Use static class in ASP.NET Web API

Question

I am posting this question using an automatic translation. Please forgive any grammatical errors.

I have built an application using the .NET framework and the ASP.net Web API. I have split the virtual path for each customer region within a site running on IIS and copied the same binary to run as separate applications. The applications run in the same application pool.

Recently, some customers have been making a very large number of requests in a matter of minutes. (I suspect a glitch in the system on the customer's end). I am thinking of adding a static class to my current application that keeps track of the number of requests per customer in a given time period and blocks them if the threshold is exceeded. From past StackOverFlow articles I have found that "information in the static class is lost if the application pool is recycled", but I have determined that this is not a problem in this case. For my purposes, I only need to be able to retain information for a few minutes.

However, I still have a few questions that I can't find answers to, so I'd like to ask you all a few questions.

1. Even if the same binary is running in the same application pool, will the static class information be kept separately for different applications?

2. Will the static constructor of a static class be executed even after the application pool is recycled?

3. Is there a problem if I reference a field in Global.asax from within a static class?

4. Is there a problem with referencing the contents of web.config from within a static class?

Attached below is the source of my experimental implementation. I plan to call the static method "ExcessiveRequestCheck.isExcessiveRequest" of this static class after the Web API receives the request and identifies the user ID.

Any advice would be sincerely appreciated.

P.S. I understand that this approach does not work well in a load balancing environment. Currently my system only runs on one virtual machine. If you are moving to the cloud or deploying a load balancer, you will probably need a different approach than this one.

public static class ExcessiveRequestCheck
{
    private static Dictionary<string, ExcessiveRequestInfo> dicExcessiveRequestCheckInfo = new Dictionary<string, ExcessiveRequestInfo>();
    private static object initLock = new object();
    private static object dicExcessiveRequestCheckInfoLock = new object();

    //If possible, I want this process to be a static constructor
    public static Dictionary<int, int> dicExcessiveRequestSkipConditions
    {
        get
        {
            lock (initLock)
            {
                if (ExcessiveRequestCheck._dicExcessiveRequestSkipConditions == null)
                {
                    //if possible, I want to set this value from Web.config.
                    ExcessiveRequestCheck._dicExcessiveRequestSkipConditions = new Dictionary<int, int>() {
                        { 5, 3 }, { 15, 5 }, { 45, 10 }, { 120, 20 }
                    };
                }
                return ExcessiveRequestCheck._dicExcessiveRequestSkipConditions;
            }
        }
    }
    private static Dictionary<int, int> _dicExcessiveRequestSkipConditions = null;

    public const int BUFFER_CLEAR_MINUTES = 5;

    public static bool isExcessiveRequest(string userId)
    {
        ExcessiveRequestCheck.refreshExcessiveRequestCheckInfo();
        lock (ExcessiveRequestCheck.dicExcessiveRequestCheckInfoLock)
        {
            if (ExcessiveRequestCheck.dicExcessiveRequestCheckInfo.ContainsKey(userId) == false)
            {
                ExcessiveRequestCheck.dicExcessiveRequestCheckInfo.Add(userId, new ExcessiveRequestInfo() { countRequest = 1 });
                return false;
            }

            bool doSkip = false;
            ExcessiveRequestCheck.dicExcessiveRequestCheckInfo[userId].countRequest++;
            foreach (KeyValuePair<int, int> pair in ExcessiveRequestCheck.dicExcessiveRequestSkipConditions)
            {
                if (ExcessiveRequestCheck.dicExcessiveRequestCheckInfo[userId].lastRequesttTime.AddSeconds(pair.Key) > DateTime.Now)
                {
                    if (ExcessiveRequestCheck.dicExcessiveRequestCheckInfo[userId].countRequest > pair.Value)
                    {
                        ExcessiveRequestCheck.dicExcessiveRequestCheckInfo[userId].wasRequestSkip = true;
                        doSkip = true;
                    }
                }
            }
            ExcessiveRequestCheck.dicExcessiveRequestCheckInfo[userId].lastRequesttTime = DateTime.Now;

            return doSkip;
        }
    }

    public static void refreshExcessiveRequestCheckInfo()
    {
        lock (ExcessiveRequestCheck.dicExcessiveRequestCheckInfoLock)
        {
            var keyList = ExcessiveRequestCheck.dicExcessiveRequestCheckInfo.Keys;
            foreach (string key in keyList)
            {
                if (ExcessiveRequestCheck.dicExcessiveRequestCheckInfo.ContainsKey(key))
                {
                    var value = ExcessiveRequestCheck.dicExcessiveRequestCheckInfo[key];
                    if (value.lastRequesttTime.AddMinutes(BUFFER_CLEAR_MINUTES) < DateTime.Now)
                    {
                        if (value.wasRequestSkip)
                        {
                            //this NLog instance was created in Global.asax.cs
                            WebApiApplication.logger.Fatal("skip request! user id=" + key);
                        }
                        ExcessiveRequestCheck.dicExcessiveRequestCheckInfo.Remove(key);
                    }
                }
            }
        }
    }
}

class ExcessiveRequestInfo
{
    public DateTime requestStartTime { get; set; } = DateTime.Now;
    public DateTime lastRequesttTime { get; set; } = DateTime.Now;
    public int countRequest { get; set; } = 0;
    public bool wasRequestSkip { get; set; } = false;
}

Answer 1

Your questions

Even if the same binary is running in the same application pool, will the static class information be kept separately for different applications?

Yes, they are separate

Will the static constructor of a static class be executed even after the application pool is recycled?

Yes, the static constructor is guaranteed to be called before any of the static methods are executed

Is there a problem if I reference a field in Global.asax from within a static class?

No more than accessing it from anywhere else

Is there a problem with referencing the contents of web.config from within a static class?

No more than accessing it from anywhere else

Your general approach

DoS

If you're trying to mitigate a denial-of-service attack or credential stuffing attack, your approach probably won't work, since requests to your service will still result in load being added to your server, and if they are performing a credential stuffing attack, it'll fill up your dictionary with millions of entries and possibly cause your application to crash.

If you want to mitigate a denial-of-service attack effectively, you will probably need a more network-oriented solution, such as a smart firewall or a WAF.

Rate limiting

If on the other hand you are attempting to throttle specific users' activities (i.e. rate limiting), again, your approach probably isn't the greatest, because it does not support load balancing-- your list is held in in-process memory. For per-user rate limiting you will probably need to track user activity in a central data store accessible to all of your servers.

Static constructors

As a general rule, you should try to avoid static constructors, or keep them very simple, as a failure in a static constructor will cause your entire application to fail to start. Be careful!

Answer 2

even if the same binary is running in the same application pool, will the static class information be kept separately for different applications?

If by different applications, you mean separate web sites? yes, it will be kept separate to each web site you have running for that app pool.

Will the static constructor of a static class be executed even after the application pool is recycled?

Hum, that's a bit confusing. The constructor will only be executed if you call the class and that given constructor. Since there is never a instance of the class created, then the "initialize/new" event is never used nor triggered. So, any method with parameters will run and work fine - including the constructor. I would suggest that there is not some "event" that gets triggered on first use - it would not and does not make sense in the context of a static class, since you never create an instance. So, if you have some methods with parameters then fine. So, constructor in the context of new instance of the class makes no sense - (did not even think that is possible with static). There is no concept of "new" event that triggers, so I fail to see how this issue can ever matter.

Is there a problem if I reference a field in Global.asax from within a static class?

Well, values in that class are global to ALL users. But, those values can go out of scope just about any old time you please. As a result, ZERO use of public members is practial. While a app-pool re-start will re-set those class values? They can go out of scope just about any old time. They are global to all and every user. So, persisting values, or attempting to persit values in a static class is NOT a viable choice for production code. You can have methods (code) in that class, but any public persisting values really can't be relied upon to persist correctly. I'm not 100% sure, but even just general .net garbage collection would likely cause a re-set.

If you need this information to persist, then you can't use static, you have to create a instance of that class and persist it in session(). And session is per user. A static class public values will apply to EVERY user - not just the current user. In effect those values are global to all users - but without any real ccontrol or garrutee that the values will persit - you have no control over this and thus you can't adopt this concpet and design for any system of practial value.

Is there a problem with referencing the contents of web.config from within a static class?

Reading values? No problem. Update or modify values? - a MASSIVE different issue. You modify web.config, that will trigger a app pool restart.

So, you free to read any file - text files, xml or whatever, and that includes web.config. As long as you not modify such files, then no problems.

The main issue here?

It simple not practical to assume, or build a design in which public static class values are to persist. The ZERO control you have when such values may go out of scope is somthing you have ZERO control over, and thus such designs can't use nor rely on values persisting.

And of course on many web hosting systems? They are now adopting cloud computing. This means from one post back to the next, you might be using a different server, and again, that means such values can't persist in memory, since from one post-back to the next, or one web service call to the next? You may well be hitting a different server anyway (and they don't share memory) (so, this suggests say using SQL server based sessions, or at the very least persisting such values in a database).

In fact, if you need such persisting values and data? Then use a database. The WHOLE idea of web based software is you do NOT have state between post-backs. And you are attempting to go even down a worse road, but hope on a wing and a prayer that some global values "might" and "sort of" and "maybe" will persist between calls to the web site. Answer: You really can't do this with any realm of reliably.

So, most of your questions don't really matter. What matters is these values are to persist, and you can't rely on such a design. If you need some persisting values, then you have to adopt a system and design that supports that concept (viewstate, cookies, or session()).

Now, I suppose you can give this a try, and then come back with a detailed report and how your experience turned out. But, there are too many pit falls, and without any code or system control over persisting values in memory, I don't think I would go down this road.

In web land, it makes next to no sense to have public variables that you attempt to persist in a static class. You can have code, you can have cool methods, you can use session(). But, the concept of persisting values in static class is a design choice that does not make sense, and can't be relied upon.

Web software is assumed to be state-less, and that VERY much is the assumption you have to make in regards to a static class, or in fact general use of such code.