Mailer

Mailer is a Micro Service responsible for sending asynchronous emails with synchronous acknowledgment.

Description

This code base provides a micro service capable of sending mails asynchronously. It has been developed using spring boot framework and uses Apache Kafka for messaging.

To make sure things are not over engineered, for starters it uses single consumer and single producer to take care of mail requests.

Requirement

REST API with synchronous acknowledgment with only one method for sending new Mail
Mail with attachment should be possible. (Attachment Content will be provided in the request by a URI pointing to the actual document binaries)
Queuing until successful response from SMTP Server. Max Retry configurable.
No Authentication required.

Tools used

Spring boot framework to write microservice
Kafka for stream processing
Spring Retry to automate retry behavior for failed operation
Embedded kafka for kafka integration testing
Junit 4 for unit tests
Swagger for API documentation
Maven for build
Google guava to use commons library
Spring mail to send mail
Mockito for unit tests mocks
HttpURLConnection used to download attachment binaries
HMailServer used as a local web mail server.

Architecture

Scalability

Scalability aspect has been covered since this microservice can scale vertically or horizontally by either spin off more instances with load balancer or introduce more consumers which deal with mail sending asynchronously.

Future Improvements

Number of consumers can be increased since email sending can be run in parallel without any fuss
Better failure notification mechanism to make sure sender aware of all failures
New API can be introduced to give statistics of sent/failed mails
New Health API can be introduced to monitor health of Kafka producers, Consumers, Mail service etc.
Caching can be introduced to avoid multiple downloads of same attachment.
Code can be modified such that consumers can be increased based on number of requests.
Freemaker template can be used when sending html content
Mail controller and producer can be combined into a micro service and group of consumers can be put into another microservice and multiple instances can be deployed with load balancers

Mail Server

HmailServer used for development in local machine with a configurable domain and user accounts
Webmail used as web mail client to check mails and attachments

Swagger

Use http://localhost:9090/swagger-ui.html to access swagger once application deployed

Kafka and Zookeeper

Follow Kafka Setup in Windows to set up kafka in local
Follow Zookeeper Setup in Windows to set up zookeeper

Integration Tests

Integration tests require running kafka and zookeeper in place.
With default configuration, Mailer expects Kafka in port 9092 and Zookeeper in port 2181
Ignore annotation in integration tests needs to be removed before running them in local.

Assumptions

If a mail failed after n(configurable in application.properties) retries, its considered a failure.
Caching is not implemented inside application
Attachment can either be an direct url(https://google.com/robots.txt) or a generic one(https://drive.google.com/uc?id=1OjVXW2xK9eHcBAjJJygtcyYtiQBwX-tt&export=download)
If attachment URI is a direct url, file name will be received from url itself
If attachment URI is a generic one, file name will be received from content-disposition header.
If attachment fetching failed without any exception, it will throw exception and mail will not be sent
Failed records/attempts are need not be persisted in any form inside the application since kafka will store them in memory and recovery even after a complete crash is possible
Simplistic approach followed during development towards delivery
Retry to download attachment is not implemented
Its assumed that request contains exact html/text content to be delivered and it doesn't require any processing.
As requirement suggests Mailer is asynchronous, thus controller send data to producer and send acceptance acknowledgment.
As requirement suggests mail controller is synchronous API.

Others

Number of retries to send mail can be configured via mailer.retry.maxAttempts in application.properties
Time gap between retries can be configured via mailer.retry.backOffDelay in application.properties

vin0010 / Mailer