Day 21

Today, I:

• finished reading DDIA chapter 7 on transactions and wrote DDIA Chapter 7: Transactions for the DDIA collection
• watched Transactions, ACID, and Isolation Levels — DDIA Chapter 7 — live walkthrough of ACID, isolation level names vs semantics, Postgres MVCC and vacuum/autovacuum, and InnoDB undo logs for snapshots
• learned how to write a simple generic concurrent test where two workers update different records in parallel and both updates persist without a lost update
• started reading DDIA chapter 8 on the trouble with distributed systems and wrote DDIA Chapter 8: The Trouble with Distributed Systems, will finish it tomorrow
• read AWS Innovation Sandbox on AWS and noted the reference architecture for account access, web ui hosting, event-driven workflows, and sandbox account governance
• read AWS Builder Center’s AI won’t replace developers, but ignoring it will cost you

Generic Ruby/Rails RSpec snippet

# spec/models/job_concurrency_spec.rb
it "keeps both updates without lost update" do
  job = Job.create!(state: { "a" => { "status" => "pending" }, "b" => { "status" => "pending" } })
  ready = Queue.new
  go = Queue.new

  threads = %w[a b].map do |key|
    Thread.new do
      ActiveRecord::Base.connection_pool.with_connection do
        local = Job.find(job.id)
        ready << true
        go.pop
        local.update_node_status!(key, "succeeded")
      end
    end
  end

  2.times { ready.pop }
  2.times { go << true }
  threads.each(&:join) # wait for both threads to finish

  result = Job.find(job.id)
  expect(result.state.dig("a", "status")).to eq("succeeded")
  expect(result.state.dig("b", "status")).to eq("succeeded")
end

# app/models/job.rb
def update_node_status!(key, status)
  with_lock do
    data = state.deep_dup
    data[key]["status"] = status
    update!(state: data)
  end
end

AWS Innovation Sandbox on AWS (quick summary)

• uses IAM Identity Center to let admins/managers/sandbox users access sandbox accounts with centralized authn/authz
• serves the web ui through CloudFront + S3, with API traffic protected by WAF and routed via API Gateway
• executes workflow logic with Lambda, EventBridge, and Step Functions for account lifecycle and automation
• stores state/config in DynamoDB and AppConfig, with a clear separation between control plane and managed sandbox accounts