10 minutes introduction to Gluster Eventing Feature

Gluster Eventing is the new feature as part of Gluster.Next
initiatives, it provides close to realtime notification and alerts for
the Gluster cluster state changes.

Websockets APIs to consume events will be added later. Now we emit
events via another popular mechanism called "Webhooks".(Many popular
products provide notifications via Webhooks Github, Atlassian,
Dropbox, and many more)

Webhooks are similar to callbacks(over HTTP), on event Gluster will
call the Webhook URL(via POST) which is configured. Webhook is a web server
which listens on a URL, this can be deployed outside of the
Cluster. Gluster nodes should be able to access this Webhook server on
the configured port. We will discuss about adding/testing webhook
later.

Example Webhook written in python,

from flask import Flask, request

app = Flask(__name__)

@app.route("/listen", methods=["POST"])
def events_listener():
    gluster_event = request.json
    if gluster_event is None:
        # No event to process, may be test call
        return "OK"

    # Process gluster_event
    # {
    #  "nodeid": NODEID,
    #  "ts": EVENT_TIMESTAMP,
    #  "event": EVENT_TYPE,
    #  "message": EVENT_DATA
    # }
    return "OK"

app.run(host="0.0.0.0", port=9000)

Eventing feature is not yet available in any of the releases, patch is
under review in upstream master(http://review.gluster.org/14248). If anybody interested in trying it
out can cherrypick the patch from review.gluster.org

git clone http://review.gluster.org/glusterfs
cd glusterfs
git fetch http://review.gluster.org/glusterfs refs/changes/48/14248/5
git checkout FETCH_HEAD
git checkout -b <YOUR_BRANCH_NAME>
./autogen.sh
./configure
make
make install

Start the Eventing using,

gluster-eventing start

Other commands available are stop, restart, reload and
status. gluster-eventing --help for more details.

Now Gluster can send out notifications via Webhooks. Setup a web
server listening to a POST request and register that URL to Gluster
Eventing. Thats all.

gluster-eventing webhook-add <MY_WEB_SERVER_URL>

For example, if my webserver is running at http://192.168.122.188:9000/listen
then register using,

gluster-eventing webhook-add ``http://192.168.122.188:9000/listen``

We can also test if web server is accessible from all Gluster nodes
using webhook-test subcommand.

gluster-eventing webhook-test http://192.168.122.188:9000/listen

With the initial patch only basic events are covered, I will add more
events once this patch gets merged. Following events are available
now.

Volume Create
Volume Delete
Volume Start
Volume Stop
Peer Attach
Peer Detach

Created a small demo to show this eventing feature, it uses Web server
which is included with the patch for Testing.(laptop hostname is sonne)

/usr/share/glusterfs/scripts/eventsdash.py --port 8080

Login to Gluster node and start the eventing,

gluster-eventing start
gluster-eventing webhook-add http://sonne:8080/listen

And then login to VM and run Gluster commands to probe/detach peer,
volume create, start etc and Observe the realtime notifications for
the same where eventsdash is running.

Example,

ssh root@fvm1
gluster peer attach fvm2
gluster volume create gv1 fvm1:/bricks/b1 fvm2:/bricks/b2 force
gluster volume start gv1
gluster volume stop gv1
gluster volume delete gv1
gluster peer detach fvm2

Demo also includes a Web UI which refreshes its UI automatically when
something changes in Cluster.(I am still fine tuning this UI, not yet
available with the patch. But soon will be available as seperate repo
in my github)