Statuses

Statuses is an experimental, extremely simple-minded microblogging infrastructure for internal use, basically created to have a small yet meaningful Clojure app to play with. Don't expect too much, certainly not something ready for production unless you're mainly interested in fiddling around with stuff.

How to for developers

Make sure you have a reasonably recent Java runtime. Statuses has been tested with JDK 7, but should work with JDK 6 as well (and possibly even JDK 5).
Grab Leiningen (we've just upgraded to the Leiningen 2 preview and suggest you use this) and install according to its instructions
Clone this repository
Use lein run to start the app
Access it at http://localhost:8080. Some sample data is auto-generated if the "database" is not found
If you want to created a deployable artifact, use lein uberjar to create a stand-alone .jar file that you can run with java -jar <name>

Dependencies

Statuses uses

Compojure for URI routing
Hiccup for HTML and XML templating
clj-time for time-related stuff
clojure.data.json for JSON support

Note that all these dependencies are fetched automatically when you use lein run for the first time, and are put into the uberjar for deployment.

Design notes

Persistence

Given its very limited initial requirements at @innoQ (and to support its usability as a self-contained example), the most significant design decision within Statuses is that no database is used. Instead, all of the status updates are kept in memory and are persisted every minute. This works suprisingly well in terms of code simplicity, but will obviously scale only within limits. But if you assume 15k are needed for every 100 status updates, even a million of them would fit in 150MB of JSON (and probably something similar in memory, haven't checked yet).

If scaling problems arise, it's likely going to be because writing the DB to disk takes too long (though even that might be doubtful given today's disk speeds). Should that become a problem, a solution might be to combine the in-memory DB with an event-driven model, where the full memory dump is only written very rarely, but every status update is persisted immediately (essentially a transaction log). The full state could thus be restored from the events stored, adding yet another buzzword and turning this an event-sourcing model. But seeing how far one can get without actually using a "real" database is part of the experiement.

So there are currently no plans to change the db model, even though refactoring the persistence into a Clojure protocol might be reasonable to allow others to implement different backends.

Modules

Currenly, the structure of the whole program is still very simplistic and should probably be refactored soon. At the moment, code is split across the following namespaces and matching files:

statuses.server: the server main entry point
statuses.backend.core: the in-memory data structure and related functions
statuses.backend.json: JSON utility functions for converting the in-memory DB to and from JSON
statuses.backend.persistence: functions to write and read the DB to/from disk
statuses.backend.time: utility functions for handling the conversion of (localized) time stamps
statuses.views.common: layouts (using Twitter Bootstrap) for the UI
statuses.views.main: the actual UI elements for displaying HTML (and JSON)
statuses.views.atom: function to provide an Atom 1.0 feed of status updates