Elector

Description

Elector is an Erlang application that automatically detects all nodes inside the distributed Erlang cluster and chooses the leader node. The elections are started automatically when the Elector application is started or when a node joins or leaves the cluster. Elector also allows you to run pre- and post-election hooks that will be triggered when the election process is started and finished.

The default election strategy is to choose the node with the highest uptime.

Features

• Automatic election process on startup or when node joins/leaves the cluster
• Ability to configure pre and post election hooks that will be called before and after the election process. These hooks must return within 1 second to avoid timeouts.
• Provides 4 built in election strategies (time based) and allows you to define your own strategy implementation (See elector_strategy_behaviour module for reference)
• Provides quorum option to detect split brain scenarios
• Provides option to define the election delay in milliseconds before the election process is started automatically and queue the election calls

Configuration

Elector supports the following configurations:

• election_delay - The delay in milliseconds before the new election starts. This value is used when automatic election is triggered either by node join/leave or startup. Default value is 1 second (1000).

• strategy_module - The module that is used for the election strategy implementation. Available options are: elector_rt_high_strategy and elector_rt_low_strategy. Feel free to write your own strategy module that implements the elector_strategy_behaviour module. Default value is elector_rt_high_strategy which chooses the node with the highest runtime.

• pre_election_hooks - A list of hooks/function calls that will be triggered exactly before the node is starting the election. Expects a list of tuples with the following format: {Module, Function, Args}. Default value is [].

• post_election_hooks - A list of hooks/function calls that will be triggered exactly after the election process. Expects a list of tuples with the following format: {Module, Function, Args}. Default value is [].

• quorum_size - The number of nodes (including the local node) that should be available in the cluster before the election process is started. Do not set it to 0 as it will disable the election process, leave empty or 1 if you want to run the election process even if there are no other nodes in the cluster. Default value is 1.

• candidate_node - Boolean indicating if this node should be a candidate for the leader election. Default value is true.

• hooks_execution - Atom indicating if the pre and post election hooks should be executed on the node starting the election process or on all nodes. The election process will be started automatically by the global commission process. The node who starts the commission process is not known and can be any node in the cluster. Available values are: local and global. Default value is global.

• automatic_elections - Boolean indicating if the automatic election process should be started when node joins or leaves the cluster. Default value is true.

• candidate_cache_refresh_interval - The interval in seconds for periodic candidate cache refresh. Set to 0 to disable periodic refresh (recommended - uses only event-based updates on node join/leave). Default value is 0.

Keep in mind to use the same configuration for all nodes in the cluster!

Guides

Installation for Elixir application

1. Add {:elector, "~> 0.3.2"} under the deps in the mix.exs file:

defp deps do
    [
        {:elector, "~> 0.3.2"}
    ]
end

1. Add elector under the extra_applications in the mix.exs file:

def application do
    [
        extra_applications: [:elector],
        mod: {MyApp, []}
    ]
end

Installation for Erlang application

1. Add elector to the deps in the rebar.config file:

{deps, [{"elector", "0.3.2"}]}.

1. Add elector to the applications list in the myapp.app.src file:

{applications, [elector]}.

Examples

Start election manually

Elixir

> alias :elector, as Elector
> Elector.elect_sync() # Start election synchronously
{:ok, :election_finished}
> Elector.elect() # Start election asynchronously
{:ok, :election_started}

Erlang

> elector:elect_sync(). % Start election synchronously
{ok, election_finished}
> elector:elect(). % Start election asynchronously
{ok, election_started}

Get current leader

Elixir

> alias :elector, as Elector
> Elector.get_leader()
{:ok, :example_node}

Erlang

> elector:get_leader().
{ok, example_node}

See the elector module for more.

https://hexdocs.pm/elector/

High level overview

Elector is a distributed application that utilizes the Erlang global module to spawn a singleton process named elector_commission. The commission process is responsible for starting the election process and keeping track of the nodes in the cluster. The election process is started by the commission process, which calls the elect/1 function from the strategy module. This function gathers the necessary information from the cluster and decides which node should become the leader.

Each node spawns the following local processes:

• elector_overviewer process is responsible for starting the commission and monitor it across the cluster.
• elector_candidate process holding the candidate information that describes if the node is a candidate for the leader election.
• elector_state process holding the selected leader information.

All processes are supervised by the elector_sup supervisor.

Setup Elector locally

Setup the elector locally and run the application:

• docker-compose up -d
• docker exec -it elector_elector_1 sh
• rebar3 compile
• erl -sname local -setcookie cookievalue -pa ./_build/default/lib/elector/ebin -eval "application:start(elector)"

Run tests:

• rebar3 compile && ct_run -dir test -logdir test_logs -pa ./_build/default/lib/elector/ebin -setcookie cookievalue

Generate documentation:

• rebar3 ex_doc