Class: Fog::Cache

Inherits:
Object
  • Object
show all
Defined in:
lib/fog/core/cache.rb

Overview

A generic cache mechanism for fog resources. This can be for a server, security group, etc.

Currently this is a on-disk cache using yml files per-model instance, however there is nothing in the way of extending this to use various other cache backends.

Basic functionality

set the namespace where this cache will be stored:

Fog::Cache.namespace_prefix = “service-account-foo-region-bar”

cache to disk:

# after dumping, there will be a yml file on disk:
resouce.cache.dump

# you can load cached data in from a different session
Fog::Cache.load(Fog::Compute::AWS::Server, compute)

# you can also expire cache (removes cached data assocaited with the resources of this model associated to the service passed in).
Fog::Cache.expire_cache!(Fog::Compute::AWS::Server, compute)

More detailed flow/usage

Normally, you would have a bunch of resources you want to cache/reload from disk. Every fog model has a cache object injected to accomplish this. So in order to cache a server for exmaple you would do something like this:

# note this is necessary in order to segregate usage of cache between various providers regions and accounts.
# if you are using one account/region/etc only, you still must set it. 'default' will do.
Fog::Cache.namespace_prefix = "prod-emea-eu-west-1"

s = security_groups.sample; s.name # => "default"
s.cache.dump # => 2371

Now it is on disk:

shai@adsk-lappy ~ % tree ~/.fog-cache/prod-emea-eu-west-1/

/Users/shai/.fog-cache/prod-emea-eu-west-1/
  └── fog_compute_aws_real
    └── fog_compute_aws_securitygroup
     ├── default-90928073d9d5d9b4e7545e88aee7ec4f.yml

You can do the same with a SecurityGroup, Instances, Elbs, etc.

Note that when loading cache from disk, you need to pass the appropriate model class, and service associated with it. Service is passed in is so that the service/connection details can be loaded into the loaded instances so they can be re-queried, etc. Model is passed in so we can find the cache data associated to that model in the namespace of cache this session is using: Will try to load all resources associated to those. If you had 1 yml file, or 100, it would load whatever it could find. As such, the normal usage of dumping would be do it on a collection:

load_balancers.each {|elb| elb.cache.dump }

In order to load the cache into a different session with nothing but the service set up, use like so: As mentioned, will load all resources associated to the model_klass and service passed in.

instances = Fog::Cache.load(Fog::Compute::AWS::Server, compute)
instances.first.id # => "i-0569a70ae6f47d229"

Note that if there is no cache located for the model class and service passed to ‘Fog::Cache.load` you will get an exception you can handle (for example, to load the resources for the fisrt time):

Fog::Cache.expire_cache!(Fog::Compute::AWS::SecurityGroup, compute)
# ... now there is no SecurityGroup cache data. So, if you tried to load it, you would get an exception:

Fog::Cache.load(Fog::Compute::AWS::SecurityGroup, compute)
  rescue Fog::Cache::CacheNotFound => e
    puts "could not find any cache data for security groups on #{compute}"
    get_resources_and_dump

Extending cache backends

Currently this is on-disk using yml. If need be, this could be extended to other cache backends:

Find references of yaml in this file, split out to strategy objects/diff backends etc.

Defined Under Namespace

Classes: CacheDir, CacheNotFound

Constant Summary collapse

SANDBOX =

where different caches per service api keys, regions etc, are stored see the namespace_prefix= method.

"~/.fog-cache"

Instance Attribute Summary collapse

Class Method Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(model) ⇒ Cache

Returns a new instance of Cache.



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# File 'lib/fog/core/cache.rb', line 180

def initialize(model)
  @model = model
end

Instance Attribute Details

#model ⇒ Object (readonly)

when a resource is used such as ‘server.cache.dump` the model klass is passed in so that it can be identified from a different session.



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# File 'lib/fog/core/cache.rb', line 98

def model
  @model
end

Class Method Details

.create_namespace(model_klass, service) ⇒ Object

creates on-disk cache of this specific model_klass and @service



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# File 'lib/fog/core/cache.rb', line 141

def self.create_namespace(model_klass, service)
  FileUtils.mkdir_p(self.namespace(model_klass, service))
end

.expire_cache!(model_klass, service) ⇒ Object

Expires cache - this does not expire all cache associated. Instead, this will remove all on-disk cache of this specific model_klass and and @service



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# File 'lib/fog/core/cache.rb', line 147

def self.expire_cache!(model_klass, service)
  FileUtils.rm_rf(namespace(model_klass, service))
end

.load(model_klass, service) ⇒ Object

Loads cache associated to the model_klass and service into memory.

If no cache is found, it will raise an error for handling:

rescue Fog::Cache::CacheNotFound
  set_initial_cache

Raises:



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# File 'lib/fog/core/cache.rb', line 107

def self.load(model_klass, service)
  cache_files = Dir.glob("#{namespace(model_klass, service)}/*")

  raise CacheNotFound if cache_files.empty?

  loaded = cache_files.map do |path|
    model_klass = Object.const_get(load_cache(path)[:model_klass])
    model_klass.new(load_cache(path)[:attrs])
  end

  collection_klass = load_cache(cache_files.sample)[:collection_klass] &&
                     Object.const_get(load_cache(cache_files.sample)[:collection_klass])

  loaded.each do |i|
    # See https://github.com/fog/fog-aws/issues/354#issuecomment-286789702
    i.collection = collection_klass.new(:service => service) if collection_klass
    i.instance_variable_set(:@service, service)
  end

  # uniqe-ify based on the total of attributes. duplicate cache can exist due to
  # `model#identity` not being unique. but if all attributes match, they are unique
  # and shouldn't be loaded again.
  uniq_loaded = loaded.uniq { |i| i.attributes }
  if uniq_loaded.size != loaded.size
    Fog::Logger.warning("Found duplicate items in the cache. Expire all & refresh cache soon.")
  end

  # Fog models created, free memory of cached data used for creation.
  @memoized = nil

  uniq_loaded
end

.load_cache(path) ⇒ Object

loads yml cache from path on disk, used to initialize Fog models.



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# File 'lib/fog/core/cache.rb', line 153

def self.load_cache(path)
  @memoized ||= {}
  return @memoized[path] if @memoized[path]
  @memoized[path] = YAML.load(File.read(path))
end

.namespace(model_klass, service) ⇒ Object

The path/namespace where the cache is stored for a specific model_klass and @service.

Raises:



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# File 'lib/fog/core/cache.rb', line 168

def self.namespace(model_klass, service)

  raise CacheDir.new("Must set an explicit identifier/name for this cache. Example: 'serviceX-regionY'") unless namespace_prefix

  ns = File.join(SANDBOX, namespace_prefix, service.class.to_s, model_klass.to_s)
  ns = safe_path(ns)
end

.namespace_prefix ⇒ Object 



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# File 'lib/fog/core/cache.rb', line 163

def self.namespace_prefix
  @namespace_prefix
end

.namespace_prefix=(name) ⇒ Object 



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# File 'lib/fog/core/cache.rb', line 159

def self.namespace_prefix=(name)
  @namespace_prefix = name
end

.safe_path(klass) ⇒ Object 



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# File 'lib/fog/core/cache.rb', line 176

def self.safe_path(klass)
  klass.to_s.gsub("::", "_").downcase
end

Instance Method Details

#dump ⇒ Object

Dump a Fog::Model resource. Every fog model/instance now has a cache method/object injected in. as such you can use the #dump method to save the attributes and metadata of that instance as cache which can be re-used in some other session.



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# File 'lib/fog/core/cache.rb', line 187

def dump
  if !File.exist?(self.class.namespace(model.class, model.service))
    self.class.create_namespace(model.class, model.service)
  end

  data = { :identity => model.identity,
                 :model_klass => model.class.to_s,
                 :collection_klass => model.collection && model.collection.class.to_s,
                 :attrs => model.attributes }

  File.open(dump_to, "w") { |f| f.write(YAML.dump(data)) }
end

#dump_to ⇒ Object

the location of where to save this fog model/instance to.



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# File 'lib/fog/core/cache.rb', line 201

def dump_to
  # some fog models have an identity field that is duplicate.
  # duplicate identities can mean the cache for that already exists.
  # this means cache duplication is possible.
  #
  # see "dumping two models that have duplicate identity" test case.
  name = "#{self.class.namespace(model.class, model.service)}/#{model.identity}-#{SecureRandom.hex}.yml"
end