Identity Map in Rails3.1
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2022-07-04 18:38:39
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Identity Map是Rails3.1的又一个新特性。
一、什么是Identity Map
下面的代码是ActiveRecord在未开启Identify Map时的表现:
上面的user和same_user是都是id为1的User对象,但是却是两个对象。
下面开启Identity Map:
当ActiveRecord开启了Identity Map后,每一个model会维持一个Map,每次实例化对象之前会通过主键去这个model对应的Map里找到对应的对象。如果存在,直接从Map里返回这个对象,否则才进行查询,实例化对象后放到Map里。
二、和Query Cache/Cache Money的差别
听起来和ActiveRecord的query cache或是cache money类似。实际上差别蛮大的:
从减少db查询方面和query cache一样,缓存池是在一次请求结束后销毁。各个请求之间不共享,而cache money是全局的,缓存结果可以被持续利用。
query cache是根据sql来做主键,而identity map和cache money是根据model的主键。
Identity Map还具有减少内存使用和保持一致性的作用。
三、Identity Map的优点
1.一致性:一个model对象保持完全唯一的引用,避免了更新同一个对象时候产生的不一致现象。
下面的例子是未开启Identity Map时,同一条记录的num被两次增加1,但是结果只增加了1.造成最终结果和实际不一致。而开启了Identity Map的结果最终是正确的。
2.节省内存:不会重复创建相同的对象。
3.提高速度:和ActiveRecord的query cache一样,减少db hits
四、存在的问题
目前默认还是未开启的。因为最近发现的在处理关联时的问题:
ps:rails 3.1的rails console默认可以打印sql日志,不需要hack irbrc了,cool~
一、什么是Identity Map
引用
Identity Map is a design pattern that:
Ensures that each object gets loaded only once by keeping every loaded object in a map. Looks up objects using the map when referring to them.
http://www.martinfowler.com/eaaCatalog/identityMap.html
Ensures that each object gets loaded only once by keeping every loaded object in a map. Looks up objects using the map when referring to them.
http://www.martinfowler.com/eaaCatalog/identityMap.html
下面的代码是ActiveRecord在未开启Identify Map时的表现:
>> user = User.first User Load (0.3ms) SELECT "users".* FROM "users" LIMIT 1 >> same_user = User.find 1 User Load (11.3ms) SELECT "users".* FROM "users" WHERE "users"."id" = ? LIMIT 1 [["id", 1]] >> user == same_user => true >> user.object_id == same_user.object_id => false
上面的user和same_user是都是id为1的User对象,但是却是两个对象。
下面开启Identity Map:
>> ActiveRecord::IdentityMap.enabled = true >> user = User.first User Load (0.4ms) SELECT "users".* FROM "users" LIMIT 1 >> same_user = User.find 1 User Loaded From Identity Map (id: 1) >> user == same_user => true >> user.object_id == same_user.object_id => true
当ActiveRecord开启了Identity Map后,每一个model会维持一个Map,每次实例化对象之前会通过主键去这个model对应的Map里找到对应的对象。如果存在,直接从Map里返回这个对象,否则才进行查询,实例化对象后放到Map里。
二、和Query Cache/Cache Money的差别
听起来和ActiveRecord的query cache或是cache money类似。实际上差别蛮大的:
从减少db查询方面和query cache一样,缓存池是在一次请求结束后销毁。各个请求之间不共享,而cache money是全局的,缓存结果可以被持续利用。
query cache是根据sql来做主键,而identity map和cache money是根据model的主键。
Identity Map还具有减少内存使用和保持一致性的作用。
三、Identity Map的优点
1.一致性:一个model对象保持完全唯一的引用,避免了更新同一个对象时候产生的不一致现象。
下面的例子是未开启Identity Map时,同一条记录的num被两次增加1,但是结果只增加了1.造成最终结果和实际不一致。而开启了Identity Map的结果最终是正确的。
>> user = User.first User Load (0.3ms) SELECT "users".* FROM "users" LIMIT 1 >> same_user = User.find 1 User Load (12.2ms) SELECT "users".* FROM "users" WHERE "users"."id" = ? LIMIT 1 [["id", 1]] >> user.num = user.num + 1 => 1 >> user.save (0.9ms) UPDATE "users" SET "updated_at" = '2011-06-12 09:46:32.722893', "num" = 1 WHERE "users"."id" = 1 => true >> same_user.num = same_user.num + 1 => 1 >> same_user.save (0.4ms) UPDATE "users" SET "updated_at" = '2011-06-12 09:46:53.093752', "num" = 1 WHERE "users"."id" = 1 => true >> user.reload => #<User id: 1, name: nil, num: 1, created_at: "2011-06-12 03:00:30", updated_at: "2011-06-12 09:46:53">#这里结果不一致了 >> ActiveRecord::IdentityMap.enabled = true => true >> same_user = User.find 1 User Load (0.2ms) SELECT "users".* FROM "users" WHERE "users"."id" = ? LIMIT 1 [["id", 1]] => #<User id: 1, name: nil, num: 1, created_at: "2011-06-12 03:00:30", updated_at: "2011-06-12 09:46:53"> >> user = User.first User Load (0.3ms) SELECT "users".* FROM "users" LIMIT 1 User Loaded From Identity Map (id: 1) => #<User id: 1, name: nil, num: 1, created_at: "2011-06-12 03:00:30", updated_at: "2011-06-12 09:46:53"> >> same_user.num = same_user.num + 1 => 2 >> same_user.save (0.4ms) UPDATE "users" SET "updated_at" = '2011-06-12 09:51:59.203676', "num" = 2 WHERE "users"."id" = 1 >> user.num = user.num + 1 => 3 >> user.save (0.5ms) UPDATE "users" SET "updated_at" = '2011-06-12 09:52:21.643577', "num" = 3 WHERE "users"."id" = 1 #加了两次1结果为3,正确!
2.节省内存:不会重复创建相同的对象。
3.提高速度:和ActiveRecord的query cache一样,减少db hits
四、存在的问题
目前默认还是未开启的。因为最近发现的在处理关联时的问题:
# Active Record Identity Map does not track associations yet. For example: # comment = @post.comments.first # comment.post = nil # @post.comments.include?(comment) #=> true # # Ideally, the example above would return false, removing the comment object from the # post association when the association is nullified. This may cause side effects, as # in the situation below, if Identity Map is enabled: # # Post.has_many :comments, :dependent => :destroy # # comment = @post.comments.first # comment.post = nil # comment.save # Post.destroy(@post.id) # # Without using Identity Map, the code above will destroy the @post object leaving # the comment object intact. However, once we enable Identity Map, the post loaded # by Post.destroy is exactly the same object as the object @post. As the object @post # still has the comment object in @post.comments, once Identity Map is enabled, the # comment object will be accidently removed. # # This inconsistency is meant to be fixed in future Rails releases.
ps:rails 3.1的rails console默认可以打印sql日志,不需要hack irbrc了,cool~