• Want to learn more about me? Check out http://t.co/cP3dSJKU and sign up for your own at http://t.co/jdsytvcu #
• More notes on #Java #Concurrency – lectures by @venkat_s at @nofluff http://t.co/OwKGwXnR #
• Several weeks late, here are @CodeAcademy week 8 notes: http://t.co/G9ZxXvr6 #Ruby #Rails #
• @puredanger Thanks for the follow #
• @cardoni I wish my life was more like yours #
• I am using the great pry gem (https://t.co/1LhNoK8v) that I found out about at last night's @ChicagoRuby hack night #
• Great page on #Ruby loops and iterators http://t.co/C0U4QaYY #

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I attended the No Fluff Just Stuff conference here in Chicago back in November 2011. I took  a few notes on some of the lectures. Here are my notes on some of the lectures on concurrency. I may post in a while with notes from more lectures, as well as some more thoughts on concurrency.

Venkat: concuurrency without pain in pure Java
Why Concurrency?
Java gave us a consistent API for multi-threading
Errors are worse on multi-core machines – multiple levels of cache in application
To get concurrency correct, you have to be God.
That model is the synchronize and suffer model
2 other models: Software Transaction Memory and Actors

public class Account {
    private int balance;
    public Account
}

Synchronized helps you cross the memory barrier synchronizing main and working memory
Locking and race condition is only part of the story
Do not synchronize the accounts, synchronize the transactions
You should also synchronize in the class using the Account classes

synchronized(from) {
    synchronized(to) {
        // do stuff
    }
}

There could be livelock or deadlock
But you could wait a long time.
But you cannot tell synchronized() to timeout.

So instead in Account, use a ReentrantLock
in Account.deposit method, use ReentrantLock.tryLock(long, TimeUnit)

This makes code more complex. synchronized blocks are pretty easy.
Don’t forget to call Reentrant.unlock()
But what if there is an exception before that? Put it in a finally.
This is why we don’t use raw JDBC

Deadlock: I have lock on A, I need a lock on B. You have a lock on B, you need a lock on A.
You could impose an order
If someone introduces a new method, all your locking could be ruined. It’s like a bathroom with a lot of doors. You got to lock every one.
Concurrency on JDK is like a mother-in-law: It’s waiting for you to fail.
95% of Java is broken for concurrency, and we don’t know it yet.

STM: Software Transaction Memory
Everything is immutable (almost everything)
Managed Mutable Identity is also in Clojure
Sharing is good, mutability is good, shared mutability is bad
Separates state from identity
identity points to state
Like stock price: $600 at 1:00 PM, $610 at 1:10
The price at 1PM on November 11, 2011 will always be $600
We can flip a switch to point identity to a new value
Immutability means no locks

the problem is not that it is failing, but that it is misbehaving quietly and you do not know it.
MMI fails loudly if you mess with it outside of a transaction
transaction thread runs in a cone of silence
final change is visible to outside world

If there is a collision, a transaction could repeat
Clojure is written in Java – it’s another JAR, and you can use it in Java

change int in Account to clojure.lang.Ref

private Ref balance
public Account(final int initialBalance) {
    balance = new Ref( initialBalance );
}

int will be replaced with java.lang.Integer
You can only put immutable stuff in there.
Also

public void deposit(final int amount) {
    LockingTransaction.runInTransaction( new Callable< Void >() {
        public Void call() throws Exception() {
        if (amount > 0) { balance.set( getBalance() + amount ) }
        return null;
        }
    });
}
public int getBalance()   { return (Integer) balance.deref(); }

Clojure:

(def balance (ref 0))
(println @balance)
(dosynch (ref-set balance 1) )
(println @balance)

You could still have race conditions. The AccountService that does the transfer still needs a transaction to wrap around the transactions in the Account classes
A transaction in STM could repeat 100,000 times – so do not print, send email

STM: we don’t see any locks (so no deadlocks)
2 drawbacks:
1. STM requires state be immutable – greate for Clojure – the only language where concurrency is safe
2. Code could ge slow or fail due to retries
Hard to be functional in Java
Another option: Isolated mutability – only one thread can touch it at a time
Actors and active objects
An actor can receive many messages at once, but they only work on one at a time
Akka library – now we use Scala’s implementation
Akka gives us STM and Actors

ActorRef ratcliff = Actors.actorOf(HollywoodActor.class).start();
ratcliff.sendOneWay( "Potter" );
ratcliff.stop();

HollywoodActor extends UntypedActor() {
    public void onReceive( final Object message )
}

So we passed a message. We did not call a method directly.
Actors do not hold Threads hostage
Actors could fail
Each Actor is sequential
Could deadlock with 2-way messaging

JDK concurrency APIs are assembly language of concurrency
—
But can Java web developers do this?

Ted Neward Guava:
Get rid of mutable state

A few people I know tweeted to a Rails commit on Github that “Added a generator option to remove the public/index.html file when generating a new Rails application.” The people that tweeted about it said it was about time that this was included.

I think a good option for the scaffold generator would be to explicitly list the routes for each of the created actions in the controller instead of just adding

resources :things

to the config/routes.rb  file.

It would generate this in the config/routes.rb file:

  get '/things' => 'things#index', :as => 'things'
  post '/things' => 'things#create'
  get '/things/new' => 'things#new', :as => 'new_thing'
  get '/things/:id/edit' => 'things#edit', :as => 'edit_thing'
  get '/things/:id' => 'things#show', :as => 'thing'
  put '/things/:id' => 'things#update'
  delete '/things/:id' => 'things#destroy'

I ran rake routes with each, and they matched.

2012-02-28_08.18.14
undefined method `model name` for NilClass:Class
Means there is a problem with the model
So instantiate it in the controller from the session

How to override application.html.erb for a controller or action

For orders controller, app/views/layouts/order.html.erb
To specify layout:
in controller:

layout 'application',  nly => :index

or in def new:

def new
   @cart = Cart.find(session[:cart_id])
   @order = Order.new
   render :layout => 'orders'
 end

we need to encrypt the credit card number. Look in config/application.rb
We will look at
config.filter_parameters +=[:password]
This will filter password out of the log
So we can add to the array
config.filter_parameters +=[:password, :card_number]

asset pipeline: We will talk about it throughout the week. Look in the assets folder. Images, javascript and stylesheets.
It takes all the javascript and puts it in one file, so the browser only has to make one request
Same for css
For css, you should go to application.css and specify files and/or directories

*= require_self
 *= require_tree

require_tree will do everything aplhabetically. You can put them in your own order.

*= require brands
 *= require products
 *= require reviews
 *= require scaffolds
 *= require users

In development mode, there is no asset pipeline. You use it in production.

In config/environments/development.rb

config.assets.debug=false

if you set that to false, you won’t see the GET requests in the logs for EVERY js and css file

Jeff can get to other order pages as well as his own
We have authentication. What about authorization?
In orders controller:

def show
   @order = @user.orders.find(params[:id])
 end

WHat if that ID is not in the order table?

def show
   @order = @user.orders.find(params[:id])
   if order.nil?
     redirect_to root_url, :notice => "Nice try"
   end
 end

You could do this:

redirect_to root_url, :notice => "Nice try" if order.nil?

But then it looks like redirect is the default.
Or

redirect_to root_url, :notice => "Nice try" unless order

Moving this to a one-liner could make it a but confusing
You could also do

if order.blank?
   redirect_to root_url, :notice => "Nice try"
 end

or

unless @order
   redirect_to root_url, :notice => "Nice try"
 end

Stay at same level of abstraction throughout the html.erb files
if you have render tags with low-level html, that could look bad

Class method that makes an ActiveRecord call
Use the scope facility
Instead of:

def self.most_recent
   order("updated_at desc").limit(3)
 end

scope :most_recent, order("updated_at desc").limit(3)

scope creates a class-level method, and returns rows
You cannot just return one row

Maybe we want to pass a param from the view to the class-level method or scope

Easy for method.
For a scope:

create a lambda: create an unnamed method on-the-fly
 scope :most_recent, lambda { |n| order("updated at desc").limit(n) }

Orders Controller:
We have a before_filter
We have model callbacks in cart_item.rb
Those are sort of like lambdas as well.
We do not call it, Rails calls it for us at some point.

Let’s do a scope on price for products:

scope :expensive, where('price > 50')

Then in the view you could chain the scopes

Lambda with a default value for min:

scope :expensive, lambda { |min=80| where('price > #{min}') }

Getting the data you want:

.find
 .find_by_*
 .where(hash)
 .where("SQL")
 .where("SQL", ....)
.where("color = BLUE''") # sql syntax - single = sign is comparison
 .where("color like 'BLUE'")

You can also use % for wildcards

.where("name like 'M%'")
 .where("name like ?", "M%")

In Products Controller:

@products = Product.where("name like '#{params[:search]}'")

or

@products = Product.where("name like '#{params[:search]}%'")

So why not do this? Jeff will explain
You could get SQL injection
So do this:

a = params[:search]
 @products = Product.where("name like '%#a%'")

Use the question mark placeholder

@products = Product.where("name like ?", params[:search])

ActiveRecord will create the SQL for you
Fuzzy search:

@products = Product.where("name like ?", "%#{params[:search]}%")

Then in the controller you could chain the methods

@products = @products.limit(1000).order('name asc')

It is not until you get to the @products.each (or a .all) in the view that you run the query

Should you use the products controller for searching, or make a search controller?
the form_tag goes to ‘/products’ with a get, which will go to the index action
—-
New app:
User Story Analysis
1. Identify resources
2. Create Models
3. Implement business rules
4. Create User Interface

Some people go in the opposite order

Air Academy User story
1. As a visitor, list the flights

In Rails 3.2, when you generate model, if you do not specify the types it will assume that it is a string
rails g model Airport code:string city:string
ericm@finance:~/ruby/ca_files/air$ more db/migrate/20120228174012_create_airports.rb

class CreateAirports < ActiveRecord::Migration
  def change
    create_table :airports do |t|
      t.string :code
      t.string :city

      t.timestamps
    end
    add_index :airports, :code
  end
end

rails g model Airport code:string city:string

emacsnw db/migrate/20120228174012_create_airports.rb

For flights, we need two airports, not just one departure_airport_id, arrival_airport_id

class Flight
  belongs_to :departure_airport, :class_name => 'Airport'
  belongs_to :arrivale_airport, :class_name => 'Airport'
end

If you say belongs_to :blah, the table needs a column blah_id
If that is the only belongs_to, Rails will assume there is a Blah model

rails g model Flight departure_airport_id:integer, arrival_airport_id:integer number:string distance:integer duration:integer departs_at:time

2012-03-01_08.38.49
Pagination:
Kaminari, Will_paginate, roll your own

User story: As a user, I want to make a reservation
As opposed to doing something “as a visitor”
Different permissions, etc
So for “As a user” that user story needs an account
User story analysis
1. Identify resources
2. Create models
3. Implement business rules
4. Create user interface

reservation model has user, a flight, chosen departure date, credit card number
Associations:
user can have many reservations
flight has many reservations
reservation belongs to flight and belongs to user

rails g scaffold Reservation user_id:integer flight_id:integer credit_card_number:string departs_on:date

departs_on for dates, departs_at for times

How to decide to model or scaffold? How much of scaffold will you use?

add_index on the reservation table
add_index is outside the create_table block

add_index :reservations, :user_id
 add_index :reservations, :flight_id

Good to have indexes for foreign keys
Then run

rake db:migrate

rake db:migrate:reset will drop database and recreate tables
rake db:seed
You could do

rake db:migrate:reset db:seed

rails g model User first last email passwod_digest

——————
Now JC from DevMynd is talking
database: structured set of data on a computer, accessible in various ways
five categories: relational, graph, key-value, document, column family databases
Relational database: built around relational theory (first-order predicate calculus), schema defined in tables of columns and rows, data is related through matching keys
Graph: based on graph theory. These DBs use nodes, properties and edges to describe a “web” of data. They can be powerful for complex ancestral queries (Amazon: People who bought X bought Y)
Key-value: primary way of retrieving objects is by a single key. What can be stored as a value varies.
Redis is used for queing systems
Document database: storing document-oriented, semi-structured data. Normally within a loose schema and the ability to store/retrieve nested structures
Column-family database: inverse of relational model. Big table.
Choosing data store can be pretty complex.
Considerations: indexing, querying, scaling, modeling, mapping, analyzing, recovering
Indexes: a parallel description of your data optimized for fast lookup
If you will ever do an order_by or a conditional query
Finding by department and last name is different than index that finds by last name and first name
Querying: getting info from db. Do we use a template object, a query language, or a string key?
Mongo can use query syntax, or you can pass in a template doc
scaling: scale up (faster, bigger hardware) or out (distributing data, indices and queries across more machines, aggregating the results)
mongo can shard well, so can redis
modeling: is your schema fixed or flexible? a few large things, lots of small things
Mapping (ORM) – ActiveRecord for RDBMS, Mongoid for MongoDB, Redis ruby driver
how will you interact with db?
analysis: what does the database provide?
recovery: relational: replication, push log to another system
Mongo: backup and restore tools
Heroku, other cloud backups are not that great

optimization: this is usually the first bottleneck
universal: careful indexing, duplication
relational: de-normalization, pivoting, materlialized views
normalized: each concept in a different table, less repetition
pivot: making rows into columns, and vice versa
materialized views:
non-relational: nest relationships, parallel queries
downside of indexing: it can take a LOT of space, plus index might be held in memory instead of disk
Put query logic in your model, not your controller
So if you change the datastore, you can just change it in the model class
relational tables good for complex joins
Think carefully about uniqueness and nullability early on

——————
Back to Jeff
For flight page, give some info about flight
the user is part of the session
reservation for
So in reservation controller, in new method: @user = User.find(session[:user_id])
You could do it from show page for a flight
@user = User.find()
Get stuff from flight show page
Reuse the flight_details partial

But we could get a Nil
So in the reservations controller
We will do something in def new
We could have a link from flight show page
@flight.id) %>
So in new,

@flight = Flight.find(params[:flight_id])

So we want to put that link in an if on the flight show page
Put a filter on the reservations new controller

before_filter :require_login,  nly => :new
   def require_login
   if session[:user_id].blank?
     redirect_to root_url, notice: "Get your act together"
   end
 end

@flight.id) %>

In reservation form:

in reservation controller.new:

@reservation.flight = @flight

validation on reservations:

create method in reservationcontroller:

def create
   @reservation = Reservation.new()
   @reservation.user = User.find(session[:user_id])
 end

Sending email in Rails – it could depend on your host

rails g mailer ReservationMailer
 rails g mailer ReservationMailer

In our reservation controller, we will use this in respond_to block

email = ReservationMailer.confirmation(@reservation)
 email.deliver

in ReservationMailer

def confirmation(reservation)
   @reservation = reservation
   mail(:subject => "Thanks for your reservation",
   :from => "reservations@academyair.com",
   :to => reservation.user.email )
 end

So in app/views/reservation_mailer/confirmation.text.erb
Hello, ,
Thanks for sending us $300.

Flight details
———————
Flight number:

• @tomordonez I will probably post about it later in the week – I would say things are looking good for Rails #
• @devmyndsoftware Thanks for the follow #
• @rails_apps Thanks for the follow #
• @karmajunkie Thanks for the follow #
• Just once I would like to boot up my Win7 laptop and NOT be asked to install another Adobe update #
• @ryanbriones @TheTakeaway is a daily news podcast with short segments – I like some of Bloomberg's stuff as well #
• @cardoni @dhh It was a great keynote #
• @CodeAcademy I met @mhartl of @railstutorial at #RailsConf http://t.co/9dt5NXBP I told him we all do his tutorial #
• @ryanbriones I was thinking of @BloombergNews http://t.co/UuB1FmdP Media company started by Mayor Mike before he became mayor of NYC #
• @RailsConf presentation by @JEG2 now online: http://t.co/hIecbbTO #
• @rubygeekdotcom Thanks for the tip #
• @devmyndsoftware Thanks for the follow #
• @damon Thanks for the follow – will you be at Lone Star Ruby Conf this year? I might go #
• @rubygeekdotcom Is there a Flash player alternative for Windows? #
• @RailsConf presentations: http://t.co/1eQk6Q6l #
• Watch this link for the great @RailsConf presentation by @karmajunkie: http://t.co/Tsj77iob #
• @rubygeekdotcom Where are you moving to? Are you leaving Austin, or are you just moving further south in Austin? #
• @RudyOnRails Is that @briankung backflipping? #
• @briankung The force is strong with you #
• @cardoni Is your company using MySQL? it seems like PostGeSQL is gaining in the Ruby world #
• @arvindang Not to be pedantic, but I must point out that if @briankung were a ninja, there would be no picture. #

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• I am in Austin for #RailsConf #
• @timtyrrell I am in town for #RailsConf – we should get together while it's on #
• @hashrocket @BohConf @RailsConf @joshuadavey Can anyone show up to BohConf? #
• @RayHightower Are you in Austin for #RailsConf #
• @redsquirrel @ryanbriones – I am in Austin for #RailsConf – how many Groupon people are here? #
• @cardoni congrats on the job at @RECSOLU #
• @vcabansag Are you going to get your pic taken with BC? #
• Hanging with the @hashrocket guys at #RailsConf #
• @briankung said "@codeacademy students are all smarter than me!" – Is that a growth-mindset statement, or a fixed-mindset statement? #
• The WiFi at #RailsConf is not the best network I have ever been on. I know there are a lot of people here, but still, it's a dev conf! #
• @umtrey Why not ask them? #
• Working on @rails_apps while at #RailsConf http://t.co/kYlvweHH with mentions of @karmajunkie @JEG2 and @blowmage #
• @timtyrrell I may join you – are you going to the EngineYard/CabForward drinkup tonight? #
• Thanks to @karmajunkie for getting me back into my laptop #
• @JaretManuel Thanks for the link #
• @vcabansag I shook his hand back in 2007 – I saw Bill and Hillary in Iowa. I got a couple of pix but none of me with them #
• @tomordonez No stickers. I am too busy loading up on shirts. Maybe next year. #
• @cardoni @tomordonez when/where was this taken? #
• Here I am with Hartl of @RailsTutorial at @RailsConf in #Austin http://t.co/9dt5NXBP #
• @timtyrrell Maybe I would see you in August at Lone Star Ruby Conf #
• @tomordonez I told @mhartl that everyone at @CodeAcademy does @RailsTutorial – more than once #
• @briankung Good to know I am not the only non-Apple Rails guy in Chicago #

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I will probably post more about RailsConf when I get back. So far I like the presentations that I have been to. Here are thoughts on a few presentations.

One of the best was “Use the Source, Luke: High fidelity data with event sourcing”, by Keith Gaddis (Twitter account here). Here is the synopsis:

Ever run into a really gnarly data problem and wished you had a do-over? Tired of wrestling with ActiveRecord to model a really complex domain? Looking for a good way to echo state changes to external systems? Then grab a cup of joe and settle in for a look at event-sourcing your data.

Event-sourced data uses Plain Old Ruby Objects (POROs) to model your data and exclusively uses events to mutate state on those objects. By serializing the events, the state of your data can be recreated for any point in time, and outside listeners can create specialized purposeful datastores of the data, enabling complex business requirements with fewer hassles. We’ll also touch on other architectural patterns like DCI and CQRS that play well with this idea

Someone afterward said that it was the best presentation they saw at the conference. People were still asking him questions and discussing the topic 30 minutes later. I will certainly look for the slides and video for this one.

Another good one was “Presenters and Decorators: A Code Tour” by Mike Moore (look at his Twitter account). The idea is that you use presenters and decorators when you have a lot of if statements in your views and templates.

I also liked “Ten Things You Didn’t Know Rails Could Do With Rails” (with 32 bonus stuff) by the always-entertaining James Edward Gray (Twitter account here).

I just spent an hour doing one of the apps at RailsApps (Twitter account here).

• @briankung So how do you change your mindset? #
• I will be in Austin next week for @railsconf http://t.co/la0s9rVO #
• @ryanmcoon @bryonfinke Do you really think that GRPN is doomed? Would that be bad for the Chicago Rails community? #

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I will be in Austin for a week for RailsConf. There may not be too many posts. I may do some tweets. I will have a few hours after I get there to post the night before the conference. Perhaps I will post on RSpec.

• @RudyOnRails Why are you going to EIU? #
• @railstutorial Is the 3.2 version finalized online? #

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