redis-logging.txt

Understanding Ohm Internals through Logging

Benefits

Ohm is actually a very thin wrapper for Redis, and most of the design patterns implemented in Ohm are based on the prescribed patterns for using Redis.

If you take a little time to grok the internals of Ohm and Redis, the more effective you will be in weilding it efficiently.

Keys

Most key-value stores have standardized on a structured design pattern for organizing data. Let’s fire up a console:

>> irb -r ohm -r logger

Ohm.redis.client.logger = Logger.new(STDOUT)

class Post < Ohm::Model
  attribute :title
end

Post.create(:title => “Grokking Ohm”)

After executing Post.create(…), you’ll see a lot of output from the logger. Let’s go through every line in detail.

Post.create

Generate a new Post:id.

*Lock Post:2 (see SETNX for an explanation of the locking design pattern).

Add the newly generated ID 2 to the Post:all SET.

Start transaction.

Delete HASH Post:2.

Set the attributes.

End transaction.

Release the lock.

Sets and Lists

Let’s do a little SET and LIST manipulation and see what Ohm does. Here’s the code for this section:

class User < Ohm::Model
  collection :posts, Post
end

class Post < Ohm::Model
  attribute :title
  reference :user, User

  list :comments, Comment
end

class Comment < Ohm::Model
end

User.create

Generate a new ID for the User.

Lock User:9.

Add this new user to the SET User:all.

Release the lock.

Post.create(:title => “Foo”, :user => User.create)

Generate an ID for this Post

Lock Post:3

Add the ID 3 to the SET Post:all

Start transaction

Remove existing Post:3 HASH

Assign the attributes to Post:3

End transaction

Add the ID of this post to the SET index (more on this below)

Book keeping for Post:3 indices

Release lock

post.comments << Comment.create

Generate an ID for this comment

Lock Comment:1

Add this comment to the SET Comment:all

Release lock

Append the comment to the LIST Post:3:comments

Understanding indexes

reference :user, User

is actually more or less the same as:

attribute :userid index :userid

def user User[user_id] end

def userid=(userid) self.userid = userid end

To further explain the example above, let’s run through the commands that was issued. Remember that we had a user_id of 9 and a post_id of 3.

OQ== is Base64 of *9, with newlines removed. The postid 3 is added effectively to Post:userid:9, if we ignore the encoding.*

Just keep track that Post:user_id:OQ== was created

Get all *posts with user_id *9**

What’s next?

We tackled a fair bit amount regarding Ohm internals. More or less this is the foundation of everything else found in Ohm, and other code are just built around the same fundamental concepts.

If there’s anything else you want to know, you can hang out on #ohm at irc.freenode.net, or you can visit the Ohm google group.