If a user creates a new activity and wants all their followers to see it except 1, how can this be implemented? Do we simply push the activity, and then immediately delete it from the specific follower's timeline feed? This seems like a hack.
https://github.com/GetStream/stream-js/issues/210
this use case hasn't come up before. Why would someone want everyone except one person to see a post? Do they want that person to unfollow them? Are there "rings" or levels of people to choose from when posting? If that's the case, you can create separate feeds with follows to them for those levels (and will likely need to use the TO field as well since fanout only goes 1 level deep).
There's no built in mechanism to specify which feeds to fan out to or which not to. The fanout is intended to happen as fast as possible (milliseconds) so doing those kinds of checks wouldn't be optimal. Your solution to quickly delete from that feed will work.
Related
I am trying to make an online teacher-doubt solving MERN application.
The workflow is as follows:
A user of the app clicks on the "Ask Doubt" button associated with the teacher.
The user makes the payment.
The user is then added to the queue where he/she waits for the doubts of the people ahead of of him/her to be resolved by that teacher. (Edit: The user and the teacher go into a chat room then and the others will wait in the queue)
I also want to display the number of people in the queue already in the queue so that the user can only pay if they have enough time.
I cannot guarantee the average time for each doubt session so I cannot ask the user to come in after some x amount of time.
Also, feel free to suggest some other implementation if u feel my approach isn't good.
Although there might be other ways to solve the problem too, I think using Cassandra as your database might be one of the solutions for you.
You can design the use the teacher id (or name) as your Partition Key and use the timestamp as your clustering column.
When you want to get the details of the person who is next in the queue, u can simply query the first student under that partition (teacher id).
When you are done asking, you can delete then delete the first row.
As I said before, there might be other ways, but this is certainly one of the simpler solutions if scalability is what you want.
Edit: Since you dont have a Doubt Model, you could create a queue model. Which consist of an Array with the user ids.
Then you can do queue.array.length to get the amount of people in the queue. And when they are in the chatroom or finished. You update the array by removing the user from the array.
We are using an event store that stores a single aggregate - a user's order (imagine an Amazon order than can be updated at any moment by both a client or someone in the e-commerce company before it actually gets dispatched).
For the first time we're going to allow our company's employees to see the order's history, as until now they could only see its current state.
We are now realizing that the events that form up the aggregate root don't really show the intent or what the user actually did. They only serve to build the current state of the order when applied sequencially to an empty order. The question is: should they?
Imagine a user that initially had one copy of book X and then removed it and added 2 again. Should we consider this as an event "User added 1 book" or events "User removed 1 book" + "User added 2 books" (we seem to have followed this approach)?
In some cases we have one initial event that then is followed by other events. I, developer, know for sure that all these events were triggered by a single command, but it seems incredibly brittle for me to make that kind of assumptions when generating on the fly this "order history" functionality for the user to see. But if I don't treat them, at least in the order history feature as a single action, it will seem like there were lots of order amendments when in fact there was just one, big one.
Should I have "macro" events that contain "micro events" inside? Should I just attach the command's id to the event so I can then easily inferr what event happened at the same and which ones not (an alternative would be relying on timestamps.. but that's disgusting).
What's the standard approch to deal with this kind of situations? I would like to be able to look at any time to the aggregate's history and generate this report (I don't want to build the report incrementally every time the order is updated).
Thanks
Command names should ideally be descriptive of intent. Which should mean it's possible to create event names which make the original intent clear. As a rule of thumb, the events in the event stream should be understandable to the relevant members of the business. It's a good rule of thumb. It should contain stuff like 'cartUpdated' etc.
Given the above, I would have expected that the showing the event stream should be fine. But I totally get why it may not be ideal in some circumstances. I.e. it may be too detailed. In which case maybe create a 'summeriser' read model fed the events.
It is common to include the command’s ID in the resulting events’ metadata, along with an optional correlation ID (useful for long running processes). This then makes it easier to build the order history projection. Alternatively, you could just use the event time stamps to correlate batches in whatever way you want (perhaps you might only want one entry even for multiple commands, if they happened in a short window).
Events (past tense) do not always capture human - or system - user intent. Commands (imperative mood) do. As all command data cannot always be easily retraced from the events it generated, keeping a structured log of commands looks like a good option here.
As my ubiquitous language I have some phrases like :
Feature : Display A Post
In order to be able to check mistakes in a post
As an admin or customer
I want to be able to view the post
Scenario : Display Post
When : I select a post
Then : the post should be viewed
Is that a right user story? Such scenarios may have some minimal differences at UI level. Should I violate the DRY principle and repeat the feature for another role?
Different users may need different requirements over time, and I think this is the reason we usually write user stories per the user role.So should I be worry about how the requirements may change over time for different roles or I can leave a single user story (and the same test code,production code, databse ...) with multiple roles and refactor when their requirements forced me to separate them ?
I am not sure what your problem here and will try to guess. So first, your first three lines is just a description and not real steps. This enables adding custom text that will not run.
As to your other 2 steps, it is very hard to say whether they are good or not. As you might have already noticed, you are not bound by Cucumber to have a specific scenario flow. Cucumber gives you the freedom to design and write your code the way it makes more sense to YOU and YOUR business logic.
Saying that, I see no issue in repeating similar steps to test another role. In order to make the feature file a bit more DRY you can use the Scenario Outline option. It might look something like this:
Scenario Outline: Display Post as <role>
When I select a post as <role>
Then the post should be viewed
Examples:
|role |
|role1|
|role2|
In this case, two scenarios run one after another while rolevalue changes according to the Examples list.
Now, in regards to your possible changes in future. You can't always predict what will happen in future and unless continuously changing current requirements is a normal practice for you or your team, I wouldn't worry too much about this. If sometime in future current scenarios will become obsolete, you will review them and rewrite them or add new ones accordingly.
If multiple roles are required in a feature, then that means it is an epic, not a feature. It is a must to break down each feature so it only has one role, and it can deliver a single value to a single group of users.
I think the problem here is your language which needs refinement to clarify what you want to do here and why its important.
It seems to me that as an admin looking to fix mistakes in a post that what I need to is to be able to change a post.
A similar thing applies for the customer (should that be author?). If you explore what they will do when a post has been authored with a mistake then you will probably find that different roles interact in different ways. You'll start to ask questions about what happens if the customer and the admin make fixes, and how the customer responds when the admin makes a fix that the customer doesn't like and all sorts of other scenarios.
If you do this you'll probably find that most of your duplication goes away, and you'll learn lots about the differences between customer and admin behaviour in this particular context.
I'm new to the CQRS/ES world and I have a question. I'm working on an invoicing web application which uses event sourcing and CQRS.
My question is this - to my understanding, a new command coming into the system (let's say ChangeLineItemPrice) should pass through the domain model so it can be validated as a legal command (for example, to check if this line item actually exists, the price doesn't violate any business rules, etc). If all goes well (the command is not rejected) - then the appropriate event is created and stored (for example LineItemPriceChanged)
The thing I didn't quite get is how do I keep this aggregate in memory to begin with, before trying to apply the command. If I have a million invoices in the system, should I playback the whole history every time I want to apply a command? Do I always save the event without any validations and do the validations when constructing the view models / projections?
If I misunderstood any part of the process I would appreciate your feedback.
Thanks for your help!
You are not alone, this is a common misunderstanding. Let me answer the validation part first:
There are 2 types of validation which take place in this kind of system. The first is the kind where you look for valid email addresses, numeric only or required fields. This type is done before the command is even issued. A command which contains these sorts of problems should not be raised as commands (for belt and braces you can check at the domain side but this is not a domain concern and you are better off just preventing this scenario).
The next type of validation is when it is a domain concern. It could be the kind of thing you mention where you check prices are within a set of specified parameters. This is a domain concept the business people would understand, do and be able to articulate.
The next phase is for the domain to apply the state change and raise the associated events. These are then persisted and on success, published for the rest of the app.
All of this is can be done with the aggregate in memory. The actions are coordinated with a domain service which handles the command. It loads the aggregate, apply's all it's past events (or loads a snapshot) then issues the command. On success of the command it requests all the new uncommitted events and tries to persist them. On success it publishes the new events.
As you see it only loads the events for that specific aggregate. Even with a lot of events this process is lightning fast. If performance is a problem there are strategies such as keeping aggregates in memory or snapshotting which you can apply.
To your last point about validating events. As they can only be generated by your aggregate they are trustworthy.
If you want more detail check out my overview of CQRS and ES here. And take a look at my post about how to build aggregate roots here.
Good luck - I hope they help!
It is right that you have to replay the event to 'rehydrate' the domain aggregate. But you don't have to replay all events for all invoices. If you store the entity id of the root aggregate in the events, you can just select and replay the events that with the relevant id.
Then, how do you find the relevant aggregate root id? One of the read repositories should contain the relevant information to get the id, based on a set of search criteria.
One of the basic tenets of CQRS, as I understand it, is that commands should be behaviour-centric, and have a value in the business or the UL, and not data-centric, ie., CRUD. Instead of focusing on updating a customer, we have commands like CustomerHasMoved. What if you have CRUD screens which are there to correct certain data. For example, we need to change the name of a customer which is misspelled. This doesn't really have much value in the business. Should this just be under the umbrella of an UpdateCustomer command?
I just want to put a comment on this quickly as it popped up.
It is important to note that some objects are actually CRUD and thats ok. I may not really care why a name is changing in my domain where I ship products to people and only need that data to print mailing labels. The trick is in making behavior the default and THEN reverting to a CRUD interface once you are sure you really don't care about the reasons as opposed to vice versa.
Greg
Actually, there could be various reasons to update the name of a customer. As you were saying, it could be misspelled or... you could get married and change your name to your husband's.
If you had only an UpdateCustomer command, you would loose the original intent and you would not be able to have different behaviours for each of them. If the name was misselled it could be as simple as updating the database, whereas if your customer got married you might need to notify the marketing departement so tthat they can offer a discount.
In the case that your entity is purely CRUD, that is there is no intent that you can associate with modifying the properties, then it's OK to have an UpdateEntityCommand. You can then transition slowly to something more task based
CustomerHasMoved is the event that is fired after you have updated the customers location. This event updates the read databases/cache databases. The command from the gui should be MoveCustomer or something like that. I think I would put the update of the customer name in a command like UpdateCustomer.