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Use raw time string from DB to generate ActiveRecord#cache_version
Currently, the `updated_at` field is used to generate a `cache_version`. Some database adapters return this timestamp value as a string that must then be converted to a Time value. This process requires a lot of memory and even more CPU time. In the case where this value is only being used for a cache version, we can skip the Time conversion by using the string value directly. - This PR preserves existing cache format by converting a UTC string from the database to `:usec` format. - Some databases return an already converted Time object, in those instances, we can directly use `created_at`. - The `updated_at_before_type_cast` can be a value that comes from either the database or the user. We only want to optimize the case where it is from the database. - If the format of the cache version has been changed, we cannot apply this optimization, and it is skipped. - If the format of the time in the database is not UTC, then we cannot use this optimization, and it is skipped. Some databases (notably PostgreSQL) returns a variable length nanosecond value in the time string. If the value ends in a zero, then it is truncated For instance instead of `2018-10-12 05:00:00.000000` the value `2018-10-12 05:00:00` is returned. We detect this case and pad the remaining zeros to ensure consistent cache version generation. Before: Total allocated: 743842 bytes (6626 objects) After: Total allocated: 702955 bytes (6063 objects) (743842 - 702955) / 743842.0 # => 5.4% ⚡️⚡️⚡️⚡️⚡️ Using the CodeTriage application and derailed benchmarks this PR shows between 9-11% (statistically significant) performance improvement versus the commit before it. Special thanks to @lsylvester for helping to figure out a way to preserve the usec format and for helping with many implementation details.
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2 changed files with 118 additions and 4 deletions
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@ -96,8 +96,14 @@ module ActiveRecord
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# Note, this method will return nil if ActiveRecord::Base.cache_versioning is set to
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# +false+ (which it is by default until Rails 6.0).
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def cache_version
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if cache_versioning && timestamp = try(:updated_at)
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timestamp.utc.to_s(:usec)
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return unless cache_versioning
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return unless has_attribute?("updated_at")
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timestamp = updated_at_before_type_cast
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if can_use_fast_cache_version?(timestamp)
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raw_timestamp_to_cache_version(timestamp)
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elsif timestamp = updated_at
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timestamp.utc.to_s(cache_timestamp_format)
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end
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end
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@ -151,5 +157,43 @@ module ActiveRecord
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end
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end
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end
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private
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# Detects if the value before type cast
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# can be used to generate a cache_version.
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#
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# The fast cache version only works with a
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# string value directly from the database.
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#
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# We also must check if the timestamp format has been changed
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# or if the timezone is not set to UTC then
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# we cannot apply our transformations correctly.
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def can_use_fast_cache_version?(timestamp)
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timestamp.is_a?(String) &&
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cache_timestamp_format == :usec &&
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default_timezone == :utc &&
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!updated_at_came_from_user?
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end
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# Converts a raw database string to `:usec`
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# format.
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#
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# Example:
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#
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# timestamp = "2018-10-15 20:02:15.266505"
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# raw_timestamp_to_cache_version(timestamp)
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# # => "20181015200215266505"
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#
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# Postgres truncates trailing zeros, https://bit.ly/2QUlXiZ
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# to account for this we pad the output with zeros
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def raw_timestamp_to_cache_version(timestamp)
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key = timestamp.delete("- :.")
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padding = 20 - key.length
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if padding != 0
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key << "0" * padding
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else
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key
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end
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end
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end
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end
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@ -44,10 +44,80 @@ module ActiveRecord
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test "cache_key_with_version always has both key and version" do
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r1 = CacheMeWithVersion.create
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assert_equal "active_record/cache_key_test/cache_me_with_versions/#{r1.id}-#{r1.updated_at.to_s(:usec)}", r1.cache_key_with_version
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assert_equal "active_record/cache_key_test/cache_me_with_versions/#{r1.id}-#{r1.updated_at.utc.to_s(:usec)}", r1.cache_key_with_version
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r2 = CacheMe.create
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assert_equal "active_record/cache_key_test/cache_mes/#{r2.id}-#{r2.updated_at.to_s(:usec)}", r2.cache_key_with_version
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assert_equal "active_record/cache_key_test/cache_mes/#{r2.id}-#{r2.updated_at.utc.to_s(:usec)}", r2.cache_key_with_version
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end
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test "cache_version is the same when it comes from the DB or from the user" do
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skip("Mysql2 does not return a string value for updated_at") if current_adapter?(:Mysql2Adapter)
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record = CacheMeWithVersion.create
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record_from_db = CacheMeWithVersion.find(record.id)
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assert_not_called(record_from_db, :updated_at) do
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record_from_db.cache_version
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end
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assert_equal record.cache_version, record_from_db.cache_version
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end
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test "cache_version does not truncate zeros when timestamp ends in zeros" do
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skip("Mysql2 does not return a string value for updated_at") if current_adapter?(:Mysql2Adapter)
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travel_to Time.now.beginning_of_day do
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record = CacheMeWithVersion.create
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record_from_db = CacheMeWithVersion.find(record.id)
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assert_not_called(record_from_db, :updated_at) do
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record_from_db.cache_version
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end
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assert_equal record.cache_version, record_from_db.cache_version
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end
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end
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test "cache_version calls updated_at when the value is generated at create time" do
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record = CacheMeWithVersion.create
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assert_called(record, :updated_at) do
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record.cache_version
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end
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end
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test "cache_version does NOT call updated_at when value is from the database" do
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skip("Mysql2 does not return a string value for updated_at") if current_adapter?(:Mysql2Adapter)
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record = CacheMeWithVersion.create
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record_from_db = CacheMeWithVersion.find(record.id)
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assert_not_called(record_from_db, :updated_at) do
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record_from_db.cache_version
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end
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end
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test "cache_version does call updated_at when it is assigned via a Time object" do
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record = CacheMeWithVersion.create
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record_from_db = CacheMeWithVersion.find(record.id)
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assert_called(record_from_db, :updated_at) do
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record_from_db.updated_at = Time.now
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record_from_db.cache_version
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end
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end
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test "cache_version does call updated_at when it is assigned via a string" do
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record = CacheMeWithVersion.create
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record_from_db = CacheMeWithVersion.find(record.id)
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assert_called(record_from_db, :updated_at) do
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record_from_db.updated_at = Time.now.to_s
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record_from_db.cache_version
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end
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end
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test "cache_version does call updated_at when it is assigned via a hash" do
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record = CacheMeWithVersion.create
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record_from_db = CacheMeWithVersion.find(record.id)
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assert_called(record_from_db, :updated_at) do
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record_from_db.updated_at = { 1 => 2016, 2 => 11, 3 => 12, 4 => 1, 5 => 2, 6 => 3, 7 => 22 }
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record_from_db.cache_version
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end
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end
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end
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end
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