WebFetch - Perl module to download and save information from the Web
use WebFetch;
The WebFetch module is a framework for downloading and saving information from the web, and for saving or re-displaying it. It provides a generalized interface for saving to a file while keeping the previous version as a backup. This is mainly intended for use in a cron-job to acquire periodically-updated information.
WebFetch allows the user to specify a source and destination, and the input and output formats. It is possible to write new Perl modules to the WebFetch API in order to add more input and output formats.
The currently-provided input formats are Atom, RSS, WebFetch "SiteNews" files and raw Perl data structures.
The currently-provided output formats are RSS, WebFetch "SiteNews" files, the Perl Template Toolkit, and export into a TWiki site.
Some modules which were specific to pre-RSS/Atom web syndication formats have been deprecated. Those modules can be found in the CPAN archive in WebFetch 0.10. Those modules are no longer compatible with changes in the current WebFetch API.
After unpacking and the module sources from the tar file, run
perl Makefile.PL
make
make install
Or from a CPAN shell you can simply type "install WebFetch" and it will download, build and install it for you.
install WebFetch
If you need help setting up a separate area to install the modules (i.e. if you don't have write permission where perl keeps its modules) then see the Perl FAQ.
To begin using the WebFetch modules, you will need to test your fetch operations manually, put them into a crontab, and then use server-side include (SSI) or a similar server configuration to include the files in a live web page.
Select a directory which will be the storage area for files created by WebFetch. This is an important administrative decision - keep the volatile automatically-generated files in their own directory so they'll be separated from manually-maintained files.
Choose the specific WebFetch-derived modules that do the work you want. See their particular manual/web pages for details on command-line arguments. Test run them first before committing to a crontab.
If needed, see the manual pages for crontab(1), crontab(5) and any web sites or books on Unix system administration.
Since WebFetch command lines are usually very long, the user may prefer to make one or more scripts as front-ends so crontab entries aren't so big.
Try not to run crontab entries too often - be aware if the site you're accessing has any resource constraints, and how often their information gets updated. If they request users not to access a feed more often than a certain interval, respect it. (It isn't hard to find violators in server logs.) If in doubt, try every 30 minutes until more information becomes available.
The following function definitions assume $obj is a blessed reference to a module that is derived from (inherits from) WebFetch.
$obj
This function allows a Perl module to register itself with the WebFetch API as able to perform various capabilities.
For subclasses of WebFetch, it can be called as a class method. __PACKAGE__->module_register( @capabilities );
__PACKAGE__->module_register( @capabilities );
For the $module parameter, the Perl module should provide its own name, usually via the __PACKAGE__ string.
The @capabilities array is any number of strings as needed to list the capabilities which the module performs for the WebFetch API. The currently-recognized capabilities are "cmdline", "input" and "output". "config", "filter", "save" and "storage" are reserved for future use. The function will save all the capability names that the module provides, without checking whether any code will use it.
For example, the WebFetch::Output::TT module registers itself like this: __PACKAGE__->module_register( "cmdline", "output:tt" ); meaning that it defines additional command-line options, and it provides an output format handler for the "tt" format, the Perl Template Toolkit.
__PACKAGE__->module_register( "cmdline", "output:tt" );
This function is exported into the main package. For all modules which registered with an "input" capability for the requested file format at the time this is called, it will call the run() function on behalf of each of the packages.
Generally, the new function should be inherited and used from a derived class. However, WebFetch provides an AUTOLOAD function which will catch wayward function calls from a subclass, and redirect it to the appropriate function in the calling class, if it exists.
The AUTOLOAD feature is needed because, for example, when an object is instantiated in a WebFetch::Input::* class, it will later be passed to a WebFetch::Output::* class, whose data method functions can be accessed this way as if the WebFetch object had become a member of that class.
This is called from the new function that modules inherit from WebFetch. If subclasses override it, they should still call it before completion. It takes "name" => "value" pairs which are all placed verbatim as attributes in $obj.
new
This specifies a WebFetch module (Perl class) which needs to be loaded. In case of an error, it throws an exception.
This function can be called by the main::fetch_main function provided by WebFetch or by another user function. This handles command-line processing for some standard options, calling the module-specific fetch function and WebFetch's $obj->save function to save the contents to one or more files.
main::fetch_main
The command-line processing for some standard options are as follows:
(required) the directory in which to write output files
(optional) the group ID to set the output file(s) to
(optional) the file mode (permissions) to set the output file(s) to
(optional) save a copy of the fetched info in the file named by this parameter. The contents of the file are determined by the --dest_format parameter. If --dest_format isn't defined but only one module has registered a file format for saving, then that will be used by default.
--dest_format
(optional) suppress printed warnings for HTTP errors (applies only to modules which use the WebFetch::get() function) in case they are not desired for cron outputs
(optional) print verbose debugging outputs, only useful for developers adding new WebFetch-based modules or finding/reporting a bug in an existing module
Modules derived from WebFetch may add their own command-line options that WebFetch::run() will use by defining a variable called @Options in the calling module, using the name/value pairs defined in Perl's Getopts::Long module. Derived modules can also add to the command-line usage error message by defining a variable called $Usage with a string of the additional parameters, as they should appear in the usage message.
@Options
$Usage
do_actions was added in WebFetch 0.10 as part of the WebFetch Embedding API. Upon entry to this function, $obj must contain the following attributes:
do_actions
is a reference to a hash containing the following three (required) keys:
is a reference to an array containing the names of the fetched data fields in the order they appear in the records of the data array. This is necessary to define what each field is called because any kind of data can be fetched from the web.
is a reference to a hash which maps from a key string with a "well-known" (to WebFetch) field type to a field name used in this table. The well-known names are defined as follows:
a one-liner banner or title text (plain text, no HTML tags)
URL or file path (as appropriate) to the news source
unique identifier string for the entry
a date stamp, which must be program-readable by Perl's Date::Calc module in the Parse_Date() function in order to support timestamp-related comparisons and processing that some users have requested. If the date cannot be parsed by Date::Calc, either translate it when your module captures it, or do not define this "well-known" field because it wouldn't fit the definition. (plain text, no HTML tags)
a paragraph of summary text in HTML
number of comments/replies at the news site (plain text, no HTML tags)
a name, handle or login name representing the author of the news item (plain text, no HTML tags)
a word or short phrase representing the category, topic or department of the news item (plain text, no HTML tags)
a location associated with the news item (plain text, no HTML tags)
The field names for this table are defined in the fields array.
The hash only maps for the fields available in the table. If no field representing a given well-known name is present in the data fields, that well-known name key must not be defined in this hash.
an array containing the data records. Each record is itself a reference to an array of strings which are the data fields. This is effectively a two-dimensional array or a table.
Only one table-type set of data is permitted per fetch operation. If more are needed, they should be arranged as separate fetches with different parameters.
is a reference to a hash. The hash keys are names for handler functions. The WebFetch core provides internal handler functions called fmt_handler_html (for HTML output), fmt_handler_xml (for XML output), fmt_handler_wf (for WebFetch::General format), However, WebFetch modules may provide additional format handler functions of their own by prepending "fmt_handler_" to the key string used in the actions array.
The values are array references containing "action specs", which are themselves arrays of parameters that will be passed to the handler functions for generating output in a specific format. There may be more than one entry for a given format if multiple outputs with different parameters are needed.
The presence of values in this field mean that output is to be generated in the specified format. The presence of these would have been chosed by the WebFetch module that created them - possibly by default settings or by a command-line argument that directed a specific output format to be used.
For each valid action spec, a separate "savable" (contents to be placed in a file) will be generated from the contents of the data variable.
The valid (but all optional) keys are
the value must be a reference to an array which specifies all the HTML generation (html_gen) operations that will take place upon the data. Each entry in the array is itself an array reference, containing the following parameters for a call to html_gen():
a file name or path string (relative to the WebFetch output directory unless a full path is given) for output of HTML text.
a hash reference containing optional name/value parameters for the HTML format handler.
(optional) a reference to code that, given a reference to an entry in @{$self->{data}{records}}, returns true (1) or false (0) for whether it will be included in the HTML output. By default, all records are included.
(optional) a reference to code that, given references to two entries in @{$self->{data}{records}}, returns the sort comparison value for the order they should be in. By default, no sorting is done and all records (subject to filtering) are accepted in order.
(optional) a refernce to code that, given a reference to an entry in @{$self->{data}{records}}, stores a savable representation of the string.
Additional valid keys may be created by modules that inherit from WebFetch by supplying a method/function named with "fmt_handler_" preceding the string used for the key. For example, for an "xyz" format, the handler function would be fmt_handler_xyz. The value (the "action spec") of the hash entry must be an array reference. Within that array are "action spec entries", each of which is a reference to an array containing the list of parameters that will be passed verbatim to the fmt_handler_xyz function.
When the format handler function returns, it is expected to have created entries in the $obj->{savables} array (even if they only contain error messages explaining a failure), which will be used by $obj->save() to save the files and print the error messages.
For coding examples, use the fmt_handler_* functions in WebFetch.pm itself.
This function must be provided by each derived module to perform the fetch operaton specific to that module. It will be called from new() so you should not call it directly. Your fetch function should extract some data from somewhere and place of it in HTML or other meaningful form in the "savable" array.
new()
TODO: cleanup references to WebFetch 0.09 and 0.10 APIs.
Upon entry to this function, $obj must contain the following attributes:
The name of the directory to save in. (If called from the command-line, this will already have been provided by the required --dir parameter.)
--dir
a reference to an array where the "savable" items will be placed by the $obj->fetch function. (You only need to provide an array reference - other WebFetch functions can write to it.)
In WebFetch 0.10 and later, this parameter should no longer be supplied by the fetch function (unless you wish to use 0.09 backward compatibility) because it is filled in by the do_actions after the fetch function is completed based on the data and actions variables that are set in the fetch function. (See below.)
Each entry of the savable array is a hash reference with the following attributes:
file name to save in
scalar w/ entire text or raw content to write to the file
(optional) group setting to apply to file
(optional) file permissions to apply to file
Contents of savable items may be generated directly by derived modules or with WebFetch's html_gen, html_savable or raw_savable functions. These functions will set the group and mode parameters from the object's own settings, which in turn could have originated from the WebFetch command-line if this was called that way.
html_gen
html_savable
raw_savable
Note that the fetch functions requirements changed in WebFetch 0.10. The old requirement (0.09 and earlier) is supported for backward compatibility.
In WebFetch 0.09 and earlier, upon exit from this function, the $obj->savable array must contain one entry for each file to be saved. More than one array entry means more than one file to save. The WebFetch infrastructure will save them, retaining backup copies and setting file modes as needed.
Beginning in WebFetch 0.10, the "WebFetch embedding" capability was introduced. In order to do this, the captured data of the fetch function had to be externalized where other Perl routines could access it. So the fetch function now only populates data structures (including code references necessary to process the data.)
Upon exit from the function, the following variables must be set in $obj:
is a reference to a hash which will be used by the do_actions function. (See above.)
This WebFetch utility function will get a URL and return a reference to a scalar with the retrieved contents. Upon entry to this function, $obj must contain the following attributes:
the URL to get
a flag which, when set to a non-zero (true) value, suppresses printing of HTTP request errors on STDERR
In WebFetch 0.10 and later, this should be used only in format handler functions. See do_actions() for details.
This WebFetch utility function stores pre-generated HTML in a new entry in the $obj->{savable} array, for later writing to a file. It's basically a simple wrapper that puts HTML comments warning that it's machine-generated around the provided HTML text. This is generally a good idea so that neophyte webmasters (and you know there are a lot of them in the world :-) will see the warning before trying to manually modify your automatically-generated text.
See $obj->fetch for details on the contents of the savable parameter
savable
This WebFetch utility function stores any raw content and a filename in the $obj->{savable} array, in preparation for writing to that file. (The actual save operation may also automatically include keeping backup files and setting the group and mode of the file.)
This should be used only in format handler functions. See do_actions() for details.
This adds a task for the save function to fetch a URL and save it verbatim in a file. This can be used to download links contained in a news feed.
This can be used by an output function which handles its own intricate output operation (such as WebFetch::Output::TWiki). If the savables array is empty, it would cause an error. Using this function drops a note in it which basically says that's OK.
This WebFetch utility function goes through all the entries in the $obj->{savable} array and saves their contents, providing several services such as keeping backup copies, and setting the group and mode of the file, if requested to do so.
If you call a WebFetch-derived module from the command-line run() or fetch_main() functions, this will already be done for you. Otherwise you will need to call it after populating the savable array with one entry per file to save.
directory to save files in
names and contents for files to save
When a WebFetch input object is passed to an output class, operations on $self would not usually work. WebFetch subclasses are considered to be cooperating with each other. So WebFetch provides AUTOLOAD functionality to catch undefined function calls for its subclasses. If the calling class provides a function by the name that was attempted, then it will be redirected there.
The easiest way to make a new WebFetch-derived module is to start from the module closest to your fetch operation and modify it. Make sure to change all of the following:
The fetch function is the meat of the operation. Get the desired info from a local file or remote site and place the contents that need to be saved in the savable parameter.
Be sure to catch and change them all.
The code and documentation may refer to output files by name.
Change the URL, number of links, etc as necessary.
If you need to add command-line parameters, modify both the @Options and $Usage variables. Don't forget to add documentation for your command-line options and remove old documentation for any you removed.
When adding documentation, if the existing formatting isn't enough for your changes, there's more information about Perl's POD ("plain old documentation") embedded documentation format at http://www.cpan.org/doc/manual/html/pod/perlpod.html
Add yourself as an author if you added any significant functionality. But if you used anyone else's code, retain the existing author credits in any module you modify to make a new one.
Please consider contributing any useful changes back to the WebFetch project at maint@webfetch.org.
maint@webfetch.org
WebFetch was written by Ian Kluft Send patches, bug reports, suggestions and questions to maint@webfetch.org.
Some changes in versions 0.12-0.13 (Aug-Sep 2009) were made for and sponsored by Twiki Inc (formerly TWiki.Net).
WebFetch is Open Source software distributed via the Comprehensive Perl Archive Network (CPAN), a worldwide network of Perl web mirror sites. WebFetch may be copied under the same terms and licensing as Perl itelf.
TODO: fill in these lists
2 POD Errors
The following errors were encountered while parsing the POD:
'=item' outside of any '=over'
You forgot a '=back' before '=head2'
To install WebFetch, copy and paste the appropriate command in to your terminal.
cpanm
cpanm WebFetch
CPAN shell
perl -MCPAN -e shell install WebFetch
For more information on module installation, please visit the detailed CPAN module installation guide.