Extending Applications in Tcl

11 min read

(For more resources on Tcl, see here.)

Very often applications need to be extended with additional functionalities. In many cases it is also necessary to run different code on various types of machines.

A good solution to this problem is to introduce extensibility to our application. We’ll want to allow additional modules to be deployed to specified clients—this means delivering modules to clients, keeping them up to date, and handling loading and unloading of modules.

The following sections will introduce how Starkits and Tcl’s VFS can be used to easily create modules for our application, and how these can be used to deploy additional code.

We’ll also introduce a simple implementation of handling modules, both on server side and client side. This requires creating code for downloading modules themselves, loading them, and the mechanism for telling clients what actions they should be performing.

The server will be responsible for telling clients when they should load or unload any of the modules. They will also inform clients if any of the modules are out of date. Clients will use this information to make sure modules that are loaded are consistent, with what server expects.

The following steps will be performed by the client:

  • The client sends a list of loaded modules and a list of all available modules along with their MD5 checksums to server
  • The server responds with the list of modules to load, unload, and which need to be downloaded
  • The client downloads modules that it needs to download; if the client already has downloaded all modules it needs, no action is taken
  • The client unloads modules that it should unload and loads modules that it should load; if the client has already loaded all modules it should have, no action is taken

Handling security is an additional consideration when creating pluggable applications. Clients should check if files are authentic whenever they download any code, which will be run on the client. This can be achieved using SSL and Certificate Authorities.

Starkits as extensions

One of the features Tcl offers is its VFS and using single file archives for staging entire archives. We’ve used these technologies to create our clients as single file executables.

Now we’ll reuse similar mechanisms to create modules for our application. All modules will simply be a Starkit VFS—this will allow embedding all types of files, creating complex scripts, and easily manage whatever a module contains.

Our modules will have their MD5 checksum calculated, similar to the automatic update feature. This will allow the application to easily check whether a module needs to be updated or not. In order to deliver modules to a client, the server will allow downloading modules, similar to retrieving binaries for automatic update.

Building modules

Our modules will be built similar to how binaries were built, with a minor exception. Modules will have their MD5 checksum stored as the first 32 bytes of the file; this does not cause problems for MK4 VFS package and allows easy checking of whether a package is up to date or not.

The source code for each module will be stored in the mod-<modulename> directories. For the purpose of this implementation, clients will source the load.tcl script when loading the package and will source the unload.tcl script when unloading it. It will be up to the module to initialize and finalize itself properly.

The directory and file structure in the following screenshot show how files for modules are laid out.

Extending Applications in TCL

It is based on previous example and only the mod-comm and mod-helloworld directories are new.

We’ll need to modify the build.tcl script and the add code responsible for building of modules. We can do this after the initial binaries have been built.

First let’s create the modules directory:

file mkdir modules

Then we’ll iterate over each module to build and create name of the target file:

foreach module {helloworld comm} {
set modfile [file join modules $module.kit]

W e’ll start off by creating a 32 byte file:

set fh [open $modfile w]fconfigure $fh -translation binary
puts -nonewline $fh [string repeat "" 32]close $fh

Next we’ll create the VFS, copy the source code of the module, and unmount it:

vfs::mk4::Mount $modfile $modfile
docopy mod-$module $modfile
vfs::unmount $modfile

Now, calculate the MD5 checksum of the newly created module:

set md5 [md5::md5 -hex -file $modfile]

And set the first 32 bytes to the checksum:

    set fh [open $modfile r+]    fconfigure $fh -translation binary
seek $fh 0 start
puts -nonewline $fh $md5
close $fh

We’ll create two modules—helloworld and comm. The first one will simply log “Hello world!” every 30 seconds. The second one will set up a comm interface that can be used for testing and debugging purposes.

Let’s start with creating the mod-helloworld/load.tcl script which will be responsible for initializing the helloworld module:

csa::log::info "Loading helloworld module"

namespace eval helloworld {}

proc helloworld::hello {} {
csa::log::info "Hello world!"
after cancel helloworld::hello
after 30000 helloworld::hello


Our module will log the information that it has been loaded ,and write out hello world to the log. The helloworld::hello command also schedules itself to be run every 30 seconds.

The mod-helloworld/unload.tcl script that cleans up the module looks like this:

csa::log::info "Unloading helloworld module"

after cancel helloworld::hello

namespace delete helloworld

This will log information about the unloading of a module, cancel the next invocation of the helloworld::hello command, and remove the entire helloworld namespace.

Implementing the comm module is also simple. The mod-comm/load.tcl script is as follows:

csa::log::info "Loading comm module"

package require comm

comm::comm configure -port 1992 -listen 1 -local 1

This script simply loads the comm package, sets it up to listen on port 1992, and only accepts connections on the local interface.

Unloading the package (in mod-comm/load.tcl) will configure the comm interface not to listen for incoming connections:

csa::log::info "Unloading comm module"

comm::comm configure -listen 0

As the comm package cannot be simply unloaded, the best solution is for load.tcl to configure it to listen for connections and unload.tcl to disable listening.

Server side

The server side of extensibility needs to perform several activities. First of all we need to track which clients should be using which modules. The second function is providing clients with modules to download. The third functionality is telling clients which modules they need to fetch from the server, and which ones that they need to load or unload from the environment.

Let’s start off with adding initialization of the modules directory to our server. We need to add it to src-server/main.tcl:

set csa::binariesdirectory [file join [pwd] binaries]set csa::modulesdirectory [file join [pwd] modules]
set csa::datadirectory [file join [pwd] data]

We’ll also need to load an additional script for handling this functionality:

csa::log::debug "Sourcing remaining files"
source [file join $starkit::topdir commapi.tcl]source [file join $starkit::topdir database.tcl]source [file join $starkit::topdir clientrequest.tcl]source [file join $starkit::topdir autoupdate.tcl]source [file join $starkit::topdir clientmodules.tcl]

Next we’ll also need to modify src-server/database.tcl to add support for storing the modules list. We’ll need to add a new table definition to script that creates all tables:

CREATE TABLE clientmodules (
client CHAR(36) NOT NULL,
module VARCHAR(255) NOT NULL

In order to work on the data we’ll also need commands to add or remove a module for a specified client:

proc csa::setClientModule {client module enabled} {
if {[llength [db eval
{SELECT guid FROM clients WHERE guid=$client
AND status=1}]] == 0} {
return false

db eval {DELETE FROM clientmodules WHERE
client=$client AND module=$module}

if {$enabled} {
db eval {INSERT INTO clientmodules (client, module)
VALUES($client, $module)}

return true

Our command starts off by checking if a client exists and returns immediately if it does not. In the next step, we delete any existing entries and insert a new row if we’ve been asked to enable a particular module for a specified client.

We’ll also need to be able to list modules associated with a particular client, which means executing a simple SQL query to list modules:

proc csa::getClientModules {client} {
return [lsort [db eval {SELECT module
FROM clientmodules WHERE client=$client}]]}

Then we’ll need to create the clientmodules.tcl file that will have functionality related to handling modules, and providing them to clients.

The first thing required is a function to read MD5 checksums from modules. We’ll first check if the file exists and return an empty string if it does not, otherwise we’ll read and return the first 32 bytes of the file:

proc csa::getModuleMD5 {name} {
variable modulesdirectory

set filename [file join $modulesdirectory $name]
if {![file exists $filename]} {
return ""
set fh [open $filename r] fconfigure $fh -translation binary
set md5 [read $fh 32] close $fh
return $md5

Next we’ll create a function that takes a client identifier, queries the database for modules for that client and returns a list of module-md5sum pairs, which can be treated as a dictionary – where the key is the module name and the value is its md5 checksum.

proc csa::getClientModulesMD5 {client} {
set rc [list] foreach module [getClientModules $client] {
lappend rc $module [getModuleMD5 $module] }
return $rc

Another function will handle requests for a particular module. Similar to how it is implemented for automatic updates, we’ll provide files only from a single directory and handle cases where a file does not exist, and register a prefix for the requests in TclHttpd:

proc csa::handleClientModule {sock suffix} {
variable modulesdirectory
set filename [file join $modulesdirectory
[file tail $suffix]]
log::debug "handleClientModule: File name: $filename"
if {[file exists $filename]} {
Httpd_ReturnFile $sock application/octet-stream
} else {
log::warn "handleClientModule: $filename not found"
Httpd_Error $sock 404

Url_PrefixInstall /client/module csa::handleClientModule

For communication with the clients, we’ll reuse the protocol for requesting jobs.

We also need a function that given a client identifier, request dictionary, and name of the response variable will provide information to the client. It will also return whether a client should be provided with a list of jobs or not. If a client will need to download new or updated modules first, we do not need to provide a list of jobs as the client will need to have updated modules first.

Let’s start by making sure that both the modules available on the client and the list of loaded modules has been sent to the client:

proc csa::csaHandleClientModules {guid req responsevar} {
upvar 1 $responsevar response

set ok true

if {[dict exists $req availableModules] && [dict exists $req loadedModules]} {

Then we copy the values to local variables for convenience, and get a list of modules that a client should have along with their MD5 checksums.

set rAvailable [dict get $req availableModules] set rLoaded [dict get $req loadedModules] set lAvailable [getClientModulesMD5 $guid]

We’ll also create a list of actions we want to pass back to the client— the list of modules it needs to download and the list of modules to load and unload. By default, all lists are empty and we’ll add items only if we detect that the client should perform actions:

set downloadList [list] set loadList [list] set unloadList [list]

As the first step, we’ll iterate over the modules that the client should have and check if it has them—if the client either does not have a module or its checksum differs, we’ll tell the client to download it.

foreach {module md5} $lAvailable {
if {(![dict exists $rAvailable $module]) ||
([dict get $rAvailable $module] != $md5)} {
lappend downloadList $module

After this we check if the client has already loaded this module. If not, we’ll tell him to load the module.

if {[lsearch -exact $rLoaded $module] < 0} {
lappend loadList $module

We’ll also iterate over the modules that the client currently has loaded and if any of them should not be loaded according to our list, we’ll tell the client to unload it.

foreach module $rLoaded {
if {![dict exists $lAvailable $module]} {
lappend unloadList $module

Once we’ve conducted our comparison, we tell the agent what should be done. If he needs to download any modules, we only return this information and return that there is no point in providing the list of jobs to perform:

if {[llength $downloadList] > 0} {
dict set response moduleDownload
set ok false
} else {

Otherwise if all modules on the client are updated, we provide a list of modules to load or unload if this is needed.

if {[llength $loadList] > 0} {
dict set response moduleLoad

if {[llength $unloadList] > 0} {
dict set response moduleUnload

Finally, we return whether or not we should provide the client with a list of jobs to perform or not:


return $ok

Now we’ll need to modify the csa::handleClientProtocol command in the src-server/clientrequest.tcl file to invoke our newly created csa::csaHandleClientModules command:

if {[csaHandleClientModules
$guid $req response]} {
# only specify jobs if client
# has all the modules

if {[dict exists $req joblimit]} {
set joblimit [dict get $req joblimit] } else {
set joblimit 10

dict set response jobs
[getJobs $guid $joblimit] log::debug "handleClientProtocol: Jobs:
[llength [dict get $response jobs]]"

This will cause jobs to be added only if the csaHandleClientModules command returned true.

We can also modify the csa::apihandle command in the src-server/commapi.tcl file to allow adding or removing a module from a client. The following needs to be added inside the main switch responsible for handling commands:

switch -- $cmd {
addClientModule {
lassign $command cmd client module
return [setClientModule $client $module 1]
removeClientModule {
lassign $command cmd client module
return [setClientModule $client $module 0]

These commands simply invoke the csa::setClientModule command created earlier.

Subscribe to the weekly Packt Hub newsletter

* indicates required


Please enter your comment!
Please enter your name here