sns topicに飛んで来たCloudWatchのalarmをslackに通知するためのlambda function

index.js

/*
### 前提
- slack webhookはstore parameterに保管している
- アラートの重要度ごとにhookを作る
- SEVERITYとCOLORをlambdaの環境変数に設定する
*/

const url = require('url');
const https = require('https');
const AWS = require('aws-sdk');

const severity = process.env.SEVERITY;
const color = process.env.COLOR;
let hookUrl;

exports.handler = (event, context, callback) => {
    const ssm = new AWS.SSM();
    var params = {
        Name: "/slack-" + severity,
        WithDecryption: true
    };
    ssm.getParameter(params, function(err, data) {
        if (err) {
            console.log("fail getParameter.", err, err.stack);
            callback(null);
        } else {
            hookUrl = data.Parameter['Value'];
            processEvent(event, callback);
        }
    });
};

function postMessage(message, callback) {
    const body = JSON.stringify(message);
    const options = url.parse(hookUrl);
    options.method = 'POST';
    options.headers = {
        'Content-Type': 'application/json',
        'Content-Length': Buffer.byteLength(body),
    };

    const postReq = https.request(options, (res) => {
        const chunks = [];
        res.setEncoding('utf8');
        res.on('data', (chunk) => chunks.push(chunk));
        res.on('end', () => {
            if (callback) {
                callback({
                    body: chunks.join(''),
                    statusCode: res.statusCode,
                    statusMessage: res.statusMessage,
                });
            }
        });
        return res;
    });

    postReq.write(body);
    postReq.end();
}

function processEvent(event, callback) {
    const message = JSON.parse(event.Records[0].Sns.Message);
    const env = AWSEnv(message.AWSAccountId);
    const alarmName = message.AlarmName;
    const newState = message.NewStateValue;
    const reason = message.NewStateReason;
    const time =  message.StateChangeTime;
    const slackMessage = {
        'username': 'Alert Notifier',
        'attachments': [{
            'pretext': alarmName + "* state is now `" + newState + "` ",
            'color': color,
            'title': 'See CloudWatch Logs',
            'title_link': 'https://******.console.aws.amazon.com/cloudwatch/home',
            'fields': [
                {
                    'title': 'Environment',
                    'value': env,
                    'short': true
                },
                {
                    'title': 'Reason',
                    'value': reason,
                    'short': true,
                },
                {
                    'title': 'Alarm Name',
                    'value': alarmName,
                    'short': true
                },
                {
                    'title': 'Time',
                    'value': time,
                    'short': true,
                }
            ]}]
    };

    postMessage(slackMessage, (response) => {
        if (response.statusCode < 400) {
            console.info('Message posted successfully');
            callback(null);
        } else if (response.statusCode < 500) {
            console.error(`Error posting message to Slack API: ${response.statusCode} - ${response.statusMessage}`);
            callback(null);  // Don't retry because the error is due to a problem with the request
        } else {
            // Let Lambda retry
            callback(`Server error when processing message: ${response.statusCode} - ${response.statusMessage}`);
        }
    });
}

function AWSEnv(AWSAccountId) {
    if (AWSAccountId == "*******") {
        return "development";
    } else {
        return "production";
    }
}