I'm using parceler, and I'm trying to define a structure like this one:
Category 1..* Product 1..* Images
Each entity has a list, and a parent instance, like this
#Parcel
public class Category {
List<Product> products;
...
}
#Parcel
public class Product {
List<Image> images;
Category parent;
...
}
#Parcel
public class Image {
Product parent;
...
}
I want to know how performance is affected when I do:
Category c = getSampleCategory(); // With products and images
Parcels.wrap(c);
and pass a Category object as an extra between activities.
I'm noticing ANRs and I ask myself if this is one of the causes
Thanks in advance
Parceler uses annotation processing and, in turn, generated code to marshal your data to and from the Bundle / Intent / Parcel. This means that the Parcels.wrap / Parcels.unwrap calls should be very performant. I'd encourage you to run your own performance benchmark (surround you calls with timer logging) around the various parts of your code, including the wrap / unwrap calls to ensure that things are taking a reasonable amount of time. If you do find that Parceler is not meeting expectations (is a source of your ANRs) please file an issue on the Parceler issue list on Github.
Related
I'm modeling my domain using DDD/CQRS and I have a problem with naming things.
Assume that I have a Category aggregate that is a tree-like structure and I want to update the category tree:
class CategoryItemEntity {
Long guid;
String title;
List<CategoryItem> children;
Datetime createdAt;
Datetime updatedAt;
}
class CategoryItemParameterObject {
Long guid;
String title;
List<CategoryItem> children;
}
class CategoryAggregate {
Long guid;
List<CategoryItem> items;
void updateTree(List<CategoryItemParameterObject> items) {
// ...
}
}
My question is What should I call CategoryItemParameterObject? Is it a DTO or Something else? I also have a CategoryItemDTO on my read side. If I call it CategoryItemDTO would it make that confusing?
Is it ok to send commands directly to aggregates?
Is it a command? An entity?
Just thing about it's use and add to it a suffix/prefix that gives to who reads the code (not you, but who will do it 3, 6 or 12 months later) useful information about it's possible usage.
About the second question: yes, there's no rule that denies this. If the entity could handle the command itself, do it. I have an handler for a command that takes it, loads the entity and give the command to it. It works flawless and the meaning is really clear. Again, think about who will maintain the code, later, and do the thing that requires less thinking 'hops'.
Assume scenario where the service requires some global configuration to handle some request.
For example when user wants to do something it requires some global configuration to check whether the user is permited todo so.
I realize that in axon i can have command handlers that could handle commands without specified target aggregate so the handling part isn't a problem.
Problem is where i would like to have persistent storage on top of that and some invariants when trying to change the configuration. The whole idea of the configuration is that it should be consistent like aggregate in axon.
ConfigService {
#Inject
configRepository;
#Inject
eventGateway;
#CommandHandler
handle(changeConfig){
let current = configRepository.loadCurrent;
//some checks
//persist here?
eventGateway.send(confgChanged)
}
#EventHandler
on(configChanged){
//or persist here?
configRepository.saveCurrent(configChanged.data)
}
}
If I do persistance on the command handler I think I shouldn't use event handler because it would save it twice. But then when i somehow lose the config repository data i can rebuild it based on the events.
Im not sure what im missing here in the understanding of the DDD concepts, to put it simply i would like to know where to put command handler for something that is neither an aggregate nor entity.
Maybe i should create command handler that calls the Config service instead making config service the command handler.
Are you using Axon without event sourcing here?
In Axon framework it is generally good practice only to change the state of an aggregate with events. If you are going to mix state or configuration loaded from a repository with state from the event store, how will you be able to guarantee that when you replay the same events, the resulting state will be the same? The next time the aggregate is loaded, there may be different state in your configRepository, resulting in a different state and different behavior of your aggregate.
Why is this bad? Well, those same events may have been handled by eventprocessors, they may have filled query tables, they may have sent messages to other systems or done other work based on the state the system had at the time. You will have a disagreement between your query database and your aggregate.
A concrete example: Imagine your aggregate processed a command to switch an email service on. The aggregate did this by applying an EmailServiceEnabledEvent and changing its own state to 'boolean emailEnabled = true'. After a while, the aggregate gets unloaded from memory. Now you change that configurationRepository to disable switching the email service on. When the aggregate is loaded again, events from the event store are applied, but this time it loads the configuration from your repository that says it shouldn't switch the email service on. The 'boolean emailEnabled' state is left false. You send a disable email service command to the aggregate, but the command handler in the aggregate thinks the email is already disabled, and doesn't apply an EmailServiceDisabledEvent. The email service is left on.
In short: I would recommend using commands to change the configuration of your aggregate.
It seems to me that you your global configuration is either a specification or a set of rules like in a rules engine.
Unlike the patterns described in GOF book, in DDD, some building blocks/patterns are more generic and can apply to different types of object that you have.
For example an Entity is something that has a life-cycle and has an identity. The stages in the life-cycle usually are: created, persisted, reconstructed from storage, modified and then it's life cycle ends by being deleted, archived, completed etc.
A Value Object is something that doesn't have identity, (most of the time) is immutable, two instances can be compared by the equality of their properties. Value Object represent important concepts in our domains like: Money in system that deal with accounting, banking etc., Vector3 and Matrix3 in systems that do mathematical calculations and simulations like modeling systems (3dsMax, Maya), video games etc. They contain important behavior.
So everything that you need to track and has identity can be an Entity.
You can have a Specification that is an entity, a Rule that is an entity, an Event can also be an entity if it has a unique ID assigned to it. In this case you can treat them just like any another entity. You can form aggregates, have repositories and services and use EventSourcing if necessary.
On the other hand a Specification, a Rule, an Event or a Command can also be Value Objects.
Specifications and Rules can also be Domain Services.
One important thing here is also the Bounded Context. The system that updates these rules is probably in a different Bounded context than the system that applies there rules. It's also possible that this isn't the case.
Here's an example.
Let's have a system, where a Customer can buy stuff. This sytem will also have Discounts on Orders that have specific Rules.
Let's say we have rule that says that: if a Customer has made an Order with more than 5 LineItems he get's a discount. If that Order has a total price of some amount (say 1000$) he gets discount.
The percentage of the discounts can be changed by the Sales team. The Sales system has OrderDicountPolicy aggregates that it can modify. On the other hand the Ordering system only reads OrderDicountPolicy aggregates and won't be able to modify them as this is the responsibility of the Sales team.
The Sales system and the Ordering system can be part of two separate Bounded Contexts: Sales and Orders. The Orders Bounded Context depends on Sales Bounded Context.
Note: I'll skip the most implementation details and add only the relevant things to shorten and simplify this example. If it's intent is not clear, I'll edit and add more details. UUID, DiscountPercentage and Money are value objects that I'll skip.
public interface OrderDiscountPolicy {
public UUID getID();
public DiscountPercentage getDiscountPercentage();
public void changeDiscountPercentage(DiscountPercentage percentage);
public bool canApplyDiscount(Order order);
}
public class LineItemsCountOrderDiscountPolicy implements OrderDiscountPolicy {
public int getLineItemsCount() { }
public void changeLineItemsCount(int count) { }
public bool canApplyDiscount(Order order) {
return order.getLineItemsCount() > this.getLineItemsCount();
}
// other stuff from interface implementation
}
public class PriceThresholdOrderDiscountPolicy implements OrderDiscountPolicy {
public Money getPriceThreshold() { }
public void changePriceThreshold(Money threshold) { }
public bool canApplyDiscount(Order order) {
return order.getTotalPriceWithoutDiscount() > this.getPriceThreshold();
}
// other stuff from interface implementation
}
public class LineItem {
public UUID getOrderID() { }
public UUID getProductID() { }
public Quantity getQuantity { }
public Money getProductPrice() { }
public Money getTotalPrice() {
return getProductPrice().multiply(getQuantity());
}
}
public enum OrderStatus { Pending, Placed, Approced, Rejected, Shipped, Finalized }
public class Order {
private UUID mID;
private OrderStatus mStatus;
private List<LineItem> mLineItems;
private DscountPercentage mDiscountPercentage;
public UUID getID() { }
public OrderStatus getStatus() { }
public DscountPercentage getDiscountPercentage() { };
public Money getTotalPriceWithoutDiscount() {
// return sum of all line items
}
public Money getTotalPrice() {
// return sum of all line items + discount percentage
}
public void changeStatus(OrderStatus newStatus) { }
public List<LineItem> getLineItems() {
return Collections.unmodifiableList(mLineItems);
}
public LineItem addLineItem(UUID productID, Quantity quantity, Money price) {
LineItem item = new LineItem(this.getID(), productID, quantity, price);
mLineItems.add(item);
return item;
}
public void applyDiscount(DiscountPercentage discountPercentage) {
mDiscountPercentage = discountPercentage;
}
}
public class PlaceOrderCommandHandler {
public void handle(PlaceOrderCommand cmd) {
Order order = mOrderRepository.getByID(cmd.getOrderID());
List<OrderDiscountPolicy> discountPolicies =
mOrderDiscountPolicyRepository.getAll();
for (OrderDiscountPolicy policy : discountPolicies) {
if (policy.canApplyDiscount(order)) {
order.applyDiscount(policy.getDiscountPercentage());
}
}
order.changeStatus(OrderStatus.Placed);
mOrderRepository.save(order);
}
}
public class ChangeOrderDiscountPolicyPercentageHandler {
public void handle(ChangeOrderDiscountPolicyPercentage cmd) {
OrderDiscountPolicy policy =
mOrderDiscountRepository.getByID(cmd.getPolicyID());
policy.changePercentage(cmd.getDiscountPercentage());
mOrderDiscountRepository.save(policy);
}
}
You can use EventSourcing if you think that it's appropriate for some aggregates. The DDD book has a chapter on global rules and specifications.
Let's take a look what whould we do in the case of a distributed application for example using microservices.
Let's say we have 2 services: OrdersService and OrdersDiscountService.
There are couple of ways to implement this operation. We can use:
Choreography with Events
Orchestration with explicit Saga or a Process Manager
Here's how we can do it if we use Choreography with Events.
CreateOrderCommand -> OrdersService -> OrderCreatedEvent
OrderCreatedEvent -> OrdersDiscountService -> OrderDiscountAvailableEvent or OrderDiscountNotAvailableEvent
OrderDiscountAvailableEvent or OrderDiscountNotAvailableEvent -> OrdersService -> OrderPlacedEvent
In this example to place the order OrdersService will wait for OrderDiscountNotAvailableEvent or OrderDiscountNotAvailableEvent so it can apply a discount before changing the status of the order to OrderPlaced.
We can also use an explicit Saga to do Orchestration between services.
This Saga will containt the sequence of steps for the process so it can execute it.
PlaceOrderCommand -> Saga
Saga asks OrdersDiscountService to see if a discount is available for that Order.
If discount is available, Saga calls OrdersService to apply a discount
Saga calls OrdersService to set the status of the Order to OrderPlaced
Note: Steps 3 and 4 can be combined
This raises the question: *"How OrdersDiscountService get's all the necessary information for the Order to calculate discounts?"*
This can either be achieved by adding all of the information of the order in the Event that this service will receive or by having OrdersDiscountService call OrdersService to get the information.
Here's a Great video from Martin Folwer on Event Driven Architectures that discusses these approaches.
The advantage of Orchestration with a Saga is that the exact process is explicitly defined in the Saga and can be found, understood and debugged.
Having implicit processes like in the case of the Choreography with Events can be harder to understand, debug and maintain.
The downside of having Sagas is that we do define more things.
Personally, I tend to go for the explicit Saga especially for complex processes, but most of the systems I work and see use both approaches.
Here are some additional resources:
https://blog.couchbase.com/saga-pattern-implement-business-transactions-using-microservices-part/
https://blog.couchbase.com/saga-pattern-implement-business-transactions-using-microservices-part-2/
https://microservices.io/patterns/data/saga.html
The LMAX Architecture is very interesting read. It's not distributed system, but is event driven and records both incomming events/commands and outgoint events. It's an interesting way to capture everything that happend in a system or a service.
I'm learning DDD approach step by step with imaginary business domain by reading books of Eric Evans and Vaughn Vernon and I try to implement it using in my project using PHP (but it really doesn't matter here).
Recently I've been reading a lot of Aggregate, AggregateRoot and Entity patterns for models that should be defined by a domain. And, frankly, I'm not sure I understand all definitions well so I decided to ask my questions here.
At first I'd like to present my (sub)domain responsible for employees' holidays management which should make answers for my questions easier.
The most trivial case is that the Employee can be found in many Teams. When the employee decides to take few days off, he has to send a HolidaysRequest with metadata like type of holidays (like rest holidays, some days off to take care of his child, etc.), the acceptance status and of course time range when he's not going to appear in his office. Of couse HolidaysRequest should be aware of which Employee has sent the HolidaysRequest. I'd like also to find all HolidaysRequest that are sent by Employee.
I'm quite sure that things like DateRange or HolidayType are pure ValueObjects. It's quite clear for me. The problems start when I have to define boundries of entities. I may have bad practices of defining associations by nesting objects in entities, so, please, tell me finding out the definitions of responsibilities here.
What is an entity here? What should be an Aggregate and where's the place for AggregateRoot?
How to define associations between entities? E.g. an Employee can belong to multiple Teams or HolidaysRequest is authored by Employee and assigned to another Employee who can accept it. Should they be implemented as Aggregates?
Why I'm asking these questions? Because few weeks ago I've posted a question here and one of answers was to think about relations between Employee and Teams, that they should be in the single Aggreate called EmployeeInTeam but I'm not sure I understand it in proper way.
Thanks for any advice.
The main thing about DDD, is to put focus in the domain, that's why its called Domain Driven Design.
When you start asking about relationships, aggregates and entities without even deeply exploring what consists your domain, you're actually looking for database modeling instead of domain.
Please, I'm not saying you're asking wrong questions, nor criticising they, I think you're not wrong at all when trying to put in practice while studying.
I'm not DDD expert, I'm learning just like you, but I'm gonna try to help.
Start by thinking what situation's may arise about Holydays Management. When you have different rules for something, you could start by using strategies (I'm saying is the final solution).
Building a nice and meaningful domain, is very hard (at least for me). You write code. Test it. Have insights, throw your code way and rewrite it. Refactor it. In your software's lifecycle, you should put focus on domain, therefore you should be always improving it.
Start by coding (like a domain's draft) to see how it looks like. Let's exercise it. First of all, why do we need to manage this stuff? What problem are we trying to solve? Ahh, sometimes employees ask some days off, we want to control it. We may approve or not, depending on the reason they want "holyday", and how is our team status. If we decline and they still go home, we'll late decide whether we fire or discount in salary. Enforcing ubiquitous language, let's express in code this problem:
public interface IHolydayStrategy
{
bool CanTakeDaysOff(HolydayRequest request);
}
public class TakeCareOfChildren : IHolydayStrategy
{
public bool CanTakeDaysOff(HolydayRequest request)
{
return IsTotalDaysRequestedUnderLimit(request.Range.TotalDays());
}
public bool IsTotalDaysRequestedUnderLimit(int totalDays)
{
return totalDays < 3;
}
}
public class InjuredEmployee : IHolydayStrategy
{
public bool CanTakeDaysOff(HolydayRequest request)
{
return true;
}
}
public class NeedsToRelax : IHolydayStrategy
{
public bool CanTakeDaysOff(HolydayRequest request)
{
return IsCurrentPercentageOfWorkingEmployeesAcceptable(request.TeamRealSize, request.WorkingEmployees)
|| AreProjectsWithinDeadline(request.Projects);
}
private bool AreProjectsWithinDeadline(IEnumerable<Project> projects)
{
return !projects.Any(p => p.IsDeadlineExceeded());
}
private bool IsCurrentPercentageOfWorkingEmployeesAcceptable(int teamRealSize, int workingEmployees)
{
return workingEmployees / teamRealSize > 0.7d;
}
}
public class Project
{
public bool IsDeadlineExceeded()
{
throw new NotImplementedException();
}
}
public class DateRange
{
public DateTime Start { get; set; }
public DateTime End { get; set; }
public int TotalDays()
{
return End.Subtract(Start).Days;
}
public bool IsBetween(DateTime date)
{
return date > Start && date < End;
}
}
public enum HolydayTypes
{
TakeCareOfChildren,
NeedToRelax,
BankOfHours,
Injured,
NeedToVisitDoctor,
WannaVisitDisney
}
public class HolydayRequest
{
public IEnumerable<Project> Projects { get; internal set; }
public DateRange Range { get; set; }
public HolydayTypes Reason { get; set; }
public int TeamRealSize { get; internal set; }
public int WorkingEmployees { get; internal set; }
}
Here is how I quickly wrote this:
Holydays may be granted or not, depending on the situation and
reason, let's create a IHolydayStrategy.
Created an empty (propertyless) HolydayRequest class.
For each possible reason, let's create a different strategy.
If the reason is to take care of children, they can take days off if
the total days request is under a limit.
If the reason is because the employee has been injured, we have no
choice other than allowing the request.
If the reason is because they need to relax, we check if we have an
acceptable percentage of working employees, or if projects are within
deadline.
As soon as I needed some data in the strategy, I used CTRL + . to
automagically create properties in HolydayRequest.
See how I don't even know how these stuff are going to be stored/mapped? I just wrote code to solve a problem, and get piece of information needed to resolve it.
Obviously this is not the final domain, is just a draft. I might take away this code and rewrite, if needed, no feelings for it yet.
People may think it's useless to create an InjuredEmployee class just to always return true, but the point here is to make use of ubiquitous language, to make things as explicit as possible, anyone would read and understand the same thing: "Well, if we have an injured employee, they are always allowed to take days off, regardless of the team's situation and how many days they need.". One of the problems this concept in DDD solves is the misunderstanding of terms and rules between developers, product owners, domain experts, and other participants.
After this, I would start writing some tests with mock data. I might refactor code.
This "3":
public bool IsTotalDaysRequestedUnderLimit(int totalDays)
{
return totalDays < 3;
}
and this "0.7d":
private bool IsCurrentPercentageOfWorkingEmployeesAcceptable(int teamRealSize, int workingEmployees)
{
return workingEmployees / teamRealSize > 0.7d;
}
are specifications, In my point of view, which shouldn't reside in a strategy. We might apply Specification Pattern to make things decoupled.
After we get to a reasonably initial solution with passed tests, now let's think how should we store it. We might use the final defined classes (such as Team, Project, Employee) here to be mapped by an ORM.
As soon as you started writing your domain, relationships will arise between your entities, that's why I usually don't care at all how the ORM will persist my domain, and what is Aggregate at this point.
See how I didn't create an Employee class yet, even though it sounds very important. That's why we shouldn't start by creating entities and their properties, because it's the exact same thing as creating tables and fields.
Your DDD turns into Database Driven Design that way, we don't want this. Of course, eventually we'll make the Employee, but let's take step by step, create only when you need it. Don't try to start modeling everything at once, predicting all entities you're going to need. Put focus on your problem, and how to solve it.
About your questions, what is entity and what is aggregate, I think you're not asking the definition of them, but whether Employee is considered one or other, considering your domain. You'll eventually answer yourself, as soon as your domain start being revealed by your code. You'll know it when you started developing your Application Layer, which should have the responsibility of loading data and delegating to your domain. What data my domain logic expects, from where do I start querying.
I hope I helped someone.
I need to perform a search on several entities with the same string then order the results.
I've heard/read a little about FOSElasticaBundle, would this bundle be able to do it? It seems (to me) to have almost to much features for this purpose and I'm not sure it could run on a shared server (hostgator).
The other solution I can think of at the moment is doing the search "manually" (by using join and union) but I'm wondering where should I put such a function: in an existing controller, a new one, a new bundle or somewhere else?
I'm worried as well that this manual solution could come to a cost, especially on some non-indexable fields.
You would do custom entity repositories. Check out the docs. Basically this extends the default FindAll, FindOneBy, etc.
You would have a function like so:
class MyEntityRepository extends Doctrine\ORM\EntityRepository {
public function findByCustomRule(){
//this is mapped to your entity (automatically adds the select)
$queryBuilder = $this->createQueryBuilder('someAlias');
$queryBuilder->orderBy('...');
//this is mapped to any entity
$queryBuilder = $this->getEntityManager()->createQueryBuilder();
$queryBuilder->select('...');
//result
$result = $queryBuilder->getQuery()->getResult();
}
}
This class is defined in the doctrine mapping and lives inside the Entity folder.. Check the docs out and you should get a basic idea.
Currently diving into DDD and i've read most of the big blue book of Eric Evans. Quite interesting so far :)
I've been modeling some aggregates where they hold a collection of entities which expire. I've come up with a generic approach of expressing that:
public class Expirable<T>
{
public T Value { get; protected set; }
public DateTime ValidTill { get; protected set; }
public Expirable(T value, DateTime validTill)
{
Value = value;
ValidTill = validTill;
}
}
I am curious what the best way is to invalidate an Expirable (nullify or omit it when working in a set). So far I've been thinking to do that in the Repository constructor since that's the place where you access the aggregates from and acts as a 'collection'.
I am curious if someone has come up with a solution to tackle this and I would be glad to hear it :) Other approaches are also very welcome.
UPDATE 10-1-2013:
This is not DDD with the CQRS/ES approach from Greg Young. But the approach Evans had, since I just started with the book and the first app. Like Greg Young said, if you have to make good tables, you have to make a few first ;)
There are probably multiple ways to approach this, but I, personally, would solve this using the Specification pattern. Assuming object expiration is a business rule that belongs in the domain, I would have a specification in addition to the class you have written. Here is an example:
public class NotExpiredSpecification
{
public bool IsSatisfiedBy(Expirable<T> expirableValue)
{
//Return true if not expired; otherwise, false.
}
}
Then, when your repositories are returning a list of aggregates or when performing any business actions on a set, this can be utilized to restrict the set to un-expired values which will make your code expressive and keep the business logic within the domain.
To learn more about the Specification pattern, see this paper.
I've added a method to my abstract repository InvalidateExpirable. An example would be the UserRepository where I remove in active user sessions like this: InvalidateExpirable(x => x.Sessions, (user, expiredSession) => user.RemoveSession(expiredSession));.
The signature of InvalidateExpirable looks like this: protected void InvalidateExpirable<TExpirableValue>(Expression<Func<T, IEnumerable<Expirable<TExpirableValue>>>> selector, Action<T, Expirable<TExpirableValue>> remover). The method itself uses reflection to extract the selected property from the selector parameter. That property name is glued in a generic HQL query which will traverse over the set calling the remove lambda. user.RemoveSession will remove the session from the aggregate. This way the I keep the aggregate responsible for it's own data. Also in RemoveSession an domain event is raised for future cases.
See: https://gist.github.com/4484261 for an example
Works quite well sofar, I have to see how it works further down in the application though.
Have been reading up on DDD with CQRS/ES (Greg Young approach) and found a great example on the MSDN site about CQRS/ES: http://msdn.microsoft.com/en-us/library/jj554200.aspx
In this example they use the command message queue to queue a Expire message in the future, which will call the Aggregate at the specified time removing/deactivate the expirable construct from the aggregate.