I am trying to design a small app to send emails (stored in MongoDB) using
golang utilizing goroutine and channel and this is the first time i use
golang.
There are 2 types of worker in each server (Retriever and Sender). A single
retriever will periodically (every few seconds) query MongoDB, fetch new
email. Then new email will be sent to Senders via a buffered channel (has
buffer size == number of Sender workers, this might be the issue). When a
sigterm received, Retriever will exist using ctx.Done and close the email
channel. Since channel closed, Senders which range over channel will exit
as well.
go func() {
defer wg.Done()
for {
select {
case <-ctx.Done():
// received sigterm, close channel
close(emailCh)
return
case <- time.After(5 * time.Second):
// query DB every 5 seconds to get a single new email....
if newEmail := dw.getNewEmail(); newEmail != nil {
emailCh <- newEmail
}
}
}
I realize this is a bad design and the issues are:
I am using FindAndModify to fetch email every time ( I need to update
email status to sending when fetching so that the app running in other
server will not fetch the same doc). That means I am actually sending a
single email in each interval which makes it non-sense to create multiple
Sender goroutines.
the alternative I can think of right now is:
1. I could run FindAndModify query multiple times in each interval to get N
emails. But If the N is larger then buffer size, it will block the select
and blocking time depends on how many more emails I retrieved than the
buffer size. If N is too large, then sigterm received, I am not be able to
shutdown the app in time. If N is too small, I can only send at most
N/interval emails per second.
2. use an unbuffered channel without interval, (keeps polling the DB).
Whenever there is a new email, send to channel. the blocking time should be
minimal (1 email). But the app will continually poll the DB, will it create
big overhead/too many connections?
go playground link:
https://play.golang.org/p/LRQ-cnLiDfB
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