init model/mapping alerting sections

Author: forstisabella

src/connections/reverse-etl/manage-retl.md

@@ -53,9 +53,17 @@ To reset a sync:
 You can choose to replay syncs. To replay a specific sync, contact [[email protected]](mailto:[email protected]). Keep in mind that triggering a replay resyncs all records for a given sync.
 
 ## Alerting
-You can opt in to receive email, Slack, and in-app alerts about Reverse ETL sync failures and partial successes. 
+You can opt in to receive email, Slack, and in-app alerts about Reverse ETL sync failures, spikes in data ingested from your model, and fluctuations in the volume of events successfully delivered from your mapping. 
 
-To subscribe to alerts: 
+<!--- TODO: rewrite above + confrm w/ eric that the following is true--->
+
+The notification channels that you select for one alert will apply to all alerts in your workspace. 
+
+> success ""
+> If you opted to receive notifications by email, you can click **View active email addresses** to see the email addresses that are currently signed up to receive notifications. 
+
+### Failed or partially successful syncs
+To subscribe to alerts for a failed or partially successful sync: 
 1. Navigate to **Settings > User Preferences**. 
 2. Select **Reverse ETL** in the **Activity Notifications** section.
 3. Click the Reverse ETL sync status that you'd like to receive notifications for. You can select one or more of the following sync statuses:
@@ -67,5 +75,41 @@ To subscribe to alerts:
     - **Enable in-app notifications**: Select this option to see an in-app notification.
 5. Click **Create alert**.
 
-> success ""
-> If you opted to receive notifications by email, you can click **View active email addresses** to see the email addresses that are currently signed up to receive notifications. 
+### Model-level volume spike alerts
+
+You can create an alert that notifies you when the volume of events received by your source in the last 24 hours changes beyond a set percentage. For example, if you set a change percentage of 4% and your source received 100 events over the first 24 hours, Segment would notify you the following day if your source ingested fewer than 96 or more than 104 events.
+
+To receive a volume spike alert in a Slack channel, you must first create a Slack webhook. For more information about Slack webhooks, see the [Sending messages using incoming webhooks](https://api.slack.com/messaging/webhooks){:target="_blank”} documentation.
+
+1. Navigate to the model you'd like to create an alert for and select the **Alerts** tab. 
+2. Click **Create alert**. 
+3. Set a *change in event volume* percentage, or the percentage of change in event volume from your source that would prompt an alert.  <!--- TODO: add example (in this step or the section intro?)--->
+4. Select one or more of the following notification channels: 
+    - **Email**: Select this channel to receive emailed alerts at the email address that you use to sign in to Segment. 
+    - **Slack notification**: Enter a Webhook URL and a Slack channel name to receive alerts in a Slack channel. 
+    - **In-app notifications**: Select this to receive notifications in the Segment app. To view your notifications, select the bell next to your user icon in the Segment app.
+5. Toggle the **Enable alert** setting on and click **Create**. 
+
+To edit or disable your alert, navigate to your model's Alerts tab and select the Actions menu.  
+
+<!--- TODO: fix the above wording ^ --->
+
+### Mapping-level successful delivery rate fluctuations
+
+You can create an alert that notifies you when the volume of events successfully received by your mapping in the last 24 hours falls below a percentage you set. For example, if you set a percentage of 99%, Segment notifies you if your destination had a successful delivery rate of 98% or below. 
+
+To receive a successful delivery rate fluctuation alert in a Slack channel, you must first create a Slack webhook. For more information about Slack webhooks, see Slack's [Sending messages using incoming webhooks](https://api.slack.com/messaging/webhooks){:target="_blank”} documentation.
+
+To subscribe to alerts for successful delivery fluctuations at the mapping level: 
+1. Navigate to your intended mapping and select the **Alerts** tab. 
+2. Click **Create alert**. 
+3. Set an *alert threshold*, or the percentage of successfully delivered events that would prompt an alert. 
+4. Select one or more of the following notification channels: 
+    - **Email**: Select this channel to receive emailed alerts at the email address that you use to sign in to Segment. 
+    - **Slack notification**: Enter a Webhook URL and a Slack channel name to receive alerts in a Slack channel. 
+    - **In-app notifications**: Select this to receive notifications in the Segment app. To view your notifications, select the bell next to your user icon in the Segment app.
+5. Toggle the **Enable alert** setting on and click **Create**. 
+
+To edit or disable your alert, navigate to your mapping's Alerts tab and select the Actions menu.  
+
+<!--- TODO: fix the above wording ^ --->