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12.12. When One Machine Is Not Enough for Your RDBMS DataBase and mod_perl

Imagine a scenario where you start your business as a small service providing a web site. After a while your business becomes very popular, and at some point you realize that it has outgrown the capacity of your machine. Therefore, you decide to upgrade your current machine with lots of memory, a cutting-edge, super-expensive CPU, and an ultra-fast hard disk. As a result, the load goes back to normal—but not for long. Demand for your services keeps on growing, and just a short time after you've upgraded your machine, once again it cannot cope with the load. Should you buy an even more powerful and very expensive machine, or start looking for another solution? Let's explore the possible solutions for this problem.

A typical web service consists of two main software components: the database server and the web server.

A typical user-server interaction consists of accepting the query parameters entered into an HTML form and submitted to the web server by a user, converting these parameters into a database query, sending it to the database server, accepting the results of the executed query, formatting them into a nice HTML page, and sending it to a user's Internet browser or another application that created the request (e.g., a mobile phone with WAP browsing capabilities). This process is depicted in Figure 12-9.

Figure 12-9

Figure 12-9. Typical user-server interaction

This schema is known as a three-tier architecture in the computing world. In a three-tier architecture, you split up several processes of your computing solution between different machines:

Tier 1
The client, who will see the data on its screen and can give instructions to modify or process the data. In our case, an Internet browser.

Tier 2
The application server, which does the actual processing of the data and sends it back to the client. In our case, a mod_perl-enabled Apache server.

Tier 3
The database server, which stores and retrieves all the data for the application server.

We are interested only in the second and the third tiers; we don't specify user machine requirements, since mod_perl is all about server-side programming. The only thing the client should be able to do is to render the generated HTML from the response, which any simple browser will do.

12.12.2. The Problem

With the database and the web server on the same machine, you have conflicting interests.

During peak loads, Apache will spawn more processes and use RAM that the database server might have been using, or that the kernel was using on its behalf in the form of a cache. You will starve your database of resources at the time when it needs those resources the most.

Disk I/O contention produces the biggest time issue. Adding another disk won't cut I/O times, because the database is the only thing that does I/O—mod_perl processes have all their code loaded in memory (we are talking about code that does pure Perl and SQL processing). Thus, it's clear that the database is I/O- and CPU-bound (it's RAM-bound only if there are big joins to make), while mod_perl is mostly CPU- and memory-bound.

There is a problem, but it doesn't mean that you cannot run the application and the web servers on the same machine. There is a very high degree of parallelism in modern PC architecture. The I/O hardware is helpful here. The machine can do many things while a SCSI subsystem is processing a command or the network hardware is writing a buffer over the wire.

If a process is not runnable (that is, it is blocked waiting for I/O or something else), it is not using significant CPU time. The only CPU time that will be required to maintain a blocked process is the time it takes for the operating system's scheduler to look at the process, decide that it is still not runnable, and move on to the next process in the list. This is hardly any time at all. If there are two processes, one of which is blocked on I/O and the other of which is CPU-bound, the blocked process is getting 0% CPU time, the runnable process is getting 99.9% CPU time, and the kernel scheduler is using the rest.

12.12.3. The Solution

The solution is to add another machine, which allows a setup where both the database and the web server run on their own dedicated machines.

This solution has the following advantages:

Flexible hardware requirements
It allows you to scale two requirements independently.

If your httpd processes are heavily weighted with respect to RAM consumption, you can easily add another machine to accommodate more httpd processes, without changing your database machine.

If your database is CPU-intensive but your httpd doesn't need much CPU time, you can get a low-end machine for the httpd and a high-end machine with a very fast CPU for the database server.

Since your web server doesn't depend on the database server location any more, you can add more web servers hitting the same database server, using the existing infrastructure.

Database security
Once you have multiple web server boxes, the backend database becomes a single point of failure, so it's a good idea to shield it from direct Internet access—something that is harder to do when the web and database servers reside on the same machine.

It also has the following disadvantages:

Network latency
A database request from a web server to a database server running on the same machine uses Unix sockets, not the TCP/IP sockets used when the client submits the query from another machine. Unix sockets are very fast, since all the communications happen within the same box, eliminating network delays. TCP/IP socket communication totally depends on the quality and the speed of the network that connects the two machines.

Basically, you can have almost the same client-server speed if you install a very fast and dedicated network between the two machines. It might impose a cost of additional NICs, but that cost is probably insignificant compared to the speed improvement you gain.

Even the normal network that you have would probably fit as well, because the network delays are probably much smaller than the time it takes to execute the query. In contrast to the previous paragraph, you really want to test the added overhead here, since the network can be quite slow, especially at peak hours.

How do you know what overhead is a significant one? All you have to measure is the average time spent in the web server and the database server. If either of the two numbers is at least 20 times bigger than the added overhead of the network, you are all set.

To give you some numbers, if your query takes about 20 milliseconds to process and only 1 millisecond to deliver the results, it's good. If the delivery takes about half of the time the processing takes, you should start considering switching to a faster and/or dedicated network.

The consequences of a slow network can be quite bad. If the network is slow, mod_perl processes remain open, waiting for data from the database server, and eat even more RAM as new child processes pop up to handle new requests. So the overall machine performance can be worse than it was originally, when you had just a single machine for both servers.

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