flipper浅析

TL;DR

feature：功能 gate：范围的类型（Boolean、Actor、Group、Percentage） type：范围的值（true/false、id、组名、百分比）

因为判断 feature 是否启用时，需要加载该 feature 关联的所有 gate ，所以不适合一个 feature 关联大量 Actor/Group

开始探究源码

看看 Flipper.enabled? 方法到底会干些什么

[1] pry(main)> Flipper.enabled? :search
  Flipper::Adapters::ActiveRecord::Gate Load (0.1ms)  SELECT "flipper_gates".* FROM "flipper_gates" WHERE "flipper_gates"."feature_key" = ?  [["feature_key", "search"]]
=> false
[2] pry(main)> $ Flipper.enabled?

From: /home/z/datahouse/asdf/installs/ruby/2.7.5/lib/ruby/2.7.0/forwardable.rb:226:

Owner: Flipper
Visibility: public
Signature: enabled?(*args, &block)
Number of lines: 5

def #{ali}(*args, &block)
  #{pre}
  begin
    #{accessor}
  end#{method_call}
[3] pry(main)>

因为同时使用了 flipper-active_record，所以开关记录在了数据库了。

而 Flipper.enabled? 方法的定义来自 forwardable，不好查看。于是跟踪一下enabled?方法的调用栈

binding.trace_tree(htmp: 'flipper_enabled_quiz'){ Flipper.enabled? :search }

得：

![[flipper-enabled-quiz-trace-20220525.html]]

概览如此

调用 Flipper.enabled?，会取全局的 Flipper::Configuration 对象 （如果未有则 new 一个），然后再取线程缓存的 Flipper::DSL 对象（如果未有则 new 一个）。dsl 内含一个 @adapter，例如 Flipper::Adapters::ActiveRecord.new。

然后产生一个 Flipper::Feature.new(name, @adapter, instrumenter: instrumenter)，并调用 Flipper::Feature#enabled?

module Flipper
  class DSL

    def enabled?(name, *args)
      feature(name).enabled?(*args)
    end

    def feature(name)
      # ...
      @memoized_features[name.to_sym] ||= Feature.new(name, @adapter, instrumenter: instrumenter)
    end

  end
end

而 Flipper::Feature#enabled? 源码如下

# flipper-0.21.0/lib/flipper/feature.rb
module Flipper
  class Feature

    def enabled?(thing = nil)
      instrument(:enabled?) do |payload|
        values = gate_values
        thing = gate(:actor).wrap(thing) unless thing.nil?
        payload[:thing] = thing
        context = FeatureCheckContext.new(
          feature_name: @name,
          values: values,
          thing: thing
        )

        if open_gate = gates.detect { |gate| gate.open?(context) }
          payload[:gate_name] = open_gate.name
          true
        else
          false
        end
      end
    end

  end
end

看起来有点复杂，或者先看看 Flipper.enable 吧

开闭功能

Flipper.enable 经过 DSL 最终会去到 Flipper::Feature#enable 源码如下。而其他 enable_xxx 都是想将“范围”包装成 Type::XXX ，然后传给 enable

def enable(thing = true)
  instrument(:enable) do |payload|
    adapter.add self

    gate = gate_for(thing)
    wrapped_thing = gate.wrap(thing)
    payload[:gate_name] = gate.name
    payload[:thing] = wrapped_thing

    adapter.enable self, gate, wrapped_thing
  end
end

def enable_actor(actor)
  enable Types::Actor.wrap(actor)
end

def enable_group(group)
  enable Types::Group.wrap(group)
end

def enable_percentage_of_time(percentage)
  enable Types::PercentageOfTime.wrap(percentage)
end

def enable_percentage_of_actors(percentage)
  enable Types::PercentageOfActors.wrap(percentage)
end

Flipper::Feature#enable 内部会先根据 Type::XXX 找到对应的 Gates::XXX 。

（这里 @gates 数组的顺序似乎固定如此，否则影响 detect ）

def gates
  @gates ||= [
    Gates::Boolean.new,
    Gates::Actor.new,
    Gates::PercentageOfActors.new,
    Gates::PercentageOfTime.new,
    Gates::Group.new,
  ]
end

def gate_for(thing)
  gates.detect { |gate| gate.protects?(thing) } || raise(GateNotFound, thing)
end

然后 gate.wrap(thing) 返回的是 Types::XXX 。对于 enable 方法来说，这步就是把 true 转成 Types::Boolean；而对于 enable_xxx 来说，返回的就是 thing

最后，会用 adapter 保存设置。如果用的是 flipper-active_record，则保存逻辑如下

# flipper-active_record-0.24.1/lib/flipper/adapters/active_record.rb
def set(feature, gate, thing, options = {})
  clear_feature = options.fetch(:clear, false)
  @gate_class.transaction do
    clear(feature) if clear_feature
    @gate_class.where(feature_key: feature.key, key: gate.key).destroy_all
    begin
      @gate_class.create! do |g|
        g.feature_key = feature.key
        g.key = gate.key
        g.value = thing.value.to_s
      end
    rescue ::ActiveRecord::RecordNotUnique
    end
  end
  nil
end

关闭功能同理，如果用的是 flipper-active_record，则会清除数据库中的配置

检查功能

源码如下

def enabled?(thing = nil)
  instrument(:enabled?) do |payload|
    values = gate_values
    thing = gate(:actor).wrap(thing) unless thing.nil?
    payload[:thing] = thing
    context = FeatureCheckContext.new(
      feature_name: @name,
      values: values,
      thing: thing
    )

    if open_gate = gates.detect { |gate| gate.open?(context) }
      payload[:gate_name] = open_gate.name
      true
    else
      false
    end
  end
end

首先会通过 gate_values，间接调用 adapter 获取 feature 关联的各种 gate

然后 gates.detect 寻找是否有某类 gate 符合条件

各种范围类型（gate）的判断逻辑如下

# lib/flipper/gates/boolean.rb
def open?(context)
  context.values[key]
end

# lib/flipper/gates/actor.rb
# 会加载出这个feature关联的所有flipper_id
def open?(context)
  value = context.values[key]
  if context.thing.nil?
    false
  else
    if protects?(context.thing)
      actor = wrap(context.thing)
      enabled_actor_ids = value
      enabled_actor_ids.include?(actor.value)
    else
      false
    end
  end
end

# lib/flipper/gates/group.rb
# 会加载出这个feature关联的所有组名，然后利用组名关联的注册block去验证
def open?(context)
  value = context.values[key]
  if context.thing.nil?
    false
  else
    value.any? do |name|
      group = Flipper.group(name)
      group.match?(context.thing, context)
    end
  end
end

# lib/flipper/gates/percentage_of_time.rb
# 使用随机数
def open?(context)
  value = context.values[key]
  rand < (value / 100.0)
end

# lib/flipper/gates/percentage_of_actors.rb
# 计算flipper_id哈希值。随着百分比增长，原本不生效的flipper_id会变成生效
def open?(context)
  percentage = context.values[key]

  if Types::Actor.wrappable?(context.thing)
    actor = Types::Actor.wrap(context.thing)
    id = "#{context.feature_name}#{actor.value}"
    # this is to support up to 3 decimal places in percentages
    scaling_factor = 1_000
    Zlib.crc32(id) % (100 * scaling_factor) < percentage * scaling_factor
  else
    false
  end
end

适配器

flipper-active_record 适配器的注入如下

# flipper-active_record-0.21.0/lib/flipper-active_record.rb
ActiveSupport.on_load(:active_record) do
  require 'flipper/adapters/active_record'
end

# flipper-active_record-0.21.0/lib/flipper/adapters/active_record.rb
Flipper.configure do |config|
  config.adapter { Flipper::Adapters::ActiveRecord.new }
end