I know its possible to signal JBPM/BPMS externally to start a work flow. You use the signal start event, and I have that working fine.
What I'm wondering is what happens to signals that do not have a corresponding signal ref.
I am capturing certain events and sending them to JBPM and I want to have a catch all workflow or rule that if there is no corresponding workflow to start so that these event's can still be processed.
Any ideas would be much appreciated.
What are you planning to do with the events that hasn't been catch by any process?
Maybe you can define some rules to do a smarter routing of your events. It depends on how complex is your logic to decide what to do.
Related
Suppose, I have an api POST /order which invokes PlaceOrder lambda and expects response from this. PlaceOrder lambda does some works, invokes another lambda ProcessPayment lambda and expects response. Also, ProcessPayment invokes CreateInvoice lambda expecting response. Whole architecture is like a RequestResponse cycle. I woulde like to achieve that without lambda invoking another lambda as it is considered as anti-pattern. My question is what is the best design pattern to achieve this behavior within 29 seconds with event-driven architecture.
What AWS suggests: As per this official documentation, they suggests to use SQS. But regarding using SQS, I have some thoughts.
My thoughts:
At event sources architecture, I can orchestrate these lambdas with SQS, SNS etc other event sources, but in that case, the nature would not be synchronous and thus I would not get response from the api.
My other solution:
Using Step Function: I can orchestrate this workflow with step function, and I think it is more elegant solution in this synchronous calling case. But I would like to achieve
this via event sources.
How can I design this scenerio with best practices using event-based achitecture?
In an Event-Driven Architecture, the communication between producers and consumers is asynchronous by design, that's the way the architecture scales.
You can get nearly synchronous communication between 2 services in an EDA, by creating dedicated queues / channels to communicate between them, make sure they're scaled up to a level where the latency is acceptable (close to synchronous values).
This adds some complexity, because the services which need responses, have to wait in a hot-loop to get them as soon as possible, and also if messages are lost, you need to have retry policies, etc.
I think you need to focus more on the mechanics of your program and a bit less on design patterns. You need to use the design patterns that fit your use-case, the other way around will not work. In the end, you build a program to fulfill a certain task or set of tasks, so that should be your end goal.
You’re stating that you have a process order Lambda, a create invoice Lambda and a process payment Lambda. I’d say the most interesting question is what you need to get done before you return a response to the user. Maybe you can process the order, respond to the user that it is done and handle the invoicing and payments on a later moment. Typically that would mean you put a message in a SQS queue or on an SNS topic.
It could be that you need your payment to be processed before you respond to the user, because they need to be informed about the status of the payment. You could then combine both actions in a single Lambda, because there is no way to spit the two tasks from one another. Keep in mind that often another option exist where you process the order first, put a message in a queue for the process payment (as it typically is a process that involves a third party) and the front end will poll for an update on the payment status. This way you can return a response quickly and still give an update on the payment as soon as possible.
The create invoice process is typically something you would never want to synchronously invoke during order confirmation. What if your invoicing application (intern or extern) is down? Theoretically you could still process orders as long as you create the invoice at some later moment in time. If you couple everything together you make order confirmation dependent on your invoice creation process, which I would regard as an unnecessary dependency.
I would really advice against step functions for this use-case. They can be utilized for long running processes that need to keep state and ‘wake up’ at specific moments, but for this specific flow I would say they do not help and are unnecessarily complex. If you have 3 things you need to do that you cannot separate from
one another, just run them in the same Lambda.
I am new to Axon framework When I am going through it I learnt that previous events will be replayed when aggregate class is loaded. I want to know what is the necessity of replay and its side effects
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Your events is always your source of truth.
To be able to perform events on top of that aggregate, you (potentially) have a bunch of validations to be done. In order to do that, you need to know the 'current state' of your Aggregate. How to accomplish that? Replaying the events and building the current Aggregate.
You can get way more concepts on Axon Ref Guide.
You can also read about Event Sourcing!
I am not sure what you want to know about side-effects and maybe you can expand a bit more on that.
I’m a beginner at Camunda/BPMN and I want to use it to control what is going on in nodejs, mostly likely using a REST API, at least for now. (Unless folks have a better idea for how nodejs should talk to Camunda.) My goal is to deliver systems where non-programmers can update the business logic in very practical ways.
I'd like to trigger the start of perhaps more-than-one process by sending a REST message, say to reflect that "a new insurance policy has been sold" and that might trigger the instantiation of say 2 processes on Monday but perhaps on Tuesday we add a third and now the same REST API call should now trigger more activity on Wednesday. (I figure it is better for nodejs to know about events but not about the process definitions. After all, my goal is to use Camunda as a sort of business logic server for my application. The less the nodejs code needs to know, the better.)
Which REST API should I be using to express the message that, say "a new insurance policy has been sold"? When I look at:
https://docs.camunda.org/manual/develop/reference/rest/signal/post-signal/
I find it very confusing. What should "name" match in the biz process definitions? I assume I don't need an executionId? I assume I can leave out tenantId?
Would some string in the message match the ID of a start event in one or more process definitions (or what has to match what)?
When I look at a process, is there an easy way to tell what variables I need to supply to start that process running?
Should I perhaps avoid using this event-oriented style of kicking off processes and just use the POST /process-definition/key/{key}/start? It would seem to me to be better form to trigger activity with events or signals or something like that rather than to have my nodejs code know about the specific process definition by name.
Should I be using events or signals in this case?
I gather that the start event should not be a "None Start Event" but I'm not clear on what type of start event TO use if I want automatic triggering based on events or signals or something? Would a "Non-interrupting - Message Start Event" be the right sort? I'm finding this confusing.
Once I have triggered the process to start, what does nodejs need to send to step the process forward from one task in that instance to the next?
Thanks!
In order to instantiate a new workflow instance you have the following possibilities:
Start exactly one instance:
Start a workflow instance by its known "key": https://docs.camunda.org/manual/develop/reference/rest/process-definition/post-start-process-instance/
Start a workflow by a message start event: https://docs.camunda.org/manual/develop/reference/rest/message/post-message/. A message can only start one specific workflow instance, it is not allowed that this is not a unique relationship. The message start event is the one you have to use in your BPMN process model. See also https://docs.camunda.org/manual/develop/reference/bpmn20/events/message-events/. This might indeed be the better approach to make your client independent of the process definition key.
Start multiple instances:
- Start a workflow instance by a BPMN signal event: https://docs.camunda.org/manual/develop/reference/rest/signal/post-signal/. The signal name could start many instances as once.
The name of the message or name of signal would be configured in the BPMN model. Both could work for your use case.
Once a process instance is started it will move automatically execute the next steps.
Probably following this example (https://blog.bernd-ruecker.com/use-camunda-without-touching-java-and-get-an-easy-to-use-rest-based-orchestration-and-workflow-7bdf25ac198e) step by step can give you some better idea?
I am implementing a Reactor design pattern, using a single thread, for asynchronous operations using Windows Events Mechanism.
I faced a problem while trying to combine my reactor to support Windows Notifications (WM_CLOSE, WM_CREATE, WM_DEVICECHANGE...) along with the existing Windows Events.
Thus, my question is:
Is it possible to signal an event when a particular window receives a particular notification?
Thanks in advance.
No, you cannot make Windows signal an event object when particular window messages are received. You would have to catch the messages in your message loop first and then signal the event object yourself as needed.
Otherwise, re-write your message loop to use MsgWaitForMultipleObjects() so it can check for event signals and pending window messages at the same time, and then you can act according to whichever one satisfies the wait on each loop iteration. Just be aware of this gotcha:
MsgWaitForMultipleObjects is a very tricky API
if you specify bWaitAll as true, you may find that your application doesn’t wake up when you expected it to
In this situation, you would set bWaitAll to false and all is well.
This should be easy to follow, but after some reading I still can find an answer.
So, say that the user needs to change his mobile number, to accomplished that, we might have a command as: ChangedUserMobileNumber
holding the new number. The domain responsible for handling the command will perform the change in the aggregate and publish an event: UserMobilePhoneChanged
There is a subscriber for that event in another domain, which also holds the user mobile number in its aggregate but according to our software architect, events can not old any data so what we end up is rather stupid to say the least:
The Domain 1, receives the command to update the mobile number, the number is updated and one event is published, also, because the event cannot hold data, the command handler in the Domain 1 issues yet another command which is sent to Domain 2. The subscriber of that event lives in Domain 2 too, we then have a Saga to handle both the event and the command.
In terms of implementation we are using NServiceBus, so we have this saga to handle these message and in it we have this line of code, where the entity.IsMobilePhoneUpdated field stored in a saga entity is changed when the event is handeled.
bool isReady = (entity.IsMobilePhoneUpdated && entity.MobilePhoneNumber != null);
Effectively the Saga is started by both the command and the event raised in the Domain 1, and until this condition is met, the saga is kept alive.
If it was up to me, I would be sending the mobile number in the event itself, I just want to get a few other opinions on this.
Thanks
I'm not sure how a UserMobilePhoneChanged event could be useful in any way unless it contained the new phone number. User asks to change a number, the event shoots out that it has. Should be very simple indeed. Why does your architect say that events shouldn't contain any information?
In the first event based system i've designed events also had no data. I also did enforce that rule. At the time that sounded like a clever decision. After a while i realised that it was dumb, and i was making a lot of workarounds because of it. Also this caused a lot of querying form the event subscribers, even for trivial data. I had no problem changing this "rule" after i realised i'm doing it wrong.
Events should have all the data required to make them meaningful. Also they should only have the data that makes sense for that event. ( No point in having the user address in a ChangePhoneNumber message )
If your architect imposes such a restriction, it's not going to be easy to develop a CQRS system. How are the read models updated? Since the events have no data then you either query something to get the data ( the write side ? ) of find some way of sending a command to the read model ( then what's the point of publishing events? ). To fix your problem you should try to have a professional discussion with this architect, preferably including other tech heads and without offending anybody try to get him to relax this constraint.
On argument you could use is Event Sourcing. Event Sourcing is complementary to CQRS and would not make sense without events that have data. Even more when using event sourcing, the only data you have is the data stored in the events. Even if you don't actually implement event sourcing you can use it's existence as a reason for events to have data.
There is little point in finding a technical solution to a people problem.