Cloud Application Architectures : Web Application Design

I cannot possibly address the intricacies of each of the many platforms developers use in building web applications, but most of the issues you will face have nothing to do with your underlying choice of platform. Whether written in .NET, Ruby, Java, PHP, or anything else, web applications share a similar general architecture—and architecture makes or breaks an application in the cloud.

Figure 1 illustrates the generic application architecture that web applications share.

Figure 1. Most web applications share the same basic architecture

You may move around or combine the boxes a bit, but you are certain to have some kind of (most often scripting) language that generates content from a combination of templates and data pulled from a model backed by a database. The system updates the model through actions that execute transactions against the model.

1. System State and Protecting Transactions

The defining issue in moving to the cloud is how your application manages its state. Let’s look at the problem of booking a room in a hotel.

The architecture from Figure 4-1 suggests that you have represented the room and the hotel in a model. For the purposes of this discussion, it does not matter whether you have a tight separation between model, view, and data, or have mixed them to some degree. The key point is that there is some representation of the hotel and room data in your application space that mirrors their respective states in the database.

How does the application state in the application tier change between the time the user makes the request and the transaction is changed in the database?

The process might look something like this basic sequence:

1. Lock the data associated with the room.

2. Check the room to see whether it is currently available.

3. If currently available, mark it as “booked” and thus no longer available.

4. Release the lock.

1.1. The problem with memory locks

You can implement this logic in many different ways, not all of which will succeed in the cloud. A common Java approach that works well in a single-server environment but fails in a multiserver context might use the following code:

public void book(Customer customer, Room room, Date[] days)
    throws BookingException {
    synchronized( room ) { // synchronized "locks" the room object
        if( !room.isAvailable(days) ) {
            throw new BookingException("Room unavailable.");
        }
        room.book(customer, days);
    }
}

Because the code uses the Java locking keyword synchronized, no other threads in the current process can make changes to the room object.^[11] If you are on a single server, this code will work under any load supported by the server. Unfortunately, it will fail miserably in a multiserver context.

^[11] I beg Java programmers to forgive my oversimplification and technically incorrect description of the synchronized keyword. I intend the explanation to help readers in general understand what is happening in this context and not to teach people about multithreading in Java.

The problem with this example is the memory-based lock that the application grabs. If you had two clients making two separate booking requests against the same server, Java would allow only one of them to execute the synchronized block at a time. As a result, you would not end up with a double booking.

On the other hand, if you had each customer making a request against different servers (or even distinct processes on the same server), the synchronized blocks on each server could execute concurrently. As a result, the first customer to reach the room.book() call would lose his reservation because it would be overwritten by the second. Figure 2 illustrates the double-booking problem.

The non-Java way of expressing the problem is that if your transactional logic uses memory-based locking to protect the integrity of a transaction, that transaction will fail in a multiserver environment—and thus it won’t be able to take advantage of the cloud’s ability to dynamically scale application processing.

One way around this problem is to use clustering technologies or cross-server shared memory systems. Another way to approach the problem is to treat the database as the authority on the state of your system.

Figure 2. The second client overwrites the first, causing a double-booking

What If My Application Uses Memory Locks? I can hear a lot of you—especially those of you who have massively multithreaded applications—cursing me right now. If you find yourself in a situation in which you use memory-based locking and reworking the application away from that model is impractical, you can still move into the cloud. You simply won’t be able to scale your application across multiple application servers. The way around this concern is to lock everything using a shared locking mechanism—typically your database engine. The only other common alternative is to write some kind of home-grown distributed transaction management system. But why do that when your database already has one?

1.2. Transactional integrity through stored procedures

I am not a huge fan of stored procedures. A key benefit of stored procedures, however, is that they enable you to leverage the database to manage the integrity of your transactions. After all, data integrity is the main job of your database engine!

Instead of doing all of the booking logic in Java, you could leverage a MySQL stored procedure:

DELIMITER |

CREATE PROCEDURE book
(
  IN customerId BIGINT,
  IN roomId BIGINT,
  IN startDate DATE,
  IN endDate DATE,
  OUT success CHAR(1)
)
BEGIN
  DECLARE n DATE;
  DECLARE cust BIGINT;
  SET success = 'Y';
  SET n = startDate;

  bookingAttempt:
  REPEAT
    SELECT customer INTO cust FROM booking
    WHERE room_id = roomId AND booking_date = n;
    IF cust IS NOT NULL AND cust <> customerId
    THEN
      SET success = 'N';
      LEAVE bookingAttempt;
    END IF;
    UPDATE booking SET customer = customerId
    WHERE room_id = roomId AND booking_date = n;
    SET n = DATE_ADD(n, INTERVAL 1 DAY);
  UNTIL n > endDate
  END REPEAT;
  IF success = 'Y' THEN
    COMMIT;
  ELSE
    ROLLBACK;
  END IF;
END
|

					    

This method goes through each row of the booking table in your MySQL database and marks it booked by the specified customer. If it encounters a date when the room is already booked, the transaction fails and rolls back.

An example using the stored procedure follows, using Python:

def book(customerId, roomId, startDate, endDate):
    conn = getConnection();
    c = conn.cursor();
    c.execute("CALL book(%s, %s, %s, %s, @success)", \
             (customerId, roomId, startDate, endDate));
    c.execute("SELECT @success");
    row = c.fetchone();
    success = row[0];
    if success == "Y":
      return 1
    else:
      report 0

Even if you have two different application servers running two different instances of your Python application, this transaction will fail, as desired, for the second customer, regardless of the point at which the second customer’s transaction begins.

1.3. Two alternatives to stored procedures

As I noted earlier, I am not a fan of stored procedures. They have the advantage of executing faster that the same logic in an application language. Furthermore, multiserver transaction management through stored procedures is very elegant. But I have three key objections:

• Stored procedures are not portable from one database to another.

• They require an extended understanding of database programming—something that may not be available to all development teams.

• They don’t completely solve the problem of scaling transactions across application servers under all scenarios. You still need to write your applications to use them wisely, and the result may, in fact, make your application more complicated.

In addition to these core objections, I personally strongly prefer a very strict separation of presentation, business modeling, business logic, and data.

The last objection is subjective and perhaps a nasty personal quirk. The first two objections, however, are real problems. After all, how many of you reading this book have found yourselves stuck with Oracle applications that could very easily work in MySQL if it weren’t for all the stored procedures? You are paying a huge Oracle tax just because you used stored procedures to build your applications!

The second objection is a bit more esoteric. If you have the luxury of a large development staff with a diverse skill set, you don’t see this problem. If you are in a small company that needs each person to wear multiple hats, it helps to have an application architecture that requires little or no database programming expertise.

To keep your logic at the application server level while still maintaining multiserver transactional integrity, you must either create protections against dirty writes or create a lock in the database.

One of the techniques I feature—and generally recommend for its speed benefits, whatever your overall architecture—is the use of a last update timestamp and modifying agent in your updates.

The booking logic from the stored procedure essentially was an update to the booking table:

UPDATE booking SET customer = ? WHERE booking_id = ?;

If you add last_update_timestamp and last_update_user fields, that SQL would operate more effectively in a multiserver environment:

UPDATE booking
SET customer = ?, last_update_timestamp = ?, last_update_user = ?
WHERE booking_id = ? AND last_update_timestamp = ? AND last_update_user = ?;

					    

In this situation, the first client will attempt to book the room for the specified date and succeed. The second client then attempts to update the row but gets no matches since the timestamp it reads—as well as the user ID of the user on the client—will not match the values updated by the first client. The second client realizes it has updated zero rows and subsequently displays an error message. No double booking!

This approach works well as long as you do not end up structuring transactions in a way that will create deadlocks. A deadlock occurs between two transactions when each transaction is waiting on the other to release a lock. Our reservations system example is an application in which a deadlock is certainly possible.

Because we are booking a range of dates in the same transaction, poorly structured application logic could cause two clients to wait on each other as one attempts to book a date already booked by the other, and vice versa. For example, if you and I are looking to book both Tuesday and Wednesday, but for whatever reason your client first tries Wednesday and my client first tries Tuesday, we will end up in a deadlock where I wait on you to commit your Wednesday booking and you wait on me to commit my Tuesday booking.

This somewhat contrived scenario is easy to address by making sure that you move sequentially through each day. Other application logic, however, may not have as obvious a solution.

Another alternative is to create a field for managing your locks. The room table, for example, might have two extra columns for booking purposes: locked_by and locked_timestamp. Before starting the transaction that books the rooms, update the room table and commit the update. Once your booking transaction completes, release the lock by nulling out those fields prior to committing that transaction.

Because this approach requires two different database transactions, you are no longer executing the booking as a single atomic transaction. Consequently, you risk leaving an open lock that prevents others from booking any rooms on any dates. You can eliminate this problem through two tricks:

• The room is considered unlocked not only when the fields are NULL, but also when the locked_timestamp has been held for a long period of time.

• When updating the lock at the end of your booking transaction, use the locked_by and locked_timestamp fields in the WHERE clause. Thus, if someone else steals a lock out from under you, you only end up rolling back your transaction.

Both of these approaches are admittedly more complex than taking advantage of stored procedures. Regardless of what approach you use, however, the important key for the cloud is simply making sure that you are not relying on memory locking to maintain your application state integrity.

2. When Servers Fail

The ultimate architectural objective for the cloud is to set up a running environment where the failure of any given application server ultimately doesn’t matter. If you are running just one server, that failure will obviously matter at some level, but it will still matter less than losing a physical server.

One trick people sometimes use to get around the problems described in the previous section is data segmentation—also known as sharding. Figure 3 shows how you might use data segmentation to split processing across multiple application servers.

Figure 3. Supporting different hotels on different servers guarantees no double bookings

In other words, each application server manages a subset of data. As a result, there is never any risk that another server will overwrite the data. Although segmentation has its place in scaling applications, that place is not at the application server in a cloud cluster. A segmented application server cluster ultimately has a very low availability rating, as the failure of any individual server does matter.

The final corollary to all of this discussion of application state and server failure is that application servers in a cloud cannot store any state data beyond caching data. In other words, if you need to back up your application server, you have failed to create a solid application server architecture for the cloud. All state information—including binary data—belongs in the database, which must be on a persistent system.^[12]

^[12] If you have read my article “Ten MySQL Best Practices” (http://www.onlamp.com/pub/a/onlamp/2002/07/11/MySQLtips.html ), you may have noted the contradiction between my admonition against storing binary data in MySQL and what I am recommending here. You should actually be caching the binary assets on the application server so that you do not need to pull them in real time from the database server. By doing so, you will get around my objections to storing binary data in MySQL.