From 0ff55054ce65797cb1231897dfbd799718a03dc0 Mon Sep 17 00:00:00 2001
From: rhishikesh <rhishikesh@helpshift.com>
Date: Sat, 6 Mar 2021 16:32:15 +0530
Subject: [PATCH] Fault tolerant gossip implementation.

---
 demo/clojure/gossip.clj | 90 ++++++++++++++++++++++++++++++++---------
 1 file changed, 70 insertions(+), 20 deletions(-)

diff --git a/demo/clojure/gossip.clj b/demo/clojure/gossip.clj
index 2db31e1..9af9fb8 100755
--- a/demo/clojure/gossip.clj
+++ b/demo/clojure/gossip.clj
@@ -62,21 +62,46 @@
         (println input)))))
 
 
-(defn reply
+(def node-id (atom ""))
+(def node-nbrs (atom []))
+(def messages (atom #{}))
+(def gossips (atom {}))
+(def next-message-id (atom 0))
+
+
+(defn- reply
   ([src dest body]
    {:src src
     :dest dest
     :body body}))
 
 
-(def node-id (atom ""))
-(def node-nbrs (atom []))
-(def messages (atom #{}))
+(defn- send!
+  ([input]
+   (-> input
+       generate-json
+       printout))
+  ([src dest body]
+   (send! (reply src dest body))))
+
+
+(defn- gossip-loop
+  []
+  (future
+    (try
+      (loop []
+        (doseq [g @gossips]
+          (send! (second g)))
+        (Thread/sleep 1000)
+        (recur))
+      (catch Exception e
+        (printerr e)))))
+
 
 (defn- process-request
   [input]
   (let [body (:body input)
-        r-body {:msg_id (rand-int 100)
+        r-body {:msg_id (swap! next-message-id inc)
                 :in_reply_to (:msg_id body)}
         nid (:node_id body)]
     (case (:type body)
@@ -93,29 +118,52 @@
                                          :topology
                                          (keyword @node-id)]))
         (reply @node-id
-             (:src input)
-             (assoc r-body
-                    :type "topology_ok")))
+               (:src input)
+               (assoc r-body
+                      :type "topology_ok")))
 
       "broadcast"
       (do
-        ;; if we already have the message, it means
-        ;; this is a broadcast, dont rebroadcast
         (when-not (@messages (:message body))
-          (doseq [n @node-nbrs]
-            ;; we have to explicitly print this out
-            ;; because just calling reply only generates a
-            ;; map which the outer loop prints out
-            (printout (generate-json (reply @node-id
-                                            n
-                                            {:type "broadcast"
-                                             :message (:message body)})))))
+          (doseq [n @node-nbrs
+                  ;; also don't broadcast to sender
+                  :when (not= n (:src input))]
+            ;; here we need to distinguish between msg-id received
+            ;; directly from a broadcast message which will be :msg_id
+            ;; and msg-id received from a peer as part of gossip.
+            ;; We can receive gossip with msg-id 1 and message 1
+            ;; and later receive broadcast with msg-id 1 and message 3
+            ;; Also, the msg-id we send here has to be used to
+            ;; record the message for retries which is then removed
+            ;; when we receive gossip_ok message
+            (let [msg-id (or (:msg_id body)
+                             (str "g:" (:g_id body)))
+                  msg {:src @node-id
+                       :dest n
+                       :body {:type "broadcast"
+                              :message (:message body)
+                              :g_id msg-id}}]
+              (swap! gossips assoc (str n "::" msg-id) msg)
+              ;; we have to explicitly send this out
+              ;; because just calling reply only generates a
+              ;; map which the outer loop prints out
+              (send! msg))))
         (swap! messages conj (:message body))
-        (when (:msg_id body)
+        (if (:msg_id body)
           (reply @node-id
                  (:src input)
                  (assoc r-body
-                        :type "broadcast_ok"))))
+                        :type "broadcast_ok"))
+          (when (:g_id body)
+            (reply @node-id
+                   (:src input)
+                   {:type "gossip_ok"
+                    :g_id (:g_id body)}))))
+
+      "gossip_ok"
+      (do
+        (swap! gossips dissoc (str (:src input) "::" (:g_id body)))
+        nil)
 
       "read"
       (reply @node-id
@@ -128,9 +176,11 @@
 (defn -main
   "Read transactions from stdin and send output to stdout"
   []
+  (gossip-loop)
   (process-stdin (comp printout
                        generate-json
                        process-request
                        parse-json)))
 
+
 (-main)