Improve ConfigMap concept

This PR adds some missing information to the ConfigMap concept page.

Author: tengqm

content/en/docs/concepts/configuration/configmap.md

@@ -16,33 +16,45 @@ or use additional (third party) tools to keep your data private.
 {{< /caution >}}
 
 
-
 <!-- body -->
 ## Motivation
 
 Use a ConfigMap for setting configuration data separately from application code.
 
 For example, imagine that you are developing an application that you can run on your
 own computer (for development) and in the cloud (to handle real traffic).
-You write the code to
-look in an environment variable named `DATABASE_HOST`. Locally, you set that variable
-to `localhost`. In the cloud, you set it to refer to a Kubernetes
-{{< glossary_tooltip text="Service" term_id="service" >}} that exposes the database
-component to your cluster.
-
+You write the code to look in an environment variable named `DATABASE_HOST`.
+Locally, you set that variable to `localhost`. In the cloud, you set it to
+refer to a Kubernetes {{< glossary_tooltip text="Service" term_id="service" >}}
+that exposes the database component to your cluster.
 This lets you fetch a container image running in the cloud and
 debug the exact same code locally if needed.
 
+A ConfigMap is not designed to hold large chunks of data. The data stored in a
+ConfigMap cannot exeed 1 MiB. If you need to store settings that are
+larger than this limit, you may want to consider mounting a volume or use a
+separate database or file service.
+
 ## ConfigMap object
 
 A ConfigMap is an API [object](/docs/concepts/overview/working-with-objects/kubernetes-objects/)
 that lets you store configuration for other objects to use. Unlike most
-Kubernetes objects that have a `spec`, a ConfigMap has a `data` section to
-store items (keys) and their values.
+Kubernetes objects that have a `spec`, a ConfigMap has `data` and `binaryData`
+fields. These fields accepts key-value pairs as their values.  Both the `data`
+field and the `binaryData` are optional. The `data` field is designed to
+contain UTF-8 byte sequences while the `binaryData` field is designed to
+contain binary data.
 
 The name of a ConfigMap must be a valid
 [DNS subdomain name](/docs/concepts/overview/working-with-objects/names#dns-subdomain-names).
 
+Each key under the `data` or the `binaryData` field must consist of
+alphanumeric characters, `-`, `_` or `.`. The keys stored in `data` must not
+overlap with the keys in the `binaryData` field.
+
+Starting from v1.19, you can add an `immutable` field to a ConfigMap
+definition to create an [immutable ConfigMap](#configmap-immutable).
+
 ## ConfigMaps and Pods
 
 You can write a Pod `spec` that refers to a ConfigMap and configures the container(s)
@@ -62,7 +74,7 @@ data:
   # property-like keys; each key maps to a simple value
   player_initial_lives: "3"
   ui_properties_file_name: "user-interface.properties"
-  #
+
   # file-like keys
   game.properties: |
     enemy.types=aliens,monsters
@@ -94,6 +106,7 @@ when that happens. By accessing the Kubernetes API directly, this
 technique also lets you access a ConfigMap in a different namespace.
 
 Here's an example Pod that uses values from `game-demo` to configure a Pod:
+
 ```yaml
 apiVersion: v1
 kind: Pod
@@ -134,7 +147,6 @@ spec:
           path: "user-interface.properties"
 ```
 
-
 A ConfigMap doesn't differentiate between single line property values and
 multi-line file-like values.
 What matters is how Pods and other objects consume those values.
@@ -153,7 +165,6 @@ ConfigMaps can be mounted as data volumes. ConfigMaps can also be used by other
 parts of the system, without being directly exposed to the Pod. For example,
 ConfigMaps can hold data that other parts of the system should use for configuration.
 
-{{< note >}}
 The most common way to use ConfigMaps is to configure settings for
 containers running in a Pod in the same namespace. You can also use a
 ConfigMap separately.
@@ -162,16 +173,23 @@ For example, you
 might encounter {{< glossary_tooltip text="addons" term_id="addons" >}}
 or {{< glossary_tooltip text="operators" term_id="operator-pattern" >}} that
 adjust their behavior based on a ConfigMap.
-{{< /note >}}
 
 ### Using ConfigMaps as files from a Pod
 
 To consume a ConfigMap in a volume in a Pod:
 
-1. Create a config map or use an existing one. Multiple Pods can reference the same config map.
-1. Modify your Pod definition to add a volume under `.spec.volumes[]`. Name the volume anything, and have a `.spec.volumes[].configMap.name` field set to reference your ConfigMap object.
-1. Add a `.spec.containers[].volumeMounts[]` to each container that needs the config map. Specify `.spec.containers[].volumeMounts[].readOnly = true` and `.spec.containers[].volumeMounts[].mountPath` to an unused directory name where you would like the config map to appear.
-1. Modify your image or command line so that the program looks for files in that directory. Each key in the config map `data` map becomes the filename under `mountPath`.
+1. Create a ConfigMap or use an existing one. Multiple Pods can reference the
+   same ConfigMap.
+1. Modify your Pod definition to add a volume under `.spec.volumes[]`. Name
+   the volume anything, and have a `.spec.volumes[].configMap.name` field set
+   to reference your ConfigMap object.
+1. Add a `.spec.containers[].volumeMounts[]` to each container that needs the
+   ConfigMap. Specify `.spec.containers[].volumeMounts[].readOnly = true` and
+   `.spec.containers[].volumeMounts[].mountPath` to an unused directory name
+   where you would like the ConfigMap to appear.
+1. Modify your image or command line so that the program looks for files in
+   that directory. Each key in the ConfigMap `data` map becomes the filename
+   under `mountPath`.
 
 This is an example of a Pod that mounts a ConfigMap in a volume:
 
@@ -201,8 +219,8 @@ own `volumeMounts` block, but only one `.spec.volumes` is needed per ConfigMap.
 
 #### Mounted ConfigMaps are updated automatically
 
-When a config map currently consumed in a volume is updated, projected keys are eventually updated as well.
-The kubelet checks whether the mounted config map is fresh on every periodic sync.
+When a ConfigMap currently consumed in a volume is updated, projected keys are eventually updated as well.
+The kubelet checks whether the mounted ConfigMap is fresh on every periodic sync.
 However, the kubelet uses its local cache for getting the current value of the ConfigMap.
 The type of the cache is configurable using the `ConfigMapAndSecretChangeDetectionStrategy` field in
 the [KubeletConfiguration struct](https://github.com/kubernetes/kubernetes/blob/{{< param "docsbranch" >}}/staging/src/k8s.io/kubelet/config/v1beta1/types.go).
@@ -224,12 +242,13 @@ data has the following advantages:
 
 - protects you from accidental (or unwanted) updates that could cause applications outages
 - improves performance of your cluster by significantly reducing load on kube-apiserver, by
-closing watches for config maps marked as immutable.
+  closing watches for ConfigMaps marked as immutable.
+
+This feature is controlled by the `ImmutableEphemeralVolumes`
+[feature gate](/docs/reference/command-line-tools-reference/feature-gates/).
+You can create an immutable ConfigMap by setting the `immutable` field to `true`.
+For example:
 
-This feature is controlled by the `ImmutableEphemeralVolumes` [feature
-gate](/docs/reference/command-line-tools-reference/feature-gates/),
-which is enabled by default since v1.19. You can create an immutable
-ConfigMap by setting the `immutable` field to `true`. For example,
 ```yaml
 apiVersion: v1
 kind: ConfigMap
@@ -240,17 +259,13 @@ data:
 immutable: true
 ```
 
-{{< note >}}
-Once a ConfigMap or Secret is marked as immutable, it is _not_ possible to revert this change
-nor to mutate the contents of the `data` field. You can only delete and recreate the ConfigMap.
-Existing Pods maintain a mount point to the deleted ConfigMap - it is recommended to recreate
-these pods.
-{{< /note >}}
-
+Once a ConfigMap is marked as immutable, it is _not_ possible to revert this change
+nor to mutate the contents of the `data` or the `binaryData` field. You can
+only delete and recreate the ConfigMap. Because existing Pods maintain a mount point
+to the deleted ConfigMap, it is recommended to recreate these pods.
 
 ## {{% heading "whatsnext" %}}
 
-
 * Read about [Secrets](/docs/concepts/configuration/secret/).
 * Read [Configure a Pod to Use a ConfigMap](/docs/tasks/configure-pod-container/configure-pod-configmap/).
 * Read [The Twelve-Factor App](https://12factor.net/) to understand the motivation for