Mailer

The mailer consumes requests for emails from RabbitMQ. For each one, it:

1. calls the provider’s API, which sends the email;
2. publishes what became of it, mail.sent or mail.refused, for the services that asked;
3. parks a request that failed for now, which comes back after a delay, and dead-letters one that never will succeed, with why.

No request stops it, and a request delivered twice sends one email, because its MessageId is the provider’s idempotency key.

The contract

A request is published to the mail exchange with the routing key mail.send:

• its MessageId is required, and is the provider’s idempotency key;
• its CorrelationId is optional, and the events carry it back;
• its body is JSON, to, subject and text, up to 256 KiB.

The sender’s address is the mailer’s setting, so no publisher can send as anyone else.

An event is published to the mail.events topic exchange, with the request’s MessageId and CorrelationId:

• mail.sent, with the provider’s ID for the email, as provider_id;
• mail.refused, with the provider’s reason, for a request the provider will never accept.

A service that wants the events binds a queue of its own to mail.events.

The names

The mail package holds the contract and the topology’s names:

const (
	Exchange       = "mail"
	RetryExchange  = "mail.retry"
	DeadExchange   = "mail.dead"
	EventsExchange = "mail.events"
)

const (
	SendQueue  = "mail.send"
	RetryQueue = "mail.send.retry"
	DeadQueue  = "mail.send.dead"
)

const (
	SendKey    = "mail.send"
	SentKey    = "mail.sent"
	RefusedKey = "mail.refused"
)

The files it ends with

Its code is a module of its own, at example/rabbitmq/mailer:

example/rabbitmq/mailer/
├── main.go             humus.Main(app.Init, ...), with a grace period of 25s
├── app/                Init, the settings, BuildTelemetry and Faults
├── mail/               the request and the events, and the topology's names
├── provider/           the provider's client, over httpclient, and its Classify
├── worker/             the Processor: decode, send, decide, publish
├── internal/certfile/  the probes' certificate, read again when it's renewed
├── cmd/mailrequest/    a CLI that publishes a request, to try the mailer by hand
├── cases/              the test cases the container tests and kind share
├── functional/         the cases against kind
├── deploy/             definitions.json, the kustomization, the alerts and
│                       dashboard, and the kind overlay
└── Dockerfile          the mailer's and mailrequest's image

These pages set up the topology, then build provider, mail and worker, then app and main, then test it, deploy it and observe it, and end with the production-ready checklist.