server.js

var aws = require('aws-sdk'),
    http = require('http'),
    winston = require('winston'),
    url = require('url'),
    mime = require('mime');

var awsRegion = 'ap-northeast-1',
    bucketName = process.env.BUCKET,
    re_mime_matcher = /image/gi,
    re_filename_extractor = /(\w+\.(png|jpg|gif))$/gi,
    logfile = 'server.log';

var logger = new winston.Logger({
    transports: [ 
        new winston.transports.File({ filename: logfile})
    ]
});

http.createServer(function(req,res){

    req.on('data',function(data){
        try{
            var hash = JSON.parse(data.toString());
        }catch(e){
            returnResponse(res,400,logger,'Invalid json: ' + data.toString());
        }

        if(!hash.url || !validateUrl(hash.url)){
            returnResponse(res,400,logger,'Invalid url: ' + JSON.parse(hash));
        }

        fetchImage(hash.url,function(error,fetchResult){

            if(error){
                returnResponse(res,400,logger,'Could not fetch image');
            }

            uploadImageToS3(
                bucketName,
                fetchResult.filename,
                fetchResult.body,
                function(error,filename){
                    if(error) {
                        returnResponse(res,400,logger, 'Could not upload image to S3: ' + JSON.stringify(error));
                    }
                    returnResponse(res,200,logger,'uploaded: ' + filename) ;
                }
            );
        });
    });
}).listen(process.env.PORT || 3000);

function returnResponse(httpResponse, status, logger, message){

    if(status === 200){
        logger.info(message);
    }else{
        logger.error(message);
    }

    httpResponse.writeHead(status);
    httpResponse.write(message);
    httpResponse.end();
}

function validateUrl(string){
    var result = url.parse(string);
    if(result.hostname){
        return true;
    }else{
        return false;
    }
}

function fetchImage(url,callback){
    http.get(url,function(res){

        if(!res.headers['content-type'].match(re_mime_matcher)){
            callback('mime_error',null);
        }

        var imageData = '';
        res.setEncoding('binary');
        res.on('data',function(chunk){
            imageData += chunk;
        });

        res.on('end',function(){
            var filename = extractFileName(url);
            var buf = new Buffer(imageData,'binary');
            callback(null,{filename:filename,body:buf})
        });
    });
}

function uploadImageToS3(bucketName,filename,body,callback){
    var s3 = new aws.S3({region:awsRegion});
    var params = {
        ACL: 'public-read',
        Body: body,
        Bucket: bucketName,
        ContentLength: body.length,
        ContentType: mime.lookup(filename),
        Key: filename
    };
    s3.putObject(params,function(error,res){
        if(error)callback(error,null);
        else callback(null,filename);
    });
}

function extractFileName(string){
    var filename = string.match(re_filename_extractor);
    return filename[0];
}