This app is a distilled example of an approach used in a couple of riak-based applicaitons I've seen. As you may know Riak allows to contact any node with any request. Although client is greatly simplified this way, there are cases when you want to contact "the right" node for a key. This simple application on being injected into Riak node exposes routing information through a simple HTTP API.

All API endpoints piggybacks on Riak's HTTP interface, so host and port are the same.

GET /riak_preflists/ringstatus Returns a json hash table of hosts and what part of the continuum they own.

For example:
curl http://localhost:8881/riak_preflists/ringstatus
returns something like:

{"0":"[email protected]",
 "22835963083295358096932575511191922182123945984":"[email protected]",
 "45671926166590716193865151022383844364247891968":"[email protected]",
   ...
  "1415829711164312202009819681693899175291684651008":"[email protected]",
  "1438665674247607560106752257205091097473808596992":"[email protected]"
}

GET /riak_preflists/nodelists/buckets/Bucket/keys/Key Returns a list of hostnames for the Bucket and key Key in the form of JSON array.

For example:
curl http://localhost:8881/riak_preflists/nodelists/buckets/foo/keys/bar
returns ["sumerman-mbpr.local"] on my machine :)

Having to ask nodelist for every request may defeat the purpose of exposing it. But if one wants to cache it, (s)he needs a way to invalidate the cache. So this endpoint provides such a way.

GET /riak_preflists/notifications[?ping=N] Returns an endless chunked stream of either ping\n or update\n. Pings are there as a means of health-check while an update is a sign to invalidate a cache of nodelists. The optional parameter allows to specify an interval in seconds between pings (1 sec by default).

• To build for injection just issue make.
• If you want to play with it in a standalone mode issue make standalone. But beware, it will download and compile almost whole riak :)

Open the first console and cd into the app's directory, then$ ./start.sh, wait a bit until everything starts up, then (in the erlang console) issue node()

Open the second one and do the$ ./start.sh 2, wait a bit again, then type riak_core:join('first_node@name') where first_node@name is the thing returned by node() in the first console.

You can join up to 9 nodes in the same way.

API ports for the nodes are 8881, 8882, ... respectively.

Just add

-pa /path/too/riak_preflists/ebin
-s riak_preflists_app

to your vm.args file

You should see in the log something like

Application riak_preflists started on node '[email protected]'

When running into Riak, the REST API will be available under the same port as Riak's one, so 8098 by default.