When a background job starts, it's sent to the back of a queue where a worker handles it; a task clears and the other starts. I think I've got this one right except I don't understand the practical side of it in some cases. Sure, if you're a company sending out 15,000 newsletters once a week using a delayed job makes perfect sense. But when you have an application of even 100 users, in which some task is long enough to need background work (like sending/fetching emails that might take a minute) then each user will have to wait in line while another user gets cleared (in case there's a single worker).
This is the part I'm not sure I'm getting right. I'm talking about the same job, but individually for each user. Does that count as a job per user? If I have 100 users, do I need to keep 100 workers for each one's process to not get tied up?
I've tried using delayed_job to simulate that, and indeed when I sign in with a different account I have to wait until another user's email gets sent until mine is. While the plugin is swift and simple to work with, I think it's not the right approach here.
I've also tried using Ajax, but since it's an HTTP request it ties up the browser in loading mode until it gets a response from the server (even with async: true). Not sure if I ruled this one out too quickly, but I was sortof looking for a more elegant server solution.
Is there a way to achieve a background job like this? (I've heard of different, mostly commercial solutions promising little waiting time, but I'm interested in completely eliminating the queue between users). If not, is there a method to make an ajax request without waiting for a response? I realize my questions are both drastically different but both seem like an appropriate solution to this problem.
Resque is a background processing engine that can support multiple queues.
Ways you could use this:
Group your tasks into queues that make sense on their priority. If you need fast response times, use it in a 'foreground' queue. Slow? (like sending/receiving emails) can be in the 'background' queue
Have one queue per user (you will need to have many many workers for this)
This SO question also gives a way to use delayed_jobs with multiple queues/tables
The purpose of delayed_job and other message queues is to asynchronously process jobs outside of your core application. I always use a queue for sending email since I'm relying on an outside application (sometimes a third-party API like gmail) to send them and I can't guarantee available and operating efficiency.
So for your use case, even with very few users, I highly recommend offloading emails to delayed_job. This will speed up your front end (ajax) and will also give you retries upon failure. You could spin up multiple workers to process the queue, but it shouldn't be necessary with your numbers unless your calls to send mail are taking a really long time (more than a couple seconds?).
And yes in most situations I'd create separate jobs for each user even though the message might be identical. The only time I'd process them all together would be if the email application / API has bulk sending and you can reduce the number of calls significantly by sending a large payload in a few calls.
Related
In the Sidekiq wiki it talks about the need for jobs to be idempotent and transactional. Conceptually this makes sense to me, and this SO answer has what looks like an effective approach at a small scale. But it's not perfect. Jobs can disappear in the middle of running. We've noticed certain work is incomplete and when we look in the logs they cut short in the middle of the work as if the job just evaporated. Probably due to a server restart or something, but it often doesn't find its way back into the queue. super_fetch tries to address this, but it errs on the side of duplicating jobs. With that we see a lot of jobs that end up running twice simultaneously. Having a database transaction cannot protect us from duplicate work if both transactions start at the same time. We'd need locking to prevent that.
Besides the transaction, though, I haven't been able to figure out a graceful solution when we want to do things in bulk. For example, let's say I need to send out 1000 emails. Options I can think of:
Spawn 1000 jobs, which each individually start a transaction, update a record, and send an email. This seems to be the default, and it is pretty good in terms of idempotency. But it has the side effect of creating a distributed N+1 query, spamming the database and causing user facing slowdowns and timeouts.
Handle all of the emails in one large transaction and accept that emails may be sent more than once, or not at all, depending on the structure. For example:
User.transaction do
users.update_all(email_sent: true)
users.each { |user| UserMailer.notification(user).deliver_now }
end
In the above scenario, if the UserMailer loop halts in the middle due to an error or a server restart, the transaction rolls back and the job goes back into the queue. But any emails that have already been sent can't be recalled, since they're independent of the transaction. So there will be a subset of the emails that get re-sent. Potentially multiple times if there is a code error and the job keeps requeueing.
Handle the emails in small batches of, say, 100, and accept that up to 100 may be sent more than once, or not at all, depending on the structure, as above.
What alternatives am I missing?
One additional problem with any transaction based approach is the risk of deadlocks in PostgreSQL. When a user does something in our system, we may spawn several processes that need to update the record in different ways. In the past the more we've used transactions the more we've had deadlock errors. It's been a couple of years since we went down that path, so maybe more recent versions of PostgreSQL handle deadlock issues better. We tried going one further and locking the record, but then we started getting timeouts on the user side as web processes compete with background jobs for locks.
Is there any systematic way of handling jobs that gracefully copes with these issues? Do I just need to accept the distributed N+1s and layer in more caching to deal with it? Given the fact that we need to use the database to ensure idempotency, it makes me wonder if we should instead be using delayed_job with active_record, since that handles its own locking internally.
This is a really complicated/loaded question, as the architecture really depends on more factors than can be concisely described in simple question/answer formats. However, I can give a general recommendation.
Separate Processing From Delivery
start a transaction, update a record, and send an email
Separate these steps out. Better to avoid doing both a DB update and email send inside a transaction, batched or not.
Do all your logic and record updates inside transactions separately from email sends. Do them individually or in bulk or perhaps even in the original web request if it's fast enough. If you save results to the DB, you can use transactions to rollback failures. If you save results as args to email send jobs, make sure processing entire batch succeeds before enqueing the batch. You have flexibility now b/c it's a pure data transform.
Enqueue email send jobs for each of those data transforms. These jobs must do little to no logic & processing! Keep them dead simple, no DB writes -- all processing should have already been done. Only pass values to an email template and send. This is critical b/c this external effect can't be wrapped in a transaction. Making email send jobs a read-only for your system (it "writes" to email, external to your system) also gives you flexibility -- you can cache, read from replicas, etc.
By doing this, you'll separate the DB load for email processing from email sends, and they are now dealt with separately. Bugs in your email processing won't affect email sends. Email send failures won't affect email processing.
Regarding Row Locking & Deadlocks
There shouldn't be any need to lock rows at all anymore -- the transaction around processing is enough to let the DB engine handle it. There also shouldn't be any deadlocks, since no two jobs are reading and writing the same rows.
Response: Jobs that die in the middle
Say the job is killed just after the transaction completes but before the emails go out.
I've reduced the possibility of that happening as much as possible by processing in a transaction separately from email sending, and making email sending as dead simple as possible. Once the transaction commits, there is no more processing to be done, and the only things left to fail are systems generally outside your control (Redis, Sidekiq, the DB, your hosting service, the internet connection, etc).
Response: Duplicate jobs
Two copies of the same job might get pulled off the queue, both checking some flag before it has been set to "processing"
You're using Sidekiq and not writing your own async job system, so you need to consider job system failures out of your scope. What remains are your job performance characteristics and job system configurations. If you're getting duplicate jobs, my guess is your jobs are taking longer to complete than the configured job timeout. Your job is taking so long that Sidekiq thinks it died (since it hasn't reported back success/fail yet), and then spawns another attempt. Speed up or break up the job so it will succeed or fail within the configured timeout, and this will stop happening (99.99% of the time).
Unlike web requests, there's no human on the other side that will decide whether or not to retry in an async job system. This is why your job performance profile needs to be predictable. Once a system gets large enough, I'd expect completely separate job queues and workers based on differences like:
expected job run time
expected job CPU/mem/disk usage
expected job DB or other I/O usage
job read only? write only? both?
jobs hitting external services
jobs users are actively waiting on
This is a super interesting question but I'm afraid it's nearly impossible to give a "one size fits all" kind of answer that is anything but rather generic. What I can try to answer is your question of individual jobs vs. all jobs at once vs. batching.
In my experience, generally the approach of having a scheduling job that then schedules individual jobs tends to work best. So in a full-blown system I have a schedule defined in clockwork where I schedule a scheduling job which then schedules the individual jobs:
# in config/clock.rb
every(1.day, 'user.usage_report', at: '00:00') do
UserUsageReportSchedulerJob.perform_now
end
# in app/jobs/user_usage_report_scheduler_job.rb
class UserUsageReportSchedulerJob < ApplicationJob
def perform
# need_usage_report is a scope to determine the list of users who need a report.
# This could, of course, also be "all".
User.need_usage_report.each(&UserUsageReportJob.method(:perform_later))
end
end
# in app/jobs/user_usage_report_job.rb
class UserUsageReportJob < ApplicationJob
def perform(user)
# the actual report generation
end
end
If you're worried about concurrency here, tweak Sidekiq's concurrency settings and potentially the connection settings of your PostgreSQL server to allow for the desired level of concurrency. I can say that I've had projects where we've had schedulers that scheduled tens of thousands of individual (small) jobs which Sidekiq then happily took in in batches of 10 or 20 on a low priority queue and processed over a couple of hours with no issues whatsoever for Sidekiq itself, the server, the database etc.
I found a question that explains how Play Framework's await() mechanism works in 1.2. Essentially if you need to do something that will block for a measurable amount of time (e.g. make a slow external http request), you can suspend your request and free up that worker to work on a different request while it blocks. I am guessing once your blocking operation is finished, your request gets rescheduled for continued processing. This is different than scheduling the work on a background processor and then having the browser poll for completion, I want to block the browser but not the worker process.
Regardless of whether or not my assumptions about Play are true to the letter, is there a technique for doing this in a Rails application? I guess one could consider this a form of long polling, but I didn't find much advice on that subject other than "use node".
I had a similar question about long requests that blocks workers to take other requests. It's a problem with all the web applications. Even Node.js may not be able to solve the problem of consuming too much time on a worker, or could simply run out of memory.
A web application I worked on has a web interface that sends request to Rails REST API, then the Rails controller has to request a Node REST API that runs heavy time consuming task to get some data back. A request from Rails to Node.js could take 2-3 minutes.
We are still trying to find different approaches, but maybe the following could work for you or you can adapt some of the ideas, I would love to get some feedbacks too:
Frontend make a request to Rails API with a generated identifier [A] within the same session. (this identifier helps to identify previous request from the same user session).
Rails API proxies the frontend request and the identifier [A] to the Node.js service
Node.js service add this job to a queue system(e.g. RabbitMQ, or Redis), the message contains the identifier [A]. (Here you should think about based on your own scenario, also assuming a system will consume the queue job and save the results)
If the same request send again, depending on the requirement, you can either kill the current job with the same identifier[A] and schedule/queue the lastest request, or ignore the latest request waiting for the first one to complete, or other decision fits your business requirement.
The Front-end can send interval REST request to check if the data processing with identifier [A] has completed or not, then these requests are lightweight and fast.
Once Node.js completes the job, you can either use the message subscription system or waiting for the next coming check status Request and return the result to the frontend.
You can also use a load balancer, e.g. Amazon load balancer, Haproxy. 37signals has a blog post and video about using Haproxy to off loading some long running requests that does not block shorter ones.
Github uses similar strategy to handle long requests for generating commits/contribution visualisation. They also set a limit of pulling time. If the time is too long, Github display a message saying it's too long and it has been cancelled.
YouTube has a nice message for longer queued tasks: "This is taking longer than expected. Your video has been queued and will be processed as soon as possible."
I think this is just one solution. You can also take a look EventMachine gem, that helps to improve the performance, handler parallel or async request.
Since this kind of problem may involve one or more services. Think about possibility of improving performance between those services(e.g. database, network, message protocol etc..), if caching may help, try out caching frequent requests, or pre-calculate results.
Should verification emails be processed / sent in the background using some kind of background job (Resque, Delayed_Job, etc)?
The app does have a tendency to hang until a verification email is sent. However, it only hangs for a split second or two. Don't know if its worth sending over as a background task.
What are your thoughts?
The main thing to keep in mind is that this delay isn't affecting just the current user, it's also affecting other users because it's holding up the Rails process (the exact effects of this will depend on the web server you're using and your setup).
If this application doesn't have a lot of users (and won't in the future) and you don't have any background job processing at the moment, then it may not be worth adding it. Otherwise it's probably a good idea.
If it's a few split seconds then I think that you can live with it. But if it takes longer then you may move it to delayed_job unless your delayed_job is always clogged and you are too much into background jobs. In that case it may take a bit longer to send the email to user and may ruin user experience. Even in that case you could set a higher priority for sending the verification emails.
I have no doubts about the capability of delayed_job and have been personally using it for quite some time now. Even Github recommends it while releasing resque:
https://github.com/blog/542-introducing-resque
We need a background job system as serious as our web framework. I highly recommend DelayedJob to anyone whose site is not 50% background work.
I'm working on a Rails application that periodically needs to perform large numbers of IO-bound operations. These operations can be performed asynchronously. For example, once per day, for each user, the system needs to query Salesforce.com to fetch the user's current list of accounts (companies) that he's tracking. This results in huge numbers (potentially > 100k) of small queries.
Our current approach is to use ActiveMQ with ActiveMessaging. Each of our users is pushed onto a queue as a different message. Then, the consumer pulls the user off the queue, queries Salesforce.com, and processes the results. But this approach gives us horrible performance. Within a single poller process, we can only process a single user at a time. So, the Salesforce.com queries become serialized. Unless we run literally hundreds of poller processes, we can't come anywhere close to saturating the server running poller.
We're looking at EventMachine as an alternative. It has the advantage of allowing us to kickoff large numbers of Salesforce.com queries concurrently within a single EventMachine process. So, we get great parallelism and utilization of our server.
But there are two problems with EventMachine. 1) We lose the reliable message delivery we had with ActiveMQ/ActiveMessaging. 2) We can't easily restart our EventMachine's periodically to lessen the impact of memory growth. For example, with ActiveMessaging, we have a cron job that restarts the poller once per day, and this can be done without worrying about losing any messages. But with EventMachine, if we restart the process, we could literally lose hundreds of messages that were in progress. The only way I can see around this is to build a persistance/reliable delivery layer on top of EventMachine.
Does anyone have a better approach? What's the best way to reliably execute large numbers of asynchronous IO-bound operations?
I maintain ActiveMessaging, and have been thinking about the issues of a multi-threaded poller also, though not perhaps at the same scale you guys are. I'll give you my thoughts here, but am also happy to discuss further o the active messaging list, or via email if you like.
One trick is that the poller is not the only serialized part of this. STOMP subscriptions, if you do client -> ack in order to prevent losing messages on interrupt, will only get sent a new message on a given connection when the prior message has been ack'd. Basically, you can only have one message being worked on at a time per connection.
So to keep using a broker, the trick will be to have many broker connections/subscriptions open at once. The current poller is pretty heavy for this, as it loads up a whole rails env per poller, and one poller is one connection. But there is nothing magical about the current poller, I could imagine writing a poller as an event machine client that is implemented to create new connections to the broker and get many messages at once.
In my own experiments lately, I have been thinking about using Ruby Enterprise Edition and having a master thread that forks many poller worker threads so as to get the benefit of the reduced memory footprint (much like passenger does), but I think the EM trick could work as well.
I am also an admirer of the Resque project, though I do not know that it would be any better at scaling to many workers - I think the workers might be lighter weight.
http://github.com/defunkt/resque
I've used AMQP with RabbitMQ in a way that would work for you. Since ActiveMQ implements AMQP, I imagine you can use it in a similar way. I have not used ActiveMessaging, which although it seems like an awesome package, I suspect may not be appropriate for this use case.
Here's how you could do it, using AMQP:
Have Rails process send a message saying "get info for user i".
The consumer pulls this off the message queue, making sure to specify that the message requires an 'ack' to be permanently removed from the queue. This means that if the message is not acknowledged as processed, it is returned to the queue for another worker eventually.
The worker then spins off the message into the thousands of small requests to SalesForce.
When all of these requests have successfully returned, another callback should be fired to ack the original message and return a "summary message" that has all the info germane to the original request. The key is using a message queue that lets you acknowledge successful processing of a given message, and making sure to do so only when relevant processing is complete.
Another worker pulls that message off the queue and performs whatever synchronous work is appropriate. Since all the latency-inducing bits have already performed, I imagine this should be fine.
If you're using (C)Ruby, try to never combine synchronous and asynchronous stuff in a single process. A process should either do everything via Eventmachine, with no code blocking, or only talk to an Eventmachine process via a message queue.
Also, writing asynchronous code is incredibly useful, but also difficult to write, difficult to test, and bug-prone. Be careful. Investigate using another language or tool if appropriate.
also checkout "cramp" and "beanstalk"
Someone sent me the following link: http://github.com/mperham/evented/tree/master/qanat/. This is a system that's somewhat similar to ActiveMessaging except that it is built on top of EventMachine. It's almost exactly what we need. The only problem is that it seems to only work with Amazon's queue, not ActiveMQ.
I'm trying to set a reminder in a system to fire at a certain time.
This is a web based app, so it's not like it will be in memory all the time.
Ideally I'd like to avoid using a service or job on the server(mainly out of curiosity, to see if there is a more efficient way to do it)
For example, imagine how many Ebay bids are constantly ending all the times, and emails being sent out seemingly perfectly in time.
Do people recon there is just a big loop going over and over, moving items into a queue etc... Or is there something lower level helping out (stored procedures, triggers etc)
Thanks everyone.
What you have to realize about eBay - and most large database-backed websites - is that the interactions between humans and the database that come through the web server are only a part (sometimes a very small part) of the functionality of the system.
To use eBay as an example, the email that goes out when auctions expire is not handled by a web server. They are far more likely to have that scripted. In other words, there is another program running on a number of their systems that look at the database for ended auctions, do some processing on them, send emails, etc.
If I were doing something similar (albeit on a much smaller scale,) I'd have my web services built in the usual way, but have a job that is run automatically every few minutes to do the maintenance work. It would start up, look at the database for work, process anything that was required, then exit.