Golang, email generation, postgresql and a bit of S3

You will be working on a program that helps in teledentistry. Patients send us images, it goes through our processing pipeline and we make some other images from it. You will be writing the program (/usr/local/bin/composer) that takes the processed images and sends them on to the dentist. There will be a configuration file in the home directory of the user the program runs as which contains the database connectivity details, and an s3 path to put the generated emails. You read this with viper. There will also be AWS credentials in the user's home directory so that you can use the ordinary AWS golang APIs to access files from s3. Developer needs to know something about the SMTP and raw email creation (adding attachments and that sort of thing) programmatically. The program you will be writing can be invoked in one of three ways /usr/local/bin/composer /usr/local/bin/composer -a /usr/local/bin/composer -m {message_id} If it is the first version (with no arguments given), then it runs select message_id from ready_for_reply order by random() and iterates through that list of message_ids, stopping after 59 seconds have elapsed. If it is the second version (with -a) then it starts a transaction and runs the same query but doesn't stop after 59 seconds -- it just keeps going until it is finished. If it is the third version, it just operates on that one message id. For each message, it begins a transaction and runs select from_field, to_field, subject, number_of_images, plaintext_body, replied from email_headers_received for update; It does a sanity check on 'replied' (it should be false) and remembers those other fields for later. It's possible that there are no images, in which case the body of the composed email should say "The following email was received by {to_field} from {from_field} and had no images in it{plaintext_body}" Otherwise, the images can be found by running this query... select image_id, cropped_version_s3_path from images_received where source_email = {message_id}; ... and then fetching the images from s3. The composed email body should then say "The following email was received by {to_field} from {from_field} and had {number_of_images} attached.{plaintext_body}". Then the emails themselves should be attached: they images will be on s3 in the location given by cropped_version_s3_path. The subject line of the email is {subject}. The email is sent to select imagery_destination_email from clinics inner join clinic_emails_received using(clinic_id) where message_id = {message_id} The generated email gets stored in s3://{generated_emails_bucket}/{message_id} -- {generated_emails_bucket} is defined in the config file. There will be a copy of postfix running on the box which can then be used to deliver the mail, so ordinary local mail sending will work and/or using localhost as the SMTP server will work. If all this has worked, then run insert into email_replies (source_message, s3_path) values ({message_id}, s3://{generated_emails_bucket}/{message_id}); update email_headers_received set replied = true where message_id = {message_id} And finally -- if everything has worked, commit the transaction. Otherwise, abort the transaction and exit with an error message and suitable error code. Relevant schema: create table if not exists email_headers_received ( message_id uuid unique, from_field varchar not null, to_field varchar not null, subject varchar not null, plaintext_body varchar, number_of_images int not null replied boolean default false ); create table clinics ( clinic_id uuid primary key, imagery_destination_email varchar not null, ); create table ready_for_reply (message_id uuid); create table email_replies ( source_message uuid references raw_emails_received unique, s3_path varchar not null unique, when_sent timestamp default timenow() ); create table images_received ( image_id uuid primary key, cropped_version_created boolean, cropped_version_s3_path varchar );