Hermaeus

Overview

Hermaeus is a library and script that provide an archiving functionality for reddit posts. It is designed for /r/teslore, but should be able to work with any subreddit collection.

I created Hermaeus after noticing that the /r/teslore Compendium, an index of posts considered especially worth remembering, was starting to take damage as links went dead for various reasons. Given that the purpose of the Compendium was to retain these posts, having it maintain references rather than absolute copies was a problem.

Functionality

Hermaeus can be accessed from other Ruby code, and provides an interface of top-level methods on the ::Hermaeus module. Hermaeus also provides a CLI tool, mora, which can scrape indices at a wiki page or in normal posts.

Hermaeus configuration is stored in $HOME/.hermaeus/config.toml. The first time Hermaeus is invoked on a system, through Ruby or the mora script, it will create a sample configuration file and then promptly crash.

Hermaeus will continue to crash until this file is filled with values that allow it to connect to reddit. This requires registering an application with reddit (under preferences/apps) and copying the ID and Secret that reddit generates. If running Hermaeus as a “script” type, which is the option that makes the most sense, you will need to put a reddit account username and password in the file as well. Hermaeus will log in as that user, which means that if given a moderator account, it will be able to read posts that have been removed but not destroyed.

At present, the only storage backend is to save the file to disk. Certain meta attributes (original title, author, date of publishing, reddit ID) are stored in a header section, which may interfere with some Markdown processors.

Performance

Hermaeus does not cache the results of scraping index posts, and so each discrete invocation must make a fresh connection and authentication, then retrieve the index. Hermaeus is almost entirely IO-bound, as even on the teslore wiki page with 2,000 entries, Nokogiri was able to transform and parse the HTML in trivial time. Once Nokogiri has pared the index down to a list of reddit post IDs, Hermaeus is able to use batch queries to pull large sections of posts in a single network call.

At present, Hermaeus defaults to working on sets of 100 posts at a time. I have not done much investigation into the maximum batch size reddit will accept, but this has proven to be a very performant first guess.

Benchmarking

Hermaeus does not have formal benchmarking embedded in its code; this is on my to-do list.

On my server, which sits on the end of an AT&T DSL network, mora seek index takes 1.97s (user), 0.16s (system), 3% CPU, 1:03.03 total time to process 1,951 links and create 1,634 reformatted files on disk. This cost 22 network calls (one to log in, one to fetch the index, twenty batches of 100-post queries).

Hermaeus takes 3.165 seconds to process 13 links, which cost 3 network calls. The initial handshake takes comparatively little time, and most of the time is spent waiting on reddit to accumulate and transmit the posts.

These times are over-inflated by one second per batch query, as Hermaeus sleeps after each bulk call to stay under the reddit rate limiter.

Extra Features

• Hermaeus peforms HTML entity replacement on the text it fetches, so that posts with HTML entity references (’, for instance) will have the actual character inserted instead. All documents are stored in UTF-8 encoding.

• Extensibility: Hermaeus.seek yields each post it collects, allowing easy connection to user-provided code for post handling. The default action in mora is to simply pass the post data to the Hermaeus::Archivist back-end and print the name and author of the post to standard output. The yielded data is a Ruby Hash that holds the post source Markdown, HTML as compiled by reddit, and metadata attributes. The Hermaeus::Apocryphon class wraps this data and provides some helper methods for it.

Feature Requests

If you would like to use Hermaeus as an archivist script for your own projects, I am happy to take feature requests (or if you’re a developer, pull requests). I will continue to work on removing the teslore-specific magic from Hermaeus’ source code so that it can be a subreddit-agnostic crawler.

I do plan on making a website to allow access into the archive Hermaeus creates, but I would be perfectly happy to yield that project to anyone who can do better than this site.