AWS Questions: DynamoDB

Here are some questions and answers about DynamoDB, Amazon’s managed NoSQL database offering.

  • What are options for dynamically scaling DynamoDB provisioned throughput?
    • Hard to beat the options outlined in this StackOverflow post.  You can do it via scripting, the DynamicDynamoDB open source library, a lambda function, cloudwatch–lots of different ways.
  • Do DynamoDB streams support multiple readers?
  • How does optimistic concurrency control work?
    • Nicely outlined here but the long and the short of it is you need to make sure you associate a version with your items, read that version when you prepare to update, and then update if and only if the version is the same as the one you read.
  • Do you have any insight into the internals of DynamoDB?
  • How do you connect to DynamoDB?  Is there an IP address?
    • You use the SDK or CLI which connect to an endpoint in a region that you know no further details of.
  • What is the difference between eventual and strong consistency with respect to DynamoDB reads?
  • Does DynamoDB have any automatic encrypt at rest options?


AWS Questions: Elastic Load Balancer

More question answered from an AWS course.

  • Does the AWS ELB have the ability to throttle requests, to stop invalid/illegal traffic – if someone refreshes a page 10 times in 5 seconds and I want to block the unnecessary requests from the refreshes?
  • What is the availability of the ELB component?
    • I couldn’t find firm numbers, but here’s an interesting article about ELB best practices.
  • In a DDOS attack, since there is a lot of traffic to your environment, do you get charged for the additional traffic?
    • Depending on the attack type, not if you are fronted by an ELB or set up your security groups/NACLs to discard the traffic.  From the DDOS whitepaper: “When [an ELB detects certain types of attacks], it will automatically scale to absorb the additional traffic but you will not incur any additional charges.”
  • When an instance is decommissioned from an ASG, does the ELB know not to send new sessions to that ASG because the instance is getting ready to shut down?

Testing Dossier Reports in Rails

One of the things I love about developing with rails is the vast array of open source, free components that you can drop in and extend your application.  Want an invoicing system?  Way to run your javascript testsSimple admin portal?  Just drop in a gem, run bundle install and you are good to go.

One of the gems I’ve used recently was dossier, which lets you write reports in SQL or active record, and then generate them in HTML or CSV (or JSON, but I didn’t use that). One tip–if you want your CSV results to have the same formatting as your HTML results, you’ll want to follow the steps on this issue.

I wrote up a couple of SQL reports, linked them into the appropriate admin pages, and called it good. Then, the app moved on, and at one point, the schema changed. (Some of you are shaking your head, knowing what is going to happen next.) Then, the reports failed.

I had forgotten the cardinal rule–write the tests first. I confess, I wasn’t sure how to, but a bit of research revealed that it wasn’t that hard. Here’s one of my spec files.

require 'spec_helper'

describe MonthlyHoursClientReport do

  # all this does is test that the SQL is valid
  it "sql valid" do
    report =
    sql = report.sql
    sql = replace_placeholders(sql)
    expect{ActiveRecord::Base.connection.execute(sql)}.to_not raise_error

  def replace_placeholders(sql)
    sql = sql.gsub(":kitchen_id",1.to_s)

This just gets the SQL from the dossier report and tries to execute the SQL in the test database. Super simple, but enough to catch the error I encountered. If/when I get more time, I could definitely add some more tests with some data in the test db to make sure the SQL is giving correct results, but I tend to be pretty confident in my SQL queries, especially when they don’t have any group by or having clauses.

Anyway, happy testing.

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