Developer Support

part of the ArsDigita Community System, by Jon Salz

Admin interface: /www/admin/monitoring/request-info.tcl
Procedures: /packages/developer-support-procs.tcl, with support in:
- /tcl/ad-abstract-url.tcl
- /tcl/ad-defs.tcl.preload
- /tcl/ad-security.tcl.preload

The Big Picture

Software development is a big feedback loop: a developer writes something, tests it, and then repeats until the results are satisfactory. It's important to streamline this cycle by having a development environment which makes it easy to analyze what the software is doing under the hood.

Peeking Under the Hood

Our development environment previously consisted largely of Emacs, and tail -f /web/servername/log/servername-error.log. Now this has been augmented: ad_footer and ad_admin_footer now display a link entitled Developer Information. (You can use the ds_link procedure to generate the link yourself.) Following the link displays a screenful of information including:

The times that the request started and ended, and its duration (with millisecond accuracy).
The request parameters (method, url, query, headers, etc.).
The output headers, if any.
Information about all database queries performed while loading the page, including their respective durations (with millisecond accuracy).

In addition, the ClientDebug facility of AOLserver 2 has been re-implemented in the abstract URL system (which serves nearly all non-static pages). If an error occurs while serving a page, a stack trace is printed out.

Note that these nifty features pop up only when you are logged in as a site-wide administrator! Revealing this information to anyone else would pose a huge security risk.

The Report

Comments

Tired of using ns_log to instrument your code, then grokking the error log to see what's wrong with your page? Use the ds_comment routine instead:

ds_comment "Foo is $foo"

Your comment will show up at the bottom of the page, beneath the Developer Information link (but only for site-wide administrators). It will also be displayed on the Developer Information page itself.

Comments are displayed even if an error occurs in the page!

Enabling It

Add the following to your parameters/yourservername.ini file:

[ns/server/yourservername/acs/developer-support]
; remember information about connections, for developers' benefit?
EnabledP=1
; remember information about every database request?
DatabaseEnabledP=1
; remember information for which client hosts?
EnabledIPs=*
; remember this information for how long? sweep how often? (in seconds)
DataLifetime=900
DataSweepInterval=900

Note that you may not want to enable this stuff for production systems - they probably incur a slight performance hit (although this hasn't been benchmarked).

How It Works

The security subsystem registers preauth and trace filters which store relevant connection information in shared variables (nsvs). The security subsystem also renames the AOLserver ns_db procedure and registers a wrapper which aggregates information about database queries.

jsalz@mit.edu