Tag Archives: Amazon Web Services

MongoDB ReplicaSet Backup in the Cloud

MongoDB replicaset is a great way to handle scalability and redundancy. In the age of the cloud nodes are added and removed from a replicaset easily and quickly and in most cases all are created from the same image.

So how can we make sure that we are always running backup from a non MASTER replica set node?

Below is a small script that will only run backup on non master replica set node.

It will also archive and compress the backup and upload it to a Google Cloud Storage bucket. You can easily modify the last part to upload the file to an AWS S3 bucket using s3cp or s3cmd.

This is a template that works best for a typical small replica set – 2 nodes and an arbiter. You will install it on both nodes, schedule it using cron and it will only run on the non master one. Even if you flip the master role between servers the script will still work well without changing a thing.

A simple and elegant solution if I may say so myself ­čÖé

ec2ssh – Quicker SSH-ing Into Your EC2 Instances

Ever since Amazon introduced tags in EC2 I felt such a relief that I can name my instances and actually remember which one is which.

It took a while for Name tags to find its way to various aspects of the AWS Console, however, connecting to machines still requires a way to find the IP address from the console or via a command line using the EC2 API Tools.

I thought that it would be much easier for me and others to utilize the Name tags to connect more easily to the machines.

Initially, the script was a simple bash script which utilizes a the ec2-describe-instances command with a flag that matched the Name attribute, however, managed various other command line parameters such as the user name to connect with (ubuntu images, for example, users the ‘ubuntu’ user instead of ‘root’. Amazon Linux AMI uses ‘ec2user’, etc) so I’ve decided to rewrite it in Python and use┬áthe magnificent┬áBoto┬áPython library.

This allowed better argument handling as well as remove the need to install Java and the EC2 API Tools to access the EC2 API.

Grab the code here

Don’t forget to send some feedback!

 

Recommendation: Consider Submitting Messages to Amazon’s Simple Queue Service (SQS) via Simple Notification Service (SNS)

Thinking a bit more about my last post, I’d come to the conclusion that unless there is a really good excuse, one should always submit items to Amazon’s Simple Queue Service (SQS) via publishing to a topic in Simple Notification Service (SNS).

In the simplest case, you’ll publish to a topic and that topic will have a single subscriber that will post the message to the queue.

However, since SNS allows multiple subscribers you can get a couple of features free of charge without changing a single line of code. For example you can:

  • Add a temporarily an Email address to get messages going to the queue via Email for easy debugging (you can easily and quickly unsubscribe via the link at the end of the Email)
  • Add additional logging by adding an HTTP/S subscriber, getting the message and perform some logging on it
  • Notify other monitoring systems that a certain process has started
I know that from now on I’ll try to think really hard if I really need to publish directly to a queue instead of using SNS.

Using Amazon’s Simple Notification Service (SNS) & Simple Queue Service (SQS) For a Reliable Push Based Processing of Messages

I’ve recently encountered a use case where I need to reliably send a message to a worker process that should handle the following requirements:

  • Message should persist until the action it specified is done.
  • Message should be processed (or wait to be processed) when the worker processes have a bug or are down
  • Message processing should be as fast as possible – process the message ASAP.

Using Amazon’s Simple Queue Service (SQS)

SQS can provide the persistency needed. The message will be available until it is deleted (at least up to ~3 days) even if the worker processes are down or have a bug.

The problem with using SQS is that it requires polling which introduces a certain delay between the time a message is published and until it is processed. That delay can be small, a couple of seconds, but can easily be up to 30 seconds and more (depending on the limitations of SQS polling and the polling interval used).

Using Amazon’s Simple Notification Service (SNS)

SNS can provide a push mechanism to notify in near-real-time to a subscriber that a message has been published.
However, SNS can only guarantee a single delivery to each subscriber of a given topic. This means that if there was a bug or a problem processing the message and there was no specific code to save it somewhere, the message is lost.

The Solution

SQS and SNS can be combined to produce a PUSH mechanism to reliably handle messages.
The general configuration steps are:
  • Create a topic
  • Subscribe an SQS queue to that topic
  • Subscribe a worker process that work via HTTP/S to that topic (for increased┬áreliability┬áthis can be an Elastic Load Balancer (ELB) that hides a set of machines)
Then the flow goes like this:
  • Submit a message to the topic
  • SNS will publish the message to the SQS queue
  • SNS will then notify via HTTP/S POST to the handler to start processing the message
When the worker process gets the HTTP/S POST from SNS it will start polling the queue until it has no items in the queue and finish the HTTP request.
To handle failures when the worker process has a bug or is down or did not get the SNS message, a regular worker process can run and poll the queue in regular, longer, intervals to make sure all messages are processed and no one gets behind.

This solution covers the original 3 requirements of message reliability, handling cases where workers are down or have bugs and handling messages as soon as they are sent.

Monitor Your Amazon Web Services Simple Queue Service (SQS) Queue Length in Amazon CloudWatch

UPDATE (2011-07-16): I just got a newsletter Email from Amazon stating that they have added SQS and SNS to CloudWatch which allows monitor SQS queues not just for the length of the queue, but for others metrics as well, so there is no real need in my script. Unless you really really want to use it ­čÖé

All you have to do is select SQS in the metrics type drop down and you will see a set of metrics to select from for all of your existing queues.

 

 

Amazon’s CloudWatch is a great tool for monitor various aspects of your service. Last May Amazon introduced custom metrics to CloudWatch which allows sending any metrics data you wish to CloudWatch. You can then store it, plot it and also create CloudWatch Alerts based on it.

One of the things missing from CloudWatch is Simple Queue Service (SQS) monitoring, so I’ve written a small script to update a queue’s count in a CloudWatch custom metric.
Having the queue’s count in CloudWatch allows adding alerts and actions based on the queue’s length.

For example, if the queue’s length is above a certain amount of a certain period of time, one of 2 things happened:

  1. There is a bug in the code causing the worker processes that process the queue’s message to fail
  2. There is a higher than usual load on the system causing the queue fill up and get more and more messages while there aren’t enough worker processes to process these messages in reasonable time

If the load is higher than usual you can easily tell via a CloudWatch alert to add an additional machine instance running more worker processes or simply send an Email alert saying there is something wrong.

The script is very easy to use and can be run from a cron job. I’m running it as a cron job in 1 minute intervals and have set up various CloudWatch alerts to better monitor my queue.

Grab the script on Github at:   SQS Cloud Watch Queue Count.