James,
Thanks for pointing out the submit function and explaining the wrapper.
Would you specifically advise against sending the result back through a
channel? I have made a prototype (using ExecutorService) where the email is
provided together with a channel and the sender starts a small go block to
take care of the result
(let [c (chan)
(send-email! {:mail mail :channel c})
(go
(let [res (<! c]
(handle-res res)))))
Inside the worker thread, I use alt!! together with a timeout to send the
result back through the chan - so that if no one takes care of the result
within a couple of seconds - the thread doesn't hang. If I understand
correctly, go blocks are garbage collected if the channel isn't referenced,
so creating a few thousand go blocks may be safe? (And they will of course
not live that long anyway before the result is sent back).
I will look into Claypool!
Erik,
You are quite right. My plan is to save emails that must be sent (not all
need guaranteed delivery) to a database but queue them right away in a
worker thread. The DB will then be updated with delivery status. In the
interest of reducing IO, I would prefer not to have to write and read from
the DB for all emails but only in case of a crash or mail server down
something similar. So there needs to be continuous monitoring of the email
table - which requires some sort of ever running task? Would you suggest a
go block or using a scheduler (there seem to be quite a few options of
Clojure schedulers).
Thanks,
Brjánn
On Fri, 26 Oct 2018 at 07:22, Erik Assum <e...@assum.net> wrote:
> Not knowing your app nor your requirements, but it might also be an idea
> to separate concerns:
>
> Your web-app could store “emails to be sent” in a database table, and you
> could have a different process reading this table at regular intervals and
> sending mails, and also marking the emails as sent, failed, or what have
> you. Maybe even a time stamp to indicate when an email was sent.
>
> This gives you a couple of benefits:
> 1) a simpler web app
> 2) a chance to resend emails
> 3) easier testing, as you can see that the web-app stores the mails it
> should in the db, without actually sending emails
>
> HTH,
>
> Erik.
> --
> i farta
>
> 25. okt. 2018 kl. 17:46 skrev brj...@gmail.com:
>
> Hi,
>
> First, I would like to briefly present myself to the list. I'm a
> psychologist and researcher at the Karolinska Institutet in Stockholm,
> Sweden. I do research within the field of internet-based psychological
> treatment - which is a field that has grown a lot during the last 10-15
> years. I have developed my own web platform to deliver these treatments to
> patients with different mental and medical conditions. I wrote the first
> version in PHP, and not being a professional programmer, the code base is a
> big mess after > 10 years of development.
>
> Two years ago, a friend introduced me to Clojure. I had never worked with
> a lisp before, but I quickly fell completely in love with it. So many of
> the issues of programming in PHP completely disappeared! Words cannot
> express my gratitude to Rich for inventing Clojure, the core team, all
> developers out there who write extremely useful libraries, and my friend
> for introducing Clojure to me.
>
> I decided to completely rewrite my web platform in Clojure. I've chosen to
> do a gradual move from PHP to Clojure, meaning that I replace PHP code with
> Clojure code component by component and continuously test in production.
> Like I said, I'm not a professional programmer, and this venture poses a
> lot of challenges. I try to read everything I find in books and on the web,
> but when it comes to more complex issues, such as threads and async
> programming, I feel that I end up almost guessing and with a lot of trial
> and error. I have no idea how cautious one should be when launching a new
> thread (while it completely occupy the server???) and am often surprised
> when my go blocks suddenly freeze. I feel that I am at the mercy of the
> Clojure community if I want to understand these (and many other!) issues.
>
> This leads me to the subject of this email. I've decided to migrate my
> mail queue from PHP to Clojure. In PHP, it's just a cron job that executes
> every five minutes to send all emails (and actually also SMS-messages, but
> not really relevant) that have been queued.
>
> I've written a basic mock Clojure implementation with the following goals
> - All messages should be sent async, i..e, the web user should not have to
> wait while the email is being sent. -> I'm sending them in a separate
> thread.
> - I have a fear that if I have a thread dedicated only to sending emails,
> I'm wasting server resources. -> I want the thread to die 5 seconds after
> the last email in the queue has been sent.
>
> My implementation basically consists of
> - An eternal go loop that receives a notification through a channel if new
> messages have been queued
> - The go loop checks if the mail sender thread is running. If not, it
> starts it.
> - The mail sender thread dies 5 secs after the last email was sent
> - The state of the thread (running / not running) is stored in an agent to
> avoid race conditions (i.e., starting multiple threads or not starting a
> thread because it is running when its status is checked but stops right
> after).
>
> My code is here
> https://gist.github.com/brjann/2aef16849b9bd445374cb6b31efece60
>
> If any of you have had the time and energy to read this far (including the
> code), I would be very grateful for your input.
> - Is there a risk that my go block will hang?
> - Have I eliminated the risk for race conditions?
> - Do I really need to kill the thread or is there no risk for thread
> starvation on the server (I will probably
> - Could I use send instead of send-off? I guess that I am now using two
> threads, one for the sender and one each time I send a message using
> send-off.
> - Any newbie mistakes / coding style issues?
> - Could this be done in a better/simpler way???
>
> (Btw, I would be very grateful for feedback on the form of my question if
> you have any. Are there better/other forums? Could I present the question
> or code in a another manner to help you understand better?)
>
> I am happy for any replies - on the list or backchannel. And I hope that
> you feel that I have not misused your inbox (I may repeat this behavior...).
>
> Thanks,
> Brjánn
>
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