I have a requirement to send daily emails to members of the system. I would like to know if SendMail method should be in MemberService or should I create separate service class for this?
Appreciate any help.
Thanks
APL
Initially, without jumping to complexity, you may begin by placing the SendEmail method on MemberService which could have a dependency on an email service abstraction. Given that the sending of a daily email isn't a response to a domain event or even business logic handled explicitly by your domain, you can move the email sending method out of MemberService, however you still need to provide a query on a repository which returns a list of members eligible for daily emails. The sending application would likely be hosted in a scheduled process which has the sole responsibility of sending emails. As such, it isn't necessary for it to have all the other functionality associated with the member service. It only needs to pull a list of members and send emails, perhaps with a templating engine.
Related
I want to develop an Emailer microservice which provides a SendEmail command. Inside the microservice I have an aggregate in mind which represents the whole email process with the following events:
Aggregate Email:
(EmailCreated)
EmailDeliveryStarted
EmailDeliveryFailed
EmailRecipientDelivered when one of the recipients received the email
EmailRecipientDeliveryFailed when one of the recipients could not receive the email
etc.
In the background the email delivery service SendGrid is used; my microservice works like a facade for that with my own events. The incoming webhooks from SendGrid are translated to proper domain events.
The process would look like this:
Command SendEmail ==> EmailCreated
EmailCreatedHandler ==> Email.Send (to SendGrid)
Incoming webhook ==> EmailDeliveryStarted
Further webhooks ==> EmailRecipientDelivered, EmailRecipientDeliveryFailed, etc.
Of course if I'd want to replace the external webservice and it would apply other messaging strategies I would adapt to that but keep my domain model with it's events. I want to let the client not worry about the concrete email delivery strategy.
Now the crucial problem I face: I want to accept the SendEmail commands even if SendGrid is not available at that very moment, which entails storing the whole email data (with attachments) and then, with an event handler, start the sending process. On the other hand I don't want to bloat my initial EmailCreated event with this BLOB data. And I want to be able to clean up this data after SendGrid has accepted my send email request.
I could also try both sending the email to SendGrid and storing an initial EmailDeliveryStarted event in the SendEmail command. But this feels like a two-phase commit: if SendGrid accepted my call but somehow my repository was unable to store the EmailDeliveryStarted event the client would be informed that something went wrong and it tries again which would be a disaster.
So I don't know how to design my aggregate and, more important, my EmailCreated event since it should not contain the BLOB data like attachments.
I found this question interesting and it took a little bit to reflect on that.
First things first - I do not see an obligation to store the email attachments in the event. You can simply store the fully qualified name of the files attached. That would keep the event log smaller and perhaps rule out the need for "deleting" the event (and you know that, in an event source model, you should not do that).
Secondly, assuming that the project is not building an e-mail client, I don't see a need to model an e-mail as an aggregate root. I see AggregateRoots represent business-relevant domains, not for a utility task like sending an e-mail. You could model this far more easily using a database table / document that keeps track of what has been sent and what not yet. I see sending e-mails through SendGrid as a reaction to a business event, certainly to be tracked, but not an AggregateRoot in its own right.
Lastly, if you want to accept SendEmail commands also when SendGrid is offline, the aggregate emits an EmailQueued event. The EmailQueuedHandler will produce a line on the read model of the process in charge taking all the Emails in queued state and batch them for sending. If the communication with SendGrid fails, you can either:
Do nothing, the sender process will pick the email at the next attempt
Emit a EmailSendFailed, intercepted by a Handler that will increase the retry count (if you want to stop after a number of retries).
Hope that is sufficiently clear and best of luck with your project.
We've created a web application that is an a e-book reader. So one thing to keep in mind is that the domain is not exactly that of reading a physical book. We are now trying to gather users' reading behavior by storing information about e-book pages accessed by our users. Since this information goes to a data warehouse we thought raising an event from the bookcontroller is the right way to do it.
bus.Publish()
But we are not sure if it should be a publish or a send since there is really only one consumer to this event and that is our business intelligence team. We've also read that it is not advisable to publish from the web app (http://www.make-awesome.com/2010/10/why-not-publish-nservicebus-messages-from-a-web-application/). So now the alternative is to use bus.Send(RecordPageAccessedCommand)
But the above command does not change our application state in anyway. So is it truly a command? I have a feeling that the mistake we are making is using NServiebus's features (Publish,Send) and trying to equate it with what a command or event is.
Please let me know what the solution to this is.
Based on the information you provided, I would recommend "sending" to your endpoint.
Sending a command implies that the endpoint handling the message should do something. In your case, recording that the page was accessed is the thing the endpoint should do.
Publishing an event implies that you are notifying 0..n subscribers that something occurred. You could publish an event from your command handler if some other service in your system was interested in the fact that a page was accessed. The key point here is that it's not a "fact" until you've recorded it.
I've found that consumers tend to grow once data is available. Having the ability to publish an event from your command handler will make it trivial to notify new consumers without changing/risking your existing code base.
The RecordPageAccessedCommand is a command as it is commanding the system to do something, in this case, record that a page has been accessed.
If I've understood your scenario correctly. A message should be sent from your controller to the "Business intelligence Team Service" telling the system to record that a page has been accessed. This service would store this information and would be the owner/technical authority of this information.
No other services should store or require this information in its pure form, they can however subscribe to events from this service, in highly contrived scenario for example, when a user reads 1000 pages the "Business intelligence Team Service" can publish an event that a 1000 pages have been read ie Bus.Publish(), which may be handled by a billing service that gives a discount for the user on their next purchase.
The data warehouse can have access to this information stored in your "Business intelligence Team Service" as it would fall under IT/OPS.
Can any developers/architects with experience with NServiceBus offer guidance and help on the following?
We have a requirement in the business (and not a lot of money) to create a robust interface between an externally hosted application and our internal ERP's (yup, more than one).
When certain activities take place in the third party application they will send us the message. i.e. call a web service passing various fields of information in the message etc. We are not in control nor can we change this third party application.
My responsibility is creating this web service and the processing of the messages into each ERP. The third party dictates how the web service will look, but not what its responsible for. We have to accept that if they get a response back of 'success' then we at this point have taken responsibility for that message! i.e. we need to ensure as close to perfect no data loss takes place.
This is where I'm interested in the use of NServiceBus. Use it to store/accept a message at first. At this point I get lost, I can't tell what should happen, i.e. what design follows. Does another machine (process) subscribe and grab the message to process it into an ERP, if so since each ERPs integration logic differs do I make a subscriber per ERP? A message may have two destination ERP targets however, so is it best the message is sent and not subscribed to.
Obviously in the whole design, I need to have some business rules which help determine the destination ERP's and then business rules that determine what actually takes place with in each ERP. So I also have a question on BRE's but this can wait although still may be a driver for what the message has to do.
so:
Third party > web service call > store message (& return success) > determine which ERP is target > process each into ERP > mark message complete
If anything fails along the lines making sure the message does not get lost. p.s. how does MSMQ prevent loss since the whole machine may die ? is this just disk resilience etc?
Many thanks if you've read and even more for any advice.
This sounds like a perfect application for NServiceBus.
Your web service should ONLY parse the request from the third and translate it into an NServiceBus message, which it should Bus.Send(). You don't respond with a 200 status code until that message is on the Bus, at which point, you are responsible for it, and NServiceBus's built-in error/retry and error queue facilities become your best friend.
This message should be received by another endpoint, but it needs to be able to account for duplicate messages or use idempotence so that duplicates aren't a problem. If the third party hits your web service, and the message is successfully placed on the bus, but then some error prevents them from receiving the 200 response code, you will get duplicates from them.
At this point, the endpoint receiving the MessageFromWebServiceCommand message could Bus.Publish() a SomeBusinessEventHappenedEvent that contains the command data.
For each ERP, create an additional endpoint that subscribes to the SomeBusinessEventHappenedEvent and uses your business logic to decide what to do respective to that ERP. In some cases, that "something" may be "nothing". Keep idempotence in mind here too, because if the message fails it will be retried.
All the other things you're worried about (preventing loss of messages, what happened if machines die) will be taken care of thanks to NServiceBus and MSMQ being naturally resilient to such problems.
Here is a blog post, including a sample project, that shows how to receive messages from an external partner via a web service and handle them with NServiceBus, and a link straight to the sample project on GitHub:
Robust 3rd Party Integrations with NServiceBus
Project Source Code on GitHub
The scenario is a follow:
I have multiple clients in which they can register themselves on a workflow server, using WCF requests, to receive some kind of notifications. The information of the notifications will be received from an external system using another receive activity. The workflow then should get the notification information and callback all registered clients using send activity and callback correlations (the clients are exposing callback interfaces implemented in there and the end-point addresses passed initially with the registration requests). "Log-running workflow service" approach is used with a persistent storage.
Now, I'm looking for some way to correlate the incoming information of the notifications received from the external system with the persisted workflow instances created previously when the registration requests, so that all clients will be notified using end-points that already passed with the registration requests. Is WF 4.0 capable of resuming and executing multiple workflow instances when the information of the notification received without storing end-points somehow manually and go though them? If yes, how can I do that?
Also, if my approach of doing so is not correct, then please advice me about the best practice of doing such system using WCF services.
Your help is highly appreciated.
When you use request correlation with workflow services the correlation key must always match a single workflow instance, you can't have multiple workflow instances react to a single message. So you either need to multicast the message using all the different correlation keys or resume you workflow instances in some other way. That other way could be to store the request somewhere, like a SQL table, and have the workflows periodically check that location if they need to notify the client.
My WCF service(hosted as Windows Service), has some 'SendEmail' methods, which sends out emails after doing some processing.
Now, I have got another requirement where client wants to preview emails before they are being sent out, so my WCF service needs to return whole email object to calling web app.
If client is happy with emails object, they can simply click 'Send out' which will then again call WCF service to send the emails.
Because at times it can take a bit longer for emails object processingy, I do not want calling application to wait until emails object is ready.
Can anyone please guide what changes I need to make to my WCF service (which currently has all one way operation)?
Also, please guide me whether I need to go for Asynch operation or message queuing or may be a duplex contract?
Thank you!
Based on your description I think you will have to:
Change current operation from sending email to storing email (probably in database).
Add additional operation for retrieving prepared emails for current user
Add additional method to confirm sending one or more emails and removing them from storage.
The process will be:
User will trigger some http request which will result in calling your WCF service for processing (first operation)
WCF service will initiate some processing (asynchronously or firt operation will be one-way so that client doesn't have to wait).
Processing will save email somehow
Depend on duration of processing you can either use AJAX to poll WebApp which will in turn poll WCF service for prepared emails or you will create separate page which will user have to access to see prepared emails. Both methods are using second operation.
User will check prepared email(s) and trigger http request which will result in calling third operation to send those emails.
You have multiple options:
Use Ladislav's approach. Only to add that service returns a token and then client uses the token to poll until a time out or a successful response. Also server keeps these temp emails for a while and after a timeout purges them.
Use duplex communication so that server also gets a way to callback the client and does so when it has finished processing. But don't do this - and here is my view why not.
Use an Asynchronous approach. You can find nice info here.