I’ve been finding myself frequently enumerating all public and elastic IPs on
various AWS accounts fairly often in order to audit the set of services exposed
to the Internet. This is a tedious process through the AWS console UI, and doing
it through the AWS CLI interface involves an impossible to remember incantation.
Services like SendGrid, Mailgun,
or Mandrill are a great way to handle outbound
email from a web application, and each will certainly allow you to transmit
email directly to their mail servers. However, any kind of outage in between
your app server and your mail transfer service could easily result in mail
getting routed to /dev/null with no easy way to identify and resend the
lost mail. This is particularly problematic for emails sent from a worker
process since they will often keep right on churning away even if other things
are down. For some applications, it may be a perfectly acceptable risk to
drop a few emails, as hopefully such service interuptions are rare, but losing
emails would be a major problem for the project I’m working on and I suspect
To mitigate this, I’m configuring Postfix on each app server to relay out
to Mandrill. You can do the same for SendGrid or Mailgun, and while this post
is going to be mostly Mandrill-specific, since that’s what I’m using, the
equivalent for SendGrid/Mailgun ought to be pretty close as all three services
use SASL authentication for relay configurations.
This is a decent solution because Postfix, being a full mail transfer agent in
its own right, has useful things like automatic retry logic built-in. Rather
than reimplement all that in my app server, no doubt in a much less reliable
way, I’m going to set up Postfix running on each app server’s localhost.
By doing it that way, I can leverage the fact that it’s hard to screw up
communication over loopback and just outsource my email reliability problem
to software that’s been around for just shy of two decades. Postfix uses very
little in the way of resources, so it’s not a problem to run it on each app
The Opscode Postfix cookbook
is the main tool we need. I’m using OpsWorks, but this cookbook isn’t specific
to OpsWorks and should work in any Chef setup.
The custom Chef JSON for a Postfix null client relay looks like this:
The main attributes in the JSON config that we care about are the values for
smtp_tls_security_level and smtp_sasl_auth_enable. The yes value for
smtp_sasl_auth_enable switches on SASL authentication, which is how Mandrill
knows which account is sending mail. Setting the value to encrypt for
smtp_tls_security_level forces TLS for everything. We ought to just care about
encrypting outbound mail since this is a null client, but, especially in the
world in which BCP 188 was
Since we’re using SASL for authentication, postfix::sasl_auth will be the
recipe we want to use. For OpsWorks, this can go into your ‘Setup’ lifecycle
I’m using Ubuntu on EC2. The Postfix cookbook doesn’t do a great job of locating
the CA certificate bundle for that platform, so you may need to specify the
correct location for that or alternatively download a bundle and place it the
fallback location of /etc/postfix/cacert.pem. You can test that it’s working
$ telnet localhost 25
Connected to myhostname.localdomain.
Escape character is '^]'.
220 myapp.com ESMTP Postfix
220 2.0.0 Ready to start TLS
If you send STARTTLS and in response get a message like
454 4.7.0 TLS not available due to local problem, that usually means Postfix
couldn’t find your CA bundle.
Finally, verify that relaying works with sendmail.
Over the years, I’ve frequently found myself implementing systems that
follow Postel’s Law on a fairly regular basis. An extremely common
scenario that comes up when building such liberal systems and parsers
is a short-circuited sequence of searches.
Often, these search sequences will involve long if chains that
repeatedly check for nil. However, this is both inefficient and
inelegant. I’ve just (re-)discovered that Ruby has a much better way of
accomodating this style of branching logic.
Take, for example, a particularly liberal feed parser that needs to
return the title of a feed:
Admittedly, the example above would allow for completely mental mixes
of RSS, Atom, and ancient RDF, and it’s perhaps a bit contrived, but
hopefully it illustrates how much cleaner you can make this kind of
logic with some well-placed control-flow or’s. Because both or and
and have extremely low operator precedence, there’s little risk of
accidentally doing something unintended, and the use of begin; end
allows for longer blocks of short-circuited logic. Additionally,
instead of begin; end, you can also use method calls when chunks of
logic start to get too long or unwieldy.
Of course, neither or nor and are anything new, but I rarely see
them used across multiple lines, and it turns out that they’re really
useful in longer expressions.
When you’re setting up dependencies in your Gemfile, you generally
want to give gem authors the benefit of the doubt. Assume that they
will adhere to semantic versioning unless you
have explicit knowledge that this is not the case for a particular gem.
Many developers make a habit of specifying their dependencies far too
pessimistically, which tends to result in dependency conflicts,
especially in larger projects. Instead of "~> x.y.z", consider
using "~> x.y", ">= x.y.z". This compound requirement should be your
instinctive default way of specifying a version constraint.