Hoover Dam.  Arizona/Nevada border.

Web Server Cluster Management

by Philip Greenspun, part of ArsDigita Server Architecture

Power equipment at Hoover Dam, on Nevada/Arizona border As noted in "ArsDigita Server Architecture", for 99% of Web publishers, the best physical server infrastructure is the simplest to administer: one computer running the RDBMS and the Web server.

Suppose that you are forced by circumstances to adopt the larger and more complex architecture detailed in the preceding article. You will have one big RDBMS server and many small Web servers. This document describes the best ways to manage a cluster of machines arranged according to the following diagram:

Here are our objectives:

  1. simplicity in managing Web content and scripts; it would be nice to cvs update or mv Web pages in one place, without having to obtain special training
  2. user-uploaded files that are stored in the file system must be written to a common directory on the big RDBMS server box
  3. if we wish to take the Web service down for maintenance, we can quickly bring up a comebacklater server and then bring it back down again, without having some machines in the farm serving the real site and some serving "come back later" pages.

Put Web content and scripts on the big server; export via NFS

The solution to Objective 1, central simple management of Web content, is to put the Web server root on the big RDBMS server. This seems conterintuitive because the big server isn't running any Web server at all. However, it makes sense because How to get the canonical content from the big server out to the actual Web servers? Export via NFS. This is how cnn.com works (170 million hits per day). They use NFS on Solaris, which has the property that frequently accessed files are cached in RAM on the NFS client. I.e., after the first request for a given piece of Web content, there is no additional network overhead for subsequent requests. If you're not running Solaris, make sure that your Unix's NFS works this way. If not, you might need to install an alternative distributed file system such as AFS (Andrew File System, developed at Carnegie Mellon and used by MIT's Project Athena).

To summarize: the big RDBMS server is also an NFS server; each individual Web server is an NFS client with read-only access.

Put user-uploaded files in an NFS-exported directory on the big server

When a user uploads a file and it isn't put into an RDBMS large object column, you have to make sure that it goes into a directory accessible to all the Web servers. For security reasons, select a directory that is not under the Web server root. You don't want the user uploading "crack-the-system.pl" and then requesting http://yourdomain.com/uploaded-files/crack-the-system.pl to make your server run this code as a CGI script. However, like the Web content, this directory must be NFS-exported. Unlike the Web content, this directory must be writable by the NFS clients.

Maintenance Announcement Strategy

If you have one box and have to take down the Web service to upgrade Oracle or whatever "ArsDigita Server Architecture" suggests the use of a "come back later" server. If you've got 24 little Web servers, it would be a headache to go around to each one and follow the procedure outlined in the preceding document. Here are two solutions:
  1. if you're using round-robin DNS to spread users among the little Web servers, write some scripts automating the reconfiguring from production to comebacklater and vice versa (see http://www.infrastructures.org/cfengine/ for some potentially helpful software and ideas)
  2. if you're using a fancy router that gives N machines the same IP address, temporarily reconfigure the router to map that IP address to a permanent comebacklater server
Round-robin DNS seems to have started going out of favor in 1997 so let's elaborate the "fancy router" solution. Pick two of the Web server machines and add an extra IP address to each. Keep a permanently installed "comebacklater" server on both of those machines. When maintenance time rolls around, pick one of these machines to serve as the comebacklater server. Edit the message directly in the file system of that computer and then reconfigure the router to point all traffic to that normally-unused virtual server. Note that we've not shut down or brought up any Web servers. Nor have we incurred any additional hardware or rackspace expense; the permanently-running comebacklater servers are simply taking up a few MB of RAM on two existing machines.

What is actually on the little boxes then?

Each little box contains Note that there is never any need to back up these machines onto tape.

What is actually on the big box?

Note that this machine contains all the critical data that must be backed up onto tape.

Text and photos Copyright 1999 Philip Greenspun. Illustration copyright Mina Reimer

Reader's Comments

For some reason, the folks at www.infrasturctures.org no longer mirror the cfengine software referred to above. The "canonical" URL for cfengine is http://www.iu.hioslo.no/cfengine/.

-- Frank Wortner, March 29, 2000
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