From 8ee4f308a47ea613a97493b8afbe1aed02f80036 Mon Sep 17 00:00:00 2001 From: Ben Busby Date: Wed, 10 Nov 2021 12:19:37 -0700 Subject: [PATCH] Prevent same instance from being selected twice in a row Introduces a new db key "-previous" to track which instance was last selected for a particular service. This allows for filtering the list of available instances to exclude the instance that was last picked, to ensure a (slightly) more even distribution of traffic. There's still the possiblity of the following scenario, however: :service instances > 2 /:service request #1 -> instance #1 /:service request #2 -> instance #2 /:service request #3 -> instance #1 /:service request #4 -> instance #2 where there are many ignored instances for a particular service. One possible solution would be to implement the "-previous" value to be a list, rather than a single value, and push to that list until only one element is left in the original "instance" array after filtering, and then delete the "-previous" key. --- config/config.exs | 3 ++- lib/farside.ex | 29 ++++++++++++++++++++++++++--- test/farside_test.exs | 15 ++++++++++++++- update.exs | 6 +++--- 4 files changed, 45 insertions(+), 8 deletions(-) diff --git a/config/config.exs b/config/config.exs index 3b9dfe6..c522a9f 100644 --- a/config/config.exs +++ b/config/config.exs @@ -2,8 +2,9 @@ import Config config :farside, redis_conn: "redis://localhost:6379", - fallback_str: "-fallback", update_file: ".update-results", service_prefix: "service-", + fallback_suffix: "-fallback", + previous_suffix: "-previous", services_json: "services.json", index: "index.eex" diff --git a/lib/farside.ex b/lib/farside.ex index 0595e33..ea574fa 100644 --- a/lib/farside.ex +++ b/lib/farside.ex @@ -1,6 +1,7 @@ defmodule Farside do @service_prefix Application.fetch_env!(:farside, :service_prefix) - @fallback_str Application.fetch_env!(:farside, :fallback_str) + @fallback_suffix Application.fetch_env!(:farside, :fallback_suffix) + @previous_suffix Application.fetch_env!(:farside, :previous_suffix) def get_services_map do {:ok, service_list} = Redix.command(:redix, ["KEYS", "#{@service_prefix}*"]) @@ -41,12 +42,34 @@ defmodule Farside do # or fall back to the default one instance = if Enum.count(instances) > 0 do - Enum.random(instances) + if Enum.count(instances) == 1 do + # If there's only one instance, just return that one... + List.first(instances) + else + # ...otherwise pick a random one from the list, ensuring + # that the same instance is never picked twice in a row. + {:ok, previous} = + Redix.command( + :redix, + ["GET", "#{service}#{@previous_suffix}"] + ) + + instance = + Enum.filter(instances, &(&1 != previous)) + |> Enum.random() + + Redix.command( + :redix, + ["SET", "#{service}#{@previous_suffix}", instance] + ) + + instance + end else {:ok, result} = Redix.command( :redix, - ["GET", "#{service}#{@fallback_str}"] + ["GET", "#{service}#{@fallback_suffix}"] ) result diff --git a/test/farside_test.exs b/test/farside_test.exs index 6347691..786c382 100644 --- a/test/farside_test.exs +++ b/test/farside_test.exs @@ -42,15 +42,28 @@ defmodule FarsideTest do IO.puts("") Enum.map(service_names, fn service_name -> - IO.puts("/#{service_name}") conn = :get |> conn("/#{service_name}", "") |> Router.call(@opts) + first_redirect = elem(List.last(conn.resp_headers), 1) + IO.puts(" /#{service_name} (#1) -- #{first_redirect}") assert conn.state == :set assert conn.status == 302 + + + conn = + :get + |> conn("/#{service_name}", "") + |> Router.call(@opts) + + second_redirect = elem(List.last(conn.resp_headers), 1) + IO.puts(" /#{service_name} (#2) -- #{second_redirect}") + assert conn.state == :set + assert conn.status == 302 + assert first_redirect != second_redirect end) end end diff --git a/update.exs b/update.exs index 7452e3d..e0c367a 100644 --- a/update.exs +++ b/update.exs @@ -1,5 +1,5 @@ defmodule Instances do - @fallback_str Application.fetch_env!(:farside, :fallback_str) + @fallback_suffix Application.fetch_env!(:farside, :fallback_suffix) @update_file Application.fetch_env!(:farside, :update_file) @services_json Application.fetch_env!(:farside, :services_json) @service_prefix Application.fetch_env!(:farside, :service_prefix) @@ -59,13 +59,13 @@ defmodule Instances do if Enum.count(instances) > 0 do Redix.command(:redix, [ "SET", - "#{service.type}#{@fallback_str}", + "#{service.type}#{@fallback_suffix}", Enum.random(instances) ]) else Redix.command(:redix, [ "SET", - "#{service.type}#{@fallback_str}", + "#{service.type}#{@fallback_suffix}", service.fallback ]) end