Lambda and client code examples for ASG Leader detection. For more info, see https://ajbrown.org/2017/02/10/leader-election-with-aws-auto-scaling-groups.html

ASGLeaderDetection.php

<?php 
public function isLeader() {
    $isLeader = true;  // get my instance-id from the metadata service
    $instanceId = exec('curl http://169.254.169.254/latest/meta-data/instance-id');
    if (!empty($instanceId)) {
        $isLeader = false;
        $ec2 = \Aws\Ec2\Ec2Client::factory(['key' => AWS_ACCESS_KEY_ID, 'secret' => AWS_SECRET_KEY, 'region' => 'us-east-1']);
        $data = $ec2->describeInstances(array('InstanceIds' => [$instanceId]));
        if (!empty($data) && !empty($data['Reservations'])) {
            $tags = $data['Reservations'][0]['Instances'][0]['Tags'];
            foreach ($tags as $tag) {
                if ($tag['Key'] == 'app:isLeader') {
                    $isLeader = true;
                    break;
                }
            }
            $tag = null;
        }
    }
    return $isLeader;
}

ASGLeaderElection_Lambda.js

console.log('Loading ElectASGLeader');

var aws = require('aws-sdk');
var autoscaling = new aws.AutoScaling();
var ec2 = new aws.EC2();

var validStates  = [ 'Pending', 'Pending:Wait', 'Pending:Proceed', 'InService' ];
var leaderTagKey   = 'app:isLeader';
var leaderTagValue = 'true';

exports.handler = function(event, context) {
  console.log('Received event:');
  console.log(event);

  var data = event.Records[0].Sns;

  console.log( "SNS Message:", data );

  leader = null,
  json = JSON.parse(data.Message)
  ;

  //list all instances currently in the autoscaling group
  autoscaling.describeAutoScalingGroups( { 'AutoScalingGroupNames' : [json.AutoScalingGroupName] }, function(err, data) {
    if( err ) {
      console.log( 'Error loading autoscaling groups: ', err );
      context.fail();
      return;
    }

    var asg = data.AutoScalingGroups.pop();
    var candidates = [];
    var allInstanceIds = [];

    asg.Instances.forEach( function( instance ) {
      allInstanceIds.push( instance.InstanceId );
      if( validStates.indexOf( instance.LifecycleState ) >= 0 ) {
        candidates.push( instance.InstanceId );
        console.log( 'Instance ' + instance.InstanceId + ' is a candidate for leader.' );
      }
    } );

    ec2.describeInstances( { 'InstanceIds' : candidates }, function(err, data) {
      if( err ) {
        console.log( 'Error loading autoscaling groups: ', err );
        context.fail();
        return;
      }

      var leaders = [];
      var newLeader = null;

      //find all leader instances
      data.Reservations.forEach( function( reservation ) {
        reservation.Instances.forEach( function( instance ) {
          instance.Tags.forEach( function( tag ) {
            if( tag.Key == leaderTagKey ) {
              leaders.push( instance );
            }
          });
        });
      });

      //if there's already a leader, don't change anything.
      if( leaders.length == 1 ) {
        console.log( 'Retaining leader instance ' + leaders[0] );
        context.succeed( leaders[0] );
        return;

        // if there is more than one leader, keep one of them.
      } else if( leaders.length > 1 ) {
        newLeader = leaders[0];

        // if there are no leaders and the triggering instance is coming online, make it the leader.
      } else if( json.Event == 'autoscaling:EC2_INSTANCE_LAUNCH' ) {
        newLeader = json.EC2InstanceId

        //Otherwise, just pick a leader.
      } else {
        newLeader = candidates[0];
      }

      //flip the tags on all instances.
      ec2.deleteTags( { 'Resources': allInstanceIds, 'Tags': [ { 'Key': leaderTagKey } ] }, function(err, data) {
        if( err ) {
          console.log( 'Error deleting tags from non-candidate instances: ', err );
          context.fail();
          return;
        }

        console.log( 'Cleared tags on ' + allInstanceIds.length + ' insances' );

        var params = {
          'Resources': [newLeader],
          'Tags': [ { 'Key': leaderTagKey, 'Value': leaderTagValue } ]
        };
        ec2.createTags( params, function() {
          if( err ) {
            console.log( 'Error creating leader tag on leader instance: ', err );
            context.fail();
            return;
          }

          console.log( 'Successfully tagged new leader instance' );
          context.succeed( newLeader );
        } )
      });


    });

  });
};