Updated LIST optimization FAQ following review

Clarified section on ResourceVersion and added sections on
avoiding continuously-repeated queries and on the
guidelines still applying when fieldSelectors are used

Author: JohnRusk

sig-scalability/configs-and-limits/faq.md

@@ -65,22 +65,21 @@ summarized as:
 ### How should we code client applications to improve scalability?
 
 As noted above, LIST requests can be particularly expensive. So when working with lists
-that may have more than a few thousand elements, consider these guidelines:
+that may have more than a few thousand elements, consider the following guidelines. 
 
 1. When defining a new resource type (new CRD) consider expected numbers 
 of objects that will exist (numbers of CRs).  See guidelines 
 [here](https://github.com/kubernetes/enhancements/tree/master/keps/sig-api-machinery/95-custom-resource-definitions#scale-targets-for-ga).
+1. When LIST-ing, the load on etcd and API Server depends primarily on the number of objects that _exist_, not the number that are _returned_. So even if you are using a field selector to filter the list and retrieve only a small number of results, these guidelines still apply.  (The only exception is retrieving a single object by `metadata.name`, which is fast.)
 1. If your code needs to hold an up-to-date list of objects in memory,
 avoid repeated LIST calls if possible.  Instead consider using the
 `Informer` classes that are provided in most Kubernetes client 
 libraries.   Informers automatically combine LIST and WATCH functionality
 to efficiently maintain an in-memory collection.
-1. If `Informer`s don't suit your needs, try to use the API Server cache 
-when LISTing.  To use the cache you must supply a `ResourceVersion`. 
-Read the [documentation about ResourceVersions](https://kubernetes.io/docs/reference/using-api/api-concepts/#resource-versions) carefully to understand how it will affect the 
-freshness of the data you receive.
+1. If `Informer`s don't suit your needs, consider whether you really need strong consistency. Do you really need to see the most recent data, up to the _exact moment in time_ when you issued the query?  If you don't need that, set `ResourceVersion=0`. This will cause your request to be served from the API Server's cache instead of from etcd. Read the [documentation about ResourceVersions](https://kubernetes.io/docs/reference/using-api/api-concepts/#resource-versions) carefully to understand how it will affect the freshness of the data you receive. 
 1. If you can't use `Informer`s AND you can't use the API Server cache,
  then be sure to [read large lists in chunks](https://kubernetes.io/docs/reference/using-api/api-concepts/#retrieving-large-results-sets-in-chunks).
+1. Additionaly, if you cannot use `Informer`s, you should also consider how _often_ your application LISTs the resources. In particular, after you read the last object in a large list, do not _immediately_ re-query the same list. Wait a while instead. Don't list more often than you need to.
 1. Consider the number of instances of your client application which will be running. For instance,
 there is a big difference between having 
 just one controller listing objects, versus having demonsets on every node