Syn

Syn (short for synonym) is a global process registry for Erlang.

Introduction

Syn is a process registry that has the following features:

• Global (i.e. a process is uniquely identified with a Key across all the nodes of a cluster).
• Any term can be used as Key.
• Fast writes.
• Automatically handles conflict resolution (such as net splits).
• Configurable callbacks.

Notes

In any distributed system you are faced with a consistency challenge, which is often resolved by having one master arbiter performing all write operations (chosen with a mechanism of leader election), or through atomic transactions.

Syn was born for applications of the IoT field. In this context, Keys used to identify a process are often the physical object's unique identifier (for instance, its serial or mac address), and are therefore already defined and unique before hitting the system. The consistency challenge is less of a problem in this case, since the likelihood of concurrent incoming requests that would register processes with the same Key is extremely low and, in most cases, acceptable.

In addition, write speeds were a determining factor in the architecture of Syn.

Therefore, Availability has been chosen over Consistency and Syn is eventually consistent.

Install

If you're using rebar, add syn as a dependency in your project's rebar.config file:

{syn, ".*", {git, "git://github.com/ostinelli/syn.git", "master"}}

Then, get and compile your dependencies:

$ rebar get-deps
$ rebar compile

Usage

Setup

Ensure to start Syn from your application. This can be done by either providing it as a dependency in your .app file, or by starting it manually:

syn:start().

Your application will have its own logic on how to connect to the other nodes in the cluster. Once it is connected, ensure to initialize Syn (this will set up the underlying mnesia backend):

syn:init().

The recommended place to initialize Syn is in the start/2 function in your main application module, something along the lines of:

-module(myapp_app).
-behaviour(application).

-export([start/2, stop/1]).

start(_StartType, _StartArgs) ->
    %% connect to nodes
    connect_nodes(),
    %% init syn
    syn:init(),
    %% start sup
    myapp_sup:start_link().

connect_nodes() ->
	%% list of nodes contained in ENV variable `nodes`
	Nodes = application:get_env(nodes),
	%% connect to nodes
	[net_kernel:connect_node(Node) || Node <- Nodes].

Syn is then ready to register processes.

Basic

To register a process:

syn:register(Key, Pid) -> ok | {error, Error}.

Types:
	Key = any()
	Pid = pid()
	Error = taken

To register a process and attach metadata to it:

syn:register(Key, Pid, Meta) -> ok | {error, Error}.

Types:
	Key = any()
	Pid = pid()
	Meta = any()
	Error = taken

To retrieve a Pid from a Key:

syn:find_by_key(Key) -> Pid | undefined.

Types:
	Key = any()
	Pid = pid()

To retrieve a Pid from a Key with its metadata:

syn:find_by_key(Key, with_meta) -> {Pid, Meta} | undefined.

Types:
	Key = any()
	Pid = pid()
	Meta = any()

To retrieve a Key from a Pid:

syn:find_by_pid(Pid) -> Key | undefined.

Types:
	Pid = pid()
	Key = any()

To retrieve a Key from a Pid with its metadata:

syn:find_by_pid(Pid, with_meta) -> {Key, Meta} | undefined.

Types:
	Pid = pid()
	Key = any()
	Meta = any()

To unregister a previously registered Key:

syn:unregister(Key) -> ok | {error, Error}.

Types:
	Key = any()
	Error = undefined

To retrieve the count of total registered processes running in the cluster:

syn:count() -> non_neg_integer().

To retrieve the count of total registered processes running on a specific node:

syn:count(Node) -> non_neg_integer().

Types:
	Node = atom()

Processes are automatically monitored and removed from the registry if they die.

Options

Options can be set in the environment variable syn. You're probably best off using an application configuration file (in releases, sys.config):

{syn, [
    %% define callback function on process exit
    {process_exit_callback, [module1, function1]},

    %% define callback function on conflicting process (instead of kill)
    {conflicting_process_callback, [module2, function2]}
]}

These options are explained here below.

Callback on process exit

The process_exit_callback option allows you to specify the module and the function of the callback that will be triggered when a process exits. This callback will be called only on the node where the process was running.

The callback function is defined as:

CallbackFun = fun(Key, Pid, Meta, Reason) -> any().

Types:
	Key = any()
	Pid = pid()
	Meta = any()
	Reason = any()

The Key and Pid are the ones of the process that exited with Reason.

For instance, if you want to print a log when a process exited:

-module(my_callback).
-export([callback_on_process_exit/4]).

callback_on_process_exit(Key, Pid, Meta, Reason) ->
	error_logger:info_msg(
		"Process with Key ~p, Pid ~p and Meta ~p exited with reason ~p~n",
		[Key, Pid, Meta, Reason]
	)

Set it in the options:

{syn, [
    %% define callback function
    {process_exit_callback, [my_callback, callback_on_process_exit]}
]}

If you don't set this option, no callback will be triggered.

Conflict resolution by callback

In case of race conditions, or during net splits, a Key might be registered simultaneously on two different nodes. In this case, the cluster experiences a naming conflict.

When this happens, Syn will resolve this conflict by choosing a single process. Syn will discard the processes running on the node the conflict is being resolved on, and by default will kill it by sending a kill signal with exit(Pid, kill).

If this is not desired, you can set the conflicting_process_callback option to instruct Syn to trigger a callback, so that you can perform custom operations (such as a graceful shutdown). In this case, the process will not be killed by Syn, and you'll have to decide what to do with it. This callback will be called only on the node where the process is running.

The callback function is defined as:

CallbackFun = fun(Key, Pid, Meta) -> any().

Types:
	Key = any()
	Pid = pid()
	Meta = any()

The Key, Pid and Meta are the ones of the process that is to be discarded.

For instance, if you want to send a shutdown message to the discarded process:

-module(my_callback).
-export([callback_on_conflicting_process/3]).

callback_on_conflicting_process(_Key, Pid, _Meta) ->
	Pid ! shutdown

Set it in the options:

{syn, [
	%% define callback function
	{conflicting_process_callback, [my_callback, callback_on_conflicting_process]}
]}

Important Note: The conflict resolution method SHOULD be defined in the same way across all nodes of the cluster. Having different conflict resolution options on different nodes can have unexpected results.

Internals

Under the hood, Syn performs dirty reads and writes into a distributed in-memory Mnesia table, replicated across all the nodes of the cluster.

To automatically handle conflict resolution, Syn implements a specialized and simplified version of the mechanisms used in Ulf Wiger's unsplit framework.

Contributing

So you want to contribute? That's great! Please follow the guidelines below. It will make it easier to get merged in.

Before implementing a new feature, please submit a ticket to discuss what you intend to do. Your feature might already be in the works, or an alternative implementation might have already been discussed.

Do not commit to master in your fork. Provide a clean branch without merge commits. Every pull request should have its own topic branch. In this way, every additional adjustments to the original pull request might be done easily, and squashed with git rebase -i. The updated branch will be visible in the same pull request, so there will be no need to open new pull requests when there are changes to be applied.

Ensure to include proper testing. To run Syn tests you simply have to be in the project's root directory and run:

$ make tests