concurrency

module WriteAmplificationGuard
  extend ActiveSupport::Concern

  def update_if_changed(attrs)
    changed = attrs.each_with_object({}) do |(key, value), acc|
      cast = self.class.type_for_attribute(key.to_s).cast(value)

class AddOptimisticLockingToDocuments < ActiveRecord::Migration[7.1]
  def change
    add_column :documents, :lock_version, :integer, null: false, default: 0
    add_index :documents, :updated_at
  end
end

class AddUniqueIndexToTags < ActiveRecord::Migration[7.1]
  disable_ddl_transaction!

  def up
    execute <<~SQL
      UPDATE tags SET name = LOWER(TRIM(name)) WHERE name IS NOT NULL;

import SwiftUI

struct PostsView: View {
    @StateObject private var viewModel = PostsViewModel()

    var body: some View {

-- Row-level locks with SELECT FOR UPDATE
BEGIN;

SELECT * FROM inventory
WHERE product_id = 123
FOR UPDATE;  -- Locks selected rows

use parking_lot::Mutex;
use std::sync::Arc;
use std::thread;

fn main() {
    let counter = Arc::new(Mutex::new(0));

module RetryableTransaction
  module_function

  RETRIABLE = [ActiveRecord::Deadlocked, ActiveRecord::LockWaitTimeout].freeze

  def run(max_retries: 3, base_delay: 0.05, &block)

-- Basic transaction
BEGIN;

UPDATE accounts SET balance = balance - 100 WHERE id = 1;
UPDATE accounts SET balance = balance + 100 WHERE id = 2;

class CreateMaintenanceTasks < ActiveRecord::Migration[7.0]
  def change
    create_table :maintenance_tasks do |t|
      t.string :name, null: false
      t.string :state, null: false, default: "pending"
      t.datetime :started_at

class CreateEmailDeliveries < ActiveRecord::Migration[7.1]
  def change
    create_table :email_deliveries do |t|
      t.string :dedupe_key, null: false
      t.string :mailer, null: false
      t.string :action, null: false

class CircuitBreaker
  class CircuitOpenError < StandardError; end

  INCREMENT = <<~LUA.freeze
    local n = redis.call('INCR', KEYS[1])
    if n == 1 then redis.call('EXPIRE', KEYS[1], ARGV[1]) end

package cache

import (
  "context"
  "sync/atomic"
  "time"