In my C# web application I need to save some items in the application cache and then retrieve those items by key. To save those items I'm using the following code:
HttpContext.Current.Items["urlname"] = "something";
That saves the item fine in the Items hashtable. However I'm not able to get the value of "urlname" in another location in the application.
Could anybody tell me what I'm doing wrong or maybe there's a better way of doing this?
Thanks.
[EDIT] Ok, now I know that the Items hashtable is a storage with very short lifetime. The contents will be deleted once the response is sent to the browser. So, how can I save my data in other way?
HttpContext.Current.Items is a per request store.
You should use HttpContext.Current.Session if you want to store and access values across multiple pages.
Have a read of this - https://web.archive.org/web/20201202215202/https://www.4guysfromrolla.com/articles/060904-1.aspx
Other thing to consider is something like memcached - CouchBase have a Windows implementation
Related
I'm trying to create simple web-app using grails.
Now, I need create new session when user opens same page in different tabs to avoid displaying same data in all opened tabs.
is it possible to define that page was opened in new tab? if it possible how create new session in controller action?.
or maybe there is a way to get something like browser tab-id?
You seem to misunderstand how a session works and they are assigned.
A session is per browser (and domain/host).
So, even though you can create a new session in a controller action it won't help because that will become the session for all the tabs of the browser and the previous session(s) will be invalidated/abandoned.
There is no such thing as a browser tab id.
You'll need to address the root issue which is causing your data affinity to be based on a browser session. Make it based on something else. (Just a general suggestion since this isn't part of your questions and you haven't provided any details.)
Here is my thoughts on this.
What you are trying to accomplish may appear simple but you will need some mechanism to capture who each session be whether it be a spring security username or actual http session id and to then store with that what controller actions they have visited so far and to keep this consistently updated whilst checking it over and over again.
Something as simple as
[
['10001':[controller:'someController', 'someAction'],[controller:'someController1', 'someAction1'],
],
['10002':[controller:'someController', 'someAction'],[controller:'someController1', 'someAction1']
]
Where '10001' is your key of your map and is your session id then it contains a list of internal maps of places visited that you capture and try to work out if they been there already - basically the question here is....
Where is the AI to say if they have seen someAction1 they should see action2 and what happens when they seen action1 and action2 and so on an ever ending loop of and what next ?
Either way you could do all that as a session variable that contains a map like above - the issue you will hit will be concurrent map (where it gets updated and read at the same time).
So you will then need to look over and into using concurrent hashmaps to get around such issues.
Either way the problem with all of above is the consistent logic to figure out if they have seen all possible options then what next ?
I think you are far better off thinking of it from a different point of view as in base it on timestamp and move the query or whatever it is to randomly generate a different output based on that timestamp since that is always going to change regardless of the user
I'm trying to learn how to use Parse and while it's very simple, it's also... not? Perhaps I'm just missing something, but it seems like Parse requires a lot of client-side code, and even sending multiple requests for a single request. For example, in my application I have a small photo gallery that each user has. The images are stored on Parse and obtained from parse when needed.
I want to make sure that a user can not store any more than 15 images in their gallery at a time, I also want these images to be ordered by an index.
Currently it seems like the only viable option is to perform the following steps on the client:
Execute a query/request to get the amount of pictures stored.
If the amount is less than 15, then execute a request to upload the picture.
Once the picture is uploaded, execute a request that stores an object linking the user that uploaded the PFFile.
This is a total of 3 or? 6 requests just to upload a file, depending on if a "response" is considered a request by parse too. This also does not provide any way to order the pictures in the gallery. Would I have to create a custom field called "index" and set that to the number of photos received in the first query + 1?
It's worse than you think: to create the picture you must create a file, save it, then save a reference to the file in an object and save that, too.
But it's also better than you think: this sort of network usage is expected in a connected app, and some of it can be mitigated with additional logic on the server ("cloud code" in parse parlance).
First, in your app, consider a simple data model where _User has an array of images (represented, say, by an "UserImage" custom class). If you keep this relationship as an array of pointers on user, than a user's images can be fetched eagerly, when the app starts, so you'll know the image count as a fact along with the user. The UserImage object will have a file reference in it, so you can optionally fetch the image data and just hold the lighter metadata with the current user.
Ordering is a more ephemeral idea. One doesn't order objects as they are saved, but rather as they are retrieved. Queries can be ordered according to any attribute, and even more to the point, since you're retrieving all 15 images, you should consider ordering them for presentation a function of the UI, not the data.
Finally, parse limits your app not by transaction count, but by transaction rate, with a free limit low enough to serve plenty of users.
If I lock my phone while running my application and unlock it say after 30 minutes or 60 minutes, my screen appears blank. All my data (its a huge list compare it to a user's twitter feed) which was in an Observable collection in my ViewModel has disappeared. When I refresh I get NullReferenceException. Note that I am not handling any state save while locking and unlocking the phone. Is that the reason for the loss of my data? How can I handle it? Since there is a limit on the state data which can be saved of 4Mb Max, will it affect the functioning of my application even if I do implement it?
[Update]
I have tried the following things:
1) http://www.scottlogic.co.uk/blog/colin/2011/05/a-simple-windows-phone-7-mvvm-tombstoning-example/
2) http://www.scottlogic.co.uk/blog/colin/2011/10/a-windows-phone-7-1-mango-mvvm-tombstoning-example/
and many more.
The problem which I now face is that my application's viewModel contains an observable collection which I have binded to the UI. This observable collection is a collection of my user-defined class which contains complex data members. One of them is a dictionary. When i try to save my viewModel using XMLSerialization it throws an error as XML serialization doesn't support Dictionary.
I have also tried to write my viewmodel after Data contract serialization onto the IS during App_Deactivated and retrieve it on App_Activated. But my collection is null on resume. On opening the IS file it shows that the collection was not written onto the file. Am I missing some key ingredient in-order to solve this problem?
Note: I need my list. I cannot refresh data.
I'd suggest that this is the wrong approach.
Tombstoning is designed to allow you to save your state, not your data. You want to store the following:
The page you're currently on
The parameters, if any, that were used to get your list of data that you are currently showing
Any selection state (has the user selected a row, etc)
Any page state (is it in edit-mode, etc)
Not all of these things will apply, but it should provide you with an idea of what you should be storing.
This will be a significantly smaller set of data using simple data types rather than large chains of complex objects.
So:
Store the properties/parameters that you use to get your data
When the app resumes go get your data again using the params. If this take a while give the user some form of progress notification. If you can't accurately do this then display activity on the screen until the load finishes so the user knows that something is happening.
I have a series of pages that an end user must fill out (check boxes) and when they are finished with each page I attempt to create a List of the check boxes they selected. At the end of the series of pages, I would like to show them everything that they have selected in a confirmation page. I have noticed that between requests the information in the List<> I create on each page is not available to the final confirmation page. I've tried a few different solutions (private globals) to no avail. How would I pass data across ActionResults to accomplish displaying all the selected data on the confirmation page? Thanks.
One potential solution. Others?
The web is stateless, meaning you have to store things if you want to keep them around for later use. That's true for any web framework. You'll need to store each page's results somewhere.
Options for building a wizard:
Store all of the selected answers in session and keep building it up from page to page. The final confirmation would get the results from session.
Store them in a database.
Store results in in a cookie.
Store them in HTML5 local storage
Carry them through on each page with hidden fields. Page 2 would have Page 1's answers in hidden fields, etc.
You need to save a state between the requests. You can do this with:
Query string parameters
Session state
Hidden fields
Db (if you wanna persist the intermediate choices after each request)
Local storage
Cookies
Anything else?
I'd guess, as RyanW points out, that storing them in session state is the usual way to do it. You could however fetch all steps in one request, do some fancy JS / store the intermediary results locally and make a final post when the questionnaire is complete.
I have a trie-based word detection algorithm for a custom dictionary. Note that regular expressions are too brittle with this dictionary as entries may contain spaces, periods, etc.
I've implemented the algorithm in a local C# app that reads in the dictionary from file and stores the trie in memory (it's compact, so no RAM size issues at all). Now I would like to use this algorithm in an MVC 3 app on a cloud host like AppHarbor, with the added twist that I want a web interface to enable adding/editing words.
It's fast enough that loading the dictionary from file and building the trie every time a user uploads their text would not be an issue (< 1s on my laptop). However, if I want to enable admins to edit the dictionary via the web interface, that would seem tricky since the dictionary would potentially be getting updated while a user is trying to upload text for analysis.
What is the best strategy for storing, loading, and updating the trie in an MVC 3 app?
I'm not sure if you are looking for specific implementation details, or more conceptual ideas about how to handle but I'll throw some ideas out there for now.
Actual Trie Classes - Here is a good C# example of classes for setting up a Trie. It sounds like you already have this part figured out.
Storing: I would persist the trie data to XML unless you are already using a database and have some need to have it in a dbms. The XML will be simple to work with in the MVC application and you don't need to worry about database connectivity issues, or the added cost of a database. I would also have two versions of the trie data on the server, a production copy and a production support copy, the second for which your admin can perform transactions against.
Loading In your admin module of the application, you may implement a feature for loading the trie data into memory, the frequency of data loading depends on your application needs. It could be scheduled or available as a manual function. Like in wordpress sites, if a user should access it while updating they would receive a message that the site is undergoing maintenance. You may choose to load into memory on demand only, and keep the trie loaded at all times except for if problems occurred.
Updating - I'd have a second database (or XML file) that is used for applying updates. The method of applying updates to production would depend partially on the frequency, quantity, and time of updates. One safe method might be to store transactions entered by the admin.
For example:
trie.put("John", 112);
trie.put("Doe", 222);
trie.Remove("John");
Then apply these transactions to your production data as needed via an admin function. If needed put your site into "maint" mode. If the updates are few and fast you may be able to code the site so that it will hold all work until transactions are processed, a user might have to wait a few milliseconds longer for a result but you wouldn't have to worry about mutating data issues.
This is pretty vague but just throwing some ideas out there... if you provide comments I'll try to give more.
1 Store trie in cache:
It is not dynamic data, and caching helps us in other tasks (like concurrency access to trie by admin and user)
2 Make access to cache clear:
:
public class TrieHelper
{
public Trie MyTrie
{
get
{
if (HttpContext.Current.Cache["myTrieKey"] == null)
HttpContext.Current.Cache["myTrieKey"] = LoadTrieFromFile(); //Returns Trie object
return (Trie)HttpContext.Current.Cache["myTrieKey"];
}
}
3 Lock trie object while adding operation in progress
public void AddWordToTrie(string word)
{
var trie = MyTrie;
lock (HttpContext.Current.Cache["myTrieKey"])
{
trie.AddWord(word);
} // notify that trie object locking when write data to file is not reuired
WriteNewWordToTrieFile(word); // should lock FileWriter object
}
}
4 If editing is performs by 1 admin at a time - store trie in xml file - it will be easy to implement logic of search element, after what word your word should be added (you can create function, that will use MyTrie object in memory), and add it, using linq to xml.
I've got a kind'a the same but 10 times bigger :)
The client design it's own calendar with questions ans possible answer in the meanwhile some is online and being used by the normal user.
What I come up was something as test and deploy. The Admin enters the calendar values and set it up correctly and after he can use a Preview button to see if it's like he needs/wants, then, to make the changes valid to all end users, he need to push Deploy.
He, as an ADMIN, will know that, until he pushes the DEPLOY button, all users accessing the Calendar will have the old values. Soon he hits deploy all is set in the Database, and pushed the files he uploaded into Amazon S3 (for faster access).
I update the Cache with the new calendar and the new Calendar object is cached until the App pool says otherwise or he hit the Deploy button again.
You could do something like this.
As you are going to perform your application in the cloud environment, I'd suggest you to take a look at CQRS and durable messaging and provide some concurrency model (possibly, optimistic concurrency and intelligent conflict detection http://skillsmatter.com/podcast/design-architecture/cqrs-not-just-for-server-systems 5:00)
Also, obviously, you need to analyze your business requirements more precisely because, as Udi Dahan mentioned, race conditions are result of the lack of business analysis.