Real-time, streaming, distributed event -> metric computation.

Pulse provides near-real-time, streaming, distributed event-to-metric compuation for the Heroku platform kernel. Heroku engineers use Pulse to maintain ambient awareness of Heroku's distributed infrastructure and to quickly diagnose platform issues.

Pulse works by consuming, processing, and performing statistical calculations against the platform's unified event log stream. All Heroku components log extensively and in an easily machine-parsable format; by processing these logs, we can build various platform metrics, analytics, monitoring, auditing, and debugging tools orthogonally to the critical-path components themselves. Pulse is one such tool.

The log messages that ultimately power Pulse originate from hosts across the Heroku platform. These messages are sent to a load-balanced pool of log forwarders. The log forwarders in turn distribute messages to an internal Splunk cluster for archival and long-term analytics, and also host "Aorta" servers which provide Pulse access to the log stream.

Pulse itself is a distributed application implemented with two process types: receiver and merger. Processes of the receiver type are responsible for physically consuming a load-balanced fraction of the platform event stream, parsing each text-encoded event into a data structure, and streaming that data through the initial phase of the Pulse statistical pipeline. The receiver processes periodically broadcast "stat fragments" corresponding to partial roll-ups of the various stats that Pulse tracks, for consumption by the merger processes. To ensure that the production Pulse deployment can process the entire Heroku log stream at peak platform load, this process type is highly horizontally scaled.

The merger is a smaller set of sharded singleton processes responsible for providing a unified statistical view for the Pulse deployment. These processes continuously combine stat fragments as they arrive from the receiver processes and periodically emits the resulting "stat snapshots" as JSON into the given METRICS_URLS.

Pulse is written in Clojure and deployed to Heroku itself using the platform's native Clojure support.

Ensure that Aorta is running at AORTA_URL, then:

$ cp .env.sample .env
$ mate .env
$ export $(cat .env)
$ lein deps
$ foreman start

Ensure that Aortas are running and dyno-reachable at AORTA_URLS, and that the username in the AORTA_URLS are scoped to this particular deployment of Pulse. Then:

$ DEPLOY=production/staging/you/etc
$ heroku create pulse-$DEPLOY -s cedar

$ heroku addons:add redistogo:large -r $DEPLOY
$ heroku config:add BUILDPACK_URL=https://github.com/heroku/heroku-buildpack-clojure.git -r $DEPLOY
$ heroku config:add DEPLOY=$DEPLOY -r $DEPLOY
$ heroku config:add CLOUD=heroku.com -r $DEPLOY
$ heroku config:add FORCE_HTTPS=true -r $DEPLOY
$ heroku config:add GRAPHITE_PERIOD=21600 -r $DEPLOY
$ heroku config:add PUBLISH_THREADS=2 -r $DEPLOY
$ heroku config:add GRAPHITE_URL=... -r $DEPLOY
$ heroku config:add SCALES_URL=... -r $DEPLOY
$ heroku config:add REDIS_URL=... -r $DEPLOY
$ heroku config:add AORTA_URLS=... -r $DEPLOY
$ heroku config:add METRICS_URLS=... -r $DEPLOY

$ git push $DEPLOY master
$ heroku scale receiver=60 merger0=1 merger1=1 merger2=1 merger3=1 merger4=1 emitter=1 -r $DEPLOY