Riak Core Tutorial Part 3: Ping Command

The content of this chapter is in the `01-template` branch.

https://gitlab.com/marianoguerra/tanodb/tree/01-template

This is part of a series, see the previous one at Riak Core Tutorial Part 2: Starting

How it Works

Let's see how ping works under the covers.

Its entry point and public API is the tanodb module, that means we have to look into tanodb.erl:

-module(tanodb).
-export([ping/0]).
-ignore_xref([ping/0]).

%% @doc Pings a random vnode to make sure communication is functional
ping() ->
    % argument to chash_key has to be a two item tuple, since it comes from riak
    % and the full key has a bucket, we use a contant in the bucket position
    % and a timestamp as key so we hit different vnodes on each call
    DocIdx = riak_core_util:chash_key({<<"ping">>, term_to_binary(os:timestamp())}),
    % ask for 1 vnode index to send this request to, change N to get more
    % vnodes, for example for replication
    N = 1,
    PrefList = riak_core_apl:get_primary_apl(DocIdx, N, tanodb),
    [{IndexNode, _Type}] = PrefList,
    riak_core_vnode_master:sync_spawn_command(IndexNode, ping, tanodb_vnode_master).
DocIdx = riak_core_util:chash_key({<<"ping">>, term_to_binary(os:timestamp())}),

The line above hashes a key to decide to which vnode the call should go, a riak_core app has a fixed number of vnodes that are distributed across all the instances of your app's physical nodes, vnodes move from instance to instance when the number of instances change to balance the load and provide fault tolerance and scalability.

The call above will allow us to ask for vnodes that can handle that hashed key, let's run it in the app console to see what it does:

(tanodb@127.0.0.1)1> DocIdx = riak_core_util:chash_key({<<"ping">>, term_to_binary(os:timestamp())}).

<<126,9,218,77,97,108,38,92,0,155,160,26,161,3,200,87,134,213,167,168>>

We seem to get a binary back, in the next line we ask for a list of vnodes that can handle that hashed key:

PrefList = riak_core_apl:get_primary_apl(DocIdx, N, tanodb),

Let's run it to see what it does:

(tanodb@127.0.0.1)2> PrefList = riak_core_apl:get_primary_apl(DocIdx, 1, tanodb).

[{{730750818665451459101842416358141509827966271488, 'tanodb@127.0.0.1'},
     primary}]

We get a list with one tuple that has 3 items, a long number, something that looks like a host and an atom, let's try changing the number 1:

(tanodb@127.0.0.1)3> PrefList2 = riak_core_apl:get_primary_apl(DocIdx, 2, tanodb).

[{{730750818665451459101842416358141509827966271488,
   'tanodb@127.0.0.1'}, primary},
 {{753586781748746817198774991869333432010090217472,
   'tanodb@127.0.0.1'}, primary}]

Now we get two tuples, the first one is the same, so what this does is to return the number of vnodes that can handle the request from the hashed key by priority.

The first number is the vnode id, it's what we get on the ping response.

Next line just unpacks the pref list to get the vnode id and ignore the other part:

[{IndexNode, _Type}] = PrefList,

Finally we ask riak_core to call the ping command on the IndexNode we got back:

riak_core_vnode_master:sync_spawn_command(IndexNode, ping, tanodb_vnode_master).

Let's try it on the console:

(tanodb@127.0.0.1)5> [{IndexNode, _Type}] = PrefList.

[{{730750818665451459101842416358141509827966271488,
   'tanodb@127.0.0.1'}, primary}]

(tanodb@127.0.0.1)6> riak_core_vnode_master:sync_spawn_command(IndexNode, ping, tanodb_vnode_master).

{pong,730750818665451459101842416358141509827966271488}

You can see we get IndexNode back in the pong response, now let's try passing the second IndexNode:

(tanodb@127.0.0.1)7> [{IndexNode1, _Type1}, {IndexNode2, _Type2}] = PrefList2.

[{{730750818665451459101842416358141509827966271488,
   'tanodb@127.0.0.1'}, primary},
 {{753586781748746817198774991869333432010090217472,
   'tanodb@127.0.0.1'}, primary}]


(tanodb@127.0.0.1)9> riak_core_vnode_master:sync_spawn_command(IndexNode2, ping, tanodb_vnode_master).

{pong,753586781748746817198774991869333432010090217472}

We get the IndexNode2 back, that means that the request was sent to the second vnode instead of the first one.

But where does the command go?

Let's see the content of tanodb_vnode.erl (just the useful parts):

-module(tanodb_vnode).
-behaviour(riak_core_vnode).

-export([start_vnode/1,
         init/1,
         terminate/2,
         handle_command/3,
         is_empty/1,
         delete/1,
         handle_handoff_command/3,
         handoff_starting/2,
         handoff_cancelled/1,
         handoff_finished/2,
         handle_handoff_data/2,
         encode_handoff_item/2,
         handle_overload_command/3,
         handle_overload_info/2,
         handle_coverage/4,
         handle_exit/3]).

-record(state, {partition}).

%% API
start_vnode(I) ->
    riak_core_vnode_master:get_vnode_pid(I, ?MODULE).

init([Partition]) ->
    {ok, #state { partition=Partition }}.

%% Sample command: respond to a ping
handle_command(ping, _Sender, State) ->
    {reply, {pong, State#state.partition}, State};
handle_command(Message, _Sender, State) ->
    lager:warning("unhandled_command ~p", [Message]),
    {noreply, State}.

Let's go by parts, first we declare our module:

-module(tanodb_vnode).

We specify that we want to implement the riak_core_vnode behavior:

-behaviour(riak_core_vnode).

Behaviors in Erlang are like interfaces, a set of functions that a module must implement to satisfy the behaviour specification, you can read more in the Erlang documentation.

In this case riak_core defines a behavior with a set of functions we must implement to be a valid riak_core vnode, you can get an idea of the kind of functionality we need by looking at the exported functions:

-export([start_vnode/1,
         init/1,
         terminate/2,
         handle_command/3,
         is_empty/1,
         delete/1,
         handle_handoff_command/3,
         handoff_starting/2,
         handoff_cancelled/1,
         handoff_finished/2,
         handle_handoff_data/2,
         encode_handoff_item/2,
         handle_overload_command/3,
         handle_overload_info/2,
         handle_coverage/4,
         handle_exit/3]).

For the moment most of them have a "dummy" implementation where they just do the minimal amount of work to satisfy the behavior and not more, it's our job to change the default implementation to fit our needs.

We will have a record called state to keep info between callbacks, this is typical Erlang way of managing state so I won't cover it here:

-record(state, {partition}).

We implement the api to start the vnode:

%% API
start_vnode(I) ->
    riak_core_vnode_master:get_vnode_pid(I, ?MODULE).

Note that on init we store the Partition value on state so we can use it later, this is what I referred above as vnode id, it's the big number you saw before:

init([Partition]) ->
    {ok, #state { partition=Partition }}.

Now for the interesting part, here we have our ping command implementation, we match for ping in the Message position (the first argument):

handle_command(ping, _Sender, State) ->

Return a response with the second item in the tuple being the actual response that the caller will get where we reply with the atom pong and the partition number of this vnode, the last item in the tuple is the new state we want to have for this vnode, since we didn't change anything we pass the current value:

{reply, {pong, State#state.partition}, State};

We implement a catch all that will just log the unknown command and give no reply back:

handle_command(Message, _Sender, State) ->
    lager:warning("unhandled_command ~p", [Message]),
    {noreply, State}.

This is the roundtrip of a ping call, our task to add more commands will be:

  • Add a function on tanodb.erl that hides the internal work done to distribute the work
  • Add a new match on handle_command to match the command we added on tanodb.erl and provide a reply

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