How does flash object map work inside grails framework - grails

Flash, which is a temporary storage map for one request.
I am wondering how this is implemented on the grails core framework
In particular I'm interested in the class(es) responsible for putting the flash map in the request and then taking it out once the request has finessed processing.

Flash is actually a temporary storage map for the present request and the next request only. It won't retain the entries after the next request unless entries are repopulated in the next request (which would be current in future). Here is how it works in Grails:
FlashScope interface which extends Map itself has two methods next() and getNow() is implemented by GrailsFlashScope. All of which can be found in grails-web-mvc.
GrailsFlasScope mainly maintains two concurrent HasMap (one for current request and second for the next request) to hold the entries. It implements next() from FlashScope to do the cleaning and "restricting-to-next-request-only" part as:
a. clear current
b. make next as current
c. clear next
Next thing to focus will be GrailsWebRequestFilter (implements OncePerRequestFilter) which makes sure that there is always a single execution of request per dispatch.
All http servlet requests are filtered by GrailsWebRequestFilter. This filter sets the flash scope to next so that every time the latest and valid info is retrieved.
Now the question is how does FlashScope reconciles current and next map? Well, that is why FlashScope is extended from Map. FlashScope overrides get(key) from map to reconcile both the maps by making sure values are retrieved from next map otherwise switch to current map.
How is flash available to controllers by default? All controller inherit ControllersApi which inherits CommonWebApi.
I hope you get what you were looking for..

If you print the class of the object:
class MyController {
def index() {
println flash.getClass().name
}
}
You will see that's org.codehaus.groovy.grails.web.servlet.GrailsFlashScope. If you look at the code, there are two ConcurrentHashMap's: one for the current request and another to the next request.
To make it available, the instance is stored in the session (see registerWithSessionIfNecessary).

Related

Grails pass object to the view and back again

I have some data that I need to persist through multiple actions within my Grails app. Due to the nature of the data, I would prefer not to store the data in the session. Here is an example of what I would like to do.
class MyController{
def index(){
MyObject object = MyObject.new(params.first, params.second, params.third)
[gspObject:object]
}
def process(){
MyObject object = params.gspObject
//continue from here
}
}
In my GSP if I do
<g:form action="process" params="[gspObject:gspObject]">
Then I get the error
Cannot cast object 'net.package.MyObject#699c14d8' with class 'java.lang.String' to class 'net.package.MyObject'
My question is, If I want to get the object back that I sent to the gsp, how can I get that? Is there some kind of scope that I can save the object in that would be a little safer then session? Is there a way to pass the object into the page itself and pass it back in the next request?
Grails has many layers, but at the bottom you have plain old HTTP just like in any web app. It's a stateless protocol, and you send a text or binary response, and receive text or text + binary requests. But you can't expect to be able to send an arbitrary object to a web browser in HTML and receive it back again in the same state as when you sent it - where is this Java/Groovy JVM object going to be stored in the browser?
You have basically two options. One is to store it at the server, which is less work because it remains as the same object the whole time. The session is a good location because it's coupled to the user, is created on-demand and can automatically time out and be removed, etc. The other is to do what you're trying to do - send it to the client and receive it back - but you are going to have to serialize it from an object (which could be a complex object containing arbitrarily many other objects) and deserialize it from the format you used on the client back into Java/Groovy objects.
JSON is a good option for serialization/marshalling. You could store the stringified object in a hidden form element if your page uses a form, or in a querystring arg if you click a link from this page to the next in the workflow. Don't send all of the object's data though, only what you need to rebuild it. Anything that's available in the database should be referenced by id and reloaded.
Something like
[gspObject: object as JSON]
or
[gspObject: [first: object.first, first: object.firstsecond, ...] as JSON]
will get it in the correct format for sending, and then you can parse the JSON from the request to reinstantiate the instance.

Cancelling NSJSONSerialization - Search as you type, requests overlapping

Similar to the iPhone Facebook app search function, I am implementing search as you type functionality into my application although I have a problem when decoding the data into JSON format.
Basically what happens is because some searches take longer than others, they return at different intervals and this causes some small visual issues when the data is presenting on the screen.
I have set an NSLOG after each decode using NSJSONSerialization for the keyword 'industry'
2013-04-09 23:38:18.941 Project Name [42836:1d03] http://fooWebAddress/json/?method=search&limit=10&q=indus
2013-04-09 23:38:19.776 Project Name [42836:3e07] http://fooWebAddress/json/?method=search&limit=10&q=indu
2013-04-09 23:38:20.352 Project Name [42836:8803] http://fooWebAddress/json/?method=search&limit=10&q=indust
2013-04-09 23:38:21.814 Project Name [42836:4e03] http://fooWebAddress/json/?method=search&limit=10&q=industr
2013-04-09 23:38:23.434 Project Name [42836:8803] http://fooWebAddress/json/?method=search&limit=10&q=ind
2013-04-09 23:38:24.070 Project Name [42836:7503] http://fooWebAddress/json/?method=search&limit=10&q=industry
As you can see it is all out of order.
Does anyone have any way of stopping NSJSONSerialization for the previous connection.
Or possibly any other way to go about this problem?
Steps up to NSJSONSerialization...
NSURLRequest (initwithURL)
NSOperationQueue
NSURLConnection (asynchronous)
NSJSONSerialization
Thanks in advance.
When the user starts typing more text, you could cancel your previous connections and ignore any further delegate callbacks you receive from them. Then make the new request for the current text.
You can do this by maintaining some sort of lastRequest or lastOperation reference. When the user starts typing, call [self.lastRequestOrOperation cancel] and ignore any further notifications from that request with a check like if (request != self.lastRequest) { return; } in whatever callbacks you have.
However this has the problem that if the user keeps typing for a while you are constantly cancelling requests and they may not see any results until they have stopped typing.
A better solution would be to add sequencing so that each request is associated with an increasing sequence ID. You then only parse the result and update the UI when the sequence of the response is higher than the last one you received. If you receive any out-of-band responses from earlier, you just ignore them.
This is a much more complex issue than just being able to cancel the NSJSONSerialization. My suggestion is to use NSFetchedResultsController to populate your table view that shows the search results. Use the search term as one of the predicate variable in the NSFetchRequest attached to NSFetchedResultsController. And then, when you parse the results using NSJSONSerialization, store the results with the search term associated with that request. As soon as the search term changed (which you can detect when the user types more characters), re-create the NSFetchedResultsController and reload your table view. In addition, you can also try to cancel the call to parse the previous results if you launched it using performSelector:withObject:afterDelay. Beware that this cannot be always relied upon as the call may have been initiated by the time you are trying to cancel.
Kinda basic, but you could always maintain an nsdictionary of sub-classed NSURLRequests (sub-classed to provide a tag).
Start request - add request to dicationary with tag = array.count - 1, with key matching tag
Connection returns - is the request the most recent request, if so, parse json
Parse JSON - is the request the most recent request, if so, show results, if not, only display if there are no previous results displayed
Request handling - remove key from dictionary
most recent request = does the dictionary contain an object with a higher key value
Currently what you are doing is, you type each character and calling web-service. Why to call web-service for each letter you type. If user is type continuously, then it will increase the load, so call the web-service only when user stops for a particular interval of time. and then pass that string to call web-service or what ever method you are calling.
[NSObject cancelPerformSelectorsWithTarget:self]; // This will cancel your all req which is going to make when user typing without stopping
[self performSelector:#selector(sendSearchRequest) withObject:searchText afterDelay:0.1f]; // This will pass the string to call a web-service method, on which user hold for some time.

Where to store a Doctrine variable created in a component so that it's accessible anywhere?

Note I am referring to one request, and not several requests and sessions.
I have several components that require Doctrine user object, some are located in layout, others are located in templates. Sometimes I need that Doctrine user object in action. Currently I have added a function to sfUser class that loads that object from database, which means every time I call that function I make a call to db. I'd like to know where to store this object so that I can access it without having to query db every time I need it. Again, we're talking about a single request, not several requests or something that would require session.
Can I save it in sfContext somehow? Any other places so that it can be available everywhere?
You can store it in your model's Table class, because tables are always accessed as singletones.
class sfGuardUserTable extends PluginsfGuardUserTable
{
protected $specialUser = null;
public function getSpecialUser()
{
if (null === $this->specialUser)
{
$this->specialUser = $this->findOneById(1);
}
return $this->specialUser;
}
}
Now, you can use this in actions and components like this:
$u = sfGuardUserTable::getInstance()->getSpecialUser();
And you will always end up with one query.
you can configure Doctrine cache so that the result of this specific request is always cached. What if so good about it is that if you use, say, the APC backend, you will have it cached across requests. You also get query caching as a bonus (this is not result caching, read the link I provided carefully)!

ASP.NET MVC - Sharing Session State Between Controllers

I am still mostly unfamiliar with Inversion of Control (although I am learning about it now) so if that is the solution to my question, just let me know and I'll get back to learning about it.
I have a pair of controllers which need to a Session variable, naturally nothing too special has happen because of how Session works in the first place, but this got me wondering what the cleanest way to share related objects between two separate controllers is. In my specific scenario I have an UploadController and a ProductController which work in conjunction with one another to upload image files. As files are uploaded by the UploadController, data about the upload is stored in the Session. After this happens I need to access that Session data in the ProductController. If I create a get/set property for the Session variable containing my upload information in both controllers I'll be able to access that data, but at the same time I'll be violating all sorts of DRY, not to mention creating a, at best, confusing design where an object is shared and modified by two completely disconnected objects.
What do you suggest?
Exact Context:
A file upload View posts a file to UploadController.ImageWithpreview(), which then reads in the posted file and copies it to a temporary directory. After saving the file, another class produces a thumbnail of the uploaded image. The path to both the original file and the generated thumbnail are then returned with a JsonResult to a javascript callback which updates some dynamic content in a form on the page which can be "Saved" or "Cancelled". Whether the uploaded image is saved or it is skipped, I need to either move or delete both it and the generated thumbnail from the temporary directory. To facilitate this, UploadController keeps track of all of the upload files and their thumbnails in a Session-maintained Queue object.
Back in the View: after the form is populated with a generated thumbnail of the image that was uploaded, the form posts back to the ProductsController where the selected file is identified (currently I store the filename in a Hidden field, which I realize is a horrible vulnerability), and then copied out of the temp directory to a permanent location. Ideally, I would like to simply access the Queue I have stored in the Session so that the form does not need to contain the image location as it does now. This is how I have envisioned my solution, but I'll eagerly listen to any comments or criticisms.
A couple of solutions come to mind. You could use a "SessionState" class that maps into the request and gets/sets the info as such (I'm doing this from memory so this is unlikely to compile and is meant to convey the point):
internal class SessionState
{
string ImageName
{
get { return HttpContext.Current.Session["ImageName"]; }
set { HttpContext.Current.Session["ImageName"] = value; }
}
}
And then from the controller, do something like:
var sessionState = new SessionState();
sessionState.ImageName = "xyz";
/* Or */
var imageName = sessionState.ImageName;
Alternatively, you could create a controller extension method:
public static class SessionControllerExtensions
{
public static string GetImageName(this IController controller)
{
return HttpContext.Current.Session["ImageName"];
}
public static string SetImageName(this IController controller, string imageName)
{
HttpContext.Current.Session["ImageName"] = imageName;
}
}
Then from the controller:
this.SetImageName("xyz");
/* or */
var imageName = this.GetImageName();
This is certainly DRY. That said, I don't particularly like either of these solutions as I prefer to store as little data, if any, in session. But if you're intent is to hold onto all of this information without having to load/discern it from some other source, this is the quickest (dirtiest) way I can think of to do it. I'm quite certain there's a much more elegant solution, but I don't have all of the information about what it is you're trying to do and what the problem domain is.
Keep in mind that when storing information in the session, you will have to dehydrate/rehydrate the objects via serialization and you may not be getting the performance you think you are from doing it this way.
Hope this helps.
EDIT: In response to additional information
Not sure on where you're looking to deploy this, but processing images "real-time" is a sure fire way to be hit with a DoS attack. My suggestion to you is as follows -- this is assuming that this is public facing and anyone can upload an image:
1) Allow the user to upload an image. This image goes into the processing queue for background processing by the application or some service. Additionally, the name of the image goes into the user's personal processing queue -- likely a table in the database. Information about background processing in a web app can be found # Schedule a job in hosted web server
2) Process these images and, while processing, display a "processing graphic". You can have an ajax request on the product page that checks for images being processed and trys to reload them every X seconds.
3) While an image is being "processed", the user can opt out of processing assuming they're the one that uploaded the image. This is available either on the product page(s) that display the image or on a separate "user queue" view that will allow them to remove the image from consideration.
So, you end up with some more domain objects and those objects are managed by the queue. I'm a strong advocate of convention over configuration so the final destination of the product image(s) should be predefined. Something like:
images/products/{id}.jpg or, if a collection, images/products/{id}/{sequence}.jpg.
You then don't need to know the destination in the form. It's the same for all images.
The queue then needs to know where the temp image was uploaded and what the product id was. The queue worker pops items from the queue, processes them, and stores them accordingly.
I know this sounds a little more "structured" than what you originally intended, but I think it's a little cleaner.
Is there complete equivalence between the UploadController and ProductController?
As files are uploaded by the UploadController, data about the upload is stored in the Session. After this happens I need to access that Session data in the ProductController.
As I read that the UploadControl needs read and write access to Upload data, the ProductController needs only read.
If that's true then you can make it clear by using an immuatable wrapper around the upload information and have the UploadController put that into the session.
The Session itself is by definiton a public shared noticeboard, decouples explicit relationships at the cost of allowing anyone to get and put. You could allow the ProductController to know about the UploadController and hence remove the need for passing the upload information via the session. My instinct is that the upload info is interesting to the public, so using Session is reasonable.
I don't see any DRY violation here, we are explicitly trying to separate responsibilities.

Passing messages from the application layer to the View

Let's say I have an AddProductToCartTask with a method Execute().
Now, the task is being called from a Controller. In the Execute method , there
is a check that if not met - the action is not performed.
Let's say the message returned would be: "You do not have enough bonus to buy
this product".
I thought about launching an event when the domain validation fails - but that would mean that in the Controller I have to have all kinds of class variables that need checking in the action (to determine if I need to set an error message, or i need to redirect .. etc)
Instead of this I could have a method on the task : GetErrorMessages(). If empty
return the JSON object if not empty return the Message. Or the method could return an enum that would tell if i need to redirect or set a message or return the object ....
I'm not sure which road to take. Any input would be appreciated. How do you bubble up messages from your domain layer ?
Edit: this is mainly in an AJAX context. Even though I'm not sure it matters as it's an action that it's getting called from somewhere .
I might be misunderstanding your request, but it seems to me like you want a central messages functionality, rather than something specific to the task object. If you leave it in your task, then the task must be kept in scope and "alive" until the AJAX request.
I do something similar, though directly from the Controller.
I have a static class called Messages. It has AddMessage(), GetLastMessage(), and GetAllMessages() methods. Each one, when first called, will check the user's session variable and, if nothing is found, creates and saves a Queue<string>() object. The methods are basically just an interface to the Queue. The Queue is nice because it handles push/pop which automatically removed "viewed" messages.
My controller does:
Messages.AddMessage("Product Saved");
You could potentially do:
Messages.AddMessage(task...GetErrorMessages());
Then, from my View, I have an html helper that checks how many error messages there are and, if any, creates a <ul> with each message as a <li>.
You could just as easily have a GetMessages() controller that returns any messages as a JSON object.
James

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