AWS "CloudWatch to Slack" Lambda function

aws-cloudwatch-to-slack-lambda.js

var url = require('url');
var https = require('https');

// TODO: !!! Must be changed following Slack WebHook URL !!!
var hookUrl = 'https://hooks.slack.com/services/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';

var processEvent = function(event, context) {
    var message = JSON.parse(event.Records[0].Sns.Message);
 
    // Format Slack posting message
    var text = "<!channel> *" + message.AlarmDescription + "* state is now `" + message.NewStateValue + "`\n" +
               "```" + 
               "reason: " + message.NewStateReason + "\n" +
               "alarm: " + message.AlarmName + "\n" + 
               "time: " + message.StateChangeTime +
               "```"
               ;

    var slackMessage = {
        text: text
    };

    postMessage(slackMessage, function(response) {
        if (response.statusCode < 400) {
            console.info('Message posted!');
            context.succeed();
        } else if (response.statusCode < 500) {
            console.error("4xx error occured when processing message: " + response.statusCode + " - " + response.statusMessage);
            context.succeed(); // Don't retry when got 4xx cuz its request error
        } else {
            // Retry Lambda func when got 5xx errors
            context.fail("Server error when processing message: " + response.statusCode + " - " + response.statusMessage);
        }
    });
};

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

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

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

exports.handler = function(event, context) {
    if (hookUrl) {
        processEvent(event, context);
    } else {
        context.fail('Missing Slack Hook URL.');
    }
};