About

This is a really simple system for handling queued emails. It reads emails out of a database table, and tries to send a specified number every second (well, it delays a second between each sending, so it is upper bounded by the specified number per second, but probably will be less, accounting for latency, request time, etc). It records when a message was sent. Right now, only one process can operate (and it creates a lock file, /tmp/amazon_emailer.lock to prevent other processes from starting - see Todo for way to fix this).

If messages fail to send for any reason (this could be connectivity, blacklisted addresses, etc, but NOT bounced messages, as those are sent, but come back later), the message has its failed_count incremented, and when that hits 3, we stop trying to send it. Messages are sent in plain text unless html is true, in which case they are sent as html. Right now there is no way to send both.

Software

This is designed to work with Amazon's Simple Email Service (SES), and PostgreSQL. It wouldn't be too hard to adapt it to other backends, but I wanted to keep it extremely simple, and everything I do is with PostgreSQL.

Setup

To use this, you need to create a table amazon_email_queue with the following colums:

id serial PRIMARY KEY, to_addr text NOT NULL, to_name text, from_addr text NOT NULL, from_name text NOT NULL, subject text NOT NULL, body text NOT NULL, date timestamptz NOT NULL DEFAULT now(), sent_at timestamptz, failed_count integer NOT NULL DEFAULT 0, html boolean NOT NULL DEFAULT false

And you should put an index on sent_at and date (used to filter the messages).

Then you should create a Config.hs file, that looks like:

    {-# LANGUAGE OverloadedStrings #-}

module Config where

import Data.ByteString (ByteString)

configHost :: String
configHost = "host for postgresql"

configUser :: String
configUser = "user for postgresql"

configPass :: String
configPass = "password for postgresql"

configDB :: String
configDB = "database name"

configAccessKey :: ByteString
configAccessKey = "AWS access key"

configSecretKey :: ByteString
configSecretKey = "AWS secret key"

configLimit :: Int
configLimit = 5 -- or higher, if you have a higher rate limit

Finally, in your application, just insert messages into the amazon_email_queue table, and then should get picked up and sent out.

Todo

It would be better to eliminate the file locking, and simply grab/mark grabbed the emails in a single database query, so that many emailers could be working on the database at once without the possibility of duplicate messages. This is an easy change.