mailer

Mailer is a service for rendering and sending email based on Golang Template with the help of MJML.

Getting Started

Golang binary is built with static link. You can download it directly from the GitHub Release page or build it by yourself by cloning this repo and running make.

A Docker image is available for amd64, arm and arm64 platforms on Docker Hub: vibioh/mailer.

You can configure app by passing CLI args or environment variables (cf. Usage section). CLI override environment variables.

You'll find a Kubernetes exemple in the infra/ folder, using my app chart

MJML

In order to use the MJML converter, you need to register to MJML API for having credentials or provided a compliant API like mjml-api.

Features

• Golang templating ease-of-use and performance
• MJML conversion on-the-fly
• Read-only container
• OpenTelemetry
• Configurable logger with JSON support

Templating & fixtures

Templates are read from the -templates option dir. You can have a look at the hello world example provided in this repository. Every files with *.tmpl extension is parsed as a template.

Fixtures for each template are found from the directory where the template is. The default fixture is a file named default.json.

HTTP or AMQP Client

mailer is capable to render and send email in a synchrone manner with the HTTP endpoint. If any action has an error (parsing, rendering, converting, sending), the HTTP response will be in error.

It can also send email in an asynchronous way with AMQP. If an error occurs, the dead-letter queue will be processed every hour and a message will be processed at most 3 times before being dropped.

Sending email

The only provider implemented for sending emails is via the SMTP protocol. This quite-old protocol is the broader compatible: you can connect it to Postfix, to SMTP providers (e.g. MailGun, SendGrid) and is more resilient than an vendor-specific HTTP endpoint.

Client usage

You can reach HTTP or AMQP endpoints directly or use the provided package client to send email easily from you Golang application. You can find a full usage example in cmd/client/client.go

Endpoints

• GET /render/: list available templates, in JSON format

• GET /render/{templateName}?fixture={fixtureName}: render templateName as HTML with given fixtureName (default by default)

• GET /fixtures/{templateName}/: list available fixtures for given templateName, in JSON format

• POST /render/{templateName}?from={senderEmail}&sender={senderName}&subject={emailSubject}&to={recipient}: render {templateName} with data from JSON payload in body and send it with the given parameters. The emailSubject can be a Golang template. The to parameters can be passed multiple times.

• GET /health: healthcheck of server, always respond okStatus (default 204)

• GET /ready: checks external dependencies availability and then respond okStatus (default 204) or 503 during graceDuration when close signal is received

• GET /version: value of VERSION environment variable

Usage

The application can be configured by passing CLI args described below or their equivalent as environment variable. CLI values take precedence over environments variables.

Be careful when using the CLI values, if someone list the processes on the system, they will appear in plain-text. Pass secrets by environment variables: it's less easily visible.

Usage of mailer:
  --address              string    [server] Listen address ${MAILER_ADDRESS}
  --amqpExchange         string    [amqp] Exchange name ${MAILER_AMQP_EXCHANGE} (default "mailer")
  --amqpExclusive                  [amqp] Queue exclusive mode (for fanout exchange) ${MAILER_AMQP_EXCLUSIVE} (default false)
  --amqpInactiveTimeout  duration  [amqp] When inactive during the given timeout, stop listening ${MAILER_AMQP_INACTIVE_TIMEOUT} (default 0s)
  --amqpMaxRetry         uint      [amqp] Max send retries ${MAILER_AMQP_MAX_RETRY} (default 3)
  --amqpPrefetch         int       [amqp] Prefetch count for QoS ${MAILER_AMQP_PREFETCH} (default 1)
  --amqpQueue            string    [amqp] Queue name ${MAILER_AMQP_QUEUE} (default "mailer")
  --amqpRetryInterval    duration  [amqp] Interval duration when send fails ${MAILER_AMQP_RETRY_INTERVAL} (default 1h0m0s)
  --amqpRoutingKey       string    [amqp] RoutingKey name ${MAILER_AMQP_ROUTING_KEY}
  --amqpURI              string    [amqp] Address in the form amqps?://<user>:<password>@<address>:<port>/<vhost> ${MAILER_AMQP_URI}
  --cert                 string    [server] Certificate file ${MAILER_CERT}
  --corsCredentials                [cors] Access-Control-Allow-Credentials ${MAILER_CORS_CREDENTIALS} (default false)
  --corsExpose           string    [cors] Access-Control-Expose-Headers ${MAILER_CORS_EXPOSE}
  --corsHeaders          string    [cors] Access-Control-Allow-Headers ${MAILER_CORS_HEADERS} (default "Content-Type")
  --corsMethods          string    [cors] Access-Control-Allow-Methods ${MAILER_CORS_METHODS} (default "GET")
  --corsOrigin           string    [cors] Access-Control-Allow-Origin ${MAILER_CORS_ORIGIN} (default "*")
  --csp                  string    [owasp] Content-Security-Policy ${MAILER_CSP} (default "default-src 'self'; base-uri 'self'; style-src 'self' 'unsafe-inline' fonts.googleapis.com; font-src fonts.gstatic.com; img-src 'self' data: http://i.imgur.com grafana.com https://ketchup.vibioh.fr/images/")
  --frameOptions         string    [owasp] X-Frame-Options ${MAILER_FRAME_OPTIONS} (default "deny")
  --graceDuration        duration  [http] Grace duration when signal received ${MAILER_GRACE_DURATION} (default 30s)
  --hsts                           [owasp] Indicate Strict Transport Security ${MAILER_HSTS} (default true)
  --idleTimeout          duration  [server] Idle Timeout ${MAILER_IDLE_TIMEOUT} (default 2m0s)
  --key                  string    [server] Key file ${MAILER_KEY}
  --loggerJson                     [logger] Log format as JSON ${MAILER_LOGGER_JSON} (default false)
  --loggerLevel          string    [logger] Logger level ${MAILER_LOGGER_LEVEL} (default "INFO")
  --loggerLevelKey       string    [logger] Key for level in JSON ${MAILER_LOGGER_LEVEL_KEY} (default "level")
  --loggerMessageKey     string    [logger] Key for message in JSON ${MAILER_LOGGER_MESSAGE_KEY} (default "msg")
  --loggerTimeKey        string    [logger] Key for timestamp in JSON ${MAILER_LOGGER_TIME_KEY} (default "time")
  --mjmlPassword         string    [mjml] Secret Key or Basic Auth password ${MAILER_MJML_PASSWORD}
  --mjmlURL              string    [mjml] MJML API Converter URL ${MAILER_MJML_URL} (default "https://api.mjml.io/v1/render")
  --mjmlUsername         string    [mjml] Application ID or Basic Auth username ${MAILER_MJML_USERNAME}
  --name                 string    [server] Name ${MAILER_NAME} (default "http")
  --okStatus             int       [http] Healthy HTTP Status code ${MAILER_OK_STATUS} (default 204)
  --port                 uint      [server] Listen port (0 to disable) ${MAILER_PORT} (default 1080)
  --pprofAgent           string    [pprof] URL of the Datadog Trace Agent (e.g. http://datadog.observability:8126) ${MAILER_PPROF_AGENT}
  --pprofPort            int       [pprof] Port of the HTTP server (0 to disable) ${MAILER_PPROF_PORT} (default 0)
  --readTimeout          duration  [server] Read Timeout ${MAILER_READ_TIMEOUT} (default 5s)
  --shutdownTimeout      duration  [server] Shutdown Timeout ${MAILER_SHUTDOWN_TIMEOUT} (default 10s)
  --smtpAddress          string    [smtp] Address ${MAILER_SMTP_ADDRESS} (default "localhost:25")
  --smtpHost             string    [smtp] Plain Auth host ${MAILER_SMTP_HOST} (default "localhost")
  --smtpPassword         string    [smtp] Plain Auth Password ${MAILER_SMTP_PASSWORD}
  --smtpUsername         string    [smtp] Plain Auth Username ${MAILER_SMTP_USERNAME}
  --telemetryRate        string    [telemetry] OpenTelemetry sample rate, 'always', 'never' or a float value ${MAILER_TELEMETRY_RATE} (default "always")
  --telemetryURL         string    [telemetry] OpenTelemetry gRPC endpoint (e.g. otel-exporter:4317) ${MAILER_TELEMETRY_URL}
  --telemetryUint64                [telemetry] Change OpenTelemetry Trace ID format to an unsigned int 64 ${MAILER_TELEMETRY_UINT64} (default true)
  --templates            string    [mailer] Templates directory ${MAILER_TEMPLATES} (default "./templates/")
  --url                  string    [alcotest] URL to check ${MAILER_URL}
  --userAgent            string    [alcotest] User-Agent for check ${MAILER_USER_AGENT} (default "Alcotest")
  --writeTimeout         duration  [server] Write Timeout ${MAILER_WRITE_TIMEOUT} (default 10s)