I can see the way to create a group as:
public Task CreateGroup(string group)
{
return Groups.Add(Context.ConnectionId, group);
}
How do I create an array of groups for Ids of an entity Channel (having Id and Name properties) even though the only connection id I have is context.ConnectionId.
(Edited with Task syntax)
If i have an array of group names, i want to be able to create groups on the basis of it. Something like ...
public void CreateGroups(string[] groups)
{
Groups.AddRange(Context.ConnectionId, groups);
}
You can do something like this:
public async Task CreateGroup(string[] groups)
{
foreach (var group in groups)
{
await Groups.Add(Context.ConnectionId, group);
}
}
There is however a caveat to adding a client to many groups. The way group membership is sent between the client and the server is a groupsToken which is an encrypted and encoded list of groups a connection belong to. Once a connection belongs to many groups the groupsToken is getting really long. Since the groupsToken is sent in the query string for some operations (most notably reconnects, or in case of longPolling for each poll request) if it gets too long the requests can fail due to url being to long.
Related
Suppose I have a simple module Client defined in Client.ned along with two subclassed simple modules:
simple Client
{
parameters:
volatile int clientId;
}
simple ClientA extends Client
{
}
simple ClientB extends Client
{
}
Now what I wish to do is define a network Network with 1000 ClientA instances and 1000 Client 2 instances as its submodules. I would like each instantiation to have a clientId one bigger than the last, i.e I would like the clientId parameter to ascend with each instantiation. For example, suppose we have the following Network.ned file:
network Network
{
submodules:
clientA[1000]: ClientA {
clientId = index;
};
clientB[1000]: ClientB {
clientId = 1000 + index;
}
}
What I'm looking for is a general approach, where we don't know the number of clients that are to be instantiated beforehand or even the number of client subclasses, just that if there is an instantiated Client of some sort, it should have a clientId parameter one larger than the last instantiation.
Remove volatile from clientId declaration in Client.ned and your solution will work properly.
The main purpose of using volatile is to guarantee returning a "fresh" value of a parameter when it is reading several times. In your network the clientId is constant, so the volatile is not necessary. The side-effect of using volatile is problem with using index, and parentIndex.
Beside the above, one should be aware that using omnetpp.ini is a very convenient method of control the simulation. For example, your NED files may look like:
simple Client {
parameters:
int clientId;
}
simple ClientA extends Client { }
simple ClientB extends Client { }
network Network {
submodules:
clientA[1000]: ClientA;
clientB[1000]: ClientB;
}
And the parameters may be set in omnetpp.ini:
**.clientA[*].clientId = index()
**.clientB[*].clientId = 1000 + index()
EDIT
When the number of clients is not known sizeof() method may be used to determine this number:
**.clientA[*].clientId = index()
**.clientB[*].clientId = sizeof(clientA) + index()
Since OMNeT++'s simulator assigns an ascending ID's to every module and submodule, utilizing getId(), getIndex(), getVectorSize(), and getParentModule() allows me to access this info for each module.
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I am new to DDD and have a concern about the size of my Aggregate Root. The object graph is like the image below. (They are collections). The problem is all of the entities depend on the state of the AggregateRoot (Event). My question is: how do I break the aggregate into smaller aggregates? It's like I have a "God" like aggregate root that just manages everything.
This is very simplistic view of my domain:
and these are the rules:
An event has a number of different states. (implemented state design
pattern here).
An event has a collection of sessions. (but only 1 can be active at a time and only if the event is in the correct state).
A session has two states: Active and Ended.
A session has a collection of Guests.
A session has a collection of photos. (Maximum
of 10).
When a session is deleted. It should delete all its children.
When a session has ended and a photo is deleted it should
check to see if there are any other photos that belong to the
session. If not it should also delete the session.
When a session has ended and a photo is deleted sometimes it should throw an exception depending on the state of the event.
When a session is active and a photo is deleted. It should not worry about whether or not the session has any other photos or not.
When a session ends it must have at least 1 photo and at least 1 guest.
A photo can be updated but only if the event is in the right state.
When an event is deleted it should delete all its children.
Edit: I have divided the 1 aggregate into smaller aggregates so that Event, Session and Photo are all ARs. The issues is a session needs to perform a check on the Event AR before starting. Is it perfectly ok to inject an event object into the sessions start method Session.Start(Event #event) or will I have concurrency issues as outlined in some of the comments?
As a first step, the following 3 articles will be invaluable: http://dddcommunity.org/library/vernon_2011/
With DDD you are splitting the entities up in to boundaries where the state is valid after a single operation from an external source completes (i.e. a method call).
Think in terms of the business problem you are trying to solve - you have used the word delete a lot...
Does delete even have a place in the wording of the business experts for whom you are designing the system? Thinking in terms of the real world and not database infrastructure, unless you can create a time machine to travel back in time and stop an event from starting and therefore change history, the word delete has no real world analogy.
If you are forcing yourself to delete children on delete, that means that operation would need to become a transaction so things that may not make sense to sit inside the aggregate root are forced too (so that the state of the entity and all its children can be controlled and assured to be valid once the method call completes). Yes there are things where you can do with a transaction across multiple aggregate roots, but these are very rare situations and to be avoided if possible.
Eventual consistency is used as an alternative to transactions and reduce complexity, if you speak to the person for whom the system is being designed, you will probably find that a delay of seconds or minutes is more than acceptable. This is plenty of time to fire off an event, to which some other business logic is listening and takes necessary action. Using eventual consistency removes the headaches that come with transactions.
Photos could take up a lot of storage yes, so you would probably need a cleanup mechanism that runs after an event is marked as finished. I would probably fire off an event once the session is marked closed, a different system somewhere else would listen for this event and after 1 year (or whatever makes sense for you) remove this from a server... assuming you used an array of string[10] for your URLs.
If this is the maximum extent of your business logic, then don't only focus on DDD, it seems like this could be a good fit for Entity Framework which is essentially CRUD and has cascade deletes built in.
Edits answer
What is a photo, does it contain attributes? Is it not instead something like a Url to a photo, or a path to a picture file?
I'm not yet thinking of databases, that should be the very last thing that is thought of and the solution should be database/technology agnostic. I see the rules as:
An event has many sessions.
A Session has the following states: NotStarted, Started and Ended.
A Session has a collection of Guests, I'm going to assume these are unique (in that two guests with the same name are not the same, so a guest should be an aggregate root).
An Event has one active Session.
When there are no active Sessions, an Event can be marked as Finished.
No Sessions can be started once an Event is marked as Finished.
A session has a collection of up to 10 photos.
When a session has ended, a photo cannot be removed.
A Session can not start if there are no Guests A Session can not end if there are no Photos.
You cannot return the Session directly, as a user of your code may call Start() on the session, you will need someway of checking with the Event that this cannot be started, so you can chain up to the root this is why I pass in the event to the Session. If you don't like this way, then just put the methods that manipulate the Session on the Event (so everything is accessed via the Event, which is enforcing all the rules).
In the simplest case, I see the photo as a string (value object) in the Session entity. As a first stab I would do something like this:
// untested, do not know if will compile!
public class Event
{
List<Session> sessions = new List<Session>();
bool isEventClosed = false;
EventId NewSession(string description, string speaker)
{
if(isEventClosed==true)
throw new InvalidOperationException("cannot add session to closed event");
// create a new session, what will you use for identity, string, guid etc
var sessionId = new SessionId(); // in this case autogenerate a guid inside this class
this.sessions.Add(new Session(sessionId, description, speaker));
}
Session GetSession(EventId id)
{
reutrn this.sessions.FirstOrDefault(x => x.id == id);
}
bool CanStartSession(Session session)
{
// TO DO: do a check session is in our array!!
if(this.isEventClosed == true)
return false;
foreach(var session in sessions)
{
if(session.IsStarted()==true)
return false;
}
return true;
}
}
public class Session
{
List<GuestId> guests = new List<GuestId>(); // list of guests
List<string> photoUrls = new List<string>(); // strings to photo urls
readonly SessionId id;
DateTime started = null;
DateTime ended = null;
readonly Event parentEvent;
public Session(Event parent, SessionId id, string description, string speaker)
{
this.id = id;
this.parentEvent = parent;
// store all the other params
}
void AddGuest(GuestId guestId)
{
this.guests.Add(guestId);
}
void RemoveGuest(GuestId guestId)
{
if(this.IsEnded())
throw new InvalidOperationException("cannot remove guest after event has ended");
}
void AddPhoto(string url)
{
if(this.photos.Count>10)
throw new InvalidOperationException("cannot add more than 10 photos");
this.photos.Add(url);
}
void Start()
{
if(this.guests.Count == 0)
throw new InvalidOperationException("cant start session without guests");
if(CanBeStarted())
throw new InvalidOperationException("already started");
if(this.parentEvent.CanStartSession()==false)
throw new InvalidOperationException("another session at our event is already underway or the event is closed");
this.started = DateTime.UtcNow;
}
void End()
{
if(IsEnded()==true)
throw new InvalidOperationException("session already ended");
if(this.photos.length==0)
throw new InvalidOperationException("cant end session without photos");
this.ended = DateTime.UtcNow;
// can raise event here that session has ended, see mediator/event-hander pattern
}
bool CanBeStarted()
{
return (IsStarted()==false && IsEnded()==false);
}
bool IsStarted()
{
return this.started!=null;
}
bool IsEnded()
{
return this.ended!=null;
}
}
No warranty on the above, and may well need to change over time as the understanding evolves and as you see better ways to re-factor the code.
A guest cannot be removed once a session has ended - this logic has been added with a simple test.
Talk about deletion of guests and leaving sessions with 0 guests - you have stated that guests cannot be removed once an event has ended... by allowing that to happen at any point would be in violation of that business rule, so it can't ever happen, ever. Besides, using the term to delete a person in your problem space makes no sense as people cannot be deleted, they existed and will always have a record that they existed. This database term delete belongs in the database, not in this domain model as you have described it.
Is this.parentEvent.CanStartSession()==false safe? No it is not multithread safe, but commands would be ran independently, perhaps in parallel, each in their own thread:
void HandleStartSessionCommand(EventId eventId, SessionId sessionId)
{
// repositories etc, have been provided in constructor
var event = repository.GetById(eventId);
var session = event.GetSession(sessionId);
session.Start();
repository.Save(session);
}
If we were using event sourcing then inside the repository it is writing the stream of changed events in a transaction, and the aggregate root's current version is used so we can detect any changes. So in terms of event sourcing, a change to the Session would indeed be a change to its parent aggregate root, since it doesn't make sense to refer to a Session event in its own right (it will always be a Event event, it cannot exist independently). Obviously the code I have given in my example is not event sourced but could be written as so.
If event sourcing is not used then depending on the transaction implementation, you could wrap the command handler in a transaction as a cross cutting concern:
public TransactionalCommandHandlerDecorator<TCommand>
: ICommandHandler<TCommand>
{
private ICommandHandler<TCommand> decoratedHandler;
public TransactionalCommandHandlerDecorator(
ICommandHandler<TCommand> decoratedHandler)
{
this.decoratedHandler = decoratedHandler;
}
public void Handle(TCommand command)
{
using (var scope = new TransactionScope())
{
this.decoratedHandler.Handle(command);
scope.Complete();
}
}
}
In short, we are using the infrastructure implementation to provide concurrency safety.
I've only began with DDD and currently trying to grasp the ways to do different things with it. I'm trying to design it using asynchronous events (no event-sourcing yet) with CQRS. Currently I'm stuck with validation of commands. I've read this question: Validation in a Domain Driven Design , however, none of the answers seem to cover complex validation across different aggregate roots.
Let's say I have these aggregate roots:
Client - contains list of enabled services, each service can have a value-object list of discounts and their validity.
DiscountOrder - an order to enable more discounts on some of the services of given client, contains order items with discount configuration.
BillCycle - each period when bills are generated is described by own billcycle.
Here's the usecase:
Discount order can be submitted. Each new discount period in discount order should not overlap with any of BillCycles. No two discounts of same type can be active at the same time on one service.
Basically, using Hibernate in CRUD style, this would look something similar to (java code, but question is language-agnostic):
public class DiscountProcessor {
...
#Transactional
public void processOrder(long orderId) {
DiscOrder order = orderDao.get(orderId);
BillCycle[] cycles = billCycleDao.getAll();
for (OrderItem item : order.getItems()) {
//Validate billcycle overlapping
for (BillCycle cycle : cycles) {
if (periodsOverlap(cycle.getPeriod(), item.getPeriod())) {
throw new PeriodsOverlapWithBillCycle(...);
}
}
//Validate discount overlapping
for (Discount d : item.getForService().getDiscounts()) {
if (d.getType() == item.getType() && periodsOverlap(d.getPeriod(), item.getPeriod())) {
throw new PeriodsOverlapWithOtherItems(...);
}
}
//Maybe some other validations in future or stuff
...
}
createDiscountsForOrder(order);
}
}
Now here are my thoughts on implementation:
Basically, the order can be in three states: "DRAFT", "VALIDATED" and "INVALID". "DRAFT" state can contain any kind of invalid data, "VALIDATED" state should only contain valid data, "INVALID" should contain invalid data.
Therefore, there should be a method which tries to switch the state of the order, let's call it order.validate(...). The method will perform validations required for shift of state (DRAFT -> VALIDATED or DRAFT -> INVALID) and if successful - change the state and transmit a OrderValidated or OrderInvalidated events.
Now, what I'm struggling with, is the signature of said order.validate(...) method. To validate the order, it requires several other aggregates, namely BillCycle and Client. I can see these solutions:
Put those aggregates directly into the validate method, like
order.validateWith(client, cycles) or order.validate(new
OrderValidationData(client, cycles)). However, this seems a bit
hackish.
Extract the required information from client and cycle
into some kind of intermediate validation data object. Something like
order.validate(new OrderValidationData(client.getDiscountInfos(),
getListOfPeriods(cycles)).
Do validation in a separate service
method which can do whatever it wants with whatever aggregates it
wants (basically similar to CRUD example above). However, this seems
far from DDD, as method order.validate() will become a dummy state
setter, and calling this method will make it possible to bring an
order unintuitively into an corrupted state (status = "valid" but
contains invalid data because nobody bothered to call validation
service).
What is the proper way to do it, and could it be that my whole thought process is wrong?
Thanks in advance.
What about introducing a delegate object to manipulate Order, Client, BillCycle?
class OrderingService {
#Injected private ClientRepository clientRepository;
#Injected private BillingRepository billRepository;
Specification<Order> validSpec() {
return new ValidOrderSpec(clientRepository, billRepository);
}
}
class ValidOrderSpec implements Specification<Order> {
#Override public boolean isSatisfied(Order order) {
Client client = clientRepository.findBy(order.getClientId());
BillCycle[] billCycles = billRepository.findAll();
// validate here
}
}
class Order {
void validate(ValidOrderSpecification<Order> spec) {
if (spec.isSatisfiedBy(this) {
validated();
} else {
invalidated();
}
}
}
The pros and cons of your three solutions, from my perspective:
order.validateWith(client, cycles)
It is easy to test the validation with order.
#file: OrderUnitTest
#Test public void should_change_to_valid_when_xxxx() {
Client client = new ClientFixture()...build()
BillCycle[] cycles = new BillCycleFixture()...build()
Order order = new OrderFixture()...build();
subject.validateWith(client, cycles);
assertThat(order.getStatus(), is(VALID));
}
so far so good, but there seems to be some duplicate test code for DiscountOrderProcess.
#file: DiscountProcessor
#Test public void should_change_to_valid_when_xxxx() {
Client client = new ClientFixture()...build()
BillCycle[] cycles = new BillCycleFixture()...build()
Order order = new OrderFixture()...build()
DiscountProcessor subject = ...
given(clientRepository).findBy(client.getId()).thenReturn(client);
given(cycleRepository).findAll().thenReturn(cycles);
given(orderRepository).findBy(order.getId()).thenReturn(order);
subject.processOrder(order.getId());
assertThat(order.getStatus(), is(VALID));
}
#or in mock style
#Test public void should_change_to_valid_when_xxxx() {
Client client = mock(Client.class)
BillCycle[] cycles = array(mock(BillCycle.class))
Order order = mock(Order.class)
DiscountProcessor subject = ...
given(clientRepository).findBy(client.getId()).thenReturn(client);
given(cycleRepository).findAll().thenReturn(cycles);
given(orderRepository).findBy(order.getId()).thenReturn(order);
given(client).....
given(cycle1)....
subject.processOrder(order.getId());
verify(order).validated();
}
order.validate(new OrderValidationData(client.getDiscountInfos(),
getListOfPeriods(cycles))
Same as the above one, you still need to prepare data for both OrderUnitTest and discountOrderProcessUnitTest. But I think this one is better as order is not tightly coupled with Client and BillCycle.
order.validate()
Similar to my idea if you keep validation in the domain layer. Sometimes it is just not any entity's responsibility, consider domain service or specification object.
#file: OrderUnitTest
#Test public void should_change_to_valid_when_xxxx() {
Client client = new ClientFixture()...build()
BillCycle[] cycles = new BillCycleFixture()...build()
Order order = new OrderFixture()...build();
Specification<Order> spec = new ValidOrderSpec(clientRepository, cycleRepository);
given(clientRepository).findBy(client.getId()).thenReturn(client);
given(cycleRepository).findAll().thenReturn(cycles);
subject.validate(spec);
assertThat(order.getStatus(), is(VALID));
}
#file: DiscountProcessor
#Test public void should_change_to_valid_when_xxxx() {
Order order = new OrderFixture()...build()
Specification<Order> spec = mock(ValidOrderSpec.class);
DiscountProcessor subject = ...
given(orderingService).validSpec().thenReturn(spec);
given(spec).isSatisfiedBy(order).thenReturn(true);
given(orderRepository).findBy(order.getId()).thenReturn(order);
subject.processOrder(order.getId());
assertThat(order.getStatus(), is(VALID));
}
Do the 3 possible states reflect your domain or is that just extrapolation ? I'm asking because your sample code doesn't seem to change Order state but throw an exception when it's invalid.
If it's acceptable for the order to stay DRAFT for a short period of time after being submitted, you could have DiscountOrder emit a DiscountOrderSubmitted domain event. A handler catches the event and (delegates to a Domain service that) examines if the submit is legit or not. It would then issue a ChangeOrderState command to make the order either VALIDATED or INVALID.
You could even suppose that the change is legit by default and have processOrder() directly take it to VALIDATED, until proven otherwise by a subsequent INVALID counter-order given by the validation service.
This is not much different from your third solution or Hippoom's one though, except every step of the process is made explicit with its own domain event. I guess that with your current aggregate design you're doomed to have a third party orchestrator (as un-DDD and transaction script-esque as it may sound) that controls the process, since the DiscountOrder aggregate doesn't have native access to all information to tell if a given transformation is valid or not.
I'm trying to create a chat application using SignalR. To make it possible to send private messages I want to assign clients to a group with the name of their profileID. So I can simply call the addMessage function of the group to send to a specific client.
When I go to this page: https://github.com/SignalR/SignalR/wiki/Hubs
It tells me to add a function to the Hub called Join(). In here I can add the incomming client to a group. So I created this code:
[HubName("Chat")]
public class ChatHub : Hub
{
public Task Join()
{
if (HttpContext.Current.User.Identity.IsAuthenticated)
{
Profiel_DataHelper profiel = new Profiel_DataHelper(HttpContext.Current.User.Identity.Name);
return Groups.Add(Context.ConnectionId, profiel.ProfielID.ToString());
}
else
{
return null;
}
}
.....
When I want to call a specific client I use this code:
var context = GlobalHost.ConnectionManager.GetHubContext();
context.Clients.Group(profielidNaar).addTyptOnline(profielidVan);
But when I run the program the Join() Task is not being called at all, therefore my call to the group is also not working.
What am I doing wrong?
Join is a method on your hub that you need to call from the client. Nobody is going to call it for you and "Join" isn't a special method that gets called automatically. The documentation is showing you how to declare method that "can" be called from the client.
There's other ways to know when clients connect, reconnect and disconnect and it's detailed here:
https://github.com/SignalR/SignalR/wiki/Hubs#detecting-connect-reconnect-and-disconnect-clients-in-hubs
I'm using SignalR's latest version and it's great. However, I've recently hit an interesting scaling issue: it seems that SignalR sends all the client's groups from the client in the query string. My system requires that a person join numerous groups, which represent all the projects that the user is subscribed to, and receive notifications on any of them.
this large number of groups (and me using GUIDs for ids) has caused the querystring to reach its maximum size, and SignalR to stop working.
This is what Fiddler shows in the request webform (using long polling for the Azure bug):
transport longPolling
connectionId bbed6f35-8379-4be3-ac28-ef3e618382ae
connectionData [{"name":"jethub"}]
messageId 85
groups ["JetHub.f9f81bcc-8417-46bd-bae5-c4134972601d","JetHub.5139a8de-04c2-48da-9427-39666e52fabd","JetHub.6b12e333-4d22-47c3-8587-7a9ad5026184","JetHub.252ea279-7a71-40e7-b03c-7d63e69f07ab","JetHub.a4843a77-1e6d-4693-b3de-b392ef465410","JetHub.27feb53a-3c2d-4b11-92f7-dbdffb874b25","JetHub.8840dfcf-e6be-4b72-965b-b282a60446e8","JetHub.bf7d3301-6fc0-4499-bee8-fe22f1bc2281","JetHub.655cba0e-7f72-402c-b80b-dcb740546163","JetHub.85d817e2-67a3-4291-b564-5320598339f6","JetHub.e3079263-3f6e-4a54-ad88-0dfc5dd2ce18","JetHub.33f00a67-9b05-4293-8119-4617e2fed9b0","JetHub.6323cfe8-fb81-4716-b553-79b9d72641a5","JetHub.b4359f8a-030a-4ac9-aacd-c05b42163bcc", ... many more]
I know I can increase the querystring size in IIS, but is there a better way to manage groups, or should I try to create my server-side grouping scheme and broadcast to each client separately? can PersistenConnections help in that regard?
Thanks.
As groups are roundtripped via the query string in SignalR 0.5.3, you have the following options:
a) increase the maximum query string size
b) use shorter group names
c) handle grouping yourself on the server and broadcast seperately to each user
PersistentConnections won't help here as the Hub API is built on top of them, so you'll run into the same problem.
I know that this has already been answered to your satisfaction, but there is a way of managing groups on the server without sending messages to each client individually. You can implement your own HubDispatcher:
using System.Collections.Generic;
using SignalR;
using SignalR.Hubs;
namespace My.Hubs
{
public class MyHubDispatcher : HubDispatcher
{
public MyHubDispatcher() : base("/myhubs") { }
protected override Connection CreateConnection(string connectionId, IEnumerable<string> signals, IEnumerable<string> groups)
{
//ex: IEnumerable<string> myGroups = new string[] { "MyHub.MyGroup", "MyHub.MyOtherGroup", "MyOtherHub.MyGroup" };
IEnumerable<string> myGroups = GetGroups(connectionId);
return base.CreateConnection(connectionId, signals, myGroups);
}
}
}
You can then set up routing like any other PersistentConnection:
using System.Web;
using System.Web.Routing;
using SignalR;
namespace My
{
// Note: For instructions on enabling IIS6 or IIS7 classic mode,
// visit http://go.microsoft.com/?LinkId=9394801
public class Application : HttpApplication
{
protected void Application_Start()
{
RouteTable.Routes.MapConnection<Hubs.MyHubDispatcher>("myhubs", "myhubs/{*operation}");
RouteConfig.RegisterRoutes(RouteTable.Routes);
}
}
}
Then you can use groups on you Hubs like you normally would:
using SignalR.Hubs;
namespace My.Hubs
{
public class MyHub : Hub
{
public void AlertClients(string id, int duration)
{
Clients["MyGroup"].Alert("MyGroup");
Clients["MyOtherGroup"].Alert("MyOtherGroup");
}
}
}
If you using the JS client you can simply include the script at ~/myhubs/hubs instead of ~/signalr/hubs. If you are using the .NET client you just use new Client.Hubs.HubConnection("http://foo/myhubs", useDefaultUrl: false);