TIL - The Tcl ICE Library

TIL Manual

1. Introduction

TIL is a collection of Tcl-only libraries and utilities of general type. Most of these libraries and utilities originates from a number of projects where interaction was a key issue and where the applications being developed and deployed would run continuously for days and, thus, need to perform some level of introspection to check the liveness of the system in question.

In general, TIL has three major goals.

  1. Facilitate the development, deployment and surveillance of Tcl-based distributed applications and systems.

  2. Suppress the discrepancies between UNIX and Windows when it comes to administration and deployment.

  3. Demonstrate a design principle that consists in writing network-aware simple components. These components handle a single and well-defined task and an application consists of several of these components, assembled on one or several machines.

TIL has been repackaged as a library for general use, and some new bugs might have appeared while doing so.

2. Modules and Programs

TIL is composed both of a number of packages that can directly be used in applications and of a number of programs that demonstrate the packages, the principles behind TIL and are utilities useful enough to make their way into a library distribution.

2.1 Organisation

TIL is mainly a library of packages, and there is one directory per package, ready for a "package require" command. Additionally, the directory named "bin" contains all programs and utilities and the directory "test" a number of files and scripts to quickly test the facilities of the libraries.

2.2 TIL Utilities

The TIL utilities are only dependent on tcllib (tested with version 1.6.1) and on the TIL library. They all share a common helper module called "argutil.tcl", which is not itself part of the library. Through "argutil", the TIL utilities attempt to look for tcllib at a specific version back into the upper hierarchy. This is only a hint for package loading and can be overriden by any other Tcl means.

The utilities can grouped into three different types of applications: applications to continuously run and supervise a number of other tasks, applications to help the deployment of distributed component-based applications and miscellaneous other helper utilities.

Most of these applications support a basic command line socket-based protocol to expose their capabilities to remote clients and attain flexibility. As such, these applications attempt to exemplify one the design principle behind TIL: TIL wishes to cut down monolythic applications into small networked components, as well as making the connection between these components explicit (as opposed to a number of modern programming paradigms that attempt to hide network complexity).

These applications all act as servers, and one utility application (called "prompt.tcl") allows to interactively talk to these servers. All these applications are built on top of modules from the TIL library, so that programmers will be able to migrate these facilities easily into their own code. The command-line protocol is a line-based protocol, where the first word of the line is the command and the remaining the arguments. All servers automatically implement a number of commands, namely COMMANDS, QUIT, CLOSE, FINISH and EXIT. COMMANDS will return the full list of commands supported by the server. QUIT and CLOSE should be sent by well-behaved clients before closing the connection (but are not mandatory). FINISH will force the server to stop serving new clients and will close all its current connections. EXIT will exit the whole application behind the server. As such, the TIL really should be run in trusted environments, even though it provides for some degree of security protection through remote host allowance and denial based on IP number and name matches.

Most of these applications takes an argument called -help, which will print out some usage help for the application. Note that some of these applications suppose that TIL is currently setup and running (see the section on the "process" package!).

2.2.1 Running and Supervising Tasks Starter

The core of this group of applications is made of "starter.tcl", which implements a Tcl-level service and application. "starter" reads a number of service-list description files and will make sure that the applications that these point at will continuously run. The output of these applications is automatically logged (and timestamped) to the disk, and the log will automatically rotate regularly. Services that have crashed will be automatically restarted.

The list of files describing the services is specified via the parameter called -infiles. These are complete path to files on the disk, where some variables (contained between % signs) will be automatically replaced by the run-time value when the script is started. Supported variables are currently all the content of the tcl_platform global array, "hostname" (the fully-qualified host name of the machine, whenever possible), "progdir" and "progname" (i.e. the name of the script without directory specification). In these files, empty lines or lines starting with a "#" will be ignored. Other lines should contain a keyword, followed by the application to start and its arguments. An example service file is contained in the "test" directory and called "services.stp". The keyword specifies the type of application, and the following are currently supported:

  • The keyword "T" will indicate a Tcl only script. In that case, the script will be started by the binary tclsh contained in the same directory as the one that started starter.

  • The keyword "W" will indicate a Tk script. In that case, the script will be started by the binary wish contained in the same directory as the one that started starter.

  • The keyword "J" will indicate a Java application. In that case, starter will use one of the libraries from TIL to locate a suitable JVM installed on the current machine.

  • The keyword "S" can be used to start any other application, typically a shell command.

Starter offers the following commands to clients:

  • LIST, will return the list of currently running services. That list will be composed of the identifiers of the services, immediately followed by the process identifier of their application.

  • INFO will return complete information on the service which identifier, process identifier or name is passed as a parameter.

  • REMOVE will remove one or several services, which identifier, process identifier or name are passed as a parameter. Running instances of these services will be killed.

  • REMOVE_ALL will remove and kill all currently known services.

  • RESTART will restart one or several services, the arguments are the same as for REMOVE.

  • RESTART_ALL will restart all currently known services.

  • RELOAD will kill all currently known services, reload the service description files and restart the services.

  • WATCH will start watching the output of a given service. Any new line appended to the log file associated to the service will be sent back to the client, preceeded by the command LOG and the identifier of the service.

  • UNWATCH suppresses a log watch as installed by the previous command. Logging will still continue on disk, only socket forwarding of log lines will stop. killapp

The "killapp.tcl" application is a barebone Tcl applications with as little dependencies as possible that simplifies operations on the services that are controlled by the starter application described in section "killapp" is meant as an emergency application and starting it without any arguments will connect to the running starter and tell it to remove all or some of its services. killapp is also able to trigger the reloading of service specification files or the restarting (as opposed to suppression) of services. daemon

The daemon.tcl application provides a service that is slightly similar to starter, except that it controls a single application. Daemon is meant to be started from a UNIX-style startup script during boot-time (/etc/init.d startup script). Daemon will fork the application that it controls and write enough state to the disk to be able to kill it later on (using the -kill argument) and to make sure one and only one instance of this application is running. Daemon will also make sure to log the output of the application that it controls to the disk. The locations for the state and log file defaults to well-known places under the UNIX system file hierarchy: /var/log, /var/run, etc.

Usually, daemon will be run by the administrator of the machine but will change user so that the application that it controls is run by another user. For situations where you cannot, as a user, have access to the boot sequence and/or not enough access priviledges, daemon can also be used to be regularly started as a standard cron job. In that mode, daemon will check that the application that it controls still runs and restart it if necessary. This mode of operation consumes more computing resources but is another way of ensuring the presence of another application on a given machine. Daemon does not assume that the other application that it controls is a Tcl script, you can control any other type of application.

daemon.tcl has been tested on a Linux system and is currently dependent on the /proc file system on UNIX machines. It is also known to be working on windows machines, as long as the TIL library and the process package have been setup correctly. tclsvcd

"tclsvcd" is a companion application to daemon and an example of an init.d-style script. Copying it to your system and making it part of your booting process should ensure the starting up of services (via the starter application above) as soon as the system goes live. Apart from being Tcl-aware, the advantage of this solution is the provision of a command line protocol for remote operations on the services being put into control. crond

The "crond.tcl" application is an implementation of cron in Tcl. It complies almost entirely to the crontab syntax, but adds the possibility to load a script (and procedures) from within the crontab and call these procedures at regular intervals. This facility makes easier the storing of state in between regular operations, i.e. without requiring storing state on the disk. A list of crontab files is pointed at by the -crontab argument. The file paths obey to the same rules as the -infiles argument of the starter script above, i.e. the can contain some run-time variable contained between % signs.

In the crontab files, empty lines and lines starting with "#" will be ignored. Otherwise, the following types of lines are supported. Any line that starts with the keyword INCLUDE will trigger the sourcing of the file name passed as a second argument. Once again, this name can contain run-time variables between % signs. The script will be sourced into the cron at the global level, which might lead to insecurity. Good practice is to make use of namespaces. The other types of lines contain five initial fields specifying when to run the activity (same syntax as crontab), followed by the keyword PROC, or the keyword EXEC. PROC will called one procedure, typically sourced an INCLUDE directive line, EXEC will execute an external command.

This implementation of the cron daemon offers a number of commands to remote clients. These are:

  • LIST will return the list of the identifiers of the current activities registered at the cron daemon.

  • INFO will return full specification of a given activity.

  • DELETE will remove an activity.

  • ADD will add a new activity at run-time. This activity will not be saved in any crontab file.

2.2.2 Componentised Applications Support Distributed Parameter Storage

The core of this group of applications is "params.tcl", an application that store, set and deliver key-value pairs to remote clients. The application is able to store both persistent and volatile data, through different commands offered to clients. Persistent parameters will be saved on a local file, which will be read at each startup of the application. Volatile parameters can be initialised through specifying a number of files that will be read when the applications is starting through the command-line parameter -infiles. This parameter, as for the service starter and the cron, accepts run-time variables enclosed by % signs.

The parameter server offers a number of commands to remote clients. These are listed below. However, a higher-level package can be used to communicate with the server in a transparent manner from applications. This package is part of TIL and called param.

  • LIST will return the list of all currently known persistent and volatile parameter.

  • GET will return one or more parameters, wildcards (string match) are accepted in the names.

  • SET and UNSET will set and unset a parameter and its value. These will be volatile parameters.

  • STORE and UNSTORE do the same, while declaring the parameter as persistent over time.

  • WATCH sees to install a watch so that the client will be notified when a parameter matching the wildcard specification given as a parameter has been set. WATCH enables simplistic synchronisation between applications that share a set of common data.

  • IGNORE removes an existing parameter watch. Data Multiplexing

The multiplex.tcl application is a server that accepts any number of clients and that will forward to all other clients any data that is sent from one of its client, hence effectively multiplexing all communication between clients. The command-line parameter -back commands if data sent by one client will be sent back to the origin.

Additionally, multiplex has a liveness parameter which allows this application to automatically end after a given period of time. By putting multiplex under the control of starter and testing regularily that it is possible to connect to the multiplex server, it is possible to check that starter itself still functions as it should. Port Bridging

The bridge.tcl application is a server that accepts any number of clients on a known port and manages the subscription to other remote servers, through this known port (typically a port allowed by a firewall), on behalf of its clients. The client side implementation of the bridge capabilities is implemented as part of the permanent client library (see section 2.3.2.).

The bridge application accepts a number of commands from its clients. These are:

  • CONNECT takes two arguments and will attempt to connect a remote service. The name or IP of the host is the first argument to the command, while the port number is the second argument.

  • DISCONNECT also takes two arguments and will disconnect from a service to which the bridge was attached using the CONNECT command.

  • Any other command will be sent further to one of the services to which the bridge is connected. The command (and thus first word) sent on the socket should be composed on the name of the host, followed by a slash, followed by the port number of the remote service. Any remaining arguments on the line will blindly be sent to the remote service. Sending such a formatted command to the bridge can automatically lead to connection to the remote service. This is currently an internal parameter that could be switched off for security reasons.

2.2.3 Miscellaneous Other Utilities Generic Prompt

prompt.tcl is a generic prompt to all the serving utilities described here. This application connects to a remote server. All command lines that are entered at the prompt will be sent to the server for treatment and all input from the server will be written in the caller's window. Log Timestamping

timestamper.tcl is an application that watches a file (or the standard input) and that will write to a file (or to the standard output) each line of the input file, preceeded by the timestamp at which this line was read. When writing to files, timestamper.tcl implements log rotation facilities so that only the latest data is kept on disk. timestamper.tcl is used both by starter.tcl and by daemon.tcl (see section 2.2.1.) when logging to disk. Local and Remote URL Watching

url_watcher.tcl continuously monitors a number of remote URLs (http only so far) or local files for modifications. As soon as the modification date of a remote or local file has changed, clients that have requested for the monitoring of these URLs will be notified. url_watcher accepts a number of commands from its clients:

  • WATCH takes all the remaining arguments as local or remote URLs and starts monitoring their modification time only if the URL exists at creation time.

  • ADD acts as above but do not check for the existence of the URL, which allows to watch for files that will be created in the future.

  • REMOVE remove one or several URL monitoring.

  • CHECK forces a check of all URLs now, otherwise checking is usually done in a lazy manner at spread intervals.

  • BURST places the URL watching service in burst mode, which means that possibly many monitoring are going to be installed soon. This will have the effect of attempting to spread more evenly all remote and local checks in time.

  • ATONCE ends bursting mode. Caching of remote URL

cachectl.tcl is a utility to locally cache a number of remote URLs to the disk. The utility is able to handle any number of caches and there are very few arguments that are recognised when creating caches for the time being. cachectl is built on top of the URL caching facility from the library below. As such, it is able to smoothly handle redirects and retries on failing URLs. It currently accepts the following commands from its clients (that really should be able access the local disk!):

  • CACHE takes the name of a directory local to the program as an argument and will instantiate (or restart) a cache from that directory. The name of directory supports the "%"-syntax introduced by crond. The command sends back a CACHE command to the client, with two arguments this time: the name of the directory and an identifier for that cache. The identifier will be used in all further commands.

  • GET takes a cache identifier and a number of URLs as its arguments. All URLs will be actively fetched into the cache if necessary. Once in the cache, the command will return back to the client the command GOT with the identifier of the cache, the URL and the name of the local file (relative to the directory or with the whole path, this is option-dependent). On failure, an ERROR command, followed by the identifier of the cache and the URL will be returned.

  • INFO takes a cache identifier and a number of URL matching patterns as its arguments. It will return information for all URLs matching the patterns to the caller through a command for each URL. Each command will start with the cache identifier, followed by the URL, followed by a list of key values describing the URL state in the cache. Port number allocator

port_allocator.tcl is a utility to allocate port number for a set of remote services. It uses an external human-readable file for storing the port number that already have been allocated. The program can either be used as a continuously running service or as a one-shot port allocator helper. When it is started with one of the -servicename or -servicedescr options, it will simply allocate a new service and write the port to the database file. Otherwise, it runs as a service and accepts two commands:

  • ALLOCATE takes two arguments, one being the name of the service (typically the name of the main (Tcl) binary) and a textual description for that service. A new port number will be associated to that service and a command will be sent back to pass back the new port number to the caller. This command is composed of the keyword ALLOCATED, followed by the name of the service, followed by its port.

  • PORTINFO takes either no argument or any number of arguments. Each argument is a port number for which information is being queried. If no arguments are given, PORTINFO will return information about all known ports. For each port, PORTINFO will return one command composed as followed: the keyword PORT, followed by the port number, followed by the name of the service, followed by the description of the service in between quotes.

2.3 The TIL Library

The TIL library is composed of a number of packages and most of the documentation is in form of well-formatted comments in the source for the time being. All packages are isolated in their own name spaces and will never pollute the global name space. The TIL library requires tcllib for operating.

Most packages provide a loglevel command that control the level of logging that is associated to the package. These levels are compliant to the logger package from tcllib. All packages have a distinct log level to be able to selectively turn up or down this level whenever necessary. The default logging level is always "warn".

2.3.1 Command Server

cmdserver is a package to implement command servers that follows a simplistic protocol where remote clients send commands that start with a keyword and end with an end of line. cmdserver provides a series of tcl procedures to handle connection and disconnection of clients, as well as the mapping of incoming socket commands onto Tcl procedures. Each client connection can be associated to any number of key, value pairs that can be used by applications to store data associated to the connection. cmdserver provides a basic level of security through the fine-grain controlling of the hosts that will be able to connect to the servers.

2.3.2 Permanent Client

permclient is a package which implements the client side of connections to remote servers implemented through cmdserver. Apart from facilitating opening and closing of connections, the major feature of permclient is it ability to re-establish connection with a remote server whenever it has been lost. The permclient package will continuously attempt to reconnect on a regular basis.

permclient is also able to connect to remote servers via the bridge facility, which is a service offered by TIL. Section gives more details.

2.3.3 Error Handler

errhandler provides a facility to handle background error in applications while still letting these to continue once an error has occured. While the package is generic enough for usage anywhere, it is primarily used by the server and client packages to discover socket connection problems and re-establish connection whenever necessary.

2.3.4 DNS Resolution

dnsresolv is a further development of the module called resolv, which is part of the tcllib dns package. Since both code bases are mine, ultimately these should be merged. dnsresolv provides a facility for inverse resolution and improves slightly the algorithm used to detect the DNS server that an application should use for resolution.

2.3.5 Remote Parameter Storage

param is a package which provides a client-side implementation of the parameter server described in section It provides facilities to set parameters at the server and to get their values. It is based on active caching of the remote values of all parameters at the server, an implementation that could be changed if scalability became an issue.

param::store is the heart of the parameter storage server. It provides a glorified key,value storage structure with facilities to write and read this "database" to and from files. Reading from files support the inclusion of files. A value starting with an "@" sign in a file will be interpreted as a list which content should be obtained from another file, which name is represented by all characters that follow the "@" sign. In all files, empty lines and lines starting with #, ; or ! are ignored.

2.3.6 Cron

The cron package provides facilities to schedule procedures to be called at regular intervals in time, and follows a syntax that is similar to the traditional UNIX cron package.

The crontab package provides facilities to read extended crontab files. These files, as described in section, are very similar to crontab files, but also support the inclusion of other Tcl scripts and the calling of both external programs and of internal procedure at regular intervals.

2.3.7 Disk Utilities

The diskutil provides a number of aid in handling files and path in a cross-platform manner. The package provides facilities to handle PATH-like variable, to generate temporary file names and directories, to concatenate the content of several files, to clean up (temporary) directories, etc.

diskutil also provides an implementation for the file path resolution facility that is used in a number of TIL applications and that replaces some variables enclosed by % signs at run-time.

2.3.8 Directory and File Content Monitoring

dirwatch and filewatch are packages to watch the content of directories and files. dirwatch will monitor directories for file additions and removal and will provide appropriate callbacks. filewatch will monitor files and provide callbacks whenever these have been created or changed.

2.3.9 Log Watch

logwatch provides facilities to watch (typically growing) log files and provide callback with their content as they grow. The package purposedly opens and closes the files dynamically in order to handle properly mounted remote file systems.

2.3.10 Log File Output

outlog provides facilities to output data to log files, together with facilities to automatically rotate these log files so that only a controlled number of these files is kept.

2.3.11 Spooling Directories

spool provides facilities to treat directory structures as spool directories. A spool is a directory that contains three sub-directory with appropriate semantics: inbox, error and sent. inbox is where files arrive, sent where files that have been treated with success are moved and error is where files that have not been treated with success are moved. The spool will watch for new files arriving in the inbox, provides callbacks to the application and move the file to the sent or error sub-directories upon success or failure, as directed by the application.

2.3.12 Time Stamping

timestamp is a tiny package to implement timestamping at the millisecond.

2.3.13 Process

process is a package to handle processes external to Tcl and running on the same host. The package provides commands to kill and list these processes in a platform agnostic way.

In order for the package to function properly on Windows, you will need the "PsTools" package from Sysinternals. This package is available at the following URL (for windows NT and above only): http://www.sysinternals.com/ntw2k/freeware/pstools.shtml. Place the content of the distribution in the bin/windows sub-directory of the package for proper functioning.

2.3.14 Java

The java package provides facilities to communicate with remote Java processes that use the utf8 writing and reading methods, and for looking to an appropriate JVM on the running host.

2.3.15 Mass URL Fetching

The massgeturl package provides facilities to fetch a "massive" number of URLs simultaneously. Additionally, the package provides a number of built-in features that will ease fetching of remote URLs and files in a more generic context. For the control of fetching "en masse", the package is built upon a fetching queue and new URLs to be got are placed on the queue together with a callback command that will be called back once the URL has been fetched or once fetching attempts have failed (see below). Internally, each host features it own separate queue and massgeturl allows to control the number of total outwards connections (which minimises the burden on the Internet connection) and the total number of connections per host (which minimises the burden on the remote servers). The package will automatically follow redirects and automatically retry a number of times if required and necessary. All problems during fetching are reported back to the caller through the command being called back with information on the nature of the problem.

2.3.16 URL Information and Content Caching

Two inter-twined modules provide facilities for remote URL and local files information and content caching. "urlhead" aims at controlling information such as the MIME type, the length or the modification dates of local or remote files. Local files have been included for completion. "urlcache" aims at providing a caching mechanism similar to the one behind most Internet browsers. Remote URLs are fetched again when their remote modification date and the cached date of the local cached file differ. Both modules are built on top of the Mass URL fetching facility described in section 2.3.15., which allows them to automatically follow redirects and perform several connection attempts if necessary.

2.3.17 Tiny HTTP Server

The tiny HTTP server is aimed at providing basic HTTP server facilities. It currently only supports HTTP 1.0 operations. The module is a heavily modified version of an implementation from an unknown source that attempted to show how simple Tcl/Tk is for implementing such servers. This implementation of the module is able to server HEAD requests and also provides an implementation for directory listing. The directories that are allowed to be listed and those that have to default to an index HTML file (index.htm usually) is controllable to provide an acceptable level of security.

2.3.18 MIME Type Guessing

This library provides an implementation of the algorithm behind the Apache web server. The MIME type of a file is guessed through its extension first, then through peeking a number of bytes from the file and guessing using a number of rules, as with the UNIX file command. tcllib 1.7 now provides a very similar facility, so the content of this module is of little interest. However, it parses automatically the rule file on startup, which provides better flexibility.

2.3.19 Playlist

This library is built on top of snack and provides a number of commands to create, order and operate (play, pause, resume, etc.) on playlists.

2.3.20 RSS

The RSS library is composed of two distinct modules. The first module is an RSS parser built on top of the XML tree parser. The parser is rudimentary, but provides supports for enclosures, which was its primary goal. On top of the parser, the library provides a monitoring module that will deliver callbacks on each new and deleted item for a given RSS feed. The monitoring module is compliant with the latest specification and will follow the "ttl" directive from the feed so as to reduce the burden on the server hosting the RSS feed. Callbacks will be provided with a token as an argument in a manner that is similar to the standard http package from Tcl. The content of the item can then be accessed through an upvar command.

2.3.21 Port Number Allocation

The portsalloc library provides a deterministic way to allocate port numbers in a distributed fashion. Port numbers are generated from the string describing the service. Provided a set of processes agree on a common set of services and, thus, strings and allocate the port numbers for these services in the same order, all processes will end up with identical port numbers allocated to the services.

2.4 Testing

These packages and utilities have been tested under in a real research environment on a daily basis. However, they have been repackaged for the purpose of the TIL library of packages and utilities and some new bugs might have occured in the process. The embryo of a test suite is contained in the test sub-directory of the library. Further documentation is still needed.

theme by Chris M