Update pipeline-overview.md

Author: askcarter

labs/pipeline-overview.md

@@ -1,8 +1,41 @@
 # Pipeline Overview
 
-CI  --  tests and builds image
-CD -- takes known good images and pushes to production  
+## Definitions
+| Term | Definition |
+| --- | --- |
+| Continuous Integration (CI) |  tests and builds image |
+| Continuous Delivery (CD) | takes known good images and pushes to production  |
+
+## Pipeline
+
+![](../docs/img/pipeline-overview.png)
 
 Dev  tags commit -->[GSR] --> [GCR builds image based on tag] -> [Spin deploys off of new image] -> [k8s]
 
 ## App Explanation
+
+## The sample app
+You'll use a very simple sample application - `gceme` - as the basis for your CD pipeline. `gceme` is written in Go and is located in the `sample-app` directory in this repo. When you run the `gceme` binary on a GCE instance, it displays the instance's metadata in a pretty card:
+
+![](docs/img/info_card.png)
+
+The binary supports two modes of operation, designed to mimic a microservice. In backend mode, `gceme` will listen on a port (8080 by default) and return GCE instance metadata as JSON, with content-type=application/json. In frontend mode, `gceme` will query a backend `gceme` service and render that JSON in the UI you saw above. It looks roughly like this:
+
+```
+-----------      ------------      ~~~~~~~~~~~~        -----------
+|         |      |          |      |          |        |         |
+|  user   | ---> |   gceme  | ---> | lb/proxy | -----> |  gceme  |
+|(browser)|      |(frontend)|      |(optional)|   |    |(backend)|
+|         |      |          |      |          |   |    |         |
+-----------      ------------      ~~~~~~~~~~~~   |    -----------
+                                                  |    -----------
+                                                  |    |         |
+                                                  |--> |  gceme  |
+                                                       |(backend)|
+                                                       |         |
+                                                       -----------
+```
+Both the frontend and backend modes of the application support two additional URLs:
+
+1. `/version` prints the version of the binary (declared as a const in `main.go`)
+1. `/healthz` reports the health of the application. In frontend mode, health will be OK if the backend is reachable.