Bootstrapping ACS

ACS Documentation : ACS Core Architecture Guide : Bootstrapping ACS

Tcl code: /tcl/0-acs-init.tcl and /packages/acs-core/bootstrap.tcl

This document describes the startup (bootstrapping) process for an AOLserver running ACS.

The Big Picture

Before ACS 3.3, the ACS startup process was extremely simple: after AOLserver performed its internal initialization (reading the configuration file, loading shared libraries and module code, etc.) it scanned through the Tcl library directory (generally /web/yourservername/tcl), sourcing each file in sequence.

While this overall structure for initialization is still intact, package management has thrown a wrench into the works - there are a few extra things to do during initialization, most notably:

Examine the ACS file tree for files that should not be present in ACS (i.e., that were once part of the ACS distribution but have since been removed).
Scan the /packages directory for new packages.
Initialize enabled packages by sourcing their *-procs.tcl and *-init.tcl files.

This document examines in detail each of the steps involved in AOLserver/ACS startup.

The Startup Process

As soon as the nsd daemon is executed by the init process (or otherwise), AOLserver reads its configuration file and chroots itself if necessary. It then loads shared libraries indicated in the .ini file (e.g., the Oracle driver and nssock), and sources Tcl module files (generally in /home/aol30/modules/tcl). This step is, and has always been, the same for all AOLservers, regardless of whether they are running ACS.

Next AOLserver sources, in lexicographical order, each file in the /tcl directory. The first such file is 0-acs-init.tcl, which doesn't do much directly except to determine the ACS path root (e.g., /web/yourservername) by trimming the final component from the path to the Tcl library directory (/web/yourservername/tcl). But 0-acs-init.tcl's has an important function, namely sourcing /packages/acs-core/bootstrap.tcl, which does the following:

Initialize some NSVs used by the core. These NSVs are documented in /packages/acs-core/apm-procs.tcl - no need to worry about them unless you're an ACS core hacker.
Verify the deletion of obsolete ACS files. The /tcl directory has evolved quite a bit over the months and years, and a few files have come and gone. The /www/doc/removed-files.txt file contains a list of files which must be deleted from the AOLserver installation, at the risk of causing weird conflicts, e.g., having several security filters registered. bootstrap.tcl scans through this list, logging error messages to the log if any of these files exist.
Source *-procs.tcl files in the ACS core. We source each file matching the *-procs.tcl glob in the /packages/acs-core directory, in lexicographical order. These procedure are needed to perform any of the following steps.
Ensure that the database is available by grabbing and releasing a handle. If we can't obtain a handle, we terminate initialization (since ACS couldn't possibly start up the server without access to the database).
Register any new packages in the /packages directory. In each directory inside /packages, we look for a .info file; if we find a package that hasn't yet been registered with the package manager (i.e., it's been copied there manually), we insert information about it into the database. (The first time ACS starts up, no packages will have been registered in the database yet, so this step will registers every single package in the /packages directory.) Note that packages discovered here are initially disabled; they must be manually enabled in the package manager before they can be used.
Ensure that the acs-core package is enabled. If the ACS core isn't initialized, the server couldn't possibly be operational, so if there's no enabled version of the ACS core we simply mark the latest installed one as enabled.
Load *-procs.tcl files for enabled packages, activating their APIs.
Load *-init.tcl files for enabled packages, giving packages a chance to register filters and procedures, initialize data structures, etc.
Verify that the core has been properly initialized by checking for the existence of an NSV created by the request processor initialization code. If it's not present, the server won't be operational, so we log an error.

At this point, bootstrap.tcl is done executing. AOLserver proceeds to source the remaining files in the /tcl directory (i.e., unpackaged libraries) and begins listening for connections.

jsalz@mit.edu