Using Ncqrs, is there a way to replay every single event ever happened (all aggregate types) and feed these through my denormalizers in order to recreate the whole read model from scratch?
Edit:
I though it's be good to provide a more specific use case. I'm building this inside a ASP.NET MVC application and using Entity Framework (Code first) for working with the read models. In order to speed up development (and because I'm lazy), I want to use a database initializer that recreates the database schemas once any read model changes. Then using the initializer's seed method to repopulate them.
There is unfortunately nothing built in to do this for you (though I haven't updated the version of ncqrs I use in quite a while so perhaps that's changed). It is also somewhat non-trivial to do it since it depends on exactly what you want to do.
The way I would do it (up to this point I have not had a need) would be to:
Call to the event store to get all relevant events
Depending on what you are doing this could be all events or just the events for one aggregate root, or a subset of events for one or more aggregate roots.
Re-create the read-model in memory from scratch (to save slow and unnecessary writing)
Store the re-created read-model in place of the existing one
Call to the event store one more time to get any events that may have been missed
Repeat until there are no new events being returned
One thing to note, if you are recreating the entire read-model database from scratch I would off-line the service temporarily or queue up new events until you finish.
Again there are different ways you could approach this problem, your architecture and scenarios will probably dictate how best to do it.
We use a MsSqlServerEventStore, to replay all the events I implemented the following code:
var myEventBus = NcqrsEnvironment.Get<IEventBus>();
if (myEventBus == null) throw new Exception("EventBus is not found in NcqesEnvironment");
var myEventStore = NcqrsEnvironment.Get<IEventStore>() as MsSqlServerEventStore;
if (myEventStore == null) throw new Exception("MsSqlServerEventStore is not found in NcqesEnvironment");
var myEvents = myEventStore.GetEventsAfter(GetFirstEventIdFromEventStore(), int.MaxValue);
myEventBus.Publish(myEvents);
This will push all the events on the eventbus and the denormalizers will process all the events. The function GetFirstEventIdFromEventStore just queries the eventstore and returns the first Id from the eventstore (where SequentialId = 1)
What I ended up doing is the following. At the service startup, before any commands are being processed, if the read model has changed, I throw it away and recreate it from scratch by processing all past events in my denormalizers. This is done in the database initializer's seed method.
This was a trivial task using the MS SQL event storage as there was a method for retrieving all events. However, I'm not sure about other event storages.
Related
I am working on a Class that takes data from a csv, works with a Scheduler Object, and associates the data with the scheduler's attributes(Project, Job, Allocation, Resource, etc). I was thinking after I got everything down(ProjectList, JobList, AllocationList, ExecutionModeList, Resources) I could just pass the scheduler object into createChart.
However, I am still unsure as to where resetPanel and createChart are called( I understand that the ProjectJobPanel has these functions).
So my 2 questions are:
Where are these functions called?( I couldn't find this information in the documentation)
If i want to display my data, do I need to do anything else other than pass in the scheduler object into the "createChart" function?
resetPanel() is called by the example swing dialog when a new dataset is loaded. If updatePanel() isn't overwritten, it's also called when a new best solution is found (so for every best solution changed event).
I'm developing an iPad-app and I'm currently struggling with finding the best approach to multithreading. Let me illustrate this with a simplified example:
I have a view with 2 subviews, a directory picker and a gallery with thumbnails of all the images in the selected directory. Since 'downloading' and generating these thumbnails can take quite a while I need multithreading so the interaction and updating of the view doesn't get blocked.
This is what I already tried:
[self performSelectorInBackground:#selector(displayThumbnails:) withObject:currentFolder];
This worked fine because the users interactions didn't get blocked, however it miserably fails when the user taps on another folder while the first folder is still loading. Two threads are trying to access the same view and variables which results in messing up each others proper execution. When the users taps another folder, the displayThumbnails of the currently loading folder should get aborted. I didn't find any way to do this..
NSThreads
I tried this but struggled with almost the same problems as with the first method, I didn't find a (easy) way to cancel the ongoing method. (Yes, I know about [aThread cancel] but didn't find a way to 'resume' the thread). Maybe I should subclass NSThread and implement my own isRunning etc methods? But isn't there any better way or a third (or even fourth and fifth) option I'm overlooking?
I think this is a fairly simple example and I think there is perhaps a better solution without subclassing NSThread. So, what would you do? Your opinions please!
NSOperationQueue should work well for this task.
Another option would be plain GCD, however, if you've never worked with it, NSOperationQueue is probably the better choice since it pretty much automatically guides you to implementing things "the right way", has obvious ways for cancellation, etc.
You want to use Concurrent NSOperations to download and process images in the background. These would be managed by an NSOperationsQueue. Essentially these operations would be configured to fetch one image per operation, process it, save it in the file system, then message back to the main app in the main thread that the image is available.
There are several projects on github that you can look at that show how to do this - just search github using "Concurrent" or "NSOperation".
iOS has a really nice facility for doing background work. Grand Central Dispatch (GCD) and Blocks, but those don't let you have an object using delegate callbacks - thus NSOperation.
So you need to read up on blocks, GCD, and then look at some open source Concurrent NSOperations code. Using Concurrent NSOperations is not as simple as using blocks.
If I had this problem, I would probably go for an approach like this:
a single thread that will load the images, and causes the main thread to display results (I'm not a big fan of having thread mess around with GUI objects)
when a new directory is requested... well, it depends on how you want to manage things. Basically, a standard queue construct (condition variable and array) could be used for the main thread to tell the thread that "this directory will be needed" by passing it the path name; the thread will check the queue even when it's loading images (like after every image or so), and switch to the new directory whenever one shows up
you could make a directory-reader object that keeps all the state, and store this indexed by the path into a dictionary. When a new directory is requested, check that dictionary first, and only create a new object if there's none for this directory. That way, partially loaded directories would stick around until they are needed again, and can continue to load instead of having to start from scratch.
Pseudocode for the thread:
while (forever)
new element = nil
if we have an active directory loader
tell directory loader to load one image
if false then make directory loader inactive
lock queue condition
if queue has elements
new element = retrieve LAST element (we aren't interested in the others)
empty queue
unlock with status "empty"
else
unlock queue
else
lock queue on condition "has elements"
new element = retrieve last element
empty queue
unlock with status "empty"
if new element != nil
if directory loader for new path does not exist
setup new directory loader for new path
store in dictionary
make it the "active" one
else
make the current one the "active"
As for the directory loader, it might look something like this:
read one image:
if there are still images to read:
read, process and store one
return true
else
performSelectorOnMainThread with an "update GUI" method and the image list as parameter
return false;
This is just a quick sketch; there's some code duplication in the thread, and the way I wrote it will only update the GUI after all images have been read, instead of making them appear as we read them. You'll have to copy the current image list, or add synchronization if you want to do that.
The code:
public ChatMessage[] GetAllMessages(int chatRoomId)
{
using (ChatModelContainer context = new ChatModelContainer(CS))
{
//var temp = context.ChatMessages.ToArray();
ChatRoom cr = context.ChatRooms.FirstOrDefault(c => c.Id == chatRoomId);
if (cr == null) return null;
return cr.ChatMessages.ToArray();
}
}
The problem:
The method (part of WCF-service) returns an empty array. If I uncomment the commented line it starts working as expected. I have tried turning of lazy loading but it didnt help.
Also, when it works, I get ChatMessages with a reference to ChatRoom populated but not the ChatParticipant. They are both referenced by the ChatMessage-entity in the schema with both Id and Navigation Properties. The Ids are set and points to the right entities but on the client-side only the ChatRoom-reference has been populated.
Related questions:
Is an array the preferred method to return collections of EF-entities like this?
When making a change in my model (edmx) Im required to run the "Generate Database from Model..."-option before I can run context.CreateDatabase(). Why? I get some error message pointing to old SSDL but I cant find where the SSDL is stored. Is this created when I run this "Generate Database..."-option?
Is it safe to return entire entity-graphs to the client? Ive read some about "circular reference exeptions" but is this fixed in EF4?
How and when is references populated in EF4? If I have lazy-loading turned on I suspect only entities I touch is populated? But with lazy loading turned off, should the entire graph be populated always then?
Are there any drawbacks of using self-updating entities over ordinary entities in EF4? I dont need self-updating right now but I might do later. Can I upgrade easily or should I start with self-updating from the start?
Why cant I use entity-keys with type string?
Each of your questions needs a separate answer, but I'll try to answer them as briefly as possible.
First of all, in the code sample you provided, you get a ChatRoom object and then try to access a related object that is not included in your query (ChatMessages). If lazy loading is turned off as you had suggested, then you will need the Include("ChatMessages") call in your query, so your LINQ query should look like this:
ChatRoom cr = context.ChatRooms.Include("ChatMessages").FirstOrDefault(c => c.Id == chatRoomId);
Please ensure that your connection string is in your config file as well.
For the related questions:
You can return collections in any way you choose - I have typically done them in a List object (and I think that's the common way), but you could use arrays if you want. To return as a list, use the .ToList() method call on your query.
I don't understand what you're trying to do here, are you using code to create your database from your EDMX file or something? I've typically used a database-first approach, so I create my tables etc then update my EDMX from the database. Even if you generate your DB from your model, you shouldn't have to run CreateDatabase in code, you should be able to run the generated script against your DB. If you are using code-only then you need to dump the EDMX file.
You can generally return entity graphs to the client, should handle ok.
EF4 should only populate what you need. If you use lazy loading, it will automatically load things that you do not include in your LINQ query when you reference them and execute the query (e.g. do a ToList() operation). This won't work so well if your client is across a physical boundary (eg a service boundary) obviously :) If you don't use lazy loading, it will load what you tell it to in your query and that is all.
Self tracking entities are used for n-tier apps, where objects have to be passed across physical boundaries (eg services). They come with an overhead of generated code for each object to keep track of its changes, they also generate POCO objects which are not dependent on EF4 (but obviously contain generated code that would make the tracked changes work with the EF4 tracker). I'd say it depends on your usage, if you're building a small app that's quite self contained, and don't really care about separation for testability without the infrastructure in place, then you don't need to use self tracking entities. I say only use framework features when you need them, so if you're not writing an enterprise scale application (enterprise doesn't have to be big, but something scalable, highly testable, high quality etc) then no need to go for self tracking POCOs.
I haven't tried but you should be able to do that - that would be a candidate for a separate question if you can't get it to work :)
I was wondering if there is a way to share an NSManagedObject between two or more NSManagedObjectContext objects running in the same thread.
I have the following problem: I have one main context shared through all my code in the application and several different contexts that are created for each remote fetch request that I issue. (I created a custom class that fetches remotely and inserts all the objects found in the server in his own NSManagedObjectContext). Those fetch requests may run simultaneously since they use NSURLConnection objects that may end at different times. If the same remote object gets fetched by different connections, I will end up with duplicates at the moment of saving and merging the context with the main one. (That is, objects that have the same remote ID but a different objectID).
One possible solution would be to save (and so persist) every object as soon as it is created but I can't do that because it may have some relationships that may still have not been filled and won't validate during the save operation.
I'm really looking forward to a method that allows you to share the same non-persistent instance of an object between context. If anybody has encountered this issue and came up with a solution, I would be pleased to know!
Context cannot communicate between each other save through their stores. However, you can insert a managed object with a nil managed object context and it will be independent (albeit without relationships) of any context. You could pass that independent managed object around however you wished and insert it into a context when you needed to persist it. This is dangerous but possible.
However, if you're not running each connection on a separate thread then you don't gain anything by having multiple context. Each connection object will activate its delegate in sequence on the main thread. In this case, your easiest solution would be to use the same delegate for all the connections and let the delegate handle the insertions into a single context. To prevent duplication, just do a fetch on the remoteID and see if you get back an extant object before inserting a new object for that remoteID.
I don't think what you want to do is possible. I mean if you want to share changes between different contexts, you got to use notifications and merge it whenever did save or did change occur. But in your case, I'd say just use 1 context and save in the end. Or a less elegant way: save all the remote ids temporary in your app and check before inserting new ones. In this case, you can continue use multiple contexts and save after each didfinishloading.
Currently I am struggling with a Entity Framework issue. I have a wcf service that sits on top of the ef framework, and allows queries to the framework. At some point the user is able to request a file from the framework. The files are referenced by solution entries, so when you request a file from a solution, the reference is loaded to gain access to the file store.
That all works fine, but from that point onward, whenever you do another query that returns that solution entry, the whole file gets attached to the return result. I need some way of detaching or unloading the reference, such that the result entries will only contain an unloaded reference to the file store again.
I have tried to create a new context and query that context to retrieve information from, but when I do that, the entity in the original context is also changed.
I have tried to detach the entity from the original context and then query from the new context. That does not work either.
I have found one way of doing this. For all the non file-download queries, I detach the result entity, and send that over the wire. I am not sure if that is the best way to go about it though.
I hope someone might be able to provide some insight, thanks for the effort.
The issue you are experiencing is probably do to Change Tracking, which is on by default.
Possible Solution:
Disable Change Tracking with MergeOption.NoTracking
using (MyEntities _context = new MyEntities())
{
_context.Widgets.MergeOption = MergeOption.NoTracking;
return _context.Widgets.ToList();
}
This article may help to point you in the right direction on how to handle this issue if the solution above does not work.
I struggled with a similar issue recently. The problem was the result of the context maintaining a reference to the object I was using (obviously). Every time I made a change to an object of the same type, even when obtained with a new context (so I thought), the object was being changed.
With help from one of my peers, we determined the context was hanging around due to the way I was registering it with my IoC container (lifestyle per web request). When I changed the lifestyle to transient (which definitively provided a new instance) then changes to objects of the same type were unaffected.
Hope this helps.