update default load balance option

src/backend/load_balance.rs

@@ -0,0 +1,25 @@
+/// Constants to specify a load balance option
+pub(super) mod load_balance_options {
+  pub const FIX_TO_FIRST: &str = "none";
+  pub const ROUND_ROBIN: &str = "round_robin";
+  pub const RANDOM: &str = "random";
+  pub const STICKY_ROUND_ROBIN: &str = "sticky";
+}
+
+#[derive(Debug, Clone)]
+/// Load Balancing Option
+pub enum LoadBalance {
+  /// Fix to the first upstream. Use if only one upstream destination is specified
+  FixToFirst,
+  /// Simple round robin without session persistance
+  RoundRobin, // TODO: カウンタはここにいれる。randomとかには不要なので
+  /// Randomly chose one upstream server
+  Random,
+  /// Round robin with session persistance using cookie
+  StickyRoundRobin,
+}
+impl Default for LoadBalance {
+  fn default() -> Self {
+    Self::FixToFirst
+  }
+}