Thursday, July 4, 2013

Riakc In Five Minutes

This is a simple example using Riakc to PUT a key into a Riak database. It assumes that you already have a Riak database up and running.

First you need to install riakc. Simply do: opam install riakc. As of this writing, the latest version of riakc is 2.0.0 and the code given depends on that version.

Now, the code. The following is a complete CLI tool that will PUT a key and print back the result from Riak. It handles all errors that the library can generate as well as outputting siblings correctly.

 * This example is valid for version 2.0.0, and possibly later
open Core.Std
open Async.Std

 * Take a string of bytes and convert them to hex string
 * representation
let hex_of_string =
  String.concat_map ~f:(fun c -> sprintf "%X" (Char.to_int c))

 * An Robj can have multiple values in it, each one with its
 * own content type, encoding, and value.  This just prints
 * the value, which is a string blob
let print_contents contents =
    ~f:(fun content ->
      let module C = Riakc.Robj.Content in
      printf "VALUE: %s\n" (C.value content))

let fail s =
  printf "%s\n" s;
  shutdown 1

let exec () =
  let host = Sys.argv.(1) in
  let port = Int.of_string Sys.argv.(2) in
   * [with_conn] is a little helper function that will
   * establish a connection, run a function on the connection
   * and tear it down when done
    (fun c ->
      let module R = Riakc.Robj in
      let content  = R.Content.create "some random data" in
      let robj     = R.create [] |> R.set_content content in
       * Put takes a bucket, a key, and an optional list of
       * options.  In this case we are setting the
       * [Return_body] option which returns what the key
       * looks like after the put.  It is possible that
       * siblings were created.

let eval () =
  exec () >>| function
    | Ok (robj, key) -> begin
       * [put] returns a [Riakc.Robj.t] and a [string
       * option], which is the key if Riak had to generate
       * it
      let module R = Riakc.Robj in
       * Extract the vclock, if it exists, and convert it to
       * to something printable
      let vclock =
   ( ~f:hex_of_string (R.vclock robj))
      let key = Option.value ~default:"<none>" key in
      printf "KEY: %s\n" key;
      printf "VCLOCK: %s\n" vclock;
      print_contents (R.contents robj);
      shutdown 0
     * These are the various errors that can be returned.
     * Many of then come directly from the ProtoBuf layer
     * since there aren't really any more semantics to apply
     * to the data if it matches the PB frame.
    | Error `Bad_conn           -> fail "Bad_conn"
    | Error `Bad_payload        -> fail "Bad_payload"
    | Error `Incomplete_payload -> fail "Incomplete_payload"
    | Error `Notfound           -> fail "Notfound"
    | Error `Incomplete         -> fail "Incomplete"
    | Error `Overflow           -> fail "Overflow"
    | Error `Unknown_type       -> fail "Unknown_type"
    | Error `Wrong_type         -> fail "Wrong_type"

let () =
  ignore (eval ());
  never_returns (Scheduler.go ())

Now compile it:

ocamlfind ocamlopt -thread -I +camlp4 -package riakc -c
ocamlfind ocamlopt -package riakc -thread -linkpkg \
-o demo.native demo.cmx

Finally, you can run it: ./demo.native hostname port

...And More Detail

The API for Riakc is broken up into two modules: Riakc.Robj and Riakc.Conn with Riakc.Opts being a third helper module. Below is in reference to version 2.0.0 of Riakc.


Riakc.Robj defines a representation of an object stored in Riak. Robj is completely pure code. The API can be found here.


This is the I/O layer. All interaction with the actual database happens through this module. Riakc.Conn is somewhat clever in that it has a compile-time requirement that you have called Riakc.Robj.set_content on any value you want to PUT. This guarantees you have resolved all siblings, somehow. Its API can be found here.


Finally, various options are defined in Riakc.Opts. These are options that GET and PUT take. Not all of them are actually supported but support is planned. The API can be viewed here.

Hopefully Riakc has a fairly straight forward API. While the example code might be longer than other clients, it is complete and correct (I hope).

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