Allow multiple flushes from EF Core DbContextOutbox

[Ferchke7]

Adds a per-call MultiFlushMode option to the EF Core outbox save/flush API:

await outbox.SaveChangesAndFlushMessagesAsync(
    MultiFlushMode.AllowMultiples,
    cancellationToken);

The default behavior is unchanged. MessageContext.MultiFlushMode still defaults to OnlyOnce.

Use Cases

Chunked publishing from Minimal APIs or MVC controllers

A common pattern is to publish a large number of messages in chunks so each chunk is committed and flushed separately:

app.MapPost("/send-bulk", async (
    IDbContextOutbox<AppDbContext> outbox,
    IReadOnlyList<CreateInvoiceCommand> commands,
    CancellationToken cancellationToken) =>
{
    foreach (var chunk in commands.Chunk(500))
    {
        foreach (var command in chunk)
        {
            await outbox.PublishAsync(command);
        }

        await outbox.SaveChangesAndFlushMessagesAsync(
            MultiFlushMode.AllowMultiples,
            cancellationToken);
    }
});

Without AllowMultiples, only the first flush is honored on the scoped outbox instance.

For example, with 2,000 messages split into 4 chunks of 500:

• chunk 1: 500 messages flushed
• chunk 2: flush skipped
• chunk 3: flush skipped
• chunk 4: flush skipped

Result: only the first 500 messages are flushed.

Import jobs that save progress per batch

Background import jobs often process records in batches and commit progress after each batch:

public async Task ImportAsync(
    IDbContextOutbox<AppDbContext> outbox,
    IReadOnlyList<CustomerRow> rows,
    CancellationToken cancellationToken)
{
    foreach (var batch in rows.Chunk(100))
    {
        foreach (var row in batch)
        {
            var customer = Customer.From(row);

            outbox.DbContext.Customers.Add(customer);

            await outbox.PublishAsync(new CustomerImported
            {
                CustomerId = customer.Id
            });
        }

        await outbox.SaveChangesAndFlushMessagesAsync(
            MultiFlushMode.AllowMultiples,
            cancellationToken);
    }
}

Each batch saves EF Core changes and flushes the messages produced by that batch.