I have a durable functions app, running on a premium elastic service plan in Azure, wherein I
(a) perform a one off task that returns a potentially large number of results
(b) run some independent processing on each result from part (a)
Part (a) relies on an external database, which starts rejecting requests when I hit a certain number of concurrent requests.
Part (b) doesn't have such a third party dependency, and should theoretically be able to scale indefinitely.
I'm aware of the ability to place limits on:
The maxmimum number of instances my service plan will scale out to
The number of concurrent requests per-instance
The number of concurrent activity functions
However, using any of these options to limit (a) would also limit (b), which I'd like to leave as concurrent as possible.
Is there a way I can limit the number of concurrent invocations of activity function (a), without placing restrictions on the number of invocations of (b)?
(If all else fails I can track the number of current executions myself in storage as part of running activity (a), but I'd much prefer to either configure this, or be able to drive it from the durable functions framework, if possible - as it is already tracking the number of queued activity functions of each type.)
Is there a way I can limit the number of concurrent invocations of activity function (a), without placing restrictions on the number of invocations of (b)?
Yes, there are plenty of tools in Azure which will allow you to build publish / subscribe segregation of (a) and (b). Perhaps the mistake here is to think that the results from (a) need to be processed in-process / synchronously with the consumer which sinks / processes these results.
i.e. If there is a good chance that (b) cannot keep up with the messages retrieved from (a), then I would consider separating the task of obtaining data from (a) from the task of processing the data in (b) via a queue or log technology.
Concentrating on (b):
If (b) requires command or transaction semantics (i.e. exactly once, guaranteed), then Azure Service Bus can be used to queue commands until they can be processed, and consumers of messages can be scaled independently of the production of messages in (a), using subscriptions. Think RabbitMQ.
If (b) can handle less reliable guarantees, e.g. at-least-once semantics, then Azure Event Hubs will allow you to partition messages across multiple concurrent consumers. Think Kafka.
Other alternatives exist too, e.g. Storage queues (low cost) and Event grids (wide number of subscriber protocols).
So TL;DR, buffer the accumulation of data from the processing, if you fear that there are throughput disparities between your ability to acquire and process the data.
Interestingly enough, if the process delivering to (a) IS a queue in itself, then you need only concern yourself with the performance of (b). The golden rule of queues is to leave data on the queue if you do not have the capacity to process it (since you will just need to buffer it, again).
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I am planning to introduce Hazelcast IQueue as a distributed task queue for the project I am working on.
There are multiple stateless services will consume tasks from such queue, a question come to my mind is ---- load balancing issue.
How can I guarantee the load is relative balanced, while those stateless services consume tasks from the queue?
Queues provide fairness.
Assuming the code is a loop of IQueue.take() and process, then if take() requests come from instance A, instance B, instance A and instance A again, then instance A gets 3 messages and B gets 1.
If this is due to the messages requiring different amounts of time to process, then with sufficient messages this should even out. If instance 1 is on faster hardware than instance 2 then it won't.
If you need the instances to follow a round-robin approach then you'd need to implement this yourself.
You could use something like IAtomicLong to record whose turn is next. All processing stops if the instance whose turn is next goes offline, which might not be what you want. The CP Subsystem provides a fuller featured approach that you can use for scheduling.
I have been researching how to efficiently solve the following use case and I am struggling to find the best solution.
Basically I have a Node.js REST API which handles requests for users from a mobile application. We want some requests to launch background tasks outside of the req/res flow because they are CPU intensive or might just take a while to execute. We are trying to implement or use any existing frameworks which are able to handle different job queues in the following way (or at least compatible with the use case):
Every user has their own set job queues (there are different kind of jobs).
The jobs within one specific queue have to be executed sequentially and only one job at a time but everything else can be executed in parallel (it would be preferable if there are no queues hogging the workers or whatever is actually consuming the tasks so all queues get more or less the same priority).
Some queues might fill up with hundreds of tasks at a given time but most likely they will be empty a lot of the time.
Queues need to be persistent.
We currently have a solution with RabbitMQ with one queue for every kind of task which all the users share. The users dump tasks into the same queues which results in them filling up with tasks from a specific user for a long time and having the rest of users wait for those tasks to be done before their own start to be consumed. We have looked into priority queues but we don't think that's the way to go for our own use case.
The first somewhat logical solution we thought of is to create temporary queues whenever a user needs to run background jobs and have them be deleted when empty. Nevertheless we are not sure if having that many queues is scalable and we are also struggling with dynamically creating RabbitMQ queues, exchanges, etc (we have even read somewhere that it might be an anti-pattern?).
We have been doing some more research and maybe the way to go would be with other stuff such as Kafka or Redis based stuff like BullMQ or similar.
What would you recommend?
If you're on AWS, have you considered SQS? There is no limit on number of standard queues created, and in flight messages can reach up to 120k. This would seem to satisfy your requirements above.
While the mentioned SQS solution did prove to be very scalable our amount of polling we would need to do or use of SNS did not make the solution optimal. On the other hand implementing a self made solution via database polling was too much for our use case and we did not have the time or computational resources to consider a new database in our stack.
Luckily, we ended up finding that the Pro version of BullMQ does have a "Group" functionality which performs a round robin strategy for different tasks within a single queue. This ended up adjusting perfectly to our use case and is what we ended up using.
I have a service bus trigger function that when receiving a message from the queue will do a simple db call, and then send out emails/sms. Can I run > 1000 calls in my service bus queue to trigger a function to run simultaneously without the run time being affected?
My concern is that I queue up 1000+ messages to trigger my function all at the same time, say 5:00PM to send out emails/sms. If they end up running later because there is so many running threads the users receiving the emails/sms don't get them until 1 hour after the designated time!
Is this a concern and if so is there a remedy?
FYI - I know I can make the function run asynchronously, would that make any difference in this scenario?
1000 messages is not a big number. If your e-mail/sms service can handle them fast, the whole batch will be gone relatively quickly. Few things to know though:
Functions won't scale to 1000 parallel executions in this case. They will start with 1 instance doing ~16 parallel calls at the same time, and then observe how fast the processing goes, then maybe add a second instance, wait again etc.
The exact scaling behavior is not publicly described and can change over time. Thus, YMMV, and you need to test against your specific scenario.
Yes, make the functions async whenever you can. I don't expect a huge boost in processing speed just because of that, but it certainly won't hurt.
Bottom line: your scenario doesn't sound like a problem for Functions, but if you need very short latency, you'll have to run a test before relying on it.
I'm assuming you are talking about an Azure Service Bus Binding to an Azure Function. There should be no issue with >1000 Azure Functions firing at the same time. They are a Serverless runtime and should be able to scale greatly if you are running under a consumption model. If you are running the functions in a service plan, you may be limited by the service plan.
In your scenario you are probably more likely to overwhelm the downstream dependencies: the database and SMS sending system, before you overwhelm the Azure Functions infrastructure.
The best thing to do is to do some load testing, and monitor the exceptions coming out of the connections to the database and SMS systems.
I'm running a Windows Azure web role which, on most days, receives very low traffic, but there are some (foreseeable) events which can lead to a high amount of background work which has to be done. The background work consists of many database calls (Azure SQL) and HTTP calls to external web services, so it is not really CPU-intensive, but it requires a lot of threads which are waiting for the database or the web service to answer. The background work is triggered by a normal HTTP request to the web role.
I see two options to orchestrate this, and I'm not sure which one is better.
Option 1, Threads: When the request for the background work comes in, the web role starts as many threads as necessary (or queues the individual work items to the thread pool). In this option, I would configure a larger instance during the heavy workload, because these threads could require a lot of memory.
Option 2, Self-Invoking: When the request for the background work comes in, the web role which receives it generates a HTTP request to itself for every item of background work. In this option, I could configure several web role instances, because the load balancer of Windows Azure balances the HTTP requests across the instances.
Option 1 is somewhat more straightforward, but it has the disadvantage that only one instance can process the background work. If I want more than one Azure instance to participate in the background work, I don't see any other option than sending HTTP requests from the role to itself, so that the load balancer can delegate some of the work to the other instances.
Maybe there are other options?
EDIT: Some more thoughts about option 2: When the request for the background work comes in, the instance that receives it would save the work to be done in some kind of queue (either Windows Azure Queues or some SQL table which works as a task queue). Then, it would generate a lot of HTTP requests to itself, so that the load balancer 'activates' all of the role instances. Each instance then dequeues a task from the queue and performs the task, then fetches the next task etc. until all tasks are done. It's like occasionally using the web role as a worker role.
I'm aware this approach has a smelly air (abusing web roles as worker roles, HTTP requests to the same web role), but I don't see the real disadvantages.
EDIT 2: I see that I should have elaborated a little bit more about the exact circumstances of the app:
The app needs to do some small tasks all the time. These tasks usually don't take more than 1-10 seconds, and they don't require a lot of CPU work. On normal days, we have only 50-100 tasks to be done, but on 'special days' (New Year is one of them), they could go into several 10'000 tasks which have to be done inside of a 1-2 hour window. The tasks are done in a web role, and we have a Cron Job which initiates the tasks every minute. So, every minute the web role receives a request to process new tasks, so it checks which tasks have to be processed, adds them to some sort of queue (currently it's an SQL table with an UPDATE with OUTPUT INSERTED, but we intend to switch to Azure Queues sometime). Currently, the same instance processes the tasks immediately after queueing them, but this won't scale, since the serial processing of several 10'000 tasks takes too long. That's the reason why we're looking for a mechanism to broadcast the event "tasks are available" from the initial instance to the others.
Have you considered using Queues for distribution of work? You can put the "tasks" which needs to be processed in queue and then distribute the work to many worker processes.
The problem I see with approach 1 is that I see this as a "Scale Up" pattern and not "Scale Out" pattern. By deploying many small VM instances instead of one large instance will give you more scalability + availability IMHO. Furthermore you mentioned that your jobs are not CPU intensive. If you consider X-Small instance, for the cost of 1 Small instance ($0.12 / hour), you can deploy 6 X-Small instances ($0.02 / hour) and likewise for the cost of 1 Large instance ($0.48) you could deploy 24 X-Small instances.
Furthermore it's easy to scale in case of a "Scale Out" pattern as you just add or remove instances. In case of "Scale Up" (or "Scale Down") pattern since you're changing the VM Size, you would end up redeploying the package.
Sorry, if I went a bit tangential :) Hope this helps.
I agree with Gaurav and others to consider one of the Azure Queue options. This is really a convenient pattern for cleanly separating concerns while also smoothing out the load.
This basic Queue-Centric Workflow (QCW) pattern has the work request placed on a queue in the handling of the Web Role's HTTP request (the mechanism that triggers the work, apparently done via a cron job that invokes wget). Then the IIS web server in the Web Role goes on doing what it does best: handling HTTP requests. It does not require any support from a load balancer.
The Web Role needs to accept requests as fast as they come (then enqueues a message for each), but the dequeue part is a pull so the load can easily be tuned for available capacity (or capacity tuned for the load! this is the cloud!). You can choose to handle these one at a time, two at a time, or N at a time: whatever your testing (sizing exercise) tells you is the right fit for the size VM you deploy.
As you probably also are aware, the RoleEntryPoint::Run method on the Web Role can also be implemented to do work continually. The default implementation on the Web Role essentially just sleeps forever, but you could implement an infinite loop to query the queue to remove work and process it (and don't forget to Sleep whenever no messages are available from the queue! failure to do so will cause a money leak and may get you throttled). As Gaurav mentions, there are some other considerations in robustly implementing this QCW pattern (what happens if my node fails, or if there's a bad ("poison") message, bug in my code, etc.), but your use case does not seem overly concerned with this since the next kick from the cron job apparently would account for any (rare, but possible) failures in the infrastructure and perhaps assumes no fatal bugs (so you can't get stuck with poison messages), etc.
Decoupling placing items on the queue from processing items from the queue is really a logical design point. By this I mean you could change this at any time and move the processing side (the code pulling from the queue) to another application tier (a service tier) rather easily without breaking any part of the essential design. This gives a lot of flexibility. You could even run everything on a single Web Role node (or two if you need the SLA - not sure you do based on some of your comments) most of the time (two-tier), then go three-tier as needed by adding a bunch of processing VMs, such as for the New Year.
The number of processing nodes could also be adjusted dynamically based on signals from the environment - for example, if the queue length is growing or above some threshold, add more processing nodes. This is the cloud and this machinery can be fully automated.
Now getting more speculative since I don't really know much about your app...
By using the Run method mentioned earlier, you might be able to eliminate the cron job as well and do that work in that infinite loop; this depends on complexity of cron scheduling of course. Or you could also possibly even eliminate the entire Web tier (the Web Role) by having your cron job place work request items directly on the queue (perhaps using one of the SDKs). You still need code to process the requests, which could of course still be your Web Role, but at that point could just as easily use a Worker Role.
[Adding as a separate answer to avoid SO telling me to switch to chat mode + bypass comments length limitation] & thinking out loud :)
I see your point. Basically through HTTP request, you're kind of broadcasting the availability of a new task to be processed to other instances.
So if I understand correctly, when an instance receives request for the task to be processed, it pushes that request in some kind of queue (like you mentioned it could either be Windows Azure Queues [personally I would actually prefer that] or SQL Azure database [Not prefer that because you would have to implement your own message locking algorithm]) and then broadcast a message to all instances that some work needs to be done. Remaining instances (or may be the instance which is broadcasting it) can then see if they're free to process that task. One instance depending on its availability can then fetch the task from the queue and start processing that task.
Assuming you used Windows Azure Queues, when an instance fetched the message, it becomes unavailable to other instances immediately for some amount of time (visibility timeout period of Azure queues) thus avoiding duplicate processing of the task. If the task is processed successfully, the instance working on that task can delete the message.
If for some reason, the task is not processed, it will automatically reappear in the queue after visibility timeout period has expired. This however leads to another problem. Since your instances look for tasks based on a trigger (generating HTTP request) rather than polling, how will you ensure that all tasks get done? Assuming you get to process just one task and one task only and it fails since you didn't get a request to process the 2nd task, the 1st task will never get processed again. Obviously it won't happen in practical situation but something you might want to think about.
Does this make sense?
i would definitely go for a scale out solution: less complex, more manageable and better in pricing. Plus you have a lesser risk on downtime in case of deployment failure (of course the mechanism of fault and upgrade domains should cover that, but nevertheless). so for that matter i completely back Gaurav on this one!
What scenario is less expensive in $$$ using Windows Azure? And is it better to separate the two tasks. E-mails are rarely sent, but chat messages are posted all the time.
Having one worker role processing e-mails gotten from the Azure Queue every 10 seconds, and one worker role processing posted chat messages from the Azure every 1 second.
Having one generic worker role that processes both e-mails sending and chat messages every 1 second.
Worker roles are most efficient when run at or near to full CPU capacity- you are, after all, paying by CPU hour for them. A useful way to achieve this is to combine worker roles such that all of your background jobs end up being performed in a single role.
A great way to run single worker role type architectures is to use some sort of generic worker role pattern- basically a plugin pattern whereby the worker role reads a message off the queue and uses some metadata encoded into the message (or the name of the queu) to determine the type of processing it requires. It will then go to blob storage to retrieve the .NET assembly to perform that type of processing, instantiate this into a new appdomain and marshall the context into that assembly for processing.
This is covered in the Asynchronous Workloads session in the WIndows Azure Platform Training Kit. This also contains a hands on lab that guides you through a sample implementation of one of these types of approaches.
The folks from Lokad have a really elegant implementation including all the polish and administration mechanisms that you'd need if you did this properly. Their implementation is New BSD licensed and was the winner of the MSFT Azure Partner of the YEar award last year. It's an essential part of almost every Azure project that I build. Highly recommended and trivial to integrate. http://code.google.com/p/lokad-cloud/
So in short, I prefer a generic worker role implemented as a plugin type pattern with dynamics type loading and instantiation.
This all depends on your scaling strategy and how many instances you're going to need to run to handle your load.
If you're planning to take advantage of supported SLA (99.999 uptime) you will need at least 2 instances for every role.
Thus, if you split them up, you will need at least 4 instances. If you keep them together, you'll need at least 2.
Processing 1 email per 10s and 1 chat message per second does not sound like a lot and I don't think you'll need more than 2 instances to handle everything.
However, if processing power gets to be lop-sided (i.e. chat messages need more computing power than email messages) and total load exceeds 4 instances, i suggest splitting them up, so that you can scale the two processes separately
You're charged based on how many hours you're running and how many CPU cores you're running. So if you spin up four small VMs all doing the same thing versus two small VMs doing one thing and two doing another, the cost is the same.