Proposal: Configmaps populating environment variables (#148)

* Proposal: Configmaps populating environment variables

* Explain how conflicts are resolved

* kubectl create configmap enhancement

* Define the specific processing order, single --from-env-file

* Remove create configmap changes

To be done under a separate proposal.

* Update nits

1 files changed, 188 insertions, 0 deletions

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+# ConfigMaps as environment variables
+
+## Goal
+
+Populating environment variables of a container from an entire ConfigMap.
+
+## Design Points
+
+A container can specify a set of existing ConfigMaps to populate environment variables.
+
+There needs to be an easy way to isolate the variables introduced by a given
+ConfigMap. The contents of a ConfigMap may not be known in advance and it may
+be generated by someone or something else. Services may provide binding
+information via a ConfigMap. If you have a common service with multiple
+instances like a Message Queue or Database, there needs to be a way to
+uniquely identify and prevent collision when consuming multiple ConfigMaps in
+a single container using this feature.
+
+## Proposed Design
+
+Containers can specify a set of sources that are consumed as environment
+variables. One such source is a ConfigMap.
+Each key defined in the ConfigMap's `Data` object must be a "C" identifier. If
+an invalid key is present, the container will fail to start.
+
+Environment variables defined by a `Container` are processed in a specific
+order.  The processing order is as follows:
+
+1. All automatic service environment variables
+1. All `EnvFrom` blocks in order
+1. All `Env` blocks in order.
+
+The last value processed for any given environment variable will be the
+decided winner.  Variable references defined by an `EnvVar` struct will be
+resolved by the current values defined even if the value is replaced later.
+
+To prevent collisions amongst multiple ConfigMaps, each defined ConfigMap can
+have an optional associated prefix that is prepended to each key in the
+ConfigMap.  Prefixes must be a "C" identifier.
+
+### Kubectl updates
+
+The `describe` command will display the configmap name that have been defined as
+part of the environment variable section including the optional prefix when
+defined.
+
+### API Resource
+
+A new `EnvFromSource` type containing a `ConfigMapRef` will be added to the
+`Container` struct.
+
+```go
+// EnvFromSource represents the source of a set of ConfigMaps
+type EnvFromSource struct {
+  // A string to place in front of every key. Must be a C_IDENTIFIER.
+  // +optional
+  Prefix string `json:"prefix,omitempty"`
+  // The ConfigMap to select from
+  ConfigMapRef *LocalObjectReference `json:"configMapRef,omitempty"`
+}
+
+type Container struct {
+  // List of sources to populate environment variables in the container.
+  // The keys defined within a source must be a C_IDENTIFIER. An invalid key
+  // will prevent the container from starting. When a key exists in multiple
+  // sources, the value associated with the last source will take precedence.
+  // Values defined by an Env with a duplicate key will take precedence over 
+  // any listed source.
+  // Cannot be updated.
+  // +optional
+  EnvFrom []EnvFromSource `json:"envFrom,omitempty"`
+}
+```
+
+### Examples
+
+### Consuming `ConfigMap` as Environment Variables
+
+```yaml
+apiVersion: v1
+kind: ConfigMap
+metadata:
+  name: etcd-env-config
+data:
+  number_of_members: "1"
+  initial_cluster_state: new
+  initial_cluster_token: DUMMY_ETCD_INITIAL_CLUSTER_TOKEN
+  discovery_token: DUMMY_ETCD_DISCOVERY_TOKEN
+  discovery_url: http://etcd_discovery:2379
+  etcdctl_peers: http://etcd:2379
+  duplicate_key: FROM_CONFIG_MAP
+  REPLACE_ME: "a value"
+```
+
+This pod consumes the entire `ConfigMap` as environment variables:
+
+```yaml
+apiVersion: v1
+kind: Pod
+metadata:
+  name: config-env-example
+spec:
+  containers:
+  - name: etcd
+    image: openshift/etcd-20-centos7
+    ports:
+    - containerPort: 2379
+      protocol: TCP
+    - containerPort: 2380
+      protocol: TCP
+    env:
+    - Name: duplicate_key
+      Value: FROM_ENV
+    - Name: expansion
+      Value: $(REPLACE_ME)
+    envFrom:
+    - configMapRef:
+        name: etcd-env-config
+```
+
+The resulting environment variables will be:
+
+```
+number_of_members="1"
+initial_cluster_state="new"
+initial_cluster_token="DUMMY_ETCD_INITIAL_CLUSTER_TOKEN"
+discovery_token="DUMMY_ETCD_DISCOVERY_TOKEN"
+discovery_url="http://etcd_discovery:2379"
+etcdctl_peers="http://etcd:2379"
+duplicate_key="FROM_ENV"
+expansion="a value"
+REPLACE_ME="a value"
+```
+
+### Consuming multiple `ConfigMap` as Environment Variables
+
+```yaml
+apiVersion: v1
+kind: ConfigMap
+metadata:
+  name: env-config
+data:
+  key1: a
+  key2: b
+```
+
+This pod consumes the entire `ConfigMap` as environment variables:
+
+```yaml
+apiVersion: v1
+kind: Pod
+metadata:
+  name: config-env-example
+spec:
+  containers:
+  - name: etcd
+    image: openshift/etcd-20-centos7
+    ports:
+    - containerPort: 2379
+      protocol: TCP
+    - containerPort: 2380
+      protocol: TCP
+    envFrom:
+    - prefix: cm1_
+      configMapRef:
+        name: env-config
+    - prefix: cm2_
+      configMapRef:
+        name: env-config
+```
+
+The resulting environment variables will be:
+
+```
+cm1_key1="a"
+cm1_key2="b"
+cm2_key1="a"
+cm2_key2="b"
+```
+
+### Future
+
+Add similar support for Secrets.
+
+
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