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# frozen_string_literal: true
# ActiveModel::Serialization (https://github.com/rails/rails/blob/v5.0.7/activemodel/lib/active_model/serialization.rb#L184)
# is simple in that it recursively calls `as_json` on each object to
# serialize everything. However, for a model like a Project, this can
# generate a query for every single association, which can add up to tens
# of thousands of queries and lead to memory bloat.
#
# To improve this, we can do several things:
# 1. Use the option tree in http://api.rubyonrails.org/classes/ActiveModel/Serializers/JSON.html
# to generate the necessary preload clauses.
#
# 2. We observe that a single project has many issues, merge requests,
# etc. Instead of serializing everything at once, which could lead to
# database timeouts and high memory usage, we take each top-level
# association and serialize the data in batches.
#
# For example, we serialize the first 100 issues and preload all of
# their associated events, notes, etc. before moving onto the next
# batch. When we're done, we serialize merge requests in the same way.
# We repeat this pattern for the remaining associations specified in
# import_export.yml.
module Gitlab
module ImportExport
class FastHashSerializer
attr_reader :subject, :tree
# Usage of this class results in delayed
# serialization of relation. The serialization
# will be triggered when the `JSON.generate`
# is exected.
#
# This class uses memory-optimised, lazily
# initialised, fast to recycle relation
# serialization.
#
# The `JSON.generate` does use `#to_json`,
# that returns raw JSON content that is written
# directly to file.
class JSONBatchRelation
include Gitlab::Utils::StrongMemoize
def initialize(relation, options, preloads)
@relation = relation
@options = options
@preloads = preloads
end
def raw_json
strong_memoize(:raw_json) do
result = +''
batch = @relation
batch = batch.preload(@preloads) if @preloads
batch.each do |item|
result.concat(",") unless result.empty?
result.concat(item.to_json(@options))
end
result
end
end
def to_json(options = {})
raw_json
end
def as_json(*)
raise NotImplementedError
end
end
BATCH_SIZE = 100
def initialize(subject, tree, batch_size: BATCH_SIZE)
@subject = subject
@batch_size = batch_size
@tree = tree
end
# With the usage of `JSONBatchRelation`, it returns partially
# serialized hash which is not easily accessible.
# It means you can only manipulate and replace top-level objects.
# All future mutations of the hash (such as `fix_project_tree`)
# should be aware of that.
def execute
simple_serialize.merge(serialize_includes)
end
private
def simple_serialize
subject.as_json(
tree.merge(include: nil, preloads: nil))
end
def serialize_includes
return {} unless includes
includes
.map(&method(:serialize_include_definition))
.tap { |entries| entries.compact! }
.to_h
end
# definition:
# { labels: { includes: ... } }
def serialize_include_definition(definition)
raise ArgumentError, 'definition needs to be Hash' unless definition.is_a?(Hash)
raise ArgumentError, 'definition needs to have exactly one Hash element' unless definition.one?
key = definition.first.first
options = definition.first.second
record = subject.public_send(key) # rubocop: disable GitlabSecurity/PublicSend
return unless record
serialized_record = serialize_record(key, record, options)
return unless serialized_record
# `#as_json` always returns keys as `strings`
[key.to_s, serialized_record]
end
def serialize_record(key, record, options)
unless record.respond_to?(:as_json)
raise "Invalid type of #{key} is #{record.class}"
end
# no has-many relation
unless record.is_a?(ActiveRecord::Relation)
return record.as_json(options)
end
data = []
record.in_batches(of: @batch_size) do |batch| # rubocop:disable Cop/InBatches
# order each batch by it's primary key to ensure
# consistent and predictable ordering of each exported relation
# as additional `WHERE` clauses can impact the order in which data is being
# returned by database when no `ORDER` is specified
batch = batch.reorder(batch.klass.primary_key)
data.append(JSONBatchRelation.new(batch, options, preloads[key]).tap(&:raw_json))
end
data
end
def includes
tree[:include]
end
def preloads
tree[:preload]
end
end
end
end
|
