Provided by: tcl-thread_2.8.2-1_amd64 
NAME
ttrace - Trace-based interpreter initialization
SYNOPSIS
package require Tcl 8.4
package require Thread ?2.8?
ttrace::eval arg ?arg ...?
ttrace::enable
ttrace::disable
ttrace::cleanup
ttrace::update ?epoch?
ttrace::getscript
ttrace::atenable cmd arglist body
ttrace::atdisable cmd arglist body
ttrace::addtrace cmd arglist body
ttrace::addscript name body
ttrace::addresolver cmd arglist body
ttrace::addcleanup body
ttrace::addentry cmd var val
ttrace::getentry cmd var
ttrace::getentries cmd ?pattern?
ttrace::delentry cmd
ttrace::preload cmd
________________________________________________________________________________________________________________
DESCRIPTION
This package creates a framework for on-demand replication of the interpreter state accross threads in an
multithreading application. It relies on the mechanics of Tcl command tracing and the Tcl unknown
command and mechanism.
The package requires Tcl threading extension but can be alternatively used stand-alone within the
AOLserver, a scalable webserver from America Online.
In a nutshell, a short sample illustrating the usage of the ttrace with the Tcl threading extension:
% package require Ttrace
2.8.0
% set t1 [thread::create {package require Ttrace; thread::wait}]
tid0x1802800
% ttrace::eval {proc test args {return test-[thread::id]}}
% thread::send $t1 test
test-tid0x1802800
% set t2 [thread::create {package require Ttrace; thread::wait}]
tid0x1804000
% thread::send $t2 test
test-tid0x1804000
As seen from above, the ttrace::eval and ttrace::update commands are used to create a thread-wide
definition of a simple Tcl procedure and replicate that definition to all, already existing or later
created, threads.
USER COMMANDS
This section describes user-level commands. Those commands can be used by script writers to control the
execution of the tracing framework.
ttrace::eval arg ?arg ...?
This command concatenates given arguments and evaluates the resulting Tcl command with trace
framework enabled. If the command execution was ok, it takes necessary steps to automatically
propagate the trace epoch change to all threads in the application. For AOLserver, only newly
created threads actually receive the epoch change. For the Tcl threading extension, all threads
created by the extension are automatically updated. If the command execution resulted in Tcl
error, no state propagation takes place.
This is the most important user-level command of the package as it wraps most of the commands
described below. This greatly simplifies things, because user need to learn just this (one)
command in order to effectively use the package. Other commands, as desribed below, are included
mostly for the sake of completeness.
ttrace::enable
Activates all registered callbacks in the framework and starts a new trace epoch. The trace epoch
encapsulates all changes done to the interpreter during the time traces are activated.
ttrace::disable
Deactivates all registered callbacks in the framework and closes the current trace epoch.
ttrace::cleanup
Used to clean-up all on-demand loaded resources in the interpreter. It effectively brings Tcl
interpreter to its pristine state.
ttrace::update ?epoch?
Used to refresh the state of the interpreter to match the optional trace ?epoch?. If the optional
?epoch? is not given, it takes the most recent trace epoch.
ttrace::getscript
Returns a synthetized Tcl script which may be sourced in any interpreter. This script sets the
stage for the Tcl unknown command so it can load traced resources from the in-memory database.
Normally, this command is automatically invoked by other higher-level commands like ttrace::eval
and ttrace::update.
CALLBACK COMMANDS
A word upfront: the package already includes callbacks for tracing following Tcl commands: proc,
namespace, variable, load, and rename. Additionaly, a set of callbacks for tracing resources (object,
clasess) for the XOTcl v1.3.8+, an OO-extension to Tcl, is also provided. This gives a solid base for
solving most of the real-life needs and serves as an example for people wanting to customize the package
to cover their specific needs.
Below, you can find commands for registering callbacks in the framework and for writing callback scripts.
These callbacks are invoked by the framework in order to gather interpreter state changes, build in-
memory database, perform custom-cleanups and various other tasks.
ttrace::atenable cmd arglist body
Registers Tcl callback to be activated at ttrace::enable. Registered callbacks are activated on
FIFO basis. The callback definition includes the name of the callback, cmd, a list of callback
arguments, arglist and the body of the callback. Effectively, this actually resembles the call
interface of the standard Tcl proc command.
ttrace::atdisable cmd arglist body
Registers Tcl callback to be activated at ttrace::disable. Registered callbacks are activated on
FIFO basis. The callback definition includes the name of the callback, cmd, a list of callback
arguments, arglist and the body of the callback. Effectively, this actually resembles the call
interface of the standard Tcl proc command.
ttrace::addtrace cmd arglist body
Registers Tcl callback to be activated for tracing the Tcl cmd command. The callback definition
includes the name of the Tcl command to trace, cmd, a list of callback arguments, arglist and the
body of the callback. Effectively, this actually resembles the call interface of the standard Tcl
proc command.
ttrace::addscript name body
Registers Tcl callback to be activated for building a Tcl script to be passed to other
interpreters. This script is used to set the stage for the Tcl unknown command. Registered
callbacks are activated on FIFO basis. The callback definition includes the name of the callback,
name and the body of the callback.
ttrace::addresolver cmd arglist body
Registers Tcl callback to be activated by the overloaded Tcl unknown command. Registered
callbacks are activated on FIFO basis. This callback is used to resolve the resource and load the
resource in the current interpreter.
ttrace::addcleanup body
Registers Tcl callback to be activated by the trace::cleanup. Registered callbacks are activated
on FIFO basis.
ttrace::addentry cmd var val
Adds one entry to the named in-memory database.
ttrace::getentry cmd var
Returns the value of the entry from the named in-memory database.
ttrace::getentries cmd ?pattern?
Returns names of all entries from the named in-memory database.
ttrace::delentry cmd
Deletes an entry from the named in-memory database.
ttrace::preload cmd
Registers the Tcl command to be loaded in the interpreter. Commands registered this way will
always be the part of the interpreter and not be on-demand loaded by the Tcl unknown command.
DISCUSSION
Common introspective state-replication approaches use a custom Tcl script to introspect the running
interpreter and synthesize another Tcl script to replicate this state in some other interpreter. This
package, on the contrary, uses Tcl command traces. Command traces are registered on selected Tcl
commands, like proc, namespace, load and other standard (and/or user-defined) Tcl commands. When
activated, those traces build an in-memory database of created resources. This database is used as a
resource repository for the (overloaded) Tcl unknown command which creates the requested resource in the
interpreter on demand. This way, users can update just one interpreter (master) in one thread and
replicate that interpreter state (or part of it) to other threads/interpreters in the process.
Immediate benefit of such approach is the much smaller memory footprint of the application and much
faster thread creation. By not actually loading all necessary procedures (and other resources) in every
thread at the thread initialization time, but by deffering this to the time the resource is actually
referenced, significant improvements in both memory consumption and thread initialization time can be
achieved. Some tests have shown that memory footprint of an multithreading Tcl application went down more
than three times and thread startup time was reduced for about 50 times. Note that your mileage may vary.
Other benefits include much finer control about what (and when) gets replicated from the master to other
Tcl thread/interpreters.
SEE ALSO
thread, tpool, tsv
KEYWORDS
command tracing, introspection