Ashish Sheth's Blog

 When your webserver goes down, you need find out why the webserver went down and fix the issue. But even before you troubleshoot and fix the issue, you need to make sure your webserver is available to your users so they can continue using your website. There are lot of sophisticated solutions are there, such as failing over to a redundant web server, or creating new instance using AWS AutoScaling. However this solutions comes with their own cost and complexity and if you are small shop like us you may not be able to afford it. And sometimes the simplest solution is to restart the webserver, if you don't have a failover server or AutoScaling configured for your server. If your infrastructure is on AWS, you can use couple of AWS services to restart the webserver automatically without any manual intervention. You need to use AWS CloudWatch Logs, CloudWatch Metrics, CloudWatch Alarm and AWS SystemManager RunCommand. Note that if you are using some Metrics published directly by AWS ser...

 Rsync Algorithm:  TR-CS-96-05.dvi (cmu.edu) RSync is a utility which is used to transfer files or folders from computer to another in unix based systems. The key advantage of Rsync is once the full data is transferred to destination, only the changed bytes are transferred next time onwards, saving time and network bandwidth. The above mentioned paper describes the algorithm used to find the changed bytes and how to file data is transferred from source and recreated at destination. Here is the summary. Instead of transferring complete file, divide the file into chunks of bytes and calculate its hashes. First time all the bytes are transferred to the destination and then for each chunk of bytes a weak rolling checksum (inspired by Adler-32 Checksum) and strong checksum of each block of bytes is calculated and shared with the source. At source, the combination of rolling hash and strong hash are used to find the indexes of chunks of bytes that are same as some block of bytes des...