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Asynchronous, High-Performance Login for Web Farms

Saturday, November 10th, 2007.

Often during my consulting engagements I run into people who say, "some things just can’t be made asynchronous" even after they agree about the inherent scalability that asynchronous communications pattern bring. One often-cited example is user authentication – taking a username and password combo and authenticating it against some back-end store. For the purpose of this post, I’m going to assume a database. Also, I’m not going to be showing more advanced features like ETags to further improve the solution.

The Setup

Just so that the example is in itself secure, we’ll assume that the password is one-way hashed before being stored. Also, given a reasonable network infrastructure our web servers will be isolated in the DMZ and will have to access some application server which, in turn, will communicate with the DB. There’s also a good chance for something like round-robin load-balancing between web servers, especially for things like user login.

Before diving into the meat of it, I wanted to preface with a few words. One of the commonalities I’ve found when people dismiss asynchrony is that they don’t consider a real deployment environment, or scaling up a solution to multiple servers, farms, or datacenters.

The Synchronous Solution

In the synchronous solution, each one of our web servers will be contacting the app server for each user login request. In other words, the load on the app server and, consequently, on the database server will be proportional to the number of logins. One property of this load is its data locality, or rather, the lack of it. Given that user U logged in, the DB won’t necessarily gain any performance benefits by loading all username/password data into memory for the same page as user U. Another property is that this data is very non-volatile – it doesn’t change that often.

I won’t go to far into the synchronous solution since its been analysed numerous times before. The bottom line is that the database is the bottleneck. You could use sharding solutions. Many of the large sites have numerous read-only databases for this kind of data, with one master for updates – replicating out to the read-only replicas. That’s great if you’re using a nice cheap database like mySql (of LAMP), not so nice if you’re running Oracle or MS Sql Server.

Regardless of what you’re doing in your data tier, you’re there. Wouldn’t it be nice to close the loop in the web servers? Even if you are using Apache, that’s going to be less iron, electricity, and cooling all around. That’s what the asynchronous solution is all about – capitalizing on the low cost of memory to save on other things.

The Asynchronous Solution

In the asynchronous solution, we cache username/hashed-password pairs in memory on our web servers, and authenticate against that. Let’s analyse how much memory that takes:

Usernames are usually 12 characters or less, but let’s take an average of 32 to be sure. Using Unicode we get to 64 bytes for the username. Hashed passwords can run between 256 and 512 bits depending on the algorithm, divide by 8 and you have 64 bytes. That’s about 128 bytes altogether. So we can safely cache 8 million of these with 1GB of memory per web server. If you’ve got a million users, first of all, good for you 🙂 Second, that’s just 128 MB of memory – relatively nothing even for a cheap 2GB web server.

Also, consider the fact that when registering a new user we can check if such a username is already taken at the web server level. That doesn’t mean it won’t be checked again in the DB to account for concurrency issues, but that the load on the DB is further reduced. Other things to notice include no read-only replicas and no replication. Simple. Our web servers are the "replicas".

The Authentication Service

What makes it all work is the "Authentication Service" on the app server. This was always there in the synchronous solution. It is what used to field all the login requests from the web servers, and, of course, allowed them to register new users and all the regular stuff. The difference is that now it publishes a message when a new user is registered (or rather, is validated – all a part of the internal long-running workflow). It also allows subscribers to receive the list of all username/hashed-password pairs. It’s also quite likely that it would keep the same data in memory too.

The same message can be used to publish both single updates, and returning the full list when using NServiceBus. Let’s define the message:

[Serializable]
public class UsernameInUseMessage : IMessage
{
    private string username;
    public string Username
    {
        get { return username; }
        set { username = value; }
    }

    private byte[] hashedPassword;
    public byte[] HashedPassword
    {
        get { return hashedPassword; }
        set { hashedPassword = value; }
    }
}

And the message that the web server sends when it wants the full list:

[Serializable]
public class GetAllUsernamesMessage : IMessage
{

}

And the code that the web server runs on startup looks like this (assuming constructor injection):

 

public class UserAuthenticationServiceAgent

    public UserAuthenticationServiceAgent(IBus bus) 
    { 
        this.bus = bus;
        bus.Subscribe(typeof(UsernameInUseMessage)); 
        bus.Send(new GetAllUsernamesMessages());
    }

}

And the code that runs in the Authentication Service when the GetAllUsernamesMessage is received:

 

public class GetAllUsernamesMessageHandler : BaseMessageHandler<GetAllUsernamesMessage>
{
    public override void Handle(GetAllUsernamesMessage message)
    {
        this.Bus.Reply(Cache.GetAll<UsernameInUseMessage>());
    }
}

 

And the class on the web server that handles a UsernameInUseMessage when it arrives:

 

public class UsernameInUseMessageHandler : BaseMessageHandler<UsernameInUseMessage>
{
    public override void Handle(UsernameInUseMessage message)
    { 
        WebCache.SaveOrUpdate(message.Username, message.HashedPassword); 
    }
}

When the app server sends the full list, multiple objects of the type UsernameInUseMessage are sent in one physical message to that web server. However, the bus object that runs on the web server dispatches each of these logical messages one at a time to the message handler above.

So, when it comes time to actually authenticate a user, this the web page (or controller, if you’re doing MVC) would call:

public class UserAuthenticationServiceAgent
{
    public bool Authenticate(string username, string password)
    {
        byte[] existingHashedPassword = WebCache[username];
        if (existingHashedPassword != null)
            return existingHashedPassword == this.Hash(password);

        return false;
    }
}

 

When registering a new user, the web server would of course first check its cache, and then send a RegisterUserMessage that contained the username and the hashed password.

[Serializable]
[StartsWorkflow]
public class RegisterUserMessage : IMessage
{
    private string username;
    public string Username
    {
        get { return username; }
        set { username = value; }
    }

    private string email;
    public string Email
    {
        get { return email; }
        set { email = value; }
    }

    private byte[] hashedPassword;
    public byte[] HashedPassword
    {
        get { return hashedPassword; }
        set { hashedPassword = value; }
    }
}

 

When the RegisterUserMessage arrives at the app server, a new long-running workflow is kicked off to handle the process:

public class RegisterUserWorkflow :
    BaseWorkflow<RegisterUserMessage>, IMessageHandler<UserValidatedMessage>
{
    public void Handle(RegisterUserMessage message)
    {
        //send validation request to message.Email containing this.Id (a guid)
        // as a part of the URL
    }

    /// <summary>
    /// When a user clicks the validation link in the email, the web server
    /// sends this message (containing the workflow Id)
    /// </summary>
    /// <param name="message"></param>
    public void Handle(UserValidatedMessage message)
    {
        // write user to the DB

        this.Bus.Publish(new UsernameInUseMessage(
            message.Username, message.HashedPassword));
    }
}

That UsernameInUseMessage would eventually arrive at all the web servers subscribed.

Performance/Security Trade-Offs

When looking deeper into this workflow we realize that it could be implemented as two separate message handlers, and have the email address take the place of the workflow Id. The problem with this alternate, better performing solution has to do with security. By removing the dependence on the workflow Id, we’ve in essence stated that we’re willing to receive a UserValidatedMessage without having previously received the RegisterUserMessage.

Since the processing of the UserValidatedMessage is relatively expensive – writing to the DB and publishing messages to all web servers, a malicious user could perform a denial of service (DOS) attack without that many messages, thus flying under the radar of many detection systems. Spoofing a guid that would result in a valid workflow instance is much more difficult. Also, since workflow instances would probably be stored in some in-memory, replicated data grid the relative cost of a lookup would be quite small – small enough to avoid a DOS until a detection system picked it up.

Improved Bandwidth & Latency

The bottom line is that you’re getting much more out of your web tier this way, rather than hammering your data tier and having to scale it out much sooner. Also, notice that there is much less network traffic this way. Not such a big deal for usernames and passwords, but other scenarios built in the same way may need more data. Of course, the time it takes us to log a user in is much shorter as well since we don’t have to cross back and forth from the web server (in the DMZ) to the app server, to the db server.

The important thing to remember in this solution is doing pub/sub. NServiceBus merely provides a simple API for designing the system around pub/sub. And publishing is where you get the serious scalability. As you get more users, you’ll obviously need to get more web servers. The thing is that you probably won’t need more database servers just to handle logins. In this case, you also get lower latency per request since all work needed to be done can be done locally on the server that received the request.

ETags make it even better

For the more advanced crowd, I’ll wrap it up with the ETags. Since web servers do go down, and the cache will be cleared, what we can do is to write that cache to disk (probably in a background thread), and "tag" it with something that the server gave us along with the last UsernameInUseMessage we received. That way, when the web server comes back up, it can send that ETag along with its GetAllUsernamesMessage so that the app server will only send the changes that occurred since. This drives down network usage even more at the insignificant cost of some disk space on the web servers.

And in closing…

Even if you don’t have anything more than a single physical server today, and it acts as your web server and database server, this solution won’t slow things down. If anything, it’ll speed it up. Regardless, you’re much better prepared to scale out than before – no need to rip and replace your entire architecture just as you get 8 million Facebook users banging down your front door.

So, go check out NServiceBus and get the most out of your iron.

  
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13 Comments

  1. TechEd USA 2008 Says:

    […] performance and scalable web applications based on the principles I outlined in my previous post Asynchronous, High Performance Login for Web Farms. I’ll also be giving a more interactive session on How to Avoid a Failed SOA, and coming in […]


  2. Daniel Says:

    What if you need to lock the account after three invalid attempts to login? You can still refuse invalid usernames on web server, but you’ll need to synchronize the number of invalid logins (valid username, invalid password) between all web servers after each invalid login, and/or write it to the database. I think it’s quite common feature.


  3. udidahan Says:

    Daniel,

    Here’s how to do it:

    On each invalid login, send an InvalidLoginMessage.

    The “service-side” handles that message and writes the number of attempts. If the number of attempts is greater than N, publish an AccountLockedMessage.

    When web servers get that notification, they mark the account as locked preventing further attempts to login.

    This solution leaves a window of time open (probably less than a second) where a user may be able to attempt to login more than N times. On the other hand, the solution is much more scalable than regular locking based solutions.

    The tradeoff should be made at the system level, and not the feature level. I think that it’s more than acceptable.


  4. Daniel Says:

    I’m a different Daniel from #2, but have a related question. I’m still trying to get my head around this concept and have two questions:

    -How are foreign keys and unique constraints handled in this setup? For example, if there is a unique constraint on email in the database, but the web server allowed a user to register the same email twice. Is it expected that all such constraints should be enforced in the service in addition to the database?

    -Are there, in fact, cases where “things can’t be made asynchronous”? When designing a system, do you have any guidelines for when to/not to use nServiceBus and event-driven design?


  5. udidahan Says:

    Daniel #4,

    Foreign keys and unique constraints are still enforced in the DB. The web server, by having a copy of emails locally, can avoid calling through to the DB for user registrations which are clearly invalid. Still, the web server doesn’t add users on its own volition, but submits a request through to the DB for that.

    The point of this web tier caching is to reduce load on the DB, not to replace it entirely.

    There are cases where “things can’t be made asynchronous”, but less than you might think. I do have guidelines around them, but they fill up a 5-day course and are very much tied to understanding the specifics of your business domain.

    Hope that helps.


  6. Jai Lalwani Says:

    Udi,
    Do you’ve any such guidelines, when to and not to make things asynchronous.

    Regards,
    Jai


  7. udidahan Says:

    Jai,

    Here’s some:

    If you can ensure that the call will be local, you can often make it synchronous.

    If you suspect that a call will be remote, you should prefer to make it asynchronous / non-blocking.


  8. Messaging ROI Says:

    […] post, he follows the design I described a while back on using messaging for user management and login for a high-scale web scenario. In his comments, he agrees with the above stating: “I certainly think that a similar […]


  9. Messaging v Enterprise aplikáciách. NServiceBus. - Tomáš - DevBlog Says:

    […] DDD) pridáva praktické(nielen design) argumenty a ilustruje ich na user logon scenári – Asynchronous, High-Performance Login for Web Farms. Udi ďalej uzatvára diskusiu o zmysle messagingu v aplikáciach medzi Ayendem a Gregom Youngom […]


  10. Jørn Wildt Says:

    Back to the question about registering twice (unique constraints on email for instance): how would the GUI handle this situation?

    Scenario:

    1) User registers new name/email. This account does not exist on the webserver, so the user gets the immediate answer “Ok”.

    2) The register-new-user message arrives in the app server where a clash is detected.

    How is the user informed of this cancellation of his account?

    Thanks, Jørn


  11. udidahan Says:

    Jørn,

    Since the app server actually handles the process of registering the user, the user doesn’t receive an OK until they go through the app server (and they confirm their email address).

    The purpose of the web server cache is primarily to offload the reads from the database giving us near linear scalability in adding web servers for the login function. While it also serves us for first-level conflict detection, it isn’t the authoritative source of information.

    Does that answer your question?


  12. Scott Johnson Says:

    I am evaluating nServiceBus for an upcoming project, and I need the functionality described in this article of the BaseMessageHandler. Unfortunately, I cannot locate this reference anywhere in the 1.9 RTM I have installed. My events are currently inheriting IMessageHandler, but I need to use this.Bus.Reply(), and I can’t. Have I missed something?


  13. udidahan Says:

    Scott,

    All you need to do is define a public property on your message handler of the type IBus, and it’ll work.

    Feel free to ask these questions on the discussion group as well:

    http://tech.groups.yahoo.com/group/nservicebus/messages


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“Udi is one of the leaders in the .Net development community, one of the truly smart guys who do not just get best architectural practice well enough to educate others but drives innovation. Udi consistently challenges my thinking in ways that make me better at what I do.”

Liron Levy, Team Leader at Rafael
“I've met Udi when I worked as a team leader in Rafael. One of the most senior managers there knew Udi because he was doing superb architecture job in another Rafael project and he recommended bringing him on board to help the project I was leading.
Udi brought with him fresh solutions and invaluable deep architecture insights. He is an authority on SOA (service oriented architecture) and this was a tremendous help in our project.
On the personal level - Udi is a great communicator and can persuade even the most difficult audiences (I was part of such an audience myself..) by bringing sound explanations that draw on his extensive knowledge in the software business. Working with Udi was a great learning experience for me, and I'll be happy to work with him again in the future.”

Adam Dymitruk Adam Dymitruk, Director of IT at Apara Systems
“I met Udi for the first time at DevTeach in Montreal back in early 2007. While Udi is usually involved in SOA subjects, his knowledge spans all of a software development company's concerns. I would not hesitate to recommend Udi for any company that needs excellent leadership, mentoring, problem solving, application of patterns, implementation of methodologies and straight out solution development.
There are very few people in the world that are as dedicated to their craft as Udi is to his. At ALT.NET Seattle, Udi explained many core ideas about SOA. The team that I brought with me found his workshop and other talks the highlight of the event and provided the most value to us and our organization. I am thrilled to have the opportunity to recommend him.”

Eytan Michaeli Eytan Michaeli, CTO Korentec
“Udi was responsible for a major project in the company, and as a chief architect designed a complex multi server C4I system with many innovations and excellent performance.”


Carl Kenne Carl Kenne, .Net Consultant at Dotway AB
“Udi's session "DDD in Enterprise apps" was truly an eye opener. Udi has a great ability to explain complex enterprise designs in a very comprehensive and inspiring way. I've seen several sessions on both DDD and SOA in the past, but Udi puts it in a completly new perspective and makes us understand what it's all really about. If you ever have a chance to see any of Udi's sessions in the future, take it!”

Avi Nehama, R&D Project Manager at Retalix
“Not only that Udi is a briliant software architecture consultant, he also has remarkable abilities to present complex ideas in a simple and concise manner, and...
always with a smile. Udi is indeed a top-league professional!”

Ben Scheirman Ben Scheirman, Lead Developer at CenterPoint Energy
“Udi is one of those rare people who not only deeply understands SOA and domain driven design, but also eloquently conveys that in an easy to grasp way. He is patient, polite, and easy to talk to. I'm extremely glad I came to his workshop on SOA.”

Scott C. Reynolds Scott C. Reynolds, Director of Software Engineering at CBLPath
“Udi is consistently advancing the state of thought in software architecture, service orientation, and domain modeling.
His mastery of the technologies and techniques is second to none, but he pairs that with a singular ability to listen and communicate effectively with all parties, technical and non, to help people arrive at context-appropriate solutions. Every time I have worked with Udi, or attended a talk of his, or just had a conversation with him I have come away from it enriched with new understanding about the ideas discussed.”

Evgeny-Hen Osipow, Head of R&D at PCLine
“Udi has helped PCLine on projects by implementing architectural blueprints demonstrating the value of simple design and code.”

Rhys Campbell Rhys Campbell, Owner at Artemis West
“For many years I have been following the works of Udi. His explanation of often complex design and architectural concepts are so cleanly broken down that even the most junior of architects can begin to understand these concepts. These concepts however tend to typify the "real world" problems we face daily so even the most experienced software expert will find himself in an "Aha!" moment when following Udi teachings.
It was a pleasure to finally meet Udi in Seattle Alt.Net OpenSpaces 2008, where I was pleasantly surprised at how down-to-earth and approachable he was. His depth and breadth of software knowledge also became apparent when discussion with his peers quickly dove deep in to the problems we current face. If given the opportunity to work with or recommend Udi I would quickly take that chance. When I think .Net Architecture, I think Udi.”

Sverre Hundeide Sverre Hundeide, Senior Consultant at Objectware
“Udi had been hired to present the third LEAP master class in Oslo. He is an well known international expert on enterprise software architecture and design, and is the author of the open source messaging framework nServiceBus. The entire class was based on discussion and interaction with the audience, and the only Power Point slide used was the one showing the agenda.
He started out with sketching a naive traditional n-tier application (big ball of mud), and based on suggestions from the audience we explored different solutions which might improve the solution. Whatever suggestions we threw at him, he always had a thoroughly considered answer describing pros and cons with the suggested solution. He obviously has a lot of experience with real world enterprise SOA applications.”

Raphaël Wouters Raphaël Wouters, Owner/Managing Partner at Medinternals
“I attended Udi's excellent course 'Advanced Distributed System Design with SOA and DDD' at Skillsmatter. Few people can truly claim such a high skill and expertise level, present it using a pragmatic, concrete no-nonsense approach and still stay reachable.”

Nimrod Peleg Nimrod Peleg, Lab Engineer at Technion IIT
“One of the best programmers and software engineer I've ever met, creative, knows how to design and implemet, very collaborative and finally - the applications he designed implemeted work for many years without any problems!

Jose Manuel Beas
“When I attended Udi's SOA Workshop, then it suddenly changed my view of what Service Oriented Architectures were all about. Udi explained complex concepts very clearly and created a very productive discussion environment where all the attendees could learn a lot. I strongly recommend hiring Udi.”

Daniel Jin Daniel Jin, Senior Lead Developer at PJM Interconnection
“Udi is one of the top SOA guru in the .NET space. He is always eager to help others by sharing his knowledge and experiences. His blog articles often offer deep insights and is a invaluable resource. I highly recommend him.”

Pasi Taive Pasi Taive, Chief Architect at Tieto
“I attended both of Udi's "UI Composition Key to SOA Success" and "DDD in Enterprise Apps" sessions and they were exceptionally good. I will definitely participate in his sessions again. Udi is a great presenter and has the ability to explain complex issues in a manner that everyone understands.”

Eran Sagi, Software Architect at HP
“So far, I heard about Service Oriented architecture all over. Everyone mentions it – the big buzz word. But, when I actually asked someone for what does it really mean, no one managed to give me a complete satisfied answer. Finally in his excellent course “Advanced Distributed Systems”, I got the answers I was looking for. Udi went over the different motivations (principles) of Services Oriented, explained them well one by one, and showed how each one could be technically addressed using NService bus. In his course, Udi also explain the way of thinking when coming to design a Service Oriented system. What are the questions you need to ask yourself in order to shape your system, place the logic in the right places for best Service Oriented system.

I would recommend this course for any architect or developer who deals with distributed system, but not only. In my work we do not have a real distributed system, but one PC which host both the UI application and the different services inside, all communicating via WCF. I found that many of the architecture principles and motivations of SOA apply for our system as well. Enough that you have SW partitioned into components and most of the principles becomes relevant to you as well. Bottom line – an excellent course recommended to any SW Architect, or any developer dealing with distributed system.”

Consult with Udi

Guest Authored Books
Chapter: Introduction to SOA    Article: The Enterprise Service Bus and Your SOA

97 Things Every Software Architect Should Know



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