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Last updated: 7 Oct 2020

email-alert-api: Decision Record: MVP implementation of Digests


In January 2018 we started one of the latest major pieces of functionality that would be required to meet our MVP (minimal viable product) and allow us to launch a Notify powered iteration of Email Alert API.

This document serves to outline the architecture we have planned for this and explain the purposes of the individual components.


Product decisions

The need for a change in architecture was determined by product decisions which differed from the previous understanding of digest function explored in adr-001.

The biggest decision that influenced the change was an intention to send each user a single digest email that contained all of their topics as is consistent with Govdelivery. We had previously intended to launch with the change that each subscription is a separate email.

A change from Govdelivery is that we will store the delivery preference (immediate, daily, weekly) at a subscription level rather than a user level. This will mean that a user can be signed up to receive emails to some lists immediately and daily/weekly digests for others.


Sequence Diagram



There are two classes to represent the respective digest periods: DailyDigestInitiatorService and WeeklyDigestInitiatorService.

Their responsibility is to start the process to create digests and to ensure that multiple digests are not started concurrently.

Initially this class was named DigestSchedulerService however we felt that this was ill fitting with the classes actual responsibilities.

  • Be interfaced by a scheduling service to run at a set time appropriate for their period
  • Create a DigestRun model
  • Ensure that multiple digests for the same period are not run concurrently
  • Interface with DigestRunSubscriberQuery to determine which subscribers are due to receive emails for this digest
  • Be configurable as to what times a digest runs from and until


A model which represents a distinct run of a period of digest (daily or weekly).

Has a responsibility to persist data that is related to a full digest run for all subscribers.

We initially named this Digest however it transpired that there was already a module in Ruby standard library named Digest.

  • Store information such as:
    • Date digest is run on
    • The period the digest is for
    • The start time for the digest period
    • The end time for the digest period
  • Store a timestamp for when all the emails were created for the digest, to act as indicator that it is complete.
  • Have a unique index for digest date and period, however this would limit ability to re-run a digest in the case of a problem.


Responsible for taking a DigestRun instance and using that to determine which subscribers should receive an email for the digest period.

  • Use a DigestRun to determine which subscribers should receive an email for a digest period
  • Determine only the subscribers who will be due to be notified of at least one content change
  • Return a list of ids referencing Subscribers
  • Return an ActiveRecord scope to allow the caller to deal with this returning an unlimited output


A model which is used to associate a DigestRun with a Subscriber instance. Will mostly be used as a persisted piece of data that logs which subscribers receive an email due to a digest. It is unlikely to be needed long term.

  • Store
    • An association to DigestRun
    • An association to Subscriber
  • Store a timestamp for when it has been completed, can be used therefore to work out if a DigestRun is complete or not.


A worker entity that accepts the argument of a DigestRunSubscriber and has the responsibility to create an Email entity for the associated Subscriber.

  • Pass the DigestRunSubscriber to SubscriptionContentChangeQuery to retrieve a collection of SubscriptionContentChangeQuery::Result instances. Each of these represents a Subscription and ContentChanges associated with that subscription.
  • Interface with DigestEmailBuilder to build an Email entity
  • Create SubscriptionContent instances for each ContentChange per Subscripption
  • Mark a DigestRunSubscriber as completed
  • Mark DigestRun as completed if all DigestRunSubscriber are complete for the DigestRun


For a given DigestRunSubscriber this will determine all the ContentChanges associated with each Subscription for the digest period.

This class began as an analogue of SubscriptionMatcher - which finds Subscriptions associated with a ContentChange - we decided however that this would require an additional class to interface with DigestRunSubscriber and so decided to have this no longer be an inverse. We also decided that the name should be suffixed with Query rather than Matcher and that we should rename SubscriptionMatcher to have this suffix too.

  • Return an array containing SubscriptionContentChangeQuery::Result instances
  • The SubscriptionContentChangeQuery::Result instances will contain details of the Subscription and the ContentChange instances for that Subscription. This is to allow rendering the Email with ContentChanges in context with their Subscription.
  • Only return necessary fields to limit memory usage/execution time


A DigestEmailBuilder will take arguments of DigestRunSubscriber and a collection of SubscriptionContentChangeQuery::Result objects. It will take this data and use this to create an Email instance.

Formally this type of class was suffixed with Renderer eg EmailRenderer however we felt that the class responsibilities should include persisting an Email instance and therefore was a creation pattern.

  • Create an Email instance
  • Create an unsubscribe link
  • Be distinct from the email builder used to create immediate emails
  • Be responsible for deciding what to do with duplicate ContentChange entries

Supplementary changes

In order to implement these changes a number of existing areas were identified:

Email and EmailRenderer should be changed

We intend to rename the EmailRenderer class to reflect that it is for immediate ContentChanges. This will be renamed ImmediateEmailBuilder.

We also intend to invert the responsibility of Email calling an instance of EmailRenderer and instead of the instance of ImmediateEmailBuilder create an instance of Email

Iterating SubscriptionContent

SubscripionContent is a model that was intended to associate ContentChange entities with Subscription and Email. An absence of an email model was an indication this required processing for an immediate email.

This however becomes problematic when we introduce SubscriptionContent instances which are for digests and should not be processed as an immediate email.

We decided the way to resolve this was to introduce an additional field to SubscriptionContent which is a foreign key to DigestRunSubscriber. A null entry for this column would indicate an email is intended for immediate processing.