Question: Can someone please suggest that in a scaled Sitecore 9 environment for some reason if the Roles other than CM and CD are down, how the Websites will be impacted? 

If the xConnect, Marketing Automation, xDb Reference Data Roles are down, will the CM and CD roles will be down as well or we have any configuration to avoid such dependency?

Answer: 
With xConnect off, the website will keep working BUT if you have any xConnect/Tracker code that is not null checked, it will fail. 

`Sitecore.Tracker?.Current?......`

or 

```
if(Tracker.Current?.IsActive == true)
{
    // do stuff
}

```

All of the Analytics related actions in Sitecore forms will cause the forms to stop working. And action that happens after the failing analytics action will not run.

For Marketing Automation, Processing Engine and xConnect Search Indexer are just services and run out of channel from the website. None of the MA will run and any xConnect searching will be out of date.
**The question: What happens to the experience if xConnect is down?**

Sitecore
========

Content Delivery Role
---------------------

### Session Data

The Content Delivery Role uses the SessionEnd of the HTTP Request lifecycle to push session data to the xConnect Collection Service. When the xConnect Collection Service is down, Sitecore CD will wait until services are restored and then push data from the sessions.

If using InProc session management, and the CD server recycles it's app pool before it's able to push the session to the Collection Service, that session information is lost.

Recommended practice is to use a Session database like SQL or Redis.

### Tracker and Identification

If the xConnect Collection Service is down, Tracker and xConnect Submit batches will fail. This will mean personalization will not fire, but the Site will still work.

Assuming there's a Session Cookie, session data will still collect.

Content Management Role
-----------------------

### Analytics and Marketing Operations

If the xConnect Reference Data service is down:

* Most Analytics reporting will not show.
* Experience Profile will not show users.
* List Segments will not display
* Marketing Automation will not work.

Processing Role
---------------

If xConnect Collection Search Indexer is down, the processing pools will not hydrate and the processing server will not do anything.

If the xConnect Reference Data service is down, Processing will error out with xConnect Issues. The processing pool should remain until the problem is resolved.

Reporting Role
--------------

The Reporting Role of Sitecore does not have a dependence on xConnect. Merely the Reporting Database in SQL.

xConnect
========

Collection Service
------------------

If this service is down, no collection of data from the CD (or CM if running the xDB migration tool) will occur.

Collection Search Index Worker
------------------------------

If this service is down, the **xdb** index will not update.

Reference Data
--------------

If this service is down, Marketing Automation Engine and Sitecore Analytics will not work.

Marketing Automation Engine
---------------------------

If this service is down, Marketing Automation plans will not advance.

Marketing Automation Reporting
------------------------------

If this service is down, reporting in Marketing Automation will not work.

Cortex Processing
-----------------

If this service is down, ML Cortex processing will not work.