Mongoid Optimistic Locking

Mongoid Atomic Operations
findAndModify
Optimistic Locking
Presented on "Startup Day"

Transcript

1. Lightning talk
   Alon Burg
   @burgalon
   Optimistic Locking
   

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2. Overview
   Define the problem
   3 examples with different solution
   3rd example solution with Optimistic
   Locking
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3. Preface: Our tools
   Ruby on Rails
   Mongo (no transactions)
   Mongoid
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4. Locking: The Problem
   Multiple threads/processes
   trying to update the same DB entity
   row 1
   row 1
   row 3
   row 1
   row 2
   Table Client
   Client
   do some processing do some processing
   

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5. Example 1: Counter
   Video 1
   100 views
   Video 2
   200 views
   Video 3
   300 views
   User 1 User 2
   Increment +1
   Increment +1
   User 1: Find video 1
   User 2: Find video 1
   User 1: Update video views to 101
   User 2: Update video views to 101
   1.
   2.
   3.
   4.
   

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6. Database atomic operation
   $set - set a particular value
   $unset - delete a particular field (v1.3+)
   $inc - increment a particular value by a certain amount
   $push - append a value to an array
   $pushAll - append several values to an array
   $pull - remove a value(s) from an existing array
   $pullAll - remove several value(s) from an existing array
   $bit - bitwise operations
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   Video.find(video_id).inc(:views, 1)
   

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7. Locking: Example 2
   Job 1
   state: init
   Client
   1
   Client
   2
   state: processing
   state: processing
   Job 2
   state: init
   Job 3
   state: init
   1) Query
   ....
   2) Update
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8. Mongo findAndModify
   Queue.
   where(state: :init).
   asc(:created_at).
   find_and_modify({ "$set" => { state: false }}, new: true)
   

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9. Example 3: Blog Post
   add update
   Requires server side logic in between
   Post
   text: “Good news, anyone. The
   Swedish robot from Pi-kea is here
   with the super collider I ordered.”
   addUpdate
   “Bad news, nobody”
   addUpdate
   “The super collider exploded”
   Farnsworth Farnsworth
   

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10. More specific code for
    Blog add_update example
    #### models/post.rb
    class Post
    include Mongoid::Document
    field :text
    end
    #### controllers/posts_controller.rb
    class PostController < ApplicationController
    ## Adds an "UPDATE: ...some text..." to an existing document
    def add_update
    post = Post.find(params[:id])
    post.text += "---UPDATE--- " + params[:more_text]
    post.save
    end
    end
    

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11. Optimistic Locking
    Fetch the object.
    Modify the object locally.
    Send an update request that says "update the object to this
    new value if it still matches its old value".
    1.
    2.
    3.
    Post
    text: “Good news, anyone. The
    Swedish robot from Pi-kea is here
    with the super collider I ordered.”
    version: 1
    Farnsworth
    addUpdate
    “Bad news, nobody”
    Post
    text: “Good news, anyone. The
    Swedish robot from Pi-kea is here
    with the super collider I ordered.
    -- UPDATE --
    Bad news, nobody”
    version: 2
    1) Fetch object 2) Concatenate text
    and increment version
    3) Save to DB if version=1
    addUpdate
    “The super collider exploded”
    Farnsworth
    1.5) Fetch object
    2.5) Concatenate text
    and increment version
    3.5) Save to DB if version=1
    fails ==> retry
    Post
    text: “Good news, anyone. The
    Swedish robot from Pi-kea is here
    with the super collider I ordered.
    -- UPDATE --
    Bad news, nobody
    --UPDATE--
    The super collider exploded”
    version: 3
    4) Fetch object
    5) Concatenate text
    and increment version
    6) Save to DB if
    version=2
    

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12. Rails gem for Mongoid
    mongoid_optimistic_locking
    #### models/post.rb
    class Post
    include Mongoid::Document
    include Mongoid::OptimisticLocking
    field :text
    end
    #### controllers/posts_controller.rb
    class PostController < ApplicationController
    ## Adds an "UPDATE: ...some text..." to an existing document
    def add_update
    begin
    post = Post.find(params[:id])
    post.text += "---UPDATE--- " + params[:more_text]
    post.save
    rescue Mongoid::Errors::StaleDocument
    retry
    end
    end
    end
    

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13. Questions?
    

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14. View Slide