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Concurrency issue when updating projection version #49

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Concurrency issue when updating projection version #49

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36a9df4
Concurrency fix
Feb 9, 2023
c22ad05
Remove alias
Feb 23, 2023
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53 changes: 31 additions & 22 deletions lib/projections/ecto.ex
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Original file line number Diff line number Diff line change
Expand Up @@ -27,7 +27,6 @@ defmodule Commanded.Projections.Ecto do
- [Usage](usage.html)

"""

defmacro __using__(opts) do
opts = opts || []

Expand Down Expand Up @@ -67,29 +66,39 @@ defmodule Commanded.Projections.Ecto do

multi =
Ecto.Multi.new()
|> Ecto.Multi.run(:verify_projection_version, fn repo, _changes ->
version =
case repo.get(ProjectionVersion, projection_name, prefix: prefix) do
nil ->
repo.insert!(
%ProjectionVersion{
projection_name: projection_name,
last_seen_event_number: 0
},
prefix: prefix
)

version ->
version
|> Ecto.Multi.insert_all(
:insert_version,
ProjectionVersion,
[
%{
inserted_at: DateTime.to_naive(DateTime.utc_now()),
updated_at: DateTime.to_naive(DateTime.utc_now()),
projection_name: projection_name,
last_seen_event_number: 0
}
],
on_conflict: :nothing,
prefix: prefix
)
|> Ecto.Multi.update_all(
:update_version,
from(version in ProjectionVersion,
select: version,
where: version.projection_name == ^projection_name,
where: version.last_seen_event_number < ^event_number
),
[set: [last_seen_event_number: event_number]],
prefix: prefix
)
Comment on lines +69 to +92
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These two separate INSERT and UPDATE statements will need to be combined as a single UPSERT query otherwise there is still the possibilty of a concurrency issue due to executing two queries.

|> Ecto.Multi.run(
:verify_projection_version,
fn _repo, %{update_version: {_, versions}} ->
case versions do
[] -> {:error, :already_seen_event}
[version] -> {:ok, %{version: version}}
end

if version.last_seen_event_number < event_number do
{:ok, %{version: version}}
else
{:error, :already_seen_event}
end
end)
|> Ecto.Multi.update(:projection_version, changeset, prefix: prefix)
)

with %Ecto.Multi{} = multi <- apply(multi_fn, [multi]),
{:ok, changes} <- transaction(multi) do
Expand Down