Viewstate in a .ashx Handler?

问题

I've got a handler (list.ashx for example) that has a method that retrieves a large dataset, then grabs only the records that will be shown on any given "page" of data. We are allowing the users to do sorting on these results. So, on any given page run, I will be retrieving a dataset that I just got a few seconds/minutes ago, but reordering them, or showing the next page of data, etc.

My point is that my dataset really hasn't changed. Normally, the dataset would be stuck into the viewstate of a page, but since I'm using a handler, I don't have that convenience. At least I don't think so.

So, what is a common way to store the viewstate associated with a current user's given page when using a handler? Is there a way to take the dataset, encode it somehow and send that back to the user, and then on the next call, pass it back and then rehydrate a dataset from those bits?

I don't think Session would be a good place to store it since we might have 1000 users all viewing different datasets of different data, and that could bring the server to its knees. At least I think so.

Does anyone have any experience with this kind of situation, and can you give me any advice?

回答1:

In this situation I would use a cache with some type of user and query info as the key. The reason being is you say it is a large dataset. Right there is something you don't want to be pushing up and down the pipe constantly. Remember your server still has to received the data if it is in ViewState and handle it. I would do something like this which would cache it for a specific user and have a short expiry:

public DataSet GetSomeData(string user, string query, string sort)
{
    // You could make the key just based on the query params but figured
    // you would want the user in there as well.
    // You could user just the user if you want to limit it to one cached item
    // per user too.
    string key = string.Format("{0}:{1}", user, query);

    DataSet ds = HttpContext.Current.Cache[key] as DataSet;
    if (ds == null)
    {
        // Need to reload or get the data
        ds = LoadMyData(query);

        // Now store it and make the expiry short so it doesn't bog up your server
        // needlessly... worst case you have to retrieve it again because the data
        // has expired.
        HttpContext.Current.Cache.Insert(key, ds, null,
            DateTime.UtcNow.AddMinutes(yourTimeout), 
            System.Web.Caching.Cache.NoSlidingExpiration);
    }

    // Perform the sort or leave as default sorting and return
    return (string.IsNullOrEmpty(sort) ? ds : sortSortMyDataSet(ds, sort));
}

When you say 1000's of users, does that mean concurrent users? If your expiration time was 1 minute how many concurrent users would make that call in a minute and require sorting. I think offloading the data to something like similar to ViewState is just trading some cache memory for bandwidth and processing load of larget requests back and forth. The less you have to transmit back and forth the better in my opinion.

回答2:

Why don't you implement a server side caching?

A I understand, you're retrieving a large amount of data and then returns only necessary records from this data to different clients. So you could use HttpContext.Current.Cache property for this.

E.g. a property which encapsulates a data retrieving logic (gets from the original data store with the first request, then puts to cache and gets from cache with every next request) could be used. In this case all the necessary data manipulations (paging, etc.) may be done much more quicker than retrieving a large amount of data with the each request.

In the case when clients have different data sources (mean each client have its own data source) the solution above may also be implemented. I suppose each client has at least identifier, so you could use different caches for different clients (client identifier as a part of cache key).

回答3:

The best you could do is "grow your own" by including the serialized data set in the body of the request to the ASHX handler. Your handler would then check to see if the request does indeed have a body by checking Request.ContentLength and then reading from Request.InputStream, and if it does serializing that body back into the data set instead of reading from your database.

来源：https://stackoverflow.com/questions/2722377/viewstate-in-a-ashx-handler