[GitHub] [couchdb] nickva commented on a change in pull request #3734: Improve fabric_util get_db timeout logic

[GitBox <gi...@apache.org>]

nickva commented on a change in pull request #3734:
URL: https://github.com/apache/couchdb/pull/3734#discussion_r704838386



##########
File path: src/fabric/src/fabric_util.erl
##########
@@ -125,15 +128,40 @@ get_shard([#shard{node = Node, name = Name} | Rest], Opts, Timeout, Factor) ->
             throw(Error);
         {Ref, Reason} ->
             couch_log:debug("Failed to open shard ~p because: ~p", [Name, Reason]),
-            get_shard(Rest, Opts, Timeout, Factor)
+            FmtReason = lists:flatten(io_lib:format("~p", [Reason])),
+            get_shard(Rest, Opts, Timeout, Factor, FmtReason)
         after Timeout ->
             couch_log:debug("Failed to open shard ~p after: ~p", [Name, Timeout]),
-            get_shard(Rest, Opts, Factor * Timeout, Factor)
+            NextTimeout = min(request_timeout(), Factor * Timeout),
+            FmtReason = lists:flatten(io_lib:format("timeout ~B msec", [Timeout])),
+            get_shard(Rest, Opts, NextTimeout, Factor, FmtReason)
         end
     after
         rexi_monitor:stop(Mon)
     end.
 
+
+get_db_timeout(N, Factor, MinTimeout, MaxTimeout) ->
+    %
+    % The progression of timeouts forms a geometric series:
+    %
+    %     MaxTimeout = T + T*F + T*F^2 + T*F^3 ...
+    %
+    % Where T is the initial timeout and F is the factor. The formula for
+    % the sum is:
+    %
+    %     Sum[T * F^I, I <- 0..N] = T * (1 - F^(N + 1)) / (1 - F)
+    %
+    % Then, for a given sum and factor we can calculate the initial timeout T:
+    %
+    %     T = Sum / ((1 - F^(N+1)) / (1 - F))
+    %
+    Timeout = MaxTimeout / ((1 - math:pow(Factor, N + 1)) / (1 - Factor)),

Review comment:
       This is the opposite calculation, given the max time we are willing to wait, and the factor, calculate the initial timeout.




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