▼ |  | Web Server: PLT HTTP Server

1 | Running the Web Server
2 | Writing Servlets
3 | Extending the Web Server
4 | Troubleshooting and Tips
5 | Acknowledgements
| Index

► | 4 | Troubleshooting and Tips

4.1 | Why are my servlets not updating on the server when I change the code on disk?
4.2 | What special considerations are there for security with the Web Server?
4.3 | How do I use Apache with the PLT Web Server?
4.4 | IE ignores my CSS or behaves strange in other ways
4.5 | Can the server create a PID file?
4.6 | How do I set up the server to use HTTPS?
4.7 | How do I limit the number of requests serviced at once by the Web Server?

On this page:

4.1`` Why are my servlets not updating on the server when I change the code on disk?
4.2`` What special considerations are there for security with the Web Server?
4.3`` How do I use Apache with the PLT Web Server?
4.4`` IE ignores my CSS or behaves strange in other ways
4.5`` Can the server create a PID file?
4.6`` How do I set up the server to use HTTPS?
4.7`` How do I limit the number of requests serviced at once by the Web Server?

Version: 4.1.4
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### 4`` Troubleshooting and Tips

#### 4.1`` Why are my servlets not updating on the server when I change the code on disk?

By default, the server uses make-cached-url->servlet to load servlets from the disk. As it loads them, they are cached and the disk is not referred to for future requests. This ensures that there is a single namespace for each servlet, so that different instances can share resources, such as database connections, and communicate through the store. The default configuration of the server (meaning the dispatcher sequence used when you load a configuration file) provides a special URL to localhost that will reset the cache: "/conf/refresh-servlets". If you want the server to reload your changed servlet code, then GET this URL and the server will reload the servlet on the next request.

#### 4.2`` What special considerations are there for security with the Web Server?

The biggest problem is that a naive usage of continuations will allow continuations to subvert authentication mechanisms. Typically, all that is necessary to execute a continuation is its URL. Thus, URLs must be as protected as the information in the continuation.

Consider if you link to a public site from a private continuation URL: the Referrer field in the new HTTP request will contain the private URL. Furthermore, if your HTTP traffic is in the clear, then these URLs can be easily poached.

One solution to this is to use a special cookie as an authenticator. This way, if a URL escapes, it will not be able to be used, unless the cookie is present. For advice about how to do this well, see Dos and Don’ts of Client Authentication on the Web from the MIT Cookie Eaters.

Note: It may be considered a great feature that URLs can be shared this way, because delegation is easily built into an application via URLs.

#### 4.3`` How do I use Apache with the PLT Web Server?

You may want to put Apache in front of your PLT Web Server application. Apache can rewrite and proxy requests for a private (or public) PLT Web Server:

RewriteRule ^(.*)$ http://localhost:8080/$1 [P]

The first argument to RewriteRule is a match pattern. The second is how to rewrite the URL. The [P] flag instructs Apache to proxy the request. If you do not include this, Apache will return an HTTP Redirect response and the client should make a second request.

See Apache’s documentation for more details on RewriteRule.

#### 4.4`` IE ignores my CSS or behaves strange in other ways

In quirks mode, IE does not parse your page as XML, in particular it will not recognize many instances of "empty tag shorthand", e.g. "<img src=’...’ />", whereas the Web Server uses xml to format XML, which uses empty tag shorthand by default. You can change the default with the empty-tag-shorthand parameter: (empty-tag-shorthand 'never).

#### 4.5`` Can the server create a PID file?

The server has no option for this, but you can add it very easily. There’s two techniques.

First, if you use a UNIX platform, in your shell startup script you can use

echo $$ > PID
exec run-web-server

Using exec will reuse the same process, and therefore, the PID file will be accurate.

Second, if you want to make your own Scheme start-up script, you can write:

(require mzlib/os)
(with-output-to-file pid-file (lambda () (write (getpid))))
(start-server)

#### 4.6`` How do I set up the server to use HTTPS?

This requires an SSL certificate and private key. This is very platform specific, but we will provide the details for using OpenSSL on UNIX:

openssl genrsa -des3 -out private-key.pem 1024

This will generate a new private key, but it will have a passphrase on it. You can remove this via:

openssl rsa -in private-key.pem -out private-key.pem

chmod 400 private-key.pem

Now, we generate a self-signed certificate:

openssl req -new -x509 -nodes -sha1 -days 365 -key private-key.pem > server-cert.pem

(Each certificate authority has different instructions for generating certificate signing requests.)

We can now start the server with:

plt-web-server --ssl

The Web Server will start on port 443 (which can be overridden with the -p option) using the "private-key.pem" and "server-cert.pem" we’ve created.

#### 4.7`` How do I limit the number of requests serviced at once by the Web Server?

Refer to Limiting Requests.

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