For running something only on one instance in an ElasticBeanstalk environment. Inspired by http://blog.rotaready.com/scheduled-tasks-elastic-beanstalk-cron/

check-whether-eb-master-instance.js

"use strict";

const AWS = require('aws-sdk');
const bluebird = require('bluebird');

/**
 * @returns {Promise.<boolean>} - is on 'master' instance or not 
 */
module.exports = bluebird.coroutine(function* (opts) {
    if (opts == null) {
        opts = {
            // default to use the credentials for the ec2 instance
            credentials: new AWS.EC2MetadataCredentials(),
        };
    }

    let instanceId = yield findCurrentInstanceId(opts);
    let beanstalkEnvironmentId = yield findEbEnvironmentId(instanceId, opts);
    let instanceIds = yield findEbInstanceIds(beanstalkEnvironmentId, opts);

    // first in the list is the master
    return instanceId === instanceIds[0]
});


const findCurrentInstanceId = function (opts) {
    return requestMatadata('/latest/meta-data/instance-id', opts)
};

const findEbEnvironmentId = function (instanceId, opts) {
    let params = {
        Filters: [
            {
                Name: "resource-id",
                Values: [
                    instanceId,
                ]
            }
        ]
    };
    return requestEc2Tags(params, opts).then(tags => {
        let envIdTag = tags.find(t => t.Key === 'elasticbeanstalk:environment-id');
        if (envIdTag == null) {
            return Promise.reject(new Error('failed to find the value of "elasticbeanstalk:environment-id" tag.'));
        }
        return envIdTag.Value
    })
};

const findEbInstanceIds = function (environmentId, opts) {
    let params = {
        EnvironmentId: environmentId
    };
    return requestEbEnvironmentResources(params, opts)
        .then(envResources => envResources.Instances.map(it => it.Id))
};


const promisifyMethod = function (obj, name) {
    return bluebird.promisify(obj[name], {context: obj})
};

const requestMatadata = function (path, opts) {
    let metadata = new AWS.MetadataService(opts);
    return promisifyMethod(metadata, 'request')(path)
};

const requestEc2Tags = function (params, opts) {
    let ec2 = new AWS.EC2(opts);
    return promisifyMethod(ec2, 'describeTags')(params).then(data => data.Tags);
};

const requestEbEnvironmentResources = function (params, opts) {
    let elasticbeanstalk = new AWS.ElasticBeanstalk(opts);
    return promisifyMethod(elasticbeanstalk, 'describeEnvironmentResources')(params)
        .then(data => data.EnvironmentResources)
};