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Prevent same instance from being selected twice in a row

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Introduces a new db key "<service>-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 "<service>-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 "<service>-previous" key.
This commit is contained in:
Ben Busby 2021-11-10 12:19:37 -07:00
parent 71fb89e028
commit 8ee4f308a4
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GPG key ID: 339B7B7EB5333D14
4 changed files with 45 additions and 8 deletions
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6
update.exs
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@ -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