Add ability to join an existing cluster

This adds some basic ability to join a node to an existing cluster. It
uses a rpc layer to initiate a join request to an existing memeber. The
response indicates whether the joining node should take part in the raft
cluster and who it's peers should be.  If raft should not be started, the
peers are the addresses of the current raft members that it should delegate
consensus operations.

To keep the meta store implementation agnostic of whether it's running
a local raft or not, a consensusStrategy type was also added.

cmd/influxd/run/command.go

@@ -82,6 +82,10 @@ func (cmd *Command) Run(args ...string) error {
 		config.Meta.Hostname = options.Hostname
 	}
 
+	if options.Join != "" {
+		config.Meta.Join = options.Join
+	}
+
 	// Validate the configuration.
 	if err := config.Validate(); err != nil {
 		return fmt.Errorf("%s. To generate a valid configuration file run `influxd config > influxdb.generated.conf`.", err)

meta/config.go

@@ -37,6 +37,9 @@ type Config struct {
 	HeartbeatTimeout    toml.Duration `toml:"heartbeat-timeout"`
 	LeaderLeaseTimeout  toml.Duration `toml:"leader-lease-timeout"`
 	CommitTimeout       toml.Duration `toml:"commit-timeout"`
+
+	// The join command-line argument
+	Join string
 }
 
 func NewConfig() Config {

meta/internal/meta.proto

@@ -264,24 +264,34 @@ message Response {
 //
 //========================================================================
 
-message RPCRequest {
-    extensions 100 to max;
+enum RPCType {
+    FetchData = 1;
+    Join = 2;
+}
 
-    enum Type {
-        QueryDataRequest                = 1;
-    }
+message ResponseHeader {
+    required bool OK = 1;
+    optional string Error = 2;
+}
 
-    required Type type = 1;
+message FetchDataRequest {
 }
 
-message QueryDataRequest {
-    extend RPCRequest {
-        optional QueryDataRequest command = 101;
-    }
+message FetchDataResponse {
+    required ResponseHeader Header = 1;
+    optional bytes Data = 2;
 }
 
-message RPCResponse {
-    required bool OK = 1;
-    optional string Error = 2;
-    optional bytes Data = 3;
+message JoinRequest {
+    required string Addr = 1;
+}
+
+message JoinResponse {
+    required ResponseHeader Header = 1;
+
+
+    optional bool EnableRaft = 2;
+    // The addresses of raft peers to use if joining as a raft member. If not joining
+    // as a raft member, these are the nodes running raft.
+    repeated string Peers = 3;
 }