NAME¶
ns_ictl - Facility to control AOLserver multi-threaded Tcl interpreters
SYNOPSIS¶
ns_ictl addmodule module
ns_ictl cancel thread
ns_ictl cleanup
ns_ictl epoch
ns_ictl get
ns_ictl getmodules
ns_ictl gettraces which
ns_ictl once key script
ns_ictl oncleanup script
ns_ictl oncreate script
ns_ictl ondelete script
ns_ictl oninit script
ns_ictl package ?-exact? package ?version?
ns_ictl runtraces which
ns_ictl save script
ns_ictl threads
ns_ictl trace when script
ns_ictl update
DESCRIPTION¶
This command provides access to the internal facilities to control and configure
multi-threaded Tcl interpreters in the context of AOLserver virtual servers.
It is normally used in startup initialization scripts to define how new
interpreters are initialized when created and to support cleanup and
re-initalization between transactions (e.g., HTTP connections).
- ns_ictl addmodule module
- Add a module to the list of modules to be initialized at startup. This
command is not normally required as each module specified in the AOLserver
modules config section for the corresponding server (e.g.,
ns/server/server1/modules) is automatically added to the list.
- ns_ictl cancel thread
- Send an asynchronous interrupt request to the specified thread, cancelling
any script currently executing on any AOLserver created interpreter (note
the interrupt is not virtual-server specific). This command utilizes the
facilities of Tcl_AsyncMark to mark as ready a callback registered
with Tcl_AsyncCreate. The callback places an error message in the
interpreter result and returns TCL_ERROR to unwind the call stack.
The underlying Tcl facility has limitations, e.g., the interrupt will only
be noticed when Tcl checks via Tcl_AsyncReady calls between
commands and the interrupt can be caught with a catch command. See
the man page for Tcl_AsyncCreate for details.
- ns_ictl cleanup
- This command invokes any callbacks registered during a transaction via the
C-level Ns_TclRegisterDefer routine. Unlike callbacks registered
with the ns_ictl trace deallocate command or
Ns_TclRegisterTrace routine, these callbacks are executed only once
and there is no Tcl-level access to the underlying
Ns_TclRegisterDefer routine.
- ns_ictl epoch
- This command returns the unique id for the current duplication script for
the virtual server. The id starts as 0 when the virtual server is created
and is incremented each time a new script is saved via the ns_ictl
save command.
- ns_ictl get
- Return the current duplication script for the virtual server. This command
is useful to view the duplication script created by the initialization
script at startup.
- ns_ictl getmodules
- Return the list of modules to be initialized at startup. This list
corresponds to the names of modules specified in the virtual server
modules config section, e.g., ns/server/server1/modules unless
additional modules are added via the ns_ictl addmodule command.
- ns_ictl gettraces which
- Return the list of traces which will be invoked at the specified time. The
which argument can be one of create, delete, allocate,
deallocate, getconn, or freeconn. The traces are returned in
the order in which they will be executed. Script level traces are returns
as strings to evaluate and C-level traces are returned with strings which
specify the address of the underlying C procedure and argument.
- ns_ictl once key script
- Evaluate given script once in the virtual server. The given key is a
string name which uniquely identifies the corresponding script. This
command is useful in a Tcl package which includes one-time initialization
routines, e.g., calls to ns_register_proc or initialization of
shared variables using nsv_set (see EXAMPLES below).
- ns_ictl oncleanup script
- This command is equivalent to ns_ictl trace deallocate
script.
- ns_ictl oncreate script
- This command is equivalent to ns_ictl trace create script.
- ns_ictl ondelete script
- This command is equivalent to ns_ictl trace delete script.
- ns_ictl oninit script
- This command is equivalent to ns_ictl trace allocate script.
- ns_ictl package ?-exact? package ?version?
- This command is used to require a package in the calling interpreter and,
if successfully loaded, in all other interpreters for the virtual server.
In addition to ensuring version consistency for the package, it is
equivalent to:
set version [package require package ?version?]
ns_ictl trace allocate [list package require package
$version]
- ns_ictl runtraces which
- This command runs the requested traces. The which argument must be
one of create, delete, allocate, deallocate,
getconn, or freeconn. Direct calls to this command are not
normally necessary as the underlying C code will invoke the callbacks at
the required times. Exceptions include calling ns_ictl runtraces or
testing purposes or to mimic the normal cleanup and initialization work
performed on between transactions in a long running thread (see
EXAMPLES below).
- ns_ictl save script
- Save the given script as the duplication script, incrementing the virtual
server epoch number. This command is normally called by the bootstrap
script after constructing the script to duplicate the procedures defined
by sourcing the various module initialization script files.
- ns_ictl threads
- Return a list of all threads with interpreters for the virtual server. The
ids return are small strings which represent the underlying thread ids and
can be passed to the ns_ictl cancel command to send an asynchronous
cancel request.
- ns_ictl trace create script
- Register script to be called when an interpreter is first created.
This is useful to create procedures, require packages, or initialize other
state to be used during the lifetime of the interpreter.
- ns_ictl trace delete script
- Register script to be called before an interpreter is destroyed.
This is useful to free any resources which may have been allocated for the
interpreter during the lifetime of the interpreter.
- ns_ictl trace allocate script
- Register script to be called each time an interpreter is allocated
for use by the Ns_TclAllocateInterp routine. This is useful for
reinitializing resources which may be used during a single transaction in
the interpreter.
- ns_ictl trace deallocate script
- Register script to be called each time an interpreter is returned
after a transaction with the Ns_TclDeAllocateInterp routine. This
is useful for garbage collection, i.e., freeing any resources which may be
used during a single transaction in the interpreter.
- ns_ictl trace getconn script
- Register script to be called each time an interpreter is returned
and associated with an HTTP connection with the Ns_GetConnInterp
routine. This could be useful to define variables relative to the HTTP
request.
- ns_ictl trace freeconn script
- Register script to be called each time an HTTP connection is
closed. This could be used to log information about the request, e.g.,
timing statistics. Note that the interpreter may still be actively
evaluating a script after the connection is closed, i.e., this is not
equivalent to ns_ictl trace deallocate for connection-related
interpreters.
- ns_ictl update
- This command can be used to atomically compare the epoch of the current
duplication script with the epoch of the interpreter, evaluating the
script and updating the epoch in the interpreter if they do not match.
This command is generally registered as a callback with ns_ictl trace
allocate by the legacy initialization code.
INTERPRETER ALLOCATION¶
Tcl interpreter in AOLserver are available on demand with state specific to a
virtual server. These interpreters are also expected to be reused for multiple
transactions (e.g., HTTP connections, scheduled procedures, socket callbacks).
To support reuse, AOLserver provides the C-level
Ns_TclAllocateInterp
routine to allocate an interpreter from a per-thread cache (creating and
initializing a new interpreter if necessary) and the
Ns_TclDeAllocateInterp routine to return an interpreter to the cache
when no longer required. All interpreters in the per-thread cache are
destroyed when a thread exists.
In general, only C-level extension writers need to call the C-level API's
directly; the various Tcl-level interfaces in AOLserver (e.g.,
ADP
pages,
ns_regiseter_proc,
ns_schedule_proc,
ns_thread,
etc.) allocate and reuse interpreters using the C-level API's automatically
before invoking the cooresponding script or ADP page.
INTERPRETER TRACES¶
To ensure a consistent state of interpreters when allocated and enable cleanup
and reinitialization between transactions, each virtual server maintains a
list of callbacks to be invoked at various points in the lifetime of an
interpreter. These callbacks are generally installed when the server is
initialized at startup and then called automatically by the
Ns_TclAllocateInterp and
Ns_TclDeAllocateInterp API's at the
appropriate times and in consistent order. The
Ns_TclRegisterTrace
routine can be used to register C-level callbacks and the
ns_ictl trace
command can be used to register Tcl script callbacks. The
ns_ictl
gettraces command can be used to list all currently registered
callbacks, both at the Tcl script and C level.
Callbacks registered via the tracing facility are invoked in a specific order
depending on the type. Initialization style callbacks including
create,
allocate, and
getconn are invoked in FIFO order, with all script
callbacks invoked after all C-level callbacks. This enables extension writers
to utilize the facilities of previously initialized extensions.
Correspondingly, cleanup style callbacks including
freeconn,
deallocate, and
delete are invoked in LIFO order, with all
scripts callbacks invoked before C-level callbacks. This helps avoid the
possibility that a cleanup callback utilizes features of a previously cleaned
up extension.
In addition, the
ns_ictl package command can be used to consistently
manage the loading of a Tcl package in all interpreters for a virtual server.
This feature is mostly a convenience routine built above the generic trace
framework with additional checks to ensure version number consistency. Coupled
with
ns_ictl once, the
ns_ictl package command provides a clean
framework to utilize Tcl packages in multi-threaded AOLserver (see
EXAMPLES).
VIRTUAL SERVER TCL INITIALIZATION¶
AOLserver also supports a Tcl initialization framework for virtual servers based
on callbacks registered by loadable modules and the sourcing of scripts files
located in corresponding directories. Options to
ns_ictl to support
this framework include
save,
get,
epoch, and
update and are used in conjunction with the generic tracing facility by
the virtual server bootstrap script (normally
bin/init.tcl). The
ns_eval command also relies on this framework to support dynamic update
of the state of interpreters.
This initialization framework pre-dates the Tcl package facilities and utilizes
introspection of the state of a startup interpreter at the end of
initialization to construct a single script which attempts to duplicate the
state in subsequent interpreters. Steps taken during this initialization
include:
- 1. Load all modules in the server's module config section, e.g.,
- ns/server/server1/modules. Modules with Tcl C-level extensions
typically call the legacy Ns_TclInitInterps routine or the more
general Ns_TclRegisterTrace routine with the
NS_TCL_TRACE_CREATE flag in their module init routine to register a
callback to invoke when new interpreters are created. The callback
normally creates one or more new commands in the interpreter with
Tcl_CreateObjCommand but may perform any per-interpreter
initialization required, e.g., creating and saving private state with the
Tcl_SetAssocData facility. In addition, as modules are loaded, the
string name of the module is added to the list of known modules.
- 2. After all C modules are loaded, AOLserver creates a new Tcl
- interpreter for the virtual server, executing any trace callbacks already
registered via the loaded C modules (e.g., any Ns_TclInitInterps
callbacks) and then sources the virtual server bootstrap script, normally
bin/init.tcl. This script creates a few utility procedures and then
sources all private and public script files in directories
which correspond to loaded modules in the order in which they were loaded.
These directories are normally relative to the virtual server and to the
AOLserver installation directory, e.g., initialization script files for
the module mymod in the server1 virtual server would be
searched for in the servers/server1/modules/tcl/mymod/ and
modules/tcl/mymod/. Any init.tcl file found in each
directory is sourced first with all remaining files sourced in
alphabetical order. In addition, any files in the public directory
with identical names to files in the private directory are skipped as a
means to enable overloading of specific functionality on a per-server
basis. In practice, most modules only contain shared utility procedures
defined in the public directories and the private
directories are empty or non-existant. The script files normally contain a
mix of commands to evaluate once for server configuration (e.g., a call to
ns_register_proc to bind a Tcl procedure to an HTTP request URL)
with proc and namespace commands to provide additional
functionality in the interpreter.
- 3. After all script files have been sourced, the bootstrap script
- code then uses a collection of recursive procedures to extract the
definitions of all procedures defined in all namespaces. The definitions
are used to construct a script which attempts to duplicate the state of
the initialization interpreters. This scripts is then saved as the
per-virtual server duplication script with the ns_ictl save
command which also increments the epoch to 1. There are limits to
this approach to determine the full state, e.g., it does not attempt to
duplicate any global variables which may have been defined in the startup
scripts. Typically, startup scripts will use nsv_set or other
mechanisms to store such shared state.
- 4. The bootstrap code then uses the ns_ictl trace allocate
- command to register a callback to the ns_ictl update command each
time an interpreter is allocated for use. In practice, interpreters are
created with the default epoch of 0 and the first call to
ns_ictl update determines an out-of-date condition, evaluates the
duplication script, and increments the interpreter's epoch to 1 to
match the state created by the startup interp.
- 5. Subsequent calls the ns_eval, if any, will evaluate the
- given script and then re-generate and save the duplication script as was
done at startup, incrementing the epoch once again. In this way,
dynamic updates which are detected in other interpreters on their next
call to ns_ictl update can be supported in a limited fashion.
In practice, while generally successful, this duplication technique has
inhibited the clean use of proper Tcl package extensions and encouraged the
use of the
ns_eval command which is generally not recommended for the
non-deterministic manner in which it attempts to dynamically reconfigure a
server. Also, commands required to configure the server once (e.g., calls to
ns_register_proc) are inter-mixed with
proc commands designed to
extend functionality in all interpreters, complicating configuration
management.
As an alternative, the example below illustrates a means to more explicitly
manage configuration through a combination of direct calls to
ns_ictl trace
create and
ns_ictl once. Unfortunately, the all encompassing nature
of the legacy initialization approach makes it difficult to incrementally move
to this cleaner approach because the duplication script construction code is
unable to distinguish between state created with the newer, cleaner
ns_ictl commands and state created as a side effect of one or more
script files being sourced. As such, it is expected the legacy initialization
framework will remain in place until AOLserver 5.x when it will be removed
entirely in a non-backwards compatible move towards the cleaner API's.
EXAMPLES¶
This example illustrates the use of
ns_ictl package and
ns_ictl
once to load an AOLserver-aware Tcl package into a virtual server. The
following code could be added to the virtual server bootstrap script,
bin/init.tcl, to load
MyPkg in the virtual server:
#
# Startup code in bin/init.tcl:
#
# Load MyPkg in all interps (including this one).
#
ns_ictl package require MyPkg
This call will result in the package being loaded into the startup interpreter
in the ordinary Tcl fashion (see the
package man page for details).
Ordinary Tcl extension packages would need no modifications but packages which
utilize AOLserver-specific features or require garbage collection between
transactions could also use
ns_ictl for finer grained control. For
example, the
init.tcl script specified by the
package ifneeded
command in the
MyPkg package's
pkgIndex.tcl file could contains:
#
# Package code in lib/myPkg1.0/init.tcl:
#
#
package provide MyPkg 1.0
#
# Server init which will be executed the first time called,
# normally in the context of the startup interpreter as above.
#
ns_ictl once MyPkg {
# Register the run Tcl proc HTTP handler.
ns_register_proc /mypkg mkpkg::run
# Register a garbage collection callback.
ns_ictl trace deallocate mypkg::cleanup
}
#
# Code which will be invoked to initialize the package in
# all interpreters when required.
#
proc mypkg::run {} {
... handle /mypkg requests ...
}
proc mkpkg::cleanup {} {
... cleanup transaction resources for mypkg, e.g., db handles ...
}
SEE ALSO¶
Ns_TclAllocateInterp(3),
Ns_TclDeAllocateInterp(3),
Ns_GetConnInterp(3),
Ns_FreeConnInterp(3),
Ns_TclInitInterps(3), Ns_TclRegisterTrace(3),
Ns_TclRegisterDeferred(3), ns_atclose(n), ns_eval(n).
KEYWORDS¶
threads, interpreters, traces, initialization
