I have a parent component with many child components repeated using an {#each} ... {/each} block, which receives updates from a webhook. I would like to propagate an update (which has an ID) into the child with the same ID. At first, I passed the update message as a prop, then checked if that prop had the same ID in the children. That felt inefficient since each child would have to process the message intended for only one child.
I then decided to use the module context feature, which allowed me to avoid checking in each component (demonstrated on the Svelte REPL here). I'm pretty satisfied with this solution, but I'm wondering if there is a better or more streamlined (or official!) way to do this since I imagine this is a pretty common scenario.
Using the module script for this is a bad idea. It means you can only have one instance of the component that uses said static context variables without causing conflicts.
The proper approach is to use setContext in the parent and getContext in the child (or any descendant) to get access to a shared object.
You can use stores in the context to preserve reactivity or use something like an EventTarget instance to pass events.
Related
I am still new in DDD. In the domain model, I have an aggregate for a Component which can either consist of sub-components or itself be a sub-component of another parent Component. In other words, I have a hierarchy of components.
Two actions are needed: AddSubComponent() and AddToParent(). How to enforce the invariant that no cycle shall exist in the hierarchy when executing these 2 actions? Shall it be enforced via domain events? Or just have reference to parents and children in the aggregate and then checking recursively within the aggregate itself the nested Components if the operation would create a cycle?
Note: I am using C# and persisting the domain model using EF Core.
Edit
I solved the problem by considering each component as an aggregate. Each has a reference to its parents and children.
AddSubComponent(Component c)
Checks if c is already a sub-component and returns or only adds c as sub-component after checking with parents of the component and the children of c. Then fires a domain event which is handled by the Application layer to verify the whole graph that it contains no cycles and also to call c.AddToParent()
AddToParent(Component c)
Does the same logic as the action above but adds only to parent and also fires a domain event which is handled at the Application layer as well only to call parent.AddSubComponent().
This way calling any method would ensure the atomic consistency only for the aggregate and eventual consistency of the full hierarchy is ensured by the application layer. Also all those operations above are wrapped in a transaction. Therefore. only valid hierarchies are persisted eventually.
I would rather implement a one-way relationship in this case. When using events, like you mentioned, it would be trivial, as you can emit an event after executing AddSubComponent, so AddToParent will happen asynchronously, as a reaction. However, producing events in a system that just persist state, is not trivial, as you must avoid unreliable two-phase commits using an outbox or CDC.
If you only maintain the list of children for a parent, that won't be necessary. It might, of course, not fit to your context as cases like ensuring that a component has only one parent won't be possible inside the model itself and would require a domain service, which queries the component parents to see how many are there.
You can enforce it right in the methods themselves.
AddSubComponent
During the Add SubComponent method, check if the component you are adding already exists as a child or a child of a child.
AddToParent
Call the "AddSubComponent" of the parent itself and the check will be done as described above.
Let's have a worker thread which is accessed from a wide variety of objects. This worker object has some public slots, so anyone who connects its signals to the worker's slots can use emit to trigger the worker thread's useful tasks.
This worker thread needs to be almost global, in the sense that several different classes use it, some of them are deep in the hierarchy (child of a child of a child of the main application).
I guess there are two major ways of doing this:
All the methods of the child classes pass their messages upwards the hierarchy via their return values, and let the main (e.g. the GUI) object handle all the emitting.
All those classes which require the services of the worker thread have a pointer to the Worker object (which is a member of the main class), and they all connect() to it in their constructors. Every such class then does the emitting by itself. Basically, dependency injection.
Option 2. seems much more clean and flexible to me, I'm only worried that it will create a huge number of connections. For example, if I have an array of an object which needs the thread, I will have a separate connection for each element of the array.
Is there an "official" way of doing this, as the creators of Qt intended it?
There is no magic silver bullet for this. You'll need to consider many factors, such as:
Why do those objects emit the data in the first place? Is it because they need to do something, that is, emission is a “command”? Then maybe they could call some sort of service to do the job without even worrying about whether it's going to happen in another thread or not. Or is it because they inform about an event? In such case they probably should just emit signals but not connect them. Its up to the using code to decide what to do with events.
How many objects are we talking about? Some performance tests are needed. Maybe it's not even an issue.
If there is an array of objects, what purpose does it serve? Perhaps instead of using a plain array some sort of “container” class is needed? Then the container could handle the emission and connection and objects could just do something like container()->handle(data). Then you'd only have one connection per container.
I'm using performBlock on my NSManagedObjectContexts so that my changes happen on the right queue for the given context. My question is - if I'm making a lot of changes and calling methods from within performBlock - is there an easy way to ensure that I use objects from the proper context.
Example:
I have an activeAccount iVar ( created on the Main Queue ) that is a NSMangedObject for the current account in the application. I have some instance methods that use the activeAccount object to perform certain tasks - getting data, setting data. So my question is if I am doing something on a background NSManagedObjectContext and I call one of these shared methods - is there a pattern I can use so that in these methods I know to either use the current activeAccount iVar or get a new one. Also, if I needed to do something that requires a NSManagedObjectContext - how do I know which one to get/use.
One method I have for knowing which NSManagedObjectContext to use is I have a method that checks if it is running on the current thread - it then knows to return the main thread's context or the background thread's context. Also, if I'm on the background thread, am I allowed to read the Object ID of the activeAccount that lives on the main thread so that I can get a copy of it on the background thread? Thanks in advance.
Brian,
Thread confinement can be a tricky proposition to maintain. The key thing you need to maintain is using objects in their proper MOC. As every managed object maintains a link to both its host MOC and its object ID, this is really easy to ensure. For example:
NSManagedObjectContext *newMOC = NSManagedObjectContext.new;
newMOC.persistentStoreCoordinator = oldActiveAccount.managedObjectContext.persistentStoreCoordinator;
ActiveAccount *newActiveAccount = [newMOC objectWithID: oldActiveAccount.objectID];
Now every instance you access from newActiveAccount is created in the newMOC and is, hence, thread confined to that MOC. objectIDs are persistent. The -persistentStoreCoordinator is rarely, if ever, changed on the mainMOC. Hence, the above code is properly confined. There are issues with the above technique if the source MOC is transient. Hence, I cannot guarantee the above code works with respect to two background MOCs.
Andrew
I have to ask first, why are you having so many contexts in use at the same time?
I use one for background operations and one for main thread. If I need to create another one for discardable changes, I'll just create it and pass it on, so now my self.managedObjectContext points to the draft context. I will never let my managed objects to live in a scope where they could access a multitude of contexts.
It is not entirely clear if you are writing for iOS or OSX, but with iOS for example:
If I need to push a new view controller into navigation stack I will initialize my destination view controller's managedObjectContext ivar as well as any NSManagedObject subclass instances. Since in -prepareForSegue: I know whether I'll create a draft context or just pass on my current one, I also know whether I need to initialize those managed object instances by referencing them by their IDs from newly created context or I can just pass them on.
Now inside my view controller I can take it for granted that my managed objects are always tied to the self.managedObjectContext.
Why is the struts action class is singleton ?
Actually I am getting point that it is multithreaded. but at time when thousand of request hitting same action, and we put synchronized for preventing threading issue, then it not give good performance bcoz thread going in wait state and it take time to proced.
Is that any way to remove singleton from action class ?
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You are asking about why the Action class is a singleton but I think you also have some issues understanding thread safety so I will try to explain both.
First of all, a Struts Action class is not implemented as a singleton, the framework just uses one instance of it. But because only one instance is used to process all incoming requests, care must be taken not to do something with in the Action class that is not thread safe. But the thing is: by default, Struts Action classes are not thread safe.
Thread safety means that a piece of code or object can be safely used in a multi-threaded environment. An Action class can be safely used in a multi-threaded environment and you can have it used in a thousand threads at the same time with no issues... that is if you implement it correctly.
From the Action class JavaDoc:
Actions must be programmed in a thread-safe manner, because the controller will share the same instance for multiple simultaneous requests. This means you should design with the following items in mind:
Instance and static variables MUST NOT be used to store information related to the state of a particular request. They MAY be used to share global resources across requests for the same action.
Access to other resources (JavaBeans, session variables, etc.) MUST be synchronized if those resources require protection. (Generally, however, resource classes should be designed to provide their own protection where necessary.
You use the Struts Action by deriving it and creating your own. When you do that, you have to take care to respect the rules above. That means something like this is a NO-NO:
public class MyAction extends Action {
private Object someInstanceField;
public ActionForward execute(...) {
// modify someInstanceField here without proper synchronization ->> BAD
}
}
You don't need to synchronize Action classes unless you did something wrong with them like in the code above. The thing is that the entry point of execution into your action is the execute method.
This method receives all it needs as parameters. You can have a thousand threads executed at the same time in the execute method with no issues because each thread has its own execution stack for the method call but not for data that is in the heap (like someInstanceField) which is shared between all threads.
Without proper synchronization when modifying someInstanceField all threads will do as they please with it.
So yes, Struts 1 Action classes are not thread safe but this is in the sense that you can't safely store state in them (i.e.make them statefulf) or if you do it must be properly synchronized.
But if you keep your Action class implementation stateless you are OK, no synchronization needed and threads don't wait for one another.
Why is the struts action class is singleton ?
It's by design. Again the JavaDoc explains it:
An Action is an adapter between the contents of an incoming HTTP request and the corresponding business logic that should be executed to process this request
The request parameters are tied to the web tier and you don't want to send that type of data into your business logic classes because that will create a tight coupling
between the two layers which will then make it impossible to easily reuse your business layer.
Because transforming web objects into model objects (and I don't mean the ActionForm beans here) should be the main purpose of Action classes, they don't need to maintain any state (and shoudn't) and also, there is no reason to have more instances of these guys, all doing the same thing. Just one will do.
If you want you can safely maintain state in your model by persisting info to a database for example, or you can maintain web state by using the http session. It is wrong to maintain state in the Action classes as this introduces the need for syncronisation as explained above.
Is there a way to remove singleton from action class?
I guess you could extend Struts and override the default behavior of RequestProcessor.processActionCreate to create yourself an Action per request
but that means adding another layer on top of Struts to change its "normal" behavior. I've already seen stuff like this go bad in a few applications so I would not recommend it.
My suggestion is to keep your Action classes stateless and go for the single instance that is created for it.
If your app is new and you absolutely need statefull Actions, I guess you could go for Struts 2 (they changed the design there and the Action instances are now one per request).
But Struts 2 is very different from Struts 1 so if you app is old it might be difficult to migrate to Struts 2.
Hope this makes it clear now.
This has changed in Struts2 http://struts.apache.org/release/2.1.x/docs/comparing-struts-1-and-2.html
*Struts 2 Action objects are instantiated for each request, so there are no thread-safety issues. (In practice, servlet containers generate many throw-away objects per request, and one more object does not impose a performance penalty or impact garbage collection.) *
I don't know much about struts, but it appears that this changed in Struts 2, so perhaps you should switch to Struts 2?
It seems that all Guice's out-of-the-box Scope implementations are inherently Thread-based (or ignore Threads entirely):
Scopes.SINGLETON and Scopes.NO_SCOPE ignore Threads and are the edge cases: global scope and no scope.
ServletScopes.REQUEST and ServletScopes.SESSION ultimately depend on retrieving scoped objects from a ThreadLocal<Context>. The retrieved Context holds a reference to the HttpServletRequest that holds a reference to the scoped objects stored as named attributes (where name is derived from com.google.inject.Key).
Class SimpleScope from the custom scope Guice wiki also provides a per-Thread implementation using a ThreadLocal<Map<Key<?>, Object>> member variable.
With that preamble, my question is this: how does one go about creating a non-Thread-based Scope? It seems that something that I can use to look up a Map<Key<?>, Object> is missing, as the only things passed in to Scope.scope() are a Key<T> and a Provider<T>.
Thanks in advance for your time.
It's a bit unclear what you want - you don't want scopes that are based on threads, and you don't want scopes that ignore threads.
But yes, scopes are intended to manage the lifecycle of an object and say when an instance should be reused. So really you're asking "what are the other possibilities for re-using an instance beyond 'always use the same instance', 'never use the same instance', and 'use an instance depending on the execution environment of the current thread'?"
Here's what comes to mind:
Use the same instance for a fixed amount of time. The example here would be of a configuration file that's reloaded and reparsed every ten minutes.
Perform some network call to query whether a given object should be re-used (maybe it's a fast call to determine whether we need to reconstruct the object, but the call for reconstructing the object is slow)
Re-use the same object until some outside call comes in telling us to reload
Re-use the same object per thread, but not with a scope that's explicitly entered and left like the servlet scopes. (So one instance per thread)
A "this thread and child threads" scope that is based on an InheritableThreadLocal, not a plain ThreadLocal.
Related to that, a Scope and a threadpool-based ExecutorService that work togehter so that instances are shared between a thread and jobs it submits for background execution.
Pull instances out of a pool; this is tricky, since we'd need a good way to return objects to the pool when finished. (Maybe you could combine this idea with something like the request scope, so that objects can be returned to the pool when the request ends)
A scope that composes two or more other scopes, so for example we could get a configuration object that is re-read every 10 minutes except that the same instance is used through the lifetime of a given request.