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Last updated: 5 Aug 2021

account-api: 001-flash-messages

The flash is a special piece of per-request state in the Rails session:

flash[:notice] = "message"

Makes a :notice flash which is available to the next request:

<h1><%= flash[:notice] %></h1>

And then is cleared.

This is useful for showing success or failure messages, or other confirmation-style notices.

Flash messages and microservices

Since flash messages are in the Rails session, they cannot be used across microservices (without sharing the session encryption key). But that is a feature we would like.

For example, consider the user journey for email notifications:

  1. Click "Sign up for notifications" link on the page
  2. Sign in
  3. [in the background] The subscription is created and the user redirected back to the original page

After step 3, the user doesn't get any immediate feedback that they now have signed up. This is where a flash message would be useful.

Storing flash messages in the GOV.UK account session

The current format of the account session is:


Flash messages could be stored in the session_hash, but then apps would need to call account-api to get or set them, as the encryption key is not shared.

So instead, let's store flash messages as plaintext:

base64("#{salt}$$#{encrypt(session_hash)}") + "$$#{flash_messages.join(',')}"

To avoid decoding issues, the messages must be URL-safe and cannot contain a "$$" or a ",". For security, also don't want to store any sensitive data in plaintext. So we will instead store message identifiers (like email_subscription_created) rather than messages themselves (like Success, you have signed up to Super Sensitive Topic!).

We can enforce this by putting the flash message manipulation logic in govuk_personalisation and mandating that they use a restricted character set (e.g. [a-zA-Z0-9_-\.]+).

The following will be added to the GovukPersonalisation::AccountConcern:

attr_reader :account_flash      # an array of messages

def account_flash_add(value)    # add a value to the flash

def account_flash_keep          # keep the entire flash for the next request

The fetch_account_session_header method will be changed to set @account_flash.

The response headers will be changed to:

response.headers[ACCOUNT_SESSION_HEADER_NAME] = "#{@account_session_header}$$#{@new_account_flash.join(',')}"

Where @new_account_flash is an initially-empty array. This is because flash messages do not persist between requests. If an app needs to persist some flash messages, it can use account_flash_add or account_flash_keep to set the necessary ones.

Will account-api itself use flash messages?

In AccountSession.deserialise we will drop any flash messages.

They do not (yet) affect the functioning of account-api. If, in the future, we want to communicate flash messages from account-api to other GOV.UK microservices, we can revisit this.

Flash messages are entirely a concern for govuk_personalisation and frontend apps.

An example usage

Returning to the example of signing up for email notifications, flash messages would let us do this:

  1. Click "Sign up for notifications" link on the page
  2. Sign in
  3. [in the background] The subscription is created, a success flash message is set, and the user redirected back to the original page
  4. See a "success" banner on the page