Tài liệu Practical mod_perl-CHAPTER 12:Server Setup Strategies pptx

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Chapter 12 CHAPTER 12

Server Setup Strategies

Since the first day mod_perl was available, users have adopted various techniques

that make the best of mod_perl by deploying it in combination with other modules

and tools. This chapter presents the theory behind these useful techniques, their pros

and cons, and of course detailed installation and configuration notes so you can eas￾ily reproduce the presented setups.

This chapter will explore various ways to use mod_perl, running it in parallel with

other web servers as well as coexisting with proxy servers.

mod_perl Deployment Overview

There are several different ways to build, configure, and deploy your mod_perl￾enabled server. Some of them are:

1. One big binary (for mod_perl) and one configuration file.

2. Two binaries (one big one for mod_perl and one small one for static objects,

such as images) and two configuration files.

3. One DSO-style Apache binary and two configuration files. The first configura￾tion file is used for the plain Apache server (equivalent to a static build of

Apache); the second configuration file is used for the heavy mod_perl server, by

loading the mod_perl DSO loadable object using the same binary.

4. Any of the above plus a reverse proxy server in httpd accelerator mode.

If you are new to mod_perl and just want to set up your development server quickly,

we recommend that you start with the first option and work on getting your feet wet

with Apache and mod_perl. Later, you can decide whether to move to the second

option, which allows better tuning at the expense of more complicated administra￾tion, to the third option (the more state-of-the-art DSO system), or to the fourth

option, which gives you even more power and flexibility. Here are some of the things

to consider.

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404 | Chapter 12: Server Setup Strategies

1. The first option will kill your production site if you serve a lot of static data from

large (4–15 MB) web server processes. On the other hand, while testing you will

have no other server interaction to mask or add to your errors.

2. The second option allows you to tune the two servers individually, for maxi￾mum performance. However, you need to choose whether to run the two serv￾ers on multiple ports, multiple IPs, etc., and you have the burden of

administering more than one server. You also have to deal with proxying or

complicated links to keep the two servers synchronized.

3. With DSO, modules can be added and removed without recompiling the server,

and their code is even shared among multiple servers.

You can compile just once and yet have more than one binary, by using differ￾ent configuration files to load different sets of modules. The different Apache

servers loaded in this way can run simultaneously to give a setup such as that

described in the second option above.

The downside is that you are dealing with a solution that has weak documenta￾tion, is still subject to change, and, even worse, might cause some subtle bugs. It

is still somewhat platform-specific, and your mileage may vary.

Also, the DSO module (mod_so) adds size and complexity to your binaries.

4. The fourth option (proxy in httpd accelerator mode), once correctly configured

and tuned, improves the performance of any of the above three options by cach￾ing and buffering page results. This should be used once you have mastered the

second or third option, and is generally the preferred way to deploy a mod_perl

server in a production environment.

If you are going to run two web servers, you have the following options:

Two machines

Serve the static content from one machine and the dynamic content from

another. You will have to adjust all the links in the generated HTML pages: you

cannot use relative references (e.g., /images/foo.gif) for static objects when the

page is generated by the dynamic-content machine, and conversely you can’t use

relative references to dynamic objects in pages served by the static server. In

these cases, fully qualified URIs are required.

Later we will explore a frontend/backend strategy that solves this problem.

The drawback is that you must maintain two machines, and this can get expen￾sive. Still, for extremely large projects, this is the best way to go. When the load

is high, it can be distributed across more than two machines.

One machine and two IP addresses

If you have only one machine but two IP addresses, you may tell each server to

bind to a different IP address, with the help of the BindAddress directive in httpd.

conf. You still have the problem of relative links here (solutions to which will be

presented later in this chapter). As we will show later, you can use the 127.0.0.1

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Standalone mod_perl-Enabled Apache Server | 405

address for the backend server if the backend connections are proxied through

the frontend.

One machine, one IP address, and two ports

Finally, the most widely used approach uses only one machine and one NIC, but

binds the two servers to two different ports. Usually the static server listens on

the default port 80, and the dynamic server listens on some other, nonstandard

port.

Even here the problem of relative links is still relevant, since while the same IP

address is used, the port designators are different, which prevents you from

using relative links for both contents. For example, a URL to the static server

could be http://www.example.com/images/nav.png, while the dynamic page might

reside at http://www.example.com:8000/perl/script.pl. Once again, the solutions

are around the corner.

Standalone mod_perl-Enabled Apache

Server

The first and simplest scenario uses a straightforward, standalone, mod_perl-enabled

Apache server, as shown in Figure 12-1. Just take your plain Apache server and add

mod_perl, like you would add any other Apache module. Continue to run it at the

port it was using before. You probably want to try this before you proceed to more

sophisticated and complex techniques. This is the standard installation procedure we

described in Chapter 3.

A standalone server gives you the following advantages:

Simplicity

You just follow the installation instructions, configure it, restart the server, and

you are done.

Figure 12-1. mod_perl-enabled Apache server

Request

Response

Clients

httpd

Apache and mod_perl

example.com:80

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406 | Chapter 12: Server Setup Strategies

No network changes

You do not have to worry about using additional ports, as we will see later.

Speed

You get a very fast server for dynamic content, and you see an enormous

speedup compared to mod_cgi, from the first moment you start to use it.

The disadvantages of a standalone server are as follows:

• The process size of a mod_perl-enabled Apache server might be huge (maybe 4

MB at startup and growing to 10 MB or more, depending on how you use it)

compared to a typical plain Apache server (about 500 KB). Of course, if memory

sharing is in place, RAM requirements will be smaller.

You probably have a few dozen child processes. The additional memory require￾ments add up in direct relation to the number of child processes. Your memory

demands will grow by an order of magnitude, but this is the price you pay for

the additional performance boost of mod_perl. With memory being relatively

inexpensive nowadays, the additional cost is low—especially when you consider

the dramatic performance boost mod_perl gives to your services with every 100

MB of RAM you add.

While you will be happy to have these monster processes serving your scripts

with monster speed, you should be very worried about having them serve static

objects such as images and HTML files. Each static request served by a mod_

perl-enabled server means another large process running, competing for system

resources such as memory and CPU cycles. The real overhead depends on the

static object request rate. Remember that if your mod_perl code produces

HTML code that includes images, each of these will produce another static

object request. Having another plain web server to serve the static objects solves

this unpleasant problem. Having a proxy server as a frontend, caching the static

objects and freeing the mod_perl processes from this burden, is another solu￾tion. We will discuss both later.

• Another drawback of this approach is that when serving output to a client with a

slow connection, the huge mod_perl-enabled server process (with all of its sys￾tem resources) will be tied up until the response is completely written to the cli￾ent. While it might take a few milliseconds for your script to complete the

request, there is a chance it will still be busy for a number of seconds or even

minutes if the request is from a client with a slow connection. As with the previ￾ous drawback, a proxy solution can solve this problem. We’ll discuss proxies

more later.

Proxying dynamic content is not going to help much if all the clients are on a fast

local net (for example, if you are administering an Intranet). On the contrary, it

can decrease performance. Still, remember that some of your Intranet users

might work from home through slow modem links.

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One Plain and One mod_perl-Enabled Apache Server | 407

If you are new to mod_perl, this is probably the best way to get yourself started.

And of course, if your site is serving only mod_perl scripts (and close to zero static

objects), this might be the perfect choice for you!

Before trying the more advanced setup techniques we are going to talk about now,

it’s probably a good idea to review the simpler straightforward installation and con￾figuration techniques covered in Chapters 3 and 4. These will get you started with

the standard deployment discussed here.

One Plain and One mod_perl-Enabled

Apache Server

As mentioned earlier, when running scripts under mod_perl you will notice that the

httpd processes consume a huge amount of virtual memory—from 5 MB–15 MB,

and sometimes even more. That is the price you pay for the enormous speed

improvements under mod_perl, mainly because the code is compiled once and needs

to be cached for later reuse. But in fact less memory is used if memory sharing takes

place. Chapter 14 covers this issue extensively.

Using these large processes to serve static objects such as images and HTML docu￾ments is overkill. A better approach is to run two servers: a very light, plain Apache

server to serve static objects and a heavier, mod_perl-enabled Apache server to serve

requests for dynamically generated objects. From here on, we will refer to these two

servers as httpd_docs (vanilla Apache) and httpd_perl (mod_perl-enabled Apache).

This approach is depicted in Figure 12-2.

The advantages of this setup are:

• The heavy mod_perl processes serve only dynamic requests, so fewer of these

large servers are deployed.

• MaxClients, MaxRequestsPerChild, and related parameters can now be optimally

tuned for both the httpd_docs and httpd_perl servers (something we could not do

before). This allows us to fine-tune the memory usage and get better server per￾formance.

Now we can run many lightweight httpd_docs servers and just a few heavy

httpd_perl servers.

The disadvantages are:

• The need for two configuration files, two sets of controlling scripts (startup/

shutdown), and watchdogs.

• If you are processing log files, you will probably have to merge the two separate

log files into one before processing them.

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