Idempotent message consumers in .NET with EF Core: inbox table, duplicates and ordering

// ======================================================================
// Q: A message is delivered twice. Or out of order. What happens?
//
// Brokers provide at-least-once delivery: "exactly-once" doesn't really exist in
// practice. So the consumer must be idempotent.
// - Natural idempotency where possible: "set status = Shipped" or an upsert,
//   instead of "add 1".
// - Ordering: the queue doesn't guarantee order when there are multiple
//   consumers or retries. Solutions: Service Bus sessions (FIFO per SessionId =
//   OrderId), a Kafka partition key, or a version / sequence number in the
//   message, where the consumer ignores anything older than what it already has.
// - Dead-letter queue: alert when it isn't empty, a tool for inspection and
//   replay, and a human who looks at it.
// - Keep messages small: IDs and the necessary fields, not whole entities. For
//   large payloads: the claim-check pattern (blob storage plus a link in the
//   message).
// ======================================================================

// Idempotent consumer with an inbox table:
// unique index (MessageId, Consumer), in the same transaction as the change
public async Task HandleAsync(OrderPlaced e, string messageId, CancellationToken ct)
{
    if (await db.ProcessedMessages.AnyAsync(m => m.MessageId == messageId && m.Consumer == "erp-sync", ct))
        return;                                                  // already processed

    await using var tx = await db.Database.BeginTransactionAsync(ct);
    db.ProcessedMessages.Add(new ProcessedMessage(messageId, "erp-sync", DateTime.UtcNow));
    db.ErpExports.Add(new ErpExport(e.OrderId, ErpExportStatus.Pending));

    try
    {
        await db.SaveChangesAsync(ct);
        await tx.CommitAsync(ct);
    }
    catch (DbUpdateException ex) when (ex.IsUniqueViolation())   // our own extension (e.g. PostgreSQL 23505)
    {
        // two instances got the same message concurrently: the unique index keeps only one
    }
}