Object Publishing

Introduction

A final early Zope innovation that I want to talk about is object publishing. Object publishing is one of those ideas that seemed weird at the time. In fact, in its execution Zope would still be considered weird by many today. But in an important way the idea of publishing code on URLs is now absolutely the norm.

The Web in 1998

Let's go back to 1998, when Zope was born. Let's look at a URL:

http://example.com/dir/file.html

If you saw this URL in 1998, you knew that /dir/file.html refers to a file on the filesystem on a web server somewhere. That is, there is an actual directory dir, and an actual file file.html. Where the root of the website would be on the filesystem was up to the configuration of the web server, but once you're inside this root paths in URLs get translated to paths in the filesystem.

A URL could refer to a static HTML file as above. You could also build dynamic web applications. You'd do this by putting files in your web server that were CGI scripts. When someone accesses the file, the script would be executed and produce HTML content. So script.cgi or script.pl or script.py or something.

Zope's way

Zope was different. The path in a URL in Zope did not map to a file in the filesystem anymore, but to a piece of Python code. Zope was one of the first systems to do so. With Zope, the path was interpreted as a series of steps to a Python method. So, the path:

/a/b/c

would translate to:

root['a']['b']['c']()

or possibly:

root.a.b.c()

or a combination thereof. The concept is called object publishing and the process by which the right method to call is found is called traversal, as an object graph is traversed.

URLs served from a Zope server looked clean, without complicated URL parameters to encode state, and without file extensions. The URLs looked so clean and unadorned that in fact they sometimes confused people who were expecting a .cgi or .pl or .php or .asp or .cfm. Bizarrely enough for a little while we had an effort to bring them back so people would be less confused.

The Web today

A variation of Zope's approach is now the norm. Most web development frameworks in Python and other languages don't map URL paths to files anymore, but to code. If you use a routing system like the one in Django, for instance, a URL is not resolved to a file, but to a function that typically queries an object from the database and then represents it as HTML or JSON.

While the mapping of URL paths to code is now the norm, Zope's particular way of doing traversal is still weird to many. Zope's way fits well with the ZODB, as the ZODB is natively an object graph that can be traversed. Zope's way doesn't fit other database structures as well.

Zope's approach is weird, but it's still worth examining. A benefit of this traversing approach is that it gives URLs to models, not only views or controllers; making explicit something that is implicit in more modern routing-to-view web frameworks. This means that not only can a URL be resolved to a model (and only then a view/controller), but you can also construct a URL with just a model. This is very powerful and, in the RESTful, client-side web of today more relevant than ever. 1

This blog entry is a part of a series on Zope and my involvement with it. Previous. Next.

1

I myself have explored combining routing and traversal in my older Traject library and the so-new-oh-no-I-gotta-write-the-docs Morepath web framework.