Monorail email design


Monorail needs a strong outbound and inbound email strategy that keeps our users informed when issues require their attention. We generate a variety of outbound messages. We accept inbound email replies from human users who find it easier to post comments via email. And, we integrate with alerts monitoring via inbound email.

Our email features must scale up in several dimensions, including:

  • The number of messages generated, which is driven by the issue activity

  • The number of distinct types of notifications that the tool can generate

  • The amount of inbound replies, alerts, as well as spam and bounce notifications

  • The variety of access controls needed for sensitive issues and different types of users


Monorail is a different-time, different-place CSCW application in which users may need to work with each other on multiple occasions to resolve an issue. Furthermore, the exact set of collaborators needed to resolve a given issue is discovered as part of the work for that issue rather than being known from the start. And, each issue participant is likely to be highly multitasking across several issues and other development tasks. As is normal for issue tracking tools, we send email notifications to issue participants for each issue change. However, because participants can get a fair number of emails from us, they want to be able to filter those emails based on the reason why the email was sent to them.

Email is not an inherently secure or private technology. We trust that email is delivered to the recipient without being read by any servers along the way. However, some email addresses may be individual users and others might be mailing lists, so we “nerf” messages in cases where Monorail has no indication that the recipient is an individual. Also, it is possible to forge an email reply, so we rely on shared secrets to authenticate that an inbound message came from a given user. Because the email sender line is so vulnerable to abuse, GAE does not allow GAE apps to set that header arbitrarily. Instead, we rely on a combination of supported email senders, DNS SPF entries, and friendly From: lines.

Users sometimes make mistakes when entering email addresses, and email accounts can be shut down over time, both of these situations generate bounce emails. Continued generation of outbound emails that bounce is a waste of resources and quota.


To keep issue participants engaged, whenever an issue is created or has a new comment, we send email notifications to the issue owner, CC'd users, users who have starred the issue, and users who have subscriptions that match the new state of the issue. Monorail generates notifications for individual issue changes, as well as bulk edits, blocking notifications, and approval changes. Monorail has a special rule for “noisy” issues, which is to only generate emails when project members post comments. Also, when a date-type field has a date value for a date that has arrived, we send a “follow-up” email to remind the user to update the issue.

Monorail sends individual emails to each recipient rather than adding them all to one long Cc: line. The reason for this is so that we can personalize the content of the message to each user based on their preferences, the reason why the message was sent to them, and our level of trust of that address. Also, using individual messages allows us to authenticate replies with a secret that is shared individually with each user. And, individual emails ensure that email replies come directly back to Monorail rather than going to other issue participants. This reduces cases of duplicate emails and allows for enforcement of permissions that might have changed after an earlier notification was sent.

To keep outbound emails organized as threads in recipients' inboxes, we set a References: header message ID that is used for all messages that belong in that thread. However, as a GAE app, Monorail has no way to access the message ID of any actual outbound email. Also, any given thread participant might join the conversation late, after missing the first email message that would have anchored the thread. So, instead of using actual message IDs, we generate a synthetic message ID that represents the thread as a whole and then reference that same ID from each email.

When we send outbound emails, we include a shared secret in the References: header that an email client will echo back in the reply, much like a cookie. When we receive an inbound email, we verify that the References: header includes a value that is appropriate to the specified user and issue. One exception to this rule is that we allow inbound emails from the alert system (Monarch).

Bounces are handled by flagging the user account with a bouncing timestamp. Monorail will not attempt to send any more emails to a bouncing user account. A user can clear the bouncing timestamp by clicking a button on their user profile page.

An inbound email is first verified to check the identity of the sender and the target issue. After permissions are checked, the message body is parsed. The first few lines of the body can contain assignments to the issue summary, status, labels, and owner. The rest of the body is posted to the issue as a comment. Common email message footers and .sig elements are detected and stripped from the comment. The original email message is also stored in the DB and can be viewed by project owners.

Detailed design: Architecture

Monorail is a GAE application with several services. The default service responds to user requests, while the latency-insensitive service executes a variety of cron jobs and follow-up tasks.

Outbound email is generated in tasks that run in the latency-insensitive service so as to remove that work from the time needed to finish the user's request, and to avoid spikes in default instance allocations when many outbound emails are generated at one time. We use automatic scaling, but turnover in default instances would lower the RAM cache hit ratio.

Inbound email is currently handled in the default service. However, those requests could be routed to the latency-insensitive service in the future.

Detailed design: Domain configuration

Monorail serves HTTP requests sent to the domain as well as and other “branded” domains configured in and the GAE console. These custom domains are also used in the email address part of the From: line of outbound emails. They must be listed as monorail@DOMAIN in the email senders tab of the settings page for the GAE app.

The Reply-To: line is always set to and is not branded.

The DNS records for each custom domain must include a TXT record like v=spf1 ?all to tell other servers to trust that the email sent from a certain list of SMTP servers is legitimate for our app.

Detailed design: Key algorithms and data structures

User protocol buffers include a few booleans for the user's preferences for email notifications.

When generating a list of notification recipients, Monorail builds a data structure called a group_reason_list which pairs addr_perm_lists with reasons why those addresses are being notified. Each addr_perm_list is a list of named tuples that have fields for a project membership boolean, an email address, an optional User protocol buffer, a constant indicating whether that user has permission to reply, and a UserPrefs protocol buffer.

The group_reason_list is built up by code that is specific to each reason. A given email address might be notified for more than one reason. Then, that list is inverted to make a dictionary keyed by recipient email address that lists the reasons why that address was notified. Entries for linked accounts are then merged. And, the list of reasons is used to add a footer to the email body that lists the specific reasons for that user.

When generating an outbound email, we add a References: header to make the messages for the same issue thread together and to authenticate any reply. That header value is computed using a hash of the user's email address and the subject line of the emails (which includes the project name and issue ID number). Each part is combined with a secret value stored in Cloud Datastore and accessed via

Most outbound emails include the summary line of the issue, the details of any updates, and the text of the comment. However, there are some cases where Monorail sends a “nerfed” safe subset of information that consists only of a link to the issue and a generic message saying that the issue has been created or updated. We send a link-only notification when the issue is restricted and the recipient may be a mailing list. Monorail cannot know if an email address represents a mailing list or an individual user, so it assumes that any address corresponding to a User record which has never visited the site is a mailing list.

Monorail considers an issue to be “noisy” if the issue already has more than 100 comments and more than 100 starrers. Such issues can generate a huge amount of email that would consume our quota and have low value for most recipients. A large amount of chatter by non-members can make it harder for project members to notice the comments that are important to resolving the issue. Monorail only sends notifications for noisy issues if the comment author is a project member.

Detailed design: Code walk-throughs

Individual issue change

  1. User A posts a comment on an existing issue 123.

  2. The user's request is handled by the default GAE service. work_env coordinates the steps to update issue 123 in the database and invalidate it in caches.

  3. work_env calls PrepareAndSendIssueChangeNotification() to create a task entry that includes a reference to the new comment and a field to omit user A being notified.

  4. That task is processed by the latency-insensitive GAE service by the NotifyIssueChangeTask handler. It determines if the issue is noisy, gathers potential recipents by reasons, omits the comment author, and checks whether each recipient would be allowed to view the issue. Three different email bodies are prepared: one for non-members that has other users' email addresses obscured, one for members that reveals email addresses, and one for link-only notifications.

  5. The group reason list is then passed to MakeBulletedEmailWorkItems() which inverts the list, adds personalized footers, and converts the plain text email body into a simple HTML formatted email body. It then returns a list with a new task record for each email message to be sent.

  6. If the generation task fails for any reason, it will be retried, but no email messages are actually sent on the failed run. If the entire process up to this point succeeds, then AddAllEmailTasks() is called to enqueue each of the single message tasks.

  7. Those tasks are handled by the OutboundEmailTask handler in the latency-insensitve service. Individual tasks are used for each outbound email because sending emails can sometimes fail. Each task is automatically retried without rerunning other tasks.

Bulk issue change

The process is similar to the individual issue change process, except that a list of allowed recipients is made for each issue. Then, that list is inverted to make a list of issues that a given recipient should be notified about. This is done in the NotifyBulkChangeTask handler.

When a given recipient is to be notified of multiple issues that were changed, the email message body lists the updates made and then lists each of the affected issues. In contrast, when a given recipient is to be notified of exactly one issue, the body is formatted to look like the individual issue change notification.

Blocking change

When issue 123 is edited to make it blocked on issue 456, participants in the upstream issue (456) are notified. Likewise, when a blocking relationship is removed, another notification is sent. This is done by the NotifyBlockingChangeTask handler.

Approval issue change

This is handled by NotifyApprovalChangeTask.

TODO: needs more details.

Date-action notifications

Some date-type custom fields can be configured to send follow-up reminders when the specified date arrives.

  1. The date-action cron task runs once each day as configured in cron.yaml.

  2. The DateActionCron handler is run in the latency-insensitive GAE service. It does an SQL query to find issues that have a date-type field set to today's date and that is configured to send a notification. For each such issue, it enqueues a new task to handle that date action.

  3. Each of those tasks is handled by IssueDateActionTask which works like an individual email notification. The main difference is that issue subscribers are not notified and issue starrers are only notified if they have opted into that type of notification. The handler posts a comment to the issue, calls ComputeGroupReasonList() to compute a group reason list, calls MakeBulletedEmailWorkItems() to make individual message tasks, and calls AddAllEmailTasks() to enqueue those email tasks.

Inbound email processing

TODO: needs more details


TODO: needs more details

Detailed design: Source code locations

  • Configuration of branded domains. Also, email From-line string templates that are used to re-route email generated on the dev or staging servers.

  • businesslogic/ Internal handlers for many changes such as updating issues. It checks permissions, coordinates updates to various backends, and enqueues tasks for follow-up work such as sending notifications.

  • features/ This file has most email notification task handlers.

  • features/ Functions to compute AddrPermLists from a list of potential recipients by checking permissions and looking up user preferences. It combines these lists into an overall group reason list. Also, computes list of issue subscribers by evaluating saved queries.

  • features/ Functions to generate personalized email bodies and enqueue lists of individual email tasks based on a group reason list.

  • features/ Cron task for the date-action feature and task handler to generate the follow-up comments and email notifications.

  • features/ Handlers for inbound email messages and bounce messages.

  • features/ and features/ Parsing and processing of issue field assignment lines that can be at the top of the body of an inbound email message.

  • features/ Functions to parse emails from our alert monitoring service and then create or update issues.

  • framework/ Utility functions for parsing and generating email headers.