8ee4f308a4
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.
91 lines
2.1 KiB
Elixir
91 lines
2.1 KiB
Elixir
defmodule Farside do
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@service_prefix Application.fetch_env!(:farside, :service_prefix)
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@fallback_suffix Application.fetch_env!(:farside, :fallback_suffix)
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@previous_suffix Application.fetch_env!(:farside, :previous_suffix)
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def get_services_map do
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{:ok, service_list} = Redix.command(:redix, ["KEYS", "#{@service_prefix}*"])
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# Match service name to list of available instances
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Enum.reduce(service_list, %{}, fn service, acc ->
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{:ok, instance_list} =
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Redix.command(
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:redix,
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["LRANGE", service, "0", "-1"]
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)
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Map.put(
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acc,
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String.replace_prefix(
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service,
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@service_prefix,
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""
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),
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instance_list
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)
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end)
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end
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def pick_instance(service) do
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{:ok, instances} =
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Redix.command(
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:redix,
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[
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"LRANGE",
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"#{@service_prefix}#{service}",
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"0",
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"-1"
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]
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)
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# Either pick a random available instance,
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# or fall back to the default one
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instance =
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if Enum.count(instances) > 0 do
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if Enum.count(instances) == 1 do
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# If there's only one instance, just return that one...
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List.first(instances)
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else
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# ...otherwise pick a random one from the list, ensuring
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# that the same instance is never picked twice in a row.
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{:ok, previous} =
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Redix.command(
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:redix,
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["GET", "#{service}#{@previous_suffix}"]
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)
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instance =
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Enum.filter(instances, &(&1 != previous))
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|> Enum.random()
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Redix.command(
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:redix,
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["SET", "#{service}#{@previous_suffix}", instance]
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)
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instance
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end
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else
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{:ok, result} =
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Redix.command(
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:redix,
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["GET", "#{service}#{@fallback_suffix}"]
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)
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result
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end
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instance
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end
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def get_last_updated do
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{:ok, last_updated} =
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Redix.command(
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:redix,
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["GET", "last_updated"]
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)
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last_updated
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end
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end
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