine to durabledict's

RedisDict
and turning on
autocreate
. These settings will then apply to all newly constructed
Manager
instances. More on what a
Manager
is and how you use it later in this document.

Setup

Once the

Manager
's storage engine has been configured, you can import gutter's default
Manager
object, which is your main interface with
gutter
:

.. code:: python

from gutter.client.default import gutter

At this point the

gutter
object is an instance of the
Manager
class, which holds all methods to register switches and check if they are active. In most installations and usage scenarios, the
gutter.client.gutter
manager will be your main interface.

Using a different default Manager ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

If you would like to construct and use a different default manager, but still have it accessible via

gutter.client.gutter
, you can construct and then assign a
Manager
instance to
settings.manager.default
value:

.. code:: python

from gutter.client.settings import manager as manager_settings
from gutter.client.models import Manager

manager_settings.default = Manager({}) # Must be done before importing the default manager

from gutter.client.default import gutter

assert manager_settings.default is gutter

.. WARNING::

:warning::warning: Note that the

settings.manager.default
value must be set before importing the default
gutter
instance. :warning::warning:

Arguments

The first step in your usage of

gutter
should be to define your arguments that you will be checking switches against. An "argument" is an object which understands the business logic and object in your system (users, requests, etc) and knows how to validate, transform and extract variables from those business objects for
Switch
conditions. For instance, your system may have a
User
object that has properties like
is_admin
,
date_joined
, etc. To switch against it, you would then create arguments for each of those values.

To do that, you construct a class which inherits from

gutter.client.arguments.Container
. Inside the body of the class, you create as many class variable "arguments" that you need by using the
gutter.client.arguments
function.

.. code:: python

from gutter.client import arguments

from myapp import User

class UserArguments(arguments.Container):

COMPATIBLE_TYPE = User

name = arguments.String(lambda self: self.input.name)
is_admin = arguments.Boolean(lambda self: self.input.is_admin)
age = arguments.Value(lambda self: self.input.age)

There are a few things going on here, so let's break down what they all mean.

  1. The
    UserArgument
    class is subclassed from
    Container
    . The subclassing is required since
    Container
    implements some of the required API.
  2. The class has a bunch of class variables that are calls to
    arguments.TYPE
    , where
    TYPE
    is the type of variable this argument is. At present there are 3 types:
    Value
    for general values,
    Boolean
    for boolean values and
    String
    for string values.
  3. arguments.TYPE()
    is called with a callable that returns the value. In the above example, we'll want to make some switches active based on a user's
    name
    ,
    is_admin
    status and
    age
    .
  4. Those callables return the actual value, which is derefenced from
    self.input
    , which is the input object (in this case a
    User
    instance).
  5. Variable
    objects understand
    Switch
    conditions and operators, and implement the correct API to allow themselves to be appropriately compared.
  6. COMPATIBLE_TYPE
    declares that this argument only works with
    User
    instances. This works with the default implementation of
    applies
    in the base argument that checks if the
    type
    of the input is the same as
    COMPATIBLE_TYPE
    .

Since constructing arguments that simply reference an attribute on

self.input
is so common, if you pass a string as the first argument of
argument()
, when the argument is accessed, it will simply return that property from
self.input
. You must also pass a
Variable
to the
variable=
kwarg so gutter know what Variable to wrap your value in.

.. code:: python

from gutter.client import arguments

from myapp import User

class UserArguments(Container):

COMPATIBLE_TYPE = User

name = arguments.String('name')
is_admin = arguments.Boolean('is_admin')
age = arguments.Value('age')

Rationale for Arguments ~~~~~~~~~~~~~~~~~~~~~~~

You might be asking, why have these

Argument
objects at all? They seem to just wrap an object in my system and provide the same API. Why can't I just use my business object itself and compare it against my switch conditions?

The short answer is that

Argument
objects provide a translation layer to translate your business objects into objects that
gutter
understands. This is important for a couple reasons.

First, it means you don't clutter your business logic/objects with code to support

gutter
. You declare all the arguments you wish to provide to switches in one location (an Argument) whose single responsibility it to interface with
gutter
. You can also construct more savvy Argument objects that may be the combination of multiple business objects, consult 3rd party services, etc. All still not cluttering your main application code or business objects.

Secondly, and most importantly, Arguments return

Variable
objects, which ensure
gutter
conditions work correctly. This is mostly relevant to the percentage-based operators, and is best illustrated with an example.

Imagine you have a

User
class with an
is_vip
boolean field. Let's say you wanted to turn on a feature for only 10% of your VIP customers. To do that, you would write a condition that says, "10% of the time when I'm called with the variable, I should be true." That line of code would probably do something like this:

.. code:: python

return 0 <= (hash(variable) % 100) < 10

The issue is that if

variable = True
, then
hash(variable) % 100
will always be the same value for every
User
with
is_vip
of
True
:

.. code:: python

>>> hash(True)
1
>>> hash(True) % 100
1

This is because in Python

True
objects always have the same hash value, and thus the percentage check doesn't work. This is not the behavior you want.

For the 10% percentage range, you want it to be active for 10% of the inputs. Therefore, each input must have a unique hash value, exactly the feature the

Boolean
variable provides. Every
Variable
has known characteristics against conditions, while your objects may not.

That said, you don't absolutely have to use

Variable
objects. For obvious cases, like
use.age > some_value
your
User
instance will work just fine, but to play it safe you should use
Variable
objects. Using
Variable
objects also ensure that if you update
gutter
any new
Operator
types that are added will work correctly with your
Variable
s.

Switches

Switches encapsulate the concept of an item that is either 'on' or 'off' depending on the input. The swich determines its on/off status by checking each of its

conditions
and seeing if it applies to a certain input.

Switches are constructed with only one required argument, a

name
:

.. code:: python

from gutter.client.models import Switch

switch = Switch('my cool feature')

Switches can be in 3 core states:

GLOBAL
,
DISABLED
and
SELECTIVE
. In the
GLOBAL
state, the Switch is enabled for every input no matter what.
DISABLED
Switches are not disabled for any input, no matter what.
SELECTIVE
Switches enabled based on their conditions.

Switches can be constructed in a certain state or the property can be changed later:

.. code:: python

switch = Switch('new feature', state=Switch.states.DISABLED)
another_switch = Switch('new feature')
another_switch.state = Switch.states.DISABLED

Compounded ~~~~~~~~~~

When in the

SELECTIVE
state, normally only one condition needs be true for the Switch to be enabled for a particular input. If
switch.compounded
is set to
True
, then all of the switches conditions need to be true in order to be enabled::
switch = Switch('require alll conditions', compounded=True)

Heriarchical Switches ~~~~~~~~~~~~~~~~~~~~~

You can create switches using a specific hierarchical naming scheme. Switch namespaces are divided by the colon character (":"), and hierarchies of switches can be constructed in this fashion:

.. code:: python

parent = Switch('movies')
child1 = Switch('movies:star_wars')
child2 = Switch('movies:die_hard')
grandchild = Switch('movies:star_wars:a_new_hope')

In the above example, the

child1
switch is a child of the
"movies"
switch because it has
movies:
as a prefix to the switch name. Both
child1
and
child2
are "children of the parent
parent
switch. And
grandchild
is a child of the
child1
switch, but not the
child2
switch.

Concent ~~~~~~~

By default, each switch makes its "am I active?" decision independent of other switches in the Manager (including its parent), and only consults its own conditions to check if it is enabled for the input. However, this is not always the case. Perhaps you have a cool new feature that is only available to a certain class of user. And of those users, you want 10% to be be exposed to a different user interface to see how they behave vs the other 90%.

gutter
allows you to set a
concent
flag on a switch that instructs it to check its parental switch first, before checking itself. If it checks its parent and it is not enabled for the same input, the switch immediately returns
False
. If its parent is enabled for the input, then the switch will continue and check its own conditions, returning as it would normally.

For example:

.. code:: python

parent = Switch('cool_new_feature')
child = Switch('cool_new_feature:new_ui', concent=True)

For example, because

child
was constructed with
concent=True
, even if
child
is enabled for an input, it will only return
True
if
parent
is also enabled for that same input.

Note: Even switches in a

GLOBAL
or
DISABLED
state (see "Switch" section above) still consent their parent before checking themselves. That means that even if a particular switch is
GLOBAL
, if it has
concent
set to
True
and its parent is not enabled for the input, the switch itself will return
False
.

Registering a Switch ~~~~~~~~~~~~~~~~~~~~

Once your

Switch
is constructed with the right conditions, you need to register it with a
Manager
instance to preserve it for future use. Otherwise it will only exist in memory for the current process. Register a switch via the
register
method on a
Manager
instance:

.. code:: python

gutter.register(switch)

The Switch is now stored in the Manager's storage and can be checked if active through

gutter.active(switch)
.

Updating a Switch ~~~~~~~~~~~~~~~~~

If you need to update your Switch, simply make the changes to the

Switch
object, then call the
Manager
's
update()
method with the switch to tell it to update the switch with the new object:

.. code:: python

switch = Switch('cool switch')
manager.register(switch)

switch.name = 'even cooler switch' # Switch has not been updated in manager yet

manager.update(switch) # Switch is now updated in the manager

Since this is a common pattern (retrieve switch from the manager, then update it), gutter provides a shorthand API in which you ask the manager for a switch by name, and then call

save()
on the switch to update it in the
Manager
it was retreived from:

.. code:: python

switch = manager.switch('existing switch')
switch.name = 'a new name'  # Switch is not updated in manager yet
switch.save()  # Same as calling manager.update(switch)

Unregistering a Switch ~~~~~~~~~~~~~~~~~~~~~~

Existing switches may be removed from the Manager by calling

unregister()
with the switch name or switch instance:

.. code:: python

gutter.unregister('deprecated switch')
gutter.unregister(a_switch_instance)

Note: If the switch is part of a hierarchy and has children switches (see the "Hierarchical Switches" section above), all descendent switches (children, grandchildren, etc) will also be unregistered and deleted.

Conditions

Each Switch can have 0+ conditions, which describe the conditions under which that switch is active.

Condition
objects are constructed with three values: a
argument
,
attribute
and
operator
.

An

argument
is any
Argument
class, like the one you defined earlier. From the previous example,
UserArgument
is an argument object.
attribute
is the attribute on a argument instance that you want this condition to check.
operator
is some sort of check applied against that attribute. For instance, is the
UserArgument.age
greater than some value? Equal to some value? Within a range of values? Etc.

Let's say you wanted a

Condition
that checks if the user's age is > 65 years old? You would construct a Condition that way:

.. code:: python

from gutter.client.operators.comparable import MoreThan

condition = Condition(argument=UserArgument, attribute='age', operator=MoreThan(65))

This Condition will be true if any input instance has an

age
that is more than
65
.

Please see the

gutter.operators
for a list of available operators.

Conditions can also be constructed with a

negative
argument, which negates the condition. For example:

.. code:: python

from gutter.client.operators.comparable import MoreThan

condition = Condition(argument=UserArgument, attribute='age', operator=MoreThan(65), negative=True)

This Condition is now

True
if the condition evaluates to
False
. In this case if the user's
age
is not more than
65
.

Conditions then need to be appended to a switch instance like so:

.. code:: python

switch.conditions.append(condition)

You can append as many conditions as you would like to a switch, there is no limit.

Checking Switches as Active

As stated before, switches are checked against input objects. To do this, you would call the switch's

enabled_for()
method with a
User
instance, for instance. You may call
enabled_for()
with any input object, it will ignore inputs for which it knows nothing about. If the
Switch
is active for your input,
enabled_for
will return
True
. Otherwise, it will return
False
.

gutter.active()
API ~~~~~~~~~~~~~~~~~~~~~~~~~

A common use case of gutter is to use it during the processing of a web request. During execution of code, different code paths are taken depending on if certain switches are active or not. Often times there are multiple switches in existence at any one time and they all need to be checked against multiple arguments. To handle this use case, Gutter provides a higher-level API.

To check if a

Switch
is active, simply call
gutter.active()
with the Switch name:

.. code:: python

gutter.active('my cool feature')
>>> True

The switch is checked against some number of input objects. Inputs can be added to the

active()
check one of two ways: locally, passed in to the
active()
call or globally, configured ahead of time.

To check against local inputs,

active()
takes any number of input objects after the switch name to check the switch against. In this example, the switch named
'my cool feature'
is checked against input objects
input1
and
input2
:

.. code:: python

gutter.active('my cool feature', input1, input2)
>>> True

If you have global input objects you would like to use for every check, you can set them up by calling the Manager's

input()
method:

.. code:: python

gutter.input(input1, input2)

Now,

input1
and
input2
are checked against for every
active
call. For example, assuming
input1
and
input2
are configured as above, this
active()
call would check if the Switch was enabled for inputs
input1
,
input2
and
input3
in that order::
gutter.active('my cool feature', input3)

Once you're doing using global inputs, perhaps at the end of a request, you should call the Manager's

flush()
method to remove all the inputs:

.. code:: python

gutter.flush()

The Manager is now setup and ready for its next set of inputs.

When calling

active()
with a local inputs, you can skip checking the
Switch
against the global inputs and only check against your locally passed in inputs by passing
exclusive=True
as a keyword argument to
active()
:

.. code:: python

gutter.input(input1, input2)
gutter.active('my cool feature', input3, exclusive=True)

In the above example, since

exclusive=True
is passed, the switch named
'my cool feature'
is only checked against
input3
, and not
input1
or
input2
. The
exclusive=True
argument is not persistent, so the next call to
active()
without
exclusive=True
will again use the globally defined inputs.

Signals

Gutter provides 4 total signals to connect to: 3 about changes to Switches, and 1 about errors applying Conditions. They are all available from the

gutter.signals
module

Switch Signals ~~~~~~~~~~~~~~ There are 3 signals related to Switch changes:

  1. switch_registered
    - Called when a new switch is registered with the Manager.
  2. switch_unregistered
    - Called when a switch is unregistered with the Manager.
  3. switch_updated
    - Called with a switch was updated.

To use a signal, simply call the signal's

connect()
method and pass in a callable object. When the signal is fired, it will call your callable with the switch that is being register/unregistered/updated. I.e.:

.. code:: python

from gutter.client.signals import switch_updated

def log_switch_update(switch): Syslog.log("Switch %s updated" % switch.name)

switch_updated.connect(log_switch_updated)

Understanding Switch Changes ~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The

switch_updated
signal can be connected to in order to be notified when a switch has been changed. To know what changed in the switch, you can consult its
changes
property:

.. code:: python

>>> from gutter.client.models import Switch
>>> switch = Switch('test')
>>> switch.concent
True
>>> switch.concent = False
>>> switch.name = 'new name'
>>> switch.changes
{'concent': {'current': False, 'previous': True}, 'name': {'current': 'new name', 'previous': 'test'}}

As you can see, when we changed the Switch's

concent
setting and
name
,
switch.changes
reflects that in a dictionary of changed properties. You can also simply ask the switch if anything has changed with the
changed
property. It returns
True
or
False
if the switch has any changes as all.

You can use these values inside your signal callback to make decisions based on what changed. I.e., email out a diff only if the changes include changed conditions.

Condition Application Error Signal ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

When a

Switch
checks an input object against its conditions, there is a good possibility that the
Argument
value may be some sort of unexpected value, and can cause an exception. Whenever there is an exception raised during
Condition
checking itself against an input, the
Condition
will catch that exception and return
False
.

While catching all exceptions is generally bad form and hides error, most of the time you do not want to fail an application request just because there was an error checking a switch condition, especially if there was an error during checking a

Condition
for which a user would not have applied in the first place.

That said, you would still probably want to know if there was an error checking a Condition. To accomplish this,

gutter
-client provides a
condition_apply_error
signal which is called when there was an error checking a
Condition
. The signal is called with an instance of the condition, the input which caused the error and the instance of the Exception class itself:

.. code:: python

signals.condition_apply_error.call(condition, inpt, error)

In your connected callback, you can do whatever you would like: log the error, report the exception, etc.

Namespaces

gutter
allows the use of "namespaces" to group switches under a single umbrella, while both not letting one namespace see the switches of another namespace, but allowing them to share the same storage instance, operators and other configuration.

Given an existing vanilla

Manager
instance, you can create a namespaced manager by calling the
namespaced()
method:

.. code:: python

notifications = gutter.namespaced('notifications')

At this point,

notifications
is a copy of
gutter
, inheriting all of its:
  • storage
  • autocreate
    setting
  • Global inputs
  • Operators

It does not, however, share the same switches. Newly constructed

Manager
instances are in the
default
namespace. When
namespaced()
is called,
gutter
changes the manager's namespace to
notifications
. Any switches in the previous
default
namespace are not visible in the
notifications
namespace, and vice versa.

This allows you to have separate namespaced "views" of switches, possibly named the exact same name, and not have them conflict with each other.

Decorators

Gutter features a

@switch_active
decorator you can use to decorate your Django views. When decorated, if the switch named as the first argument of the
@switch_decorated
decorator is False, a
Http404
exception is raised. However, if you also pass a
redirect_to=
kwarg, the decorator will return a
HttpResponseRedirect
instance, redirecting to that location. If the switch is active, then the view runs as normal.

For example, here is a view decorated with

@switch_active
:

.. code:: python

from gutter.client.decorators import switch_active

@switch_active('cool_feature') def my_view(request): return 'foo'

As stated above, if the

cool_feature
switch is inactive, this view will raise a
Http404
exception.

If, however, the decorator was constructed with a

redirect_to=
kwarg:

.. code:: python

@switch_active('cool_feature', redirect_to=reverse('upsell-page'))

Then a

HttpResponseRedirect
instance will be returned, redirecting to
reverse('upsell-page')
.

Testing Utilities

If you would like to test code that uses

gutter
and have the
gutter
manager return predictable results, you can use the
switches
object from the
testutils
module.

The

swtiches
object can be used as both a context manager and a decorator. It is passed
kwargs
of switch names and their
active
return values.

For instance, with this code here, by passing

cool_feature=True
to the
switches
object as a context manager, any call to
gutter.active('cool_feature')
will return
True
. Calls to
active()
with other switch names will return their actual live switch status:

.. code:: python

from gutter.client.testutils import switches
from gutter.client.default import gutter

with switches(cool_feature=True): gutter.active('cool_feature') # True

And when using

switches
as a decorator:

.. code:: python

from gutter.client.testutils import switches
from gutter.client.default import gutter

@switches(cool_feature=True) def run(self): gutter.active('cool_feature') # True

Additionally, you may pass an alternate

Manager
instance to
switches
to use that manager instead of the default one:

.. code:: python

from gutter.client.testutils import switches
from gutter.client.models import Manager

my_manager = Manager({})

@switches(my_manager, cool_feature=True) def run(self): gutter.active('cool_feature') # True

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