would really appreciate your help, in terms of the implementation details, as well as the 2 clarifying questions I have below:
Context:
Creating an offers promotion workflow. The offer has an expiration date (We start counting down after the offer is accepted.)
User can choose to decline offer (workflow then stops)
Once offer is accepted, they will have 7 days to try and redeem cashback points. Once they meet the cashback points requirement, we will credit the the points to their account
1st question: is the below logic correct? and That I am using signals.
2nd question:, I am listening for signals in the parent workflow on say 'parentSignal' channel. Listening for "ACCEPTED", "REJECTED", "CANCELLED".
In the child workflow I am listening for "10%CASHBACK", "50%CASHBACK" "100%CASHBACK" as well as "CANCELLED" as the admin can cancel whenever he/she wants to. IS THIS THE CORRECT WAY TO TRIGGER WORKFLOWS?
How I'm thinking of writing the workflow (however, I can't seem to get it to work properly in the Cadence GUI, when I try to emit different signals)
Parent workflow (OfferWorkflow)
Listen for signal (signal received from external service)
if accept, start execute Child Workflow (cashback workflow)
if reject, end workflow
if cancelled (by admin, end workflow, cancel any cashback progress)
child workflow (cashbackWorkflow, with expiration time)
Listen for signal, once 10% of cashback requirement is met (send email)
Listen for signal, once 50% cashback is met (send email)
Listen for 100% cashback is met
// Perform credit (make call to external function)
There will be an external service that will be sending signals. The external service knows the progress. For example, if a user spends up to 10% of cashback, then we send a signal to the cadence workflow.
1st Q: Logic wise, it's correct. I don't see a problem.
2nd Q: this can work, however, you need to be careful about how to send the cancel signal to childWF. If you let external service to do it, it may be a consistent problem. If you let parentWF to do it, you need to make sure parent WF is not closed.
Overall, your design works, but it can be improved.
The main issue of your design is about using childWF. Your workflow will be much simplified, saving lots of edge cases if you don't use childWF. Conceptually, childWF is useful if your parentWF is too complicated that you want to break down, and also get a result from childWF. See here for more details about when you should use childWF: What is a good use case for a child workflow in Uber Cadence?
If not using childWF, the pseudo code becomes this:
OfferWorkflow(input)
1. init offer state(local variable object), and you can register a [query][1] handler for this object.
2. In a loop, listen for signals for operation:
2.1 accept: check state, if not accepted, then accepted, and start a timer with a future operation:
2.1.1:
when the timer fires, check state, if accepted, then end workflow(accepted->end)
2.2 reject: check state, if not accepted, then end workflow(accepted->end), if accepted, you may ignore or end workflow
2.3 cancel: end workflow(accepted->end)
2.4 10% or 59%: check state, if accepted, then send email
2.5 100% : check state, if accepted then perform credit and then change state accepted->credited.
For error handling in above, eg, 100% for a unaccepted offer, you may emit logs/metrics for monitoring.
Related
Suppose I have an aggregate containing some data and when it reaches a certain state, I'd like to take all that state and pass it to some outside service. For argument and simplicity's sake, lets just say it is an aggregate that has a list and when all items in that list are checked off, I'd like to send the entire state to some outside service. Now when I'm handling the command for checking off the last item in the list, I'll know that I'm at the end but it doesn't seem correct to send it to the outside system from the processing of the command. So given this scenario what is the recommended approach if the outside system requires all of the state of the aggregate. Should the outside system build its own copy of the data based on the aggregate events or is there some better approach?
Should the outside system build its own copy of the data based on the aggregate events.
Probably not -- it's almost never a good idea to share the responsibility of rehydrating an aggregate from its history. The service that owns the object should be responsible for rehydration.
First key idea to understand is when in the flow the call to the outside service should happen.
First, the domain model processes the command arguments, computing the update to the event history, including the ChecklistCompleted event.
The application takes that history, and saves it to the book of record
The transaction completes successfully.
At this point, the application knows that the operation was successful, but the caller doesn't. So the usual answer is to be thinking of an asynchronous operation that will do the rest of the work.
Possibility one: the application takes the history that it just saved, and uses that history to create schedule a task to rehydrate a read-only copy of the aggregate state, and then send that state to the external service.
Possibility two: you ditch the copy of the history that you have now, and fire off an asynchronous task that has enough information to load its own copy of the history from the book of record.
There are at least three ways that you might do this. First, you could have the command schedule the task as before.
Second, you could have a event handler listening for ChecklistCompleted events in the book of record, and have that handler schedule the task.
Third, you could read the ChecklistCompleted event from the book of record, and publish a representation of that event to a shared bus, and let the handler in the external service call you back for a copy of the state.
I was under the impression that one bounded context should not reach out to get state from another bounded context but rather keep local copies of the data it needed.
From my experience, the key idea is that the services shouldn't block each other -- or more specifically, a call to service B should not block when service A is unavailable. Responding to events is fundamentally non blocking; does it really matter that we respond to an asynchronously delivered event by making an asynchronous blocking call?
What this buys you, however, is independent evolution of the two services - A broadcasts an event, B reacts to the event by calling A and asking for a representation of the aggregate that B understands, A -- being backwards compatible -- delivers the requested representation.
Compare this with requiring a new release of B every time the rehydration logic in A changes.
Udi Dahan raised a challenging idea - the notion that each piece of data belongs to a singe technical authority. "Raw business data" should not be replicated between services.
A service is the technical authority for a specific business capability.
Any piece of data or rule must be owned by only one service.
So in Udi's approach, you'd start to investigate why B has any responsibility for data owned by A, and from there determine how to align that responsibility and the data into a single service. (Part of the trick: the physical view of a service can span process boundaries; in other words, a process may be composed from components that belong to more than one service).
Jeppe Cramon series on microservices is nicely sourced, and touches on many of the points above.
You should never externalise your state. Reporting on that state is a function of the read side, as it produces reports and you'll need that data to call the service. The structure of your state is plastic, and you shouldn't have an external service that relies up that structure otherwise you'll have to update both in lockstep which is a bad thing.
There is a blog that puts forward a strong argument that the process manager is the correct place to put this type of feature (calling an external service), because that's the appropriate place for orchestrating events.
I found time as the best value as event version.
I can merge perfectly independent events of different event sources on different servers whenever needed without being worry about read side event order synchronization. I know which event (from server 1) had happened before the other (from server 2) without the need for global sequential event id generator which makes all read sides to depend on it.
As long as the time is a globally ever sequential event version , different teams in companies can act as distributed event sources or event readers And everyone can always relay on the contract.
The world's simplest notification from a write side to subscribed read sides followed by a query pulling the recent changes from the underlying write side can simplify everything.
Are there any side effects I'm not aware of ?
Time is indeed increasing and you get a deterministic number, however event versioning is not only serves the purpose of preventing conflicts. We always say that when we commit a new event to the event store, we send the new event version there as well and it must match the expected version on the event store side, which must be the previous version plus exactly one. If there will be a thousand or three millions of ticks between two events - I do not really care, this does not give me the information I need. And if I have missed one event on the go is critical to know. So I would not use anything else than incremental counter, with events versioned per aggregate/stream.
In PostgreSQL, are DEFERRED triggers executed before (within) the completion of the transaction or just after it?
The documentation says:
DEFERRABLE
NOT DEFERRABLE
This controls whether the constraint can be deferred. A constraint
that is not deferrable will be checked immediately after every
command. Checking of constraints that are deferrable can be postponed
until the end of the transaction (using the SET CONSTRAINTS command).
It doesn't specify if it is still inside the transaction or out. My personal experience says that it is inside the transaction and I need it to be outside!
Are DEFERRED (or INITIALLY DEFERRED) triggers executed inside of the transaction? And if they are, how can I postpone their execution to the time when the transaction is completed?
To give you a hint what I'm after, I'm using pg_notify and RabbitMQ (PostgreSQL LISTEN Exchange) to send out messages. I process such messages in an external application. Right now I have a trigger which notifies the external app of the newly inserted records by including the record's id in the message. But in a non-deterministic way, once in a while, when I try to select a record by its id at hand, the record can not be found. That's because the transaction is not complete yet and the record is not actually added to the table. If I can only postpone the execution of the trigger for after the completion of the transaction, everything will work out.
In order to get better answers let me explain the situation even closer to the real world. The actual scenario is a little more complicated than what I explained before. The source code can be found here if anyone's interested. Becuase of reasons that I'm not gonna dig into, I have to send the notification from another database so the notification is actually sent like:
PERFORM * FROM dblink('hq','SELECT pg_notify(''' || channel || ''', ''' || payload || ''')');
Which I'm sure makes the whole situation much more complicated.
Triggers (including all sorts of deferred triggers) fire inside the transaction.
But that is not the problem here, because notifications are delivered between transactions anyway.
The manual on NOTIFY:
NOTIFY interacts with SQL transactions in some important ways.
Firstly, if a NOTIFY is executed inside a transaction, the notify
events are not delivered until and unless the transaction is
committed. This is appropriate, since if the transaction is aborted,
all the commands within it have had no effect, including NOTIFY. But
it can be disconcerting if one is expecting the notification events to
be delivered immediately. Secondly, if a listening session receives a
notification signal while it is within a transaction, the notification
event will not be delivered to its connected client until just after
the transaction is completed (either committed or aborted). Again, the
reasoning is that if a notification were delivered within a
transaction that was later aborted, one would want the notification to
be undone somehow — but the server cannot "take back" a notification
once it has sent it to the client. So notification events are only
delivered between transactions. The upshot of this is that
applications using NOTIFY for real-time signaling should try to keep
their transactions short.
Bold emphasis mine.
pg_notify() is just a convenient wrapper function for the SQL NOTIFY command.
If some rows cannot be found after a notification has been received, there must be a different cause! Go find it. Likely candidates:
Concurrent transactions interfering
Triggers doing something more or different than you think they do.
All sorts of programming errors.
Either way, like the manual suggests, keep transactions that send notifications short.
dblink
Update: Transaction control in a PROCEDURE or DO statement in Postgres 11 or later makes this a lot simpler. Just COMMIT; to (also) send waiting notifications.
Original answer (mostly for Postgres 10 or older):
PERFORM * FROM dblink('hq','SELECT pg_notify(''' || channel || ''', ''' || payload || ''')');
... which should be rewritten with format() to simplify and make the syntax secure:
PRERFORM dblink('hq', format('NOTIFY %I, %L', channel, payload));
dblink is a game-changer here, because it opens a separate transaction in the other database. This is sometimes used to fake autonomous transaction.
Does Postgres support nested or autonomous transactions?
How do I do large non-blocking updates in PostgreSQL?
dblink() waits for the remote command to finish. So the remote transaction will most probably commit first. The manual:
The function returns the row(s) produced by the query.
If you can send notification from the same transaction instead, that would be a clean solution.
Workaround for dblink
If notifications have to be sent from a different transaction, there is a workaround with dblink_send_query():
dblink_send_query sends a query to be executed asynchronously, that is, without immediately waiting for the result.
DO -- or plpgsql function
$$
BEGIN
-- do stuff
PERFORM dblink_connect ('hq', 'your_connstr_or_foreign_server_here');
PERFORM dblink_send_query('con1', format('SELECT pg_sleep(3); NOTIFY %I, %L ', 'Channel', 'payload'));
PERFORM dblink_disconnect('con1');
END
$$;
If you do this right before the end of the transaction, your local transaction gets 3 seconds (pg_sleep(3)) head start to commit. Chose an appropriate number of seconds.
There is an inherent uncertainty to this approach, since you get no error message if anything goes wrong. For a secure solution you need a different design. After successfully sending the command, chances for it to still fail are extremely slim, though. The chance that successful notifications are missed seem much higher, but that's built into your current solution already.
Safe alternative
A safer alternative would be to write to a queue table and poll it like discussed in #Bohemian's answer. This related answer demonstrates how to poll safely:
Postgres UPDATE … LIMIT 1
I'm posting this as an answer, assuming the actual problem you are trying to solve is deferring execution of an external process until after the transaction is completed (rather than the X-Y "problem" you're trying to solve using trigger Kung Fu).
Having the database tell an app to do something is a broken pattern. It's broken because:
There's no fallback if the app doesn't get the message, eg because it's down, network explodes, whatever. Even the app replying with an acknowledgment (which it can't), wouldn't fix this problem (see next point)
There's no sensible way to retry the work if the app gets the message but fails to complete it (for any of lots of reasons)
In contrast, using the database as a persistant queue, and having the app poll it for work, and take the work off the queue when work is complete, has none of the above problems.
There are lots of ways to achieve this. The one I prefer is to have some process (usually trigger on insert, update and delete) put data into a "queue" table. Have another process poll that table for work to do, and delete from the table when work is complete.
It also adds some other benefits:
The production and consumption of work is decoupled, which means you can safely kill and restart your app (which must happen from time to time, eg deploying) - the queue table will happily grow while the app is down, and will drain when the app is back up. You can even replace the app with an entirely new one
If for whatever reason you want to initiate processing of certain items, you can just manually insert rows into the queue table. I used this technique myself to initiate the processing of all items in a database that needed initialising by being put on the queue once. Importantly, I didn't need to do a perfunctory update to every row just to fire the trigger
Getting to your question, a slight delay can be introduced by adding a timestamp column to the queue table and having the poll query only select rows that are older than (say) 1 second, which gives the database time to complete its transaction
You can't overload the app. The app will read only as much work as it can handle. If your queue is growing, you need a faster app, or more apps If multiple consumers are operating, concurrency can be solved by (for example) adding a "token" column to the queue table
Queues that are backed by database tables is the basis of how persistent queues are implemented in commercial grade queue-based platforms, so the pattern is well tested, used and understood.
Leave the database to do what it does best, and the only thing it does well: Manage data. Don't try to make your database server into an app server.
I want to run a plug-in every 30 minutes, to poll an external system for changes. I am in CRM Online, so I don't have ready access to a scheduling engine.
To run the plug-in, I have a 'trigger' entity with a timezone independent date-
Updating the field also triggers a workflow, which in pseudocode has this logic:
If (Trigger_WaitUntil >= [Process-Execution Time])
{
Timeout until Trigger:WaitUntil
{
Set Trigger_WaitUntil to [Process-Execution Time] + 30 minutes
Stop Workflow with status of: Succeeded
}
}
If Trigger_WaitUntil < [Process-Execution Time])
{
Send email //Tell an admin that the recurring task has self-terminated
Stop Workflow with status of: Canceled
}
So, the behaviour I expect is that every 30 minutes, the 'WaitUntil' field gets updated (and the Plug-in and workflow get triggered again); unless the WaitUntil date is before the Execution time, in which case stop the workflow.
However, 4 hours or so later (probably 8 executions, although I haven't verified that yet) I get an infinite loop warning "This workflow job was canceled because the workflow that started it included an infinite loop. Correct the workflow logic and try again. For information about workflow".
My question is why? Do workflows have a correlation id like plug-ins, which is being carried through to the child workflow? If so, is there anyway I can prevent this, whilst maintaining the current basic mechanism of using a single trigger record to manage the schedule (I've seen other solutions in which workflows create new records, but then you've got to go round tidying up the old trigger records as well)
Yes, this behavior is well-known. The only way to implement recurring workflows without issues with infinite loops in Dynamics CRM and using only OOB features is usage of Bulk Deletion functionality. This article describes how to implement it - http://www.crmsoftwareblog.com/2012/08/using-the-bulk-deletion-process-to-schedule-recurring-workflows/
UPD: If you want to run your code every 30 mins then you will have to create 48 bulkdelete jobs with correspond startdatetime like 12:00, 12: 30, 1:00 ...
The current supported method for CRM is to use the Azure Scheduler.
Excerpt:
create a Web API application to communicate with CRM and our external
provider running on a shared (free) Azure web site and also utilize
the Azure Scheduler to manage the recurrence pattern.
The free version of the Azure Scheduler limits us to execution no more
than once an hour and a maximum of 5 jobs. If you have a lot going on
$20 a month will get you executions every minute and up to 50 jobs -
which sounds like a pretty good deal.
so if you wanted every 30 minutes, you could create two jobs, one on the half hour, and one on the hour.
The Bulk Deletion is an interesting work around and something we've used before. It creates extra work and maintenance though so I try to avoid it if possible.
I would generally recommend building a windows application and using the windows scheduling feature (I know you said you don't have a scheduler available but this is often forgotten). This approach works really well and is very easy to troubleshoot. Writing to logs and sending error email alerts is pretty easy to make it robust. The server doesn't need to be accessible externally, it only needs to reach CRM. If you had CRM on-prem, you could just use the same server.
Azure Scheduler is a great suggestion. This keeps you in the cloud which is nice.
SSIS is another option if you already have KingswaySoft or Cozy Roc in place.
You could build a workflow that creates another record and cleans up after itself; however, this is really using the wrong tool for the job. Also, it's very easy for it to fail and then not initiate the next record.
There is a solution called "Scheduled Workflow Runner". You create a FetchXML query to create a record set to run against, and point it at an on-demand workflow that you want it to run on each record.
http://alexanderdevelopment.net/post/2013/05/18/scheduling-recurring-dynamics-crm-workflows-with-fetchxml/
I have a scala client that talk to the CallFire API. I can't find anything in the documentation about having a phone number be immediately available (accept phone calls) after placing an order from the API. Here is the specific line I use: https://github.com/oGLOWo/callfire-scala-client/blob/master/src/main/scala/com/oglowo/callfire/Client.scala#L166
I need these numbers to be available when my customers purchase them. Are there any parameters that I don't know about or something that I'm doing wrong that is causing the numbers to not pick up for several minutes?
Number purchases can take several minutes to fulfil as the order is processed by the upstream number provider, which can vary according to the region and number type. As such, this is necessarily an asynchronous process.
My suggestion would be that after you create the number order, each time that it is necessary to know the status of the number you purchased, you can invoke the GetNumber operation to get status information for that number.
The most relevant field for your purposes would be the "Status" field, which indicates where in the number fulfilment process that number is. Once the status has transitioned to "Active", your number should be fully available.
Additionally, you can look at the "CallFeature" and "TextFeature" fields, in the NumberConfiguraton section of the Number resource, to see whether the number has confirmed call or text service yet, respectively.
Alternatively, you can also invoke the GetNumberOrder operation to get the status of your order. This will give you information on the status of the number order itself, but in my opinion is less useful for your purposes than querying the number status directly.
It is also worth mentioning that there are cases where the number is technically being serviced, but CallFire's number inventory hasn't yet been updated to indicate this. This can be pushed along by creating inbound traffic to the number on each of the features. That is, you might have a number which "activates" the numbers you purchase more rapidly by sending a call or text to them. This is due to the slight delay between the number being configured upstream, and CallFire's systems being notified of that fact. By sending traffic to the number, you more rapidly give CallFire's systems feedback that the number is enabled. This can save you up to a couple of minutes, if time is of the essence.
Your question has prompted me to create a feature request for CallFire internally, to add an event type to CreateSubscription for when number orders transition between statuses. This way, you could avoid having to poll for number/order statuses repeatedly, and instead we would notify your server by HTTP POST when the number order transitions to finished.