Preferences

Herald's preference system lets users control which notifications they receive and on which channels. Preferences are checked automatically during the delivery pipeline -- if a user has opted out, the notification is silently skipped.

Preference Entity01

The preference.Preference struct stores a user's notification preferences for an application:

type Preference struct {
    ID        id.PreferenceID              `json:"id"`
    AppID     string                       `json:"app_id"`
    UserID    string                       `json:"user_id"`
    Overrides map[string]ChannelPreference `json:"overrides"`
    CreatedAt time.Time                    `json:"created_at"`
    UpdatedAt time.Time                    `json:"updated_at"`
}

Preference IDs use the hprf prefix.

ChannelPreference02

The ChannelPreference struct holds per-channel opt-in/opt-out flags for a specific notification type:

type ChannelPreference struct {
    Email *bool `json:"email,omitempty"`
    SMS   *bool `json:"sms,omitempty"`
    Push  *bool `json:"push,omitempty"`
    InApp *bool `json:"inapp,omitempty"`
}

Each field is a *bool (pointer to bool) to distinguish three states:

Value	Meaning
`nil`	No preference set -- use default (opted in)
`true`	Explicitly opted in
`false`	Explicitly opted out

Per-Type Per-Channel Control03

The Overrides map is keyed by notification type slug (the template slug). This gives users granular control over each notification type on each channel:

pref := &preference.Preference{
    AppID:  "app_01abc...",
    UserID: "user_01xyz...",
    Overrides: map[string]preference.ChannelPreference{
        "auth.welcome": {
            Email: boolPtr(true),   // receive welcome emails
            InApp: boolPtr(true),   // receive welcome in-app
        },
        "auth.password-changed": {
            Email: boolPtr(true),   // receive password change emails
            SMS:   boolPtr(false),  // do NOT receive password change SMS
        },
        "marketing.weekly-digest": {
            Email: boolPtr(false),  // opt out of marketing emails
            Push:  boolPtr(false),  // opt out of marketing push
        },
    },
}

func boolPtr(b bool) *bool { return &b }

Opt-Out Check04

The IsOptedOut method checks whether a user has explicitly opted out of a specific notification type on a specific channel:

func (p *Preference) IsOptedOut(notifType string, channel string) bool {
    if p == nil || p.Overrides == nil {
        return false
    }

    cp, ok := p.Overrides[notifType]
    if !ok {
        return false
    }

    switch channel {
    case "email":
        return cp.Email != nil && !*cp.Email
    case "sms":
        return cp.SMS != nil && !*cp.SMS
    case "push":
        return cp.Push != nil && !*cp.Push
    case "inapp":
        return cp.InApp != nil && !*cp.InApp
    default:
        return false
    }
}

Key behaviors:

Returns false (not opted out) if no preference exists at all
Returns false if no override exists for the notification type
Returns false if the channel field is nil (no preference set)
Returns true only when the channel field is explicitly set to false

Default Behavior05

When no preference is set, Herald always delivers the notification. The preference system is opt-out only -- users must explicitly disable channels they do not want to receive.

This means:

A brand-new user with no preferences receives all notifications on all channels
Setting Email: boolPtr(true) is functionally equivalent to having no preference for email
The only way to prevent delivery is to set a channel to false

Upsert Semantics06

The store uses upsert semantics for SetPreference. If a preference already exists for the (app_id, user_id) pair, it is replaced:

err := store.SetPreference(ctx, pref)

This simplifies the API -- callers always provide the full preference state, and the store handles insert-or-update.

Store Interface07

type Store interface {
    GetPreference(ctx context.Context, appID string, userID string) (*Preference, error)
    SetPreference(ctx context.Context, p *Preference) error
    DeletePreference(ctx context.Context, appID string, userID string) error
}

Integration with Delivery08

During Send, Herald checks preferences before proceeding with template rendering and provider resolution:

// In Herald.Send()
if req.UserID != "" && req.Template != "" {
    pref, _ := h.store.GetPreference(ctx, req.AppID, req.UserID)
    if pref != nil && pref.IsOptedOut(req.Template, channel) {
        h.logger.Debug("herald: user opted out",
            "user_id", req.UserID,
            "template", req.Template,
            "channel", channel,
        )
        return &SendResult{Status: message.StatusSent, Error: "user opted out"}, nil
    }
}

Note that both UserID and Template must be present for the preference check to run. Direct sends without a template slug bypass preference checking.

API Endpoints09

Preferences are managed through the /preferences route group:

Method	Path	Operation
`GET`	`/preferences`	Get preferences (query: `app_id`, `user_id`)
`PUT`	`/preferences`	Update preferences (upsert)

Update Preferences Request

{
  "app_id": "app_01abc...",
  "user_id": "user_01xyz...",
  "overrides": {
    "auth.welcome": {
      "email": true,
      "inapp": true
    },
    "marketing.weekly-digest": {
      "email": false,
      "push": false
    }
  }
}

Get Preferences Response

{
  "id": "hprf_01j8x7k2...",
  "app_id": "app_01abc...",
  "user_id": "user_01xyz...",
  "overrides": {
    "auth.welcome": {
      "email": true,
      "inapp": true
    },
    "marketing.weekly-digest": {
      "email": false,
      "push": false
    }
  },
  "created_at": "2025-01-15T10:30:00Z",
  "updated_at": "2025-01-20T14:00:00Z"
}

Building a Preferences UI10

A typical preferences UI shows a matrix of notification types and channels. For each cell, display a toggle based on the ChannelPreference value:

Notification Type	Email	SMS	Push	In-App
Welcome	on	--	--	on
Password Changed	on	off	--	on
Weekly Digest	off	--	off	--

Cells with -- represent nil values (no explicit preference, defaults to on). This allows you to only show relevant channels per notification type.

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