Archive for August, 2009|Monthly archive page

Riak Demo: Stickynotes

Warning: This blog post is woefully out of date:

  1. Instead of building Riak from source, you should download a pre-compiled release from downloads.basho.com (though the source is still available, if you want).
  2. Starting Riak is now done with the bin/riak start command, not start-fresh.sh Stopping is also easily done with bin/riak stop
  3. Jiak is no longer the preferred HTTP interface. There is now one that accepts any content type, not just JSON.
  4. Without Jiak, back-linking of notes will need to be done with post-commit hooks.

There are probably other things I’ve overlooked. Really, you should just head over to the Riak wiki to get the intro.


Basho released Riak to the world just over a week ago. Some think that the docs are good enough that further explanation is not necessary. But, if you’re looking for a little more introduction, read on.

The short version of this post: Riak is as simple as downloading it and hitting the HTTP interface:

1$ hg clone http://bitbucket.org/basho/riak/
2$ cd riak
3$ make
4$ ./start-fresh.sh config/riak.erlenv

5$ curl -X PUT -H "Content-type: application/json" \
5>   http://localhost:8098/jiak/foo \
5>   -d "{\"schema\":{\"allowed_fields\":[\"bar\"],\"required_fields\":[\"bar\"],\"read_mask\":[\"bar\"],\"write_mask\":[\"bar\"]}}"

6$ curl -X PUT -H "Content-type: application/json" \
6>   http://localhost:8098/jiak/foo/baz \
6>   -d "{\"bucket\":\"foo\",\"key\":\"baz\",\"object\":{\"bar\":\"Hello World\"},\"links\":[]}"

7$ curl http://localhost:8098/jiak/foo
{"schema":{"allowed_fields":["bar"],"required_fields":["bar"],"read_mask":["bar"],"write_mask":["bar"]},"keys":["baz"]}

8$ curl http://localhost:8098/jiak/foo/baz
{"object":{"bar":"Hello World"},"vclock":"MzIwsDSwMDQyMjAyNjXQLcpMzHYwNDLXMwBCQ3SusYGZoZGxvqG+mbGJobmxkbmFsQEA","lastmod":"Wed, 12 Aug 2009 20:23:50 GMT","vtag":"6DxaqiRCDBevf03tzPZpzl","bucket":"foo","key":"baz","links":[]}

Commands 1-4 download and start Riak. Command 5 creates the bucket foo, and tells Jiak, Riak’s HTTP interface, that objects in bucket foo must have a bar field that is readable and writable. Command 6 creates the object baz in bucket foo. Command 7 gets the listing of bucket foo. Command 8 retrieves object baz in bucket foo.

Aside: Some have found it difficult to stop Riak, so I’ll throw the tip out here: killall heart. You may also have to kill the erl command afterward, but if you don’t kill heart first, Riak will just come right back.

That’s all it takes to use Riak. You can stop reading now and happily speak REST to it for the rest of your application’s lifetime.

If you’re still hanging around, though, maybe you’d be interested in a few more features that Jiak has to offer, like link-walking and field validation. To demonstrate those, I’ll spend the rest of this post describing the creation of an application on Riak.

Let’s say that I wanted to create a note-taking system, something like the fine Erlang/Mochiweb demo put together by the guys at Beebole.

There will be two kinds of objects in my system: notes and groups. The properties of a note will be the text of the note, the color, the position on the screen, and the stacking order. The properties of a group will be the name of the group and the list of notes in the group.

I’ll start by whipping up two modules to manage those buckets for me. These modules tell jiak_resource how to validate the structure of objects in these buckets (jiak_resource is the Webmachine resource that runs Riak’s HTTP interface). They’ll follow the basic structure of jiak_example.erl, which ships with Riak.

-module(groups).
-export([init/2, auth_ok/3, bucket_listable/0, allowed_fields/0,
         required_fields/0, read_mask/0, write_mask/0,
         expires_in_seconds/3, check_write/4, effect_write/4,
         after_write/4, merge_siblings/1]).

init(_Key, Context) -> {ok, Context}.

auth_ok(_Key, ReqData, Context) -> {true, ReqData, Context}.

bucket_listable() -> true.

allowed_fields()  -> [<<"name">>].
required_fields() -> allowed_fields().
read_mask()       -> allowed_fields().
write_mask()      -> allowed_fields().

expires_in_seconds(_Key, ReqData, Context) ->
    {600, ReqData, Context}.

check_write({_PutType, _Key}, JiakObject, ReqData, Context) ->
    {ObjDiffs,_} = Context:diff(),
    case lists:foldl(fun check_diff/2, [], ObjDiffs) of
        [] ->
            {{ok, JiakObject}, ReqData, Context};
        Errors ->
            {{error, list_to_binary(string:join(Errors, ", "))},
             ReqData, Context}
    end.

check_diff({<<"name">>, _, Value}, ErrorAcc) ->
    if is_binary(Value) -> ErrorAcc;
       true             -> ["name field must be a string"|ErrorAcc]
    end.

effect_write(_Key, JiakObject, ReqData, Context) ->
    {{ok, JiakObject}, ReqData, Context}.

after_write(_Key, _JiakObject, ReqData, Context) ->
    {ok, ReqData, Context}.

merge_siblings(Siblings) ->
    jiak:standard_sibling_merge(Siblings).

The groups module ensures that every group has a name field (through implementation of allowed_fields/0, required_fields/0, read_fields/0, and write_fields/0), and that the value of that field is a string (using check_write/4 and check_diff/2).

-module(notes).
-export([init/2, auth_ok/3, bucket_listable/0, allowed_fields/0,
         required_fields/0, read_mask/0, write_mask/0,
         expires_in_seconds/3, check_write/4, effect_write/4,
         after_write/4, merge_siblings/1]).

init(_Key, Context) -> {ok, Context}.

auth_ok(_Key, ReqData, Context) -> {true, ReqData, Context}.

bucket_listable() -> true.

allowed_fields() ->
    [<<"text">>, <<"x">>, <<"y">>, <<"z">>, <<"color">>].

required_fields() -> allowed_fields().
read_mask()       -> allowed_fields().
write_mask()      -> allowed_fields().

expires_in_seconds(_Key, ReqData, Context) ->
    {600, ReqData, Context}.

check_write({_PutType, Key}, JiakObject, ReqData, Context) ->
    {ObjDiffs,_} = Context:diff(),
    case lists:foldl(fun check_diff/2, [], ObjDiffs) of
        [] ->
            {{ok, JiakObject}, ReqData, Context:set_prop(key, Key)};
        Errors ->
            {{error, list_to_binary(string:join(Errors, ", "))},
             ReqData, Context}
    end.

-define(COLORS, [<<"yellow">>, <<"pink">>, <<"green">>, <<"blue">>]).

check_diff({<<"text">>, _, Value}, ErrorAcc) ->
    if is_binary(Value) -> ErrorAcc;
       true             -> ["text field must be a string"|ErrorAcc]
    end;
check_diff({Coord, _, Value}, ErrorAcc)
  when Coord==<<"x">>;Coord==<<"y">>;Coord==<<"z">> ->
    if is_integer(Value) -> ErrorAcc;
       true ->
            [io_lib:format("~s field must be an integer", [Coord])
             |ErrorAcc]
    end;
check_diff({<<"color">>, _, Value}, ErrorAcc) ->
    case lists:member(Value, ?COLORS) of
        true -> ErrorAcc;
        false ->
            [io_lib:format("color field must be one of (~s)",
                           [string:join([binary_to_list(C)||C<-?COLORS],
                                        ",")])
             |ErrorAcc]
    end.

effect_write(_Key, JiakObject, ReqData, Context) ->
    {{ok, JiakObject}, ReqData, Context}.

after_write(_Key, JiakObject, ReqData, Context) ->
    spawn(fun() ->
                  [[_, GroupKey, _]] = jiak_object:links(JiakObject, groups),
                  {ok, C} = jiak:local_client(),
                  {ok, G} = C:get(groups, GroupKey, 2),
                  Key = Context:get_prop(key),
                  C:put(jiak_object:add_link(G, notes, Key, <<"note">>), 2)
          end),
    {ok, ReqData, Context}.

merge_siblings(Siblings) ->
    jiak:standard_sibling_merge(Siblings).

The notes module ensures that all notes have a text field that is a string; x, y, and z fields that are integers; and a color field that is one of a specific list of strings (using the same functions as the groups module used).

One more interesting thing happens in the notes module. If you look at after_write/4, you’ll see that it fetches the groups object that the note links to, and adds the note to that group’s links. Jiak calls after_write/4 after the note has been stored in Riak, so what I’ve written here is effectively an automatic back-link monitor. We’ll return to the concept of links in a moment.

If I put the notes and groups modules in place, I can fire up Riak and immediately being sending HTTP requests to its Jiak interface to create and modify groups and notes. For example:

$ curl -X PUT -H "Content-type: application/json" \
>   http://127.0.0.1:8098/jiak/groups/todos \
>   -d "{\"bucket\":\"groups\",\"key\":\"todos\",\"object\":{\"name\":\"todo\"}\"links\":[]}"

$ curl -X PUT -H "Content-type: application/json" \
>   http://127.0.0.1:8098/jiak/notes/blog \
>   -d "{\"bucket\":\"notes\",\"key\":\"blog\",\"object\":{\"text\":\"finish blog post\",\"x\":0,\"y\":0,\"z\":0,\"color\":\"green\"},\"links\":[[\"groups\",\"todos\",\"open\"]]"

These two lines would create a group named todo with a note labeled finish blog post.

Now, about those links. See the ["groups","todos","open"] item in the links field of that notes object? That’s a link to the groups object named todos, and I’ve tagged it open. The affect_write/4 function in the notes module will add a link to the groups object of the form ["notes","blog","note"].

What does this get me? More than just record-keeping: I can now use the Jiak utility jaywalker to get all of the notes in the todo group with a single query:

$ curl http://127.0.0.1:8098/jiak/groups/todos/notes,_,_

You’ll recognize the first part, through /jiak/groups/todos/. The segment after that is a link query. This ones says "all notes objects, with any tag." The links are structured as {bucket},{tag},{accumulate} segments, with underscore meaning "any."

The example query will return an object with a results field that is a list of lists of results. That is, if I were to store the data returned from that query in a variable called data, my list of notes would be at data.results[0].

I’m not limited to one hop, either. If there were also person objects in my system, linked from notes objects as authors I might get all of the authors of the notes in a group with:

$ curl http://127.0.0.1:8098/jiak/groups/todos/notes,_,_/person,author,_

More information on link-walking can be found in the jaywalker_resource documentation.

The curl commands and the HTTP requests they represent are pretty simple, but I’m going to be hitting these resources from Javascript running in a browser. So, I’ll wrap it all up in a nice utility class:

function JiakClient(BaseUrl, Opts) {
    this.baseurl = BaseUrl;
    if (!(this.baseurl.slice(-1) == '/'))
        this.baseurl += '/';

    this.opts = Opts||{};
}

JiakClient.prototype.store = function(Object, Callback, NoReturnBody) {
    var req = {
        contentType: "application/json",
        dataType: "json"
    };

    if (this.opts.alwaysPost || !Object.key)
        req.type = 'POST';
    else
        req.type = 'PUT';
    
    req.url = this.baseurl+Object.bucket+'/';
    if (Object.key) req.url += Object.key;
    
    if (!(this.opts.noReturnBody || NoReturnBody))
        req.url += '?returnbody=true';

    if (typeof Callback == 'function')
        req.success = Callback;

    req.data = JSON.stringify(Object);

    return $.ajax(req);
}

JiakClient.prototype.fetch = function(Bucket, Key, Callback) {
    return $.ajax({
        url:      this.baseurl+Bucket+'/'+Key,
        dataType: "json",
        success:  Callback
    });
}

JiakClient.prototype.remove = function(Bucket, Key, Callback) {
    return $.ajax({
        type:    'DELETE',
        url:     this.baseurl+Bucket+'/'+Key,
        success: Callback
    });
}

JiakClient.prototype.walk = function(Start, Spec, Callback) {
    var req = {
        dataType: "json",
        success: Callback
    };

    if ('bucket' in Start)
        req.url = this.baseurl+Start.bucket+'/'+Start.key+'/';
    else
        req.url = this.baseurl+Start[0]+'/'+Start[1]+'/';

    for (i in Spec) {
        req.url += (Spec[i].bucket||'_')+','+
            (Spec[i].tag||'_')+','+
            ((Spec[i].acc || i == Spec.length-1) ? '1' : '_')+'/';
    }

    return $.ajax(req);
}

Now I can create the same effect as the curl commands with this code:

var J = new JiakClient("/jiak/");
J.store({
  bucket:"groups",
  key:"todos"
  object:{"name":"todo"}
  links:[]
},
function(group) {
  J.store({
    bucket:"notes",
    key:"blog",
    object:{
    },
    links:[["groups",group.key,"open"]]
  });
});

With the request for "all notes in the todo group" looking like:

J.walk({bucket:"groups", key:"todos"},
       [{bucket:"notes"}],
       function(data) {
         var notes = data.results[0];
         //do stuff...
       });

Okay, now I have my backend and frontend – I’d better fill in the middle. Any intermediary webserver could work, but because Riak comes with Webmachine included, I’ll just setup a quick Webmachine app.

$ cd $RIAK_HOME
$ deps/webmachine/scripts/new_webmachine.erl stickynotes ..
$ cp config/riak.erlenv ../stickynotes/riak-config.erlenv

I copied the Riak config (riak.erlenv) because there are two customizations I want to add to it:

{add_paths, ["../stickynotes/ebin"]}.
{jiak_buckets, [notes, groups]}.

The former puts the stickynotes ebin in Riak’s code path, so Jiak can reach the notes and groups modules I just wrote. The latter does nothing but force the atoms 'notes' and 'groups' into the Riak node so Jiak can use list_to_existing_atom/1.

To my new Webmachine app, I’ll add four things:

  1. the notes and groups modules I just wrote
  2. the static files from Beebole’s example applcation (HTML, CSS, JS) … modified a bit to load and use the jiak.js utility from above
  3. a simple static resource server
  4. a simple proxy resource to pass requests through to jiak (the couchdb_proxy from my wmexamples repo will do fine)

Once I have everything aligned (including dispatch setup correctly), I just start Riak and my Webmachine node:

$ cd $RIAK_HOME
$ ./start-fresh.sh ../stickynotes/riak-config.erlenv
$ cd ../stickynotes
$ ./start.sh

…then point my browser at http://localhost:8000/, and I see the app’s UI with an empty group ready to store some notes. I recommend opening Firebug to watch the requests fly by.

I realize that I’ve glossed over a bit of the Webmachine application stuff, but that’s because it’s mostly rehash of older posts. The better way to cover all of that material is for me to tell you to open up the demo/stickynotes directory in the Riak repo you just cloned, and read the code written there yourself. :)

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