Apache, the well-known HTTP server, is something of a poster child for open-source software: it is popular, stable, flexible, secure, portable, extensible and compliant with internet standards. I've been using Apache since it was first released, and it's a joy to work with.
But given that there are multiple open-source operating systems, editors, databases and programming languages, it shouldn't come as a surprise to hear that Apache isn't the only open-source HTTP server. What is surprising is that one of the alternatives comes from America Online, the same company that sponsors Mozilla, the open-source web browser.
AOLserver offers many of the same features as Apache: it is released under an open-source license, is easy and flexible to configure and offers an API for writing plugin modules. But AOLserver has a fundamentally different architecture from Apache, which often makes it a more efficient choice. Moreover, AOLserver includes a built-in Tcl interpreter, multiple threads, database API and database connection pooling. If your web site uses a lot of database connections, then it's worth looking into AOLserver as an alternative to Apache.
This month, we look at AOLserver as a lead-in to several articles about the open-source OpenACS (Open Architecture Community System) web application framework. While AOLserver is not an absolute requirement for OpenACS, it is the standard and expected way to configure and install the system.
AOLserver began as NaviServer, written and sold by a pioneering company called NaviSoft that offered high-quality, client- and server-side tools for web publishers. AOL has often bought companies that have developed interesting technology; in the case of NaviSoft, AOL bought them for their server- rather than for their client-side tools.
AOLserver probably would have remained relatively unknown were it not for a combination of events: AOL made binary copies of the program available at no cost via the Internet, and Philip Greenspun began work on the ArsDigita Community System. AOLserver makes it easy to create high-performance, database-backed web sites; the fact that ACS made heavy use of relational databases meant that AOLserver was a perfect fit.
But while AOLserver was free of charge, the source code still was unavailable to the general public. In 1999, Greenspun helped negotiate a deal that resulted in the release of AOLserver 3.0 under the AOLserver Public License, which is essentially identical to the Mozilla Public License.
ArsDigita itself effectively went out of business earlier this year, with its remaining staff and assets becoming part of Red Hat Software. But the original ACS software lives on in the OpenACS Project, which is based on the original Tcl version of ACS, using AOLserver and either PostgreSQL or Oracle.
AOLserver's transition to an open-source model wasn't without its problems. While the details are still a bit sketchy, a number of OpenACS developers briefly worked on their own fork of AOLserver, which they called OpenNSD, arguing that the AOLserver developers needed to be more open to community involvement. OpenNSD now appears to be dead, with the OpenACS community once again encouraging people to use AOLserver.
At the same time, ArsDigita added a number of enhancements to AOLserver that have not yet been added to the server's source code in its latest stable or development editions. Because my use of AOLserver is almost always connected to OpenACS, I will be using AOLserver 3.3ad13, available at www.openacs.org/software.adp. More advanced versions, including a development snapshot for the upcoming 4.0 version, are available at www.aolserver.com. But right now, these official versions are not guaranteed to support OpenACS.
Until earlier this year, when version 2.0 was first released to the general public, Apache was a multiprocess server. That is, a number of Apache processes would run at any given time, with each process able to handle a single HTTP transaction at a given time. Supporting ten simultaneous transactions meant having ten Apache processes running in parallel, while supporting 100 simultaneous connections meant having 100 such connections available.
Apache 2.0 changes this somewhat, allowing you to have multiple threads per process. Each thread can handle an HTTP connection, meaning that five threads in each of five processes can accept up to 25 simultaneous connections. Because threads typically consume fewer resources than processes, this effectively means that a typical PC will benefit from an increase in performance.
AOLserver, by contrast, has always been multithreaded and operates within a single process. At any given time, your computer will be running only one copy of nsd, whose name reflects the fact that it was once the NaviServer dæmon. But that one process can handle a large number of simultaneous HTTP connections. Indeed, AOL continues to push AOLserver development precisely because it can handle such a large number of simultaneous connections. It uses AOLserver on its own high-traffic web sites, including netscape.com, aol.com and digitalcity.com.
One advantage—and potential pitfall—of a multithreaded solution is the fact that the threads easily can share data structures with one another. AOLserver takes advantage of this to create a pool of database connections. Because connections in this pool are always kept open, your web applications will not have to spend time opening (or closing) them. Moreover, because it is rare for all of a server's current HTTP connections to require simultaneous access to a database, the pool can contain fewer connections than the maximum number of threads—reducing the amount of memory used by the web server and the database server. You can think of this as the database equivalent of packet switching, in which a telephone line is shared among numerous parties by taking advantage of the fact that no one needs the line 100% of the time.
AOLserver supports plugin modules, much as Apache does. There are a number of modules available, ranging from an XML parser (nsxml) to an embedded version of Python (PyWx). There are also modules for executing CGI programs, for secure connections via SSL and for individual relational databases such as MySQL, PostgreSQL and Oracle. Because OpenACS can work with either PostgreSQL or Oracle, the version of AOLserver available for download from openacs.org includes both of these modules, as well as nsxml.
Just as mod_perl allows web developers to customize Apache's configuration and responses without using C, AOLserver provides an integrated API that allows you to create custom functionality in the Tcl language. Truth be told, I personally would prefer to use Perl or Python for development, but as many AOLserver and OpenACS developers have told me over the years, Tcl “isn't that bad”, and I have managed to create a number of interesting, maintainable applications using Tcl and AOLserver in a remarkably short period of time. (I haven't yet tried the embedded Python module, in part because OpenACS requires the use of Tcl.) And, the API that AOLserver provides makes it relatively easy to work with such things as HTTP headers and HTML form values.
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