I don't think the time it takes to send a mail is relevant in this discussion. It's not like there was any choice here, if 100K users have mail notifications enabled you will have to sent 100K mails, it does not have any impact on the design of the notification module side. It sure is interesting from for mail sender module and system administration but that's a different subject.
Back to Notification module, as I said the "let's get everything and then filter it later" is really a bad idea from memory point of view (and is a big issue in current watchlist implementation) unless you don't put stuff to filter in memory but then you end up reinventing a database and then query it for each user so IMO you should go in the "let's deal with each user separately". By the way you might need to consider sending several mails depending on the number of events, to be safe. That's for the general idea. IMO you should go this way, do tests and optimize what you can in what needs to be done for sending a user mail and it's possible it's actually not that long without doing anything special (a request that only deal with one table with proper indices is very fast usually). Also since it's a background thread, speed is less critical (unless it takes a ridiculous among of time of course :)), certainly far less important that trying to not steeling all the available memory to front threads. On the more technical details Vincent talked about MimeMessageFactory API and I agree implementing a NotificationMimeMessageFactory would probably be a good idea. On Thu, Jun 1, 2017 at 9:58 AM, Vincent Massol <[email protected]> wrote: > >> On 1 Jun 2017, at 09:36, Guillaume Delhumeau <[email protected]> >> wrote: >> >> 2017-05-31 21:16 GMT+02:00 Vincent Massol <[email protected]>: >> >>> Hi, >>> >>>> On 31 May 2017, at 20:50, Guillaume Delhumeau < >>> [email protected]> wrote: >>>> >>>> 2017-05-31 15:59 GMT+02:00 Vincent Massol <[email protected]>: >>>> >>>>> Hi Guillaume, >>>>> >>>>>> On 31 May 2017, at 12:15, Guillaume Delhumeau < >>>>> [email protected]> wrote: >>>>>> >>>>>> Help me to decide! >>>>>> >>>>>> TL;DR: >>>>>> >>>>>> * I need to know if performing a query on the database for each user >>> who >>>>>> want to receive an email with all the notifications, is a scalability >>>>> issue >>>>>> (in a job context). >>>>> >>>>> Yes whenever we do a lot of queries to the DB it’s a scalability issue. >>> If >>>>> we have 100K users then it’s 100K queries for definitely a scalability >>>>> issue. >>>>> >>>> >>>> Well, in that case, I don't know if sending 100K emails is scalable too. >>> >>> The new mail system is made for that. There’s a single mail thread >>> (actually 2 but that’s a detail) and it can send an infinite number of >>> mails without slowing down XWiki. Ofc the only thing not guaranteed is how >>> long it takes to do so. But that can be fixed outside of XWiki by having a >>> proxy mail server which would accept immediately all mails sent by XWiki >>> before forwarding them to some cluster of mail servers. It may not be >>> enough though and maybe sending 100K mails to the proxy mail server would >>> already take too long. Would be interesting to have some measure of how >>> long it takes to send a single mail. I think I did some computation at some >>> point but i don’t remember the results. >>> >>> Do you mean that the notification center would execute the DB queries one >>> by one? >> >> >> Yes this is what I mean. > > ok so the issue is how a large company who wants to use XWiki could speed up > the mai delivery so that it becomes close to realtime with 100K users. > > Some questions: > * If we do one DB query per user how long would it take for 100K users? > * How long it takes to prepare 100K emails with a static fixed string content? > * How long does it take to send the 100K-prepared emails IF the receiving > mail server respond instantaneously and is on the same local machine? > > The last 2 questions is to evaluate the mail sender performance itself. If > the answer is acceptable then the large company could use a proxy mail server > as I mentioned earlier and we need to ensure that the preparation of the > emails is very fast on our side. Doing 100K queries would definitely not > scale in this case (if each query takes 30ms, then that would be 3000 > seconds, which is 50 minutes, which is about 1 hour). Since we have hourly > jobs, the previous job would not even finish before the next one triggers… > > And I’m not even considering the realtime use case. For realtime, we should > do the other way around anyway, ie find who’s subscribed to the related > notification(s). > > In any case since number of users are unbound and can reach very high values, > I don’t think we should iterate over users. There are a fixed numbers of > events during a time period and since there are a lot more reads than writes > on a wiki (more than 80% are readers), it’s a number that would stay low even > when you have 100K users. So I would always try to find events first and then > in a single query find all users having subscribed to those events. > > Thanks > -Vincent > >> >>> In this case it could work indeed and it should be left to the mail module >>> to handle that by implementing a custom MimeMessageFactory with an >>> iterator. It’s important to delegate this to the mail sender API IMO. See >>> UsersAndGroupsMimeMessageFactory for an example. AFAIR Edy refactored the >>> watchlist to use a MimeMessageFactory. >>> >>> Thanks >>> -Vincent >>> >>>> We need to find a way to do a single query (or a small fixed number of >>>>> queries independent of the # of users). >>>>> >>>>> If not possible then we may need to either: >>>>> A) Add some new table in our DB to help do that >>>>> B) Use some tool other than the DB, e.g. SOLR, etc >>>>> >>>>> Thanks >>>>> -Vincent >>>>> >>>>>> * If it's not an issue, I can implement the "naïve" solution which >>>>> requires >>>>>> less development. >>>>>> >>>>>> Full message: >>>>>> >>>>>> Status: >>>>>> * notifications are displayed on the top menu when you browse the wiki. >>>>>> * notifications are displayed differently for each individual user >>>>>> according to their preferences (filters on event type, on locations, >>>>>> etc...). >>>>>> * similar notifications are grouped together into "composite >>>>> notifications". >>>>>> * there is only a few notifications displayed (5 by default). >>>>>> >>>>>> Objective: >>>>>> * send an email periodically (every hour, every day, every week) >>>>> according >>>>>> to the user preferences with ALL events that happened during the last >>>>>> period of time, but still according to the user preferences. >>>>>> >>>>>> Inspiration: >>>>>> * the watchlist gets ALL events that happened during the last period of >>>>> time >>>>>> * then, for each user, remove the events which the user is not >>>>> interested in >>>>>> * Benefit: only one query to get the events from the database for all >>>>> users >>>>>> >>>>>> Problems: >>>>>> * in the notifications, I have introduced a NotificationFilter role the >>>>>> make possible to inject some SQL in the query to get the events >>> according >>>>>> to the user preferences. I call this "pre-filters". >>>>>> ** it means we generate a unique request for each individual user, so >>> if >>>>> we >>>>>> send a mail to 1000 users, we will have 1000 requests to the database. >>>>>> >>>>>> I wonder if it's a non-problem or a big scability issue. Because even >>> if >>>>>> the whole job that send emails take ~10 minutes, it does not matter. >>> It's >>>>>> not a realtime thing. >>>>>> >>>>>> For the records, NotificationFilter have "post-filters" too, that >>> perform >>>>>> check on the event itself (for example checking the permissions, >>> etc...). >>>>>> >>>>>> Alternatives: >>>>>> * just like the watchlist, perform a very generic query on the database >>>>> to >>>>>> get all the events that happened during the last period of time >>>>>> * then for each user, use only the "post-filters" to remove events the >>>>> user >>>>>> don't care of >>>>>> >>>>>> Problem: >>>>>> * it means the pre-filters that make sense in the notification use-case >>>>>> cannot be used for emails. Developers must be aware of this. >>>>>> * it requires some refactoring of the code that group similar >>>>> notifications. >>>>>> >>>>>> Question: >>>>>> Should I go with the "naive" solution, ie for each user get all >>>>>> notifications and send a mail, or should I go with the "only 1 query to >>>>> the >>>>>> database for all users" version? >>>>>> >>>>>> Thanks, >>>>>> >>>>>> -- >>>>>> Guillaume Delhumeau ([email protected]) >>>>>> Research & Development Engineer at XWiki SAS >>>>>> Committer on the XWiki.org project >>>>> >>>>> >>>> >>>> >>>> -- >>>> Guillaume Delhumeau ([email protected]) >>>> Research & Development Engineer at XWiki SAS >>>> Committer on the XWiki.org project >>> >>> >> >> >> -- >> Guillaume Delhumeau ([email protected]) >> Research & Development Engineer at XWiki SAS >> Committer on the XWiki.org project > -- Thomas Mortagne
