PhiloLogic4 Load Script Architecture

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Clovis and I have been doing a great deal of work lately on PhiloLogic4's document-loading process, and I feel that it's matured enough to start documenting in detail.  The best place to start is with the standard PhiloLogic4, which you can look at on github if you don't have one close at hand:

The load script works more or less like the old philoload script, with some important differences:

  1. The load script is not installed system-wide--you generally want to keep it near your data, with any other scripts. 
  2. The load script has no global configuration file--all configuration is kept separate in each copy of the script that you create.
  3. The PhiloLogic4 Parser class is fully configurable from the load script--you can change any Xpaths you want, or even supply a replacement Parser class if you need to.
  4. The load script is designed to be short, and easy to understand and modify.
The most important pieces of information in any load script are the system setup variables at the top of the file.  These will give immediate errors if they aren't set up right.  

database_root is the filesystem path to the web-accessible directory where your PhiloLogic4 database will live, like /var/www/philologic/--so your webserver process will need read access to it, and you will need write access to create the database--and don't forget to keep the slash at the end of the directory, or you'll get errors.  

url_root is the HTTP URL that the database_root directory is accessible at: be a reasonable mapping of the example above, but it will depend on your DNS setup, server configuration, and other hosting issues outside the scope of this document.

template_dir, which defaults to database_root + "/_system_dir/_install_dir/", is the directory containing all the scripts, reports, templates, and stylesheets that make up a PhiloLogic4 database application.  If you have customized behaviors or designs that you want reflected in all of the databases you build, you can keep those templates in a directory on their own where they won't get overwritten.  

(At the moment, you can't "clone" the templates from an existing database, because they actual database content can be very large, but we'd very much like to implement that feature in the future to allow for easy reloads.)

Most of the rest of the file is configuration for the Loader class, which does all of the real work, but the config is kept here, in the script, so you don't have to maintain custom classes for every database. 

For now, it's just important to know what options can be specified in the load script:
  1. default_object_level defines the type of object returned for the purpose of most navigation reports--for most database, this will be "doc", but you might want to use "div1" for dictionary or encyclopedia databases.
  2. navigable_objects is a list of the object types stored in the database and available for searching, reporting, and navigation--("doc","div1","div2","div3") is the default, but you might want to append "para" if you are parsing interesting metadata on paragraphs, like in drama.  Pages are handled separately, and don't need to be included here.
  3. filters and post_filters are lists of loader functions--their behavior and design will be documented separately, but they are basically lists of modular loader functions to be executed in order, and so shouldn't be modified carelessly.
  4. plain_text_obj is a very useful option that generates a flat text file representations of all objects of a given type, like "doc" or "div1", usually for data mining with Mallet or some other tool.
  5. extra_locals is a catch_all list of extra parameters to pass on to your database later, if you need to--think of it as a "swiss army knife" for passing data from the loader to the database at run-time.
The next section of the load script is setup for the XML Parser:

This is a bit complex, and will be explored in depth in a separate post, but the basic layout is this:
  1. xpaths is a list of 2-tuples that maps philologic object types to absolute XPaths--that is, XPaths evaluated where "." refers to the TEI document root element.  You can define multiple XPaths for the same type of object, but you will get much better and more consistent results if you do not.
  2. metadata_xpaths is a list of 3-tuples that map one or more XPaths to each metadata field defined on each object type.  These are evaluated relative to whatever XML element matched the XPath for the object type in question--so "." here refers to a doc, div1, or paragraph-level object somewhere in the xml.
  3. pseudo_empty_tags is a very obscure option for things that you want to treat as containers, even if they are encoded as self-closing tags.  
  4. suppress_tags is a list of tags in which you do not want to perform tokenization at all--that is, no words in them will be searchable via full-text search.  It does not prohibit extracting metadata from the content of those tags.
  5. word_regex and punct_regex are regular expression fragments that drive our tokenizer.  Each needs to consist of exactly one capturing subgroup so that our tokenizer can use them correctly. They are both fully unicode-aware--usually, the default \w class is fine for words, but in some cases you may need to add apostrophes and such to the word pattern.  Likewise, the punctuation regex pattern fully supports multi-byte utf-8 punctuation.  In both cases you should enter characters as unicode code points, not utf-8 byte strings.
The next section consists of just a few scary incantations that shouldn't be modified:

But the following 2 sections are where all the work gets done, and an important place to perform modifications.   First, we construct the Loader object, passing it all the configuration variables we have constructed so far:

Then we operate the Loader object step-by-step:

And that's it!  

Usually, these load functions should all be executed in the same order, but it is worth paying special attention to the load_metadata variable that is constructed right before l.parse_files is called.  This variable controls the entire parsing process, and is incredibly powerful.  Not only does it let you define any order in which to load your files, but you can also supply any document-level metadata you wish, and change the xpaths, load_filters, or parser class used per file, which can be very useful on complex or heterogeneous data sets.  However, this often requires either some source of stand-off metadata or pre-processing/parsing stage.  

For this purpose, we've added a powerful new Loader function called sort_by_metadata which integrates the functions of a PhiloLogic3 style metadata guesser and sorter, while still being modular enough to replaced entirely when necessary.  We'll describe it in more detail in a later post, but for now, you can look at the new artfl_load_script to get a sense of how to construct a more robust, fault-tolerant loader using this new function.

Up next: the architecture of the PhiloLogic Loader class itself.
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