Design

This is a quick prototype that turned out to be quite usable. The design is minimal: some home-made ORM for the feed storage, crude parallelism with the multiprocessing module and a simple plugin API using importlib.

More information about known issues and limitations in the feed2exec manual page document.

Plugin system

Plugins are documented in the Plugins section. You can also refer to the Writing new plugins section if you wish to write a new plugin or extend an existing one.

The plugin system uses a simple importlib based architecture where plugin are simple Python modules loaded at runtime based on a module path provided by the user. This pattern was inspired by a StackOverflow discussion.

The following options were also considered:

Those options were ultimately not used because they add an aditionnal dependency and are more complicated than a simple import. We also did not need plugin listing or discovery, which greatly simplifies our design.

There is some code duplication between different parts (e.g. the feed2exec.plugins.output() and feed2exec.plugins.filter() plugin interfaces, the maildir and mbox plugins, etc), but never more than twice.

Concurrent processing

The threading design may be a little clunky and is certainly less tested, which is why it is disabled by default (use --parallel to use it). There are known deadlocks issues with high concurrency scenarios (e.g. with catchup enabled).

I had multiple design in minds: the current one (multiprocessing.Pool and pool.apply_async) vs aiohttp (on the asyncio branch) vs pool.map (on the threadpoolmap branch). The aiohttp design was very hard to diagnose and debug, which made me abandon the whole thing. After reading up on Curio and Trio, I’m tempted to give async/await a try again, but that would mean completely dropping 2.7 compatibility. The pool.map design is just badly adapted, as it would load all the feed’s datastructure in memory before processing them.

Test suite

The test suite is in feed2exec/tests but also as doctest comments in some functions imported from the ecdysis project. You can run all the tests with pytest, using, for example:

pytest-3

This is also hooked into the setup.py command, so this also works:

python3 setup.py test

Note that some tests will fail in Python 2, as the code is written and tested in Python3. Furthermore, the feed output is taken from an up to date (5.2.1) feedparser version, so the tests are marked as expected to fail for lower versions. You should, naturally, run tests before submitting patches.

The test suite also uses the vcrpy module to cache HTTP requests. This tool caches HTTP requests locally so the test suite can run offline. To add a new network test, you can simply add a new test doing requests with the right decorator, and a new recording will be added to the source tree. We commit the recordings in git so the test suite actually runs offline, so be careful about the content added there. Ideally, the license of that content should be documented in debian/copyright.

betamax was also considered but requires a refactoring of all requests to use session objects. This would have the added benefit of allowing a custom user agent, so it is still considered and is a work in progress in the betamax branch. The current approach on that branch uses a global session object which is problematic: a better approach may be to encapsulate this in a FeedFetcher or simply Feed object, at which point we would end up rearchitecturing the whole feeds.py file...

Comparison

feed2exec is a fairly new and minimal program, so features you may expect from another feed reader may not be present. I chose to write a new program because, when I started, both existing alternatives were in a questionable state: feed2imap was mostly abandoned and rss2email’s maintainer was also unresponsive. Both were missing the features I was looking for, which was to unify my feed parsers in a single program: i needed something that could deliver mail, run commands and send tweets. The latter isn’t done yet, but I am hoping to complete this eventually.

The program may not be for everyone, however, so I made those comparison tables to clarify what feed2exec does compared to the alternatives.

General information:

Program Version Date SLOC Language
feed2exec 0.5 2017 1417 Python
feed2imap 1.2.5 2015 3249 Ruby
rss2email 3.9 2014 1986 Python
  • version: the version analysed
  • date: the date of that release
  • SLOC: Source Lines of Codes as counted by sloccount, only counting dominant language (e.g. excluding XML from test feeds)
  • Language: primary programming language

Delivery options:

Program Maildir Mbox IMAP SMTP sendmail exec
feed2exec
feed2imap
rss2email
  • maildir: writing to Maildir folders. r2e has a pull request to implement maildir support, but it’s not merged at the time of writing
  • IMAP: sending emails to IMAP servers
  • SMTP: delivering emails over the SMTP protocol, with authentication
  • sendmail: delivering local using the local MTA
  • exec: run arbitrary comands to run on new entries. feed2imap has a execurl parameter to execute commands, but it receives an unparsed dump of the feed instead of individual entries. rss2email has a postprocess filter that is a Python plugin that can act on indiviual (or digest) messages which could possibly be extended to support arbitrary commands, but that is rather difficult to implement for normal users.

Features:

Program Pause OPML Retry Images Filter Reply Digest
feed2exec
feed2imap
rss2email
  • pause: feed reading can be disabled temporarily by user. in feed2exec, this is implemented with the pause configuration setting. the catchup option can also be used to catchup with feed entries.
  • retry: tolerate temporary errors. For example, feed2imap will report errors only after 10 failures.
  • images: download images found in feed. feed2imap can download images and attach them to the email.
  • filter: if we can apply arbitrary filters to the feed output. feed2imap can apply filters to the unparsed dump of the feed.
  • reply: if the generated email ‘from’ header is usable to make a reply. rss2email has a use-publisher-email setting (off by default) for this, for example. feed2exec does this by default.
  • digest: possibility of sending a single email per run instead of one per entry

Note

feed2imap supports only importing OPML feeds, exporting is supported by a third-party plugin.