nhpip
DB(dis)Connection - ERROR: could not serialize access due to concurrent update
Hi,
We are using DBConnection and Ecto with a Postgres DB. Occasionally we get errors like below in the Postgres log:
2022-09-17 12:36:59.478 EDT [92184] ERROR: could not serialize access due to concurrent update
2022-09-17 12:36:59.478 EDT [92184] CONTEXT: SQL statement "update collections
set scoped_seq = scoped_seq + 1
where id = collection_id
returning scoped_seq"
PL/pgSQL function advance_seq(uuid) line 5 at SQL statement
SQL statement "select advance_seq(new.collection_id)"
PL/pgSQL function set_rev_id() line 3 at SQL statement
90% of the time this results in a Postgrex.Error exception with a code of :serialization_failure which permits us to retry the transaction.
However, in a small percentage of cases we get the same error in the Postgres log, but instead a DBConnection.ConnectionError exception with a message of:
12:17:13.217 [error] Postgrex.Protocol (#PID<0.2447.0>) disconnected: ** (DBConnection.ConnectionError) transaction rolling back
My questions are:
- Why do we sometimes get a Postgrex error and sometimes a DBConnection disconnect for what appears to be the same error?
- Why should it disconnect?
- Can we stop it from disconnecting (or force a reconnect)?
Now I can retry the transaction further up the stack, but that involves re-doing lots of data processing and doesn’t always work.
We are running latest of db_connection, ecto and postgrex.
Thanks
Marked As Solved
joey_the_snake
In general Postgrex.Error means your query went through to the database and the response from the database was an error. DBConnection.Error there wasn’t a clean send/receive to/from the db.
In your case it looks like that error comes when you try to execute a query on a transaction that is already rolling back. So this puts the connection in an erroneous state and it raises/disconnects.
A wild guess: you are retrying in the same transaction you receive the error in instead of letting the transaction completely roll back and starting a new one.
Also Liked
nhpip
Thanks, that makes total sense. It was before my time, but the implementation is kind of weird. The pseudo code looks like:
defmodule Server do
def transaction(mod, fun, args, .....) do
try do
Repo.transaction(fn ->
....
case apply(mod, fun, args) do
{:ok, rsp} -> rsp
{:error, e} -> Repo.rollback(e)
end)
rescue
e in Postgrex.Error ->
repeat_logic_if_serializatiion_error(...)
end
end
end
Now the problem is that mod, fun can also invoke Server.transaction, with a different mod, fun. Indeed it can get quite deep. So it’s possible that the exception can get caught and repeated in an inner layer.
Thanks again…
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