The Fixlet Historian is an application that allows users to visualize differences between any two versions of a fixlet, as well as inspect a site's history. It leverages the presence of existing content on sync.bigfix.com to produce this analysis.

Requirements

• python (CPython version 2.7) an interpreter for Python

• requests a Python library utility for serving HTTP requests

• python-Levenshtein a Python library for computing textual differences

• node (node.js) a JavaScript server library

• npm a package manager for node.js applications

• bower a package manager for JavaScript dependencies

Installation

1. Download the files to where you want your server to be.

   $ git clone [email protected]:bigfix/fixlet-historian
   

2. Install requests.

   $ pip install requests
   

   Go here for more information.

3. Get the application seed. There are several ways to do this:

• You can download the seed database itself fxfdata.db (around 2 GB).

  $ wget

• You can download seed_cache.txt (around 150 KB) from and then run python seed.py. The presence of this file greatly speeds up the creation of the seed database.

  $ wget $ python seed.py

• You can run python seed.py by itself. This will build both the cache file and the seed database. Be aware that this takes a very long time (about 6 hours on our systems) and should probably be run overnight.

  $ python seed.py

4. Bring the database to its current version. This will take a long time to complete (about 15 hours on our systems).

   $ python update.py
   

This step is interruptible and later resumable due to the nature of SQLite's transactional properties. The application will still run if this step is interrupted, but the database will only be partially complete. Re-run this step later to finish the update.

5. Install the backend server dependencies.

   $ npm install
   

6. Install the frontend server dependencies.

   $ cd frontend
   $ bower install
   

7. Download and run the installer for python-Levenshtein appropriate for your system here.

Usage

To run the server after it's been built, simply run node main.js.

To keep the contents of the application database up-to-date run python update.py regularly. It is advised to run this at least once a day.

Technical Details

Architecture

There are three principle components of the application: the persistent component, the server backend, and the server frontend.

Persistence

Data persistence is provided by an application database named fxfdata.db. The scripts seed.py and update.py are simply call functions from the main file dataminer.py. The file database.py provides utilities for interacting with the database, while the file fixlet_parser.py provides utilities for parsing fixlets.

The script seed.py initializes the database. It interacts with the auxiliary files gathers.txt and seed_cache.txt. gathers.txt contains a list of site URLs to target. seed_cache.txt is a serialized Python dictionary produced by finding the first versions of each fixlet file per site, and can be used to speed up building the seed database (as this is an expensive operation).

SQLite guarantees atomic updates per transaction. update.py provides atomicity on the level of fxf files. If any error occurs as a fxf file is updated, the entire transaction aborts and the script continues. As a result, the failure to update any fixlet file will not affect the update of any other fixlet file, nor will it affect the consistency of data previously recorded for that file.

Server Backend

The server backend consists of the node server main.js, as well as a dependency listing package.json and the differential analysis script diff_service.py. main.js routes requests with the library express.js. When required, the server retrieves data from the database file and passes it to the page rendering mechanism. Whenever the server needs to produce a differential, it runs the command diff_service.py, capturing its output.

Server Frontend

Requests serviced through main.js are rendered in express.js templates on the frontend. Parameters to these templates are passed from main.js and give most of the site content. Common public assets reside in the assets/ directory.

Database Design

This application uses a SQLite database to store data about sites, fixlet files, and individual fixlets. The database contains seven tables:

1. The table RevisionTypes enumerates metadata values for the state of fixlet files and revisions. It is the smallest table, static, and simply records constants from database.py.

2. The table Sites contains the names of sites and urls. It is a static and relatively small table, proportional to the number of sites provided during the creation of the database seed.

3. The table FxfFiles contains information about individual fixlet files tracked by the application. It contains each file's site and name, as well as its latest fetched version and its latest version on disk. These numbers are different because the application optimizes away storing redundant identical files (which were unchanged in some revision). This table is relatively small and relatively static.

4. The table FxfRevisions contains information about each unique version of a fixlet file which introduced a change, the nature of the change, and the URL the change was fetched from.

5. For each entry in FxfRevisions there exists another entry in FxfContents which contains the fixlet file content itself. These tables are both large, and since the FxfContents table is much larger than the FxfRevisions table the FxfRevisions table acts as a kind of index on the FxfContents table. These tables are dynamic, updated whenever a new revision is spotted on sync.bigfix.com.

6. The table Revisions contains information about each revision of a particular fixlet. It contains information about the fixlet's site, ID, version, and title, as well as the nature of the revision, when it was published, and which revision of fxf it came from.

7. For each entry in Revisions there exists an entry in RevisionContents. Like FxfContents, this table contains the actual fixlet contents as it was parsed from the fxf file, and the division of the tables reduces access time penalties resulting from large row sizes. These contents are retained in JSON form with HTML characters escaped for direct rendering on the frontend. Both these tables are large and dynamic as well.

Differential Analysis

Fixlet differentials are produced with the aid of the python-Levenshtein library.

The Levenshtein distance is the mathematical distance between two strings, formally determined by the length of the minimal sequence of single-character inserts, replacements, and removals needed to transform one string into another. The python-Levenshtein library computes this sequence deterministically.

While this sequence is precise in a mathematical sense and the library fast, for many inputs the algorithm produces operations that are not well human-readable. Human-readability is determined in large part by the contiguity of inserted and removed substrings. The Python standard library also produces text differentials (via the difflib library) but these differentials are neither very precise nor oftentimes very readable at all. However, in certain cases these differentials are much more readable than those produced by the Levenshtein algorithm.

As a result, we use a simple heuristic to determine which algorithms to use: we measure the number of transformation operations needed by each algorithm and take the one which uses fewer. This maximizes contiguity and also tends to pick the transformation which is more precise, falling back to Python's algorithm when the Levenshtein algorithm fails.

Incomplete Elements

• The server build and update processes are not particularly robust. Although we make use of SQLite's transaction system for recovery in the common case, crashes can sometimes trigger bugs that are difficult to recover from manually.

• Parsing remains brittle. Changes to the format of published fixlets will likely break the current system.

• The timestamps of publication on different revisions are inconsistent.

• It's likely that there exist better string-matching algorithms for the computation of differentials. Further research there would be useful.

• The differentials poorly handle the case in which a digest is added so that successive relevance "shifts down" from where it was inserted. It should be possible to produce these diffs more intelligently.

• The mechanism for adding new .fxf files to track has not been tested.

• Enhancements to the UI, especially to site history.

Planned Features

• Only fixlets are currently parsed. Analyses, Tasks, and other objects which are parsable can (probably) be easily added.

• Many fixlet metadata changes are not currently recorded. The differentials can be expanded to include changes to interesting metadata (e.g. fixlet titles)

• A tool to export a fixlet from a given revision.

• Formatting of content, including Relevance, ActionScript, and HTML.

Bugs

• Fixlets which move from one .fxf file to another .fxf file are marked as having a "new" revision type even though they are not. They should be marked "changed" instead.

• Certain sites are not parsed correctly on the first pass of the update script. Subsequent sweeps of the script tend to fix the issue.

• Certain fixlet IDs are duplicates, and fixlet files containing these duplicates fail to update properly.

• HTML escaping may be inconsistent.

Support

Any issues or questions regarding this software should be filed via GitHub issues.