Add script checks

README.md

@@ -1,6 +1,7 @@
 # health-checker
 
-A simple HTTP server that will return `200 OK` if the given TCP ports are all successfully accepting connections.
+A simple HTTP server that will return `200 OK` if the configured checks are all successful.  If any of the checks fail,
+it will return `HTTP 504 Gateway Not Found`. 
 
 ## Motivation
 
@@ -14,15 +15,23 @@ a single TCP port, or an HTTP(S) endpoint. As a result, our use case just isn't
 We wrote health-checker so that we could run a daemon on the server that reports the true health of the server by
 attempting to open a TCP connection to more than one port when it receives an inbound HTTP request on the given listener.
 
+Using the `--script` -option, the `health-checker` can be extended to check many other targets. One concrete exeample is monitoring
+`ZooKeeper` node status during rolling deployment. Just polling the `ZooKeeper`'s TCP client port doesn't necessarily guarantee
+that the node has (re-)joined the cluster. Using the `health-check` with a custom script target, we can 
+[monitor ZooKeeper](https://zookeeper.apache.org/doc/r3.4.8/zookeeperAdmin.html#sc_monitoring) using the 
+[4 letter words](https://zookeeper.apache.org/doc/r3.4.8/zookeeperAdmin.html#sc_zkCommands), ensuring we report health back to the 
+[Load Balancer](https://aws.amazon.com/documentation/elastic-load-balancing/) correctly.
+
 ## How It Works
 
 When health-checker is started, it will listen for inbound HTTP requests for any URL on the IP address and port specified
 by `--listener`. When it receives a request, it will attempt to open TCP connections to each of the ports specified by
-an instance of `--port`. If all TCP connections succeed, it will return `HTTP 200 OK`. If any TCP connection fails, it
-will return `HTTP 504 Gateway Not Found`. 
+an instance of `--port` and/or execute the script target specified by `--script`. If all configured checks - all TCP 
+connections and zero exit status for the script - succeed, it will return `HTTP 200 OK`. If any of the checks fail, 
+it will return `HTTP 504 Gateway Not Found`. 
 
 Configure your AWS Health Check to only pass the Health Check on `HTTP 200 OK`. Now when an HTTP Health Check request
-comes in, all desired TCP ports will be checked.
+comes in, all desired TCP ports will be checked and the script target executed.
 
 For stability, we recommend running health-checker under a process supervisor such as [supervisord](http://supervisord.org/)
 or [systemd](https://www.freedesktop.org/wiki/Software/systemd/) to automatically restart health-checker in the unlikely
@@ -46,9 +55,10 @@ health-checker [options]
 | `--listener` |  The IP address and port on which inbound HTTP connections will be accepted. | `0.0.0.0:5000`
 | `--log-level` | Set the log level to LEVEL. Must be one of: `panic`, `fatal`, `error,` `warning`, `info`, or `debug` | `info` 
 | `--help` | Show the help screen | | 
+| `--script` | Path to script to run - will PASS if it completes within 5s with a zero exit status | | 
 | `--version` | Show the program's version | | 
 
-#### Example
+#### Example 1
 
 Run a listener on port 6000 that accepts all inbound HTTP connections for any URL. When the request is received,
 attempt to open TCP connections to port 5432 and 3306. If both succeed, return `HTTP 200 OK`. If any fails, return `HTTP
@@ -58,3 +68,23 @@ attempt to open TCP connections to port 5432 and 3306. If both succeed, return `
 health-checker --listener "0.0.0.0:6000" --port 5432 --port 3306
 ```
 
+#### Example 2
+
+Run a listener on port 6000 that accepts all inbound HTTP connections for any URL. When the request is received,
+attempt to run the script. If exit code is zero, return `HTTP 200 OK`. If any other exit code, return `HTTP
+504 Gateway Not Found`.
+
+```
+health-checker --listener "0.0.0.0:6000" --script /path/to/script.sh
+```
+
+#### Example 3
+
+Run a listener on port 6000 that accepts all inbound HTTP connections for any URL. When the request is received,
+attempt to open TCP connection to port 8000 and run the script. If both succeed, return `HTTP 200 OK`. If either fails, return `HTTP
+504 Gateway Not Found`.
+
+```
+health-checker --listener "0.0.0.0:6000" --port 8000 --script /usr/local/bin/zk-health-check.sh
+```
+