Add some docs for config (#215)

* Add some docs for config

* typo

README.md

@@ -80,4 +80,26 @@ nodes: {
 5. Attach to the debugger.
     1. Using VS Code: Run and Debug -> Connect to server
     2. Using dlv cli: `dlv connect localhost:56268`
-        1. Required: Configure subsitute-path so dlv can resolve source code: `config substitute-path /build <abs path to lemming src>`
+        1. Required: Configure subsitute-path so dlv can resolve source code: `config substitute-path /build <abs path to lemming src>`
+
+## Configuration
+
+In general, Lemming should be configured through gNMI, however in some cases using flags is acceptable.
+
+When to use gNMI:
+* Config modelled in OC
+* Values that may be modified at runtime
+
+When to use flags:
+* Startup options: (eg gNMI listen port)
+* Immutable config
+* Environment specific options (location of some resource)
+
+Lemming uses Viper and pflags for configuration. Currently, only flags are supported (no env vars and no config file).
+Flags are defined in cmd/lemming/lemming.go.
+
+
+In KNE, flags are set using the `args` attribute in the topology file.
+The Lemming operator also adds some mandatory flags to lemming for ease of use. These are flags that are always set
+since they required to run lemming in a containerized environment.
+They are defined at operator/controllers/lemming_controller.go.

cmd/lemming/lemming.go

@@ -17,29 +17,29 @@ package main
 import (
 	"flag"
 
-	"github.com/openconfig/lemming"
-	"github.com/openconfig/lemming/sysrib"
 	"github.com/spf13/pflag"
 	"github.com/spf13/viper"
 	"google.golang.org/grpc/credentials"
 	"google.golang.org/grpc/credentials/insecure"
 
+	"github.com/openconfig/lemming"
+	"github.com/openconfig/lemming/sysrib"
+
 	log "github.com/golang/glog"
 )
 
 var (
-	gnmiAddr    = flag.String("gnmi", ":9339", "gNMI listen address")
-	gribiAddr   = flag.String("gribi", ":9340", "gRIBI listen address")
-	bgpPort     = flag.Uint("bgp_port", 179, "BGP listening port")
+	gnmiAddr    = pflag.String("gnmi", ":9339", "gNMI listen address")
+	gribiAddr   = pflag.String("gribi", ":9340", "gRIBI listen address")
+	bgpPort     = pflag.Uint("bgp_port", 179, "BGP listening port")
 	target      = pflag.String("target", "fakedut", "name of the fake target")
 	tlsKeyFile  = pflag.String("tls_key_file", "", "Controls whether to enable TLS for gNXI services. If unspecified, insecure credentials are used.")
 	tlsCertFile = pflag.String("tls_cert_file", "", "Controls whether to enable TLS for gNXI services. If unspecified, insecure credentials are used.")
 	zapiAddr    = pflag.String("zapi_addr", sysrib.ZAPIAddr, "Custom ZAPI address: use unix:/tmp/zserv.api for a temp.")
+	_           = pflag.Bool("enable_dataplane", false, "Controls whether to enable dataplane")
 )
 
 func main() {
-	pflag.Bool("enable_dataplane", false, "Controls whether to enable dataplane")
-
 	pflag.CommandLine.AddGoFlagSet(flag.CommandLine)
 	pflag.Parse()
 	viper.BindPFlags(pflag.CommandLine)