Cutting the AWS bill with spot instances

AWS has definitely changed the way we all approach infrastructures these days, especially here — at Mind Candy.

We’re finally not limited by the amount of available hardware, so we can get whatever amount of resources (well, nearly) we need, whenever we need, plus we get CloudFormations.

However, as exciting as spawning 100+ servers can be, as with many things, if you’re not cautious and smart, it can cost you a lot of money.

One way to save a bit of money on your AWS bill (and “a bit” is a serious understatement) is by utilising Spot Instances.

“Spot Instances allow you to name your own price for Amazon EC2 computing capacity. You simply bid on spare Amazon EC2 instances and run them whenever your bid exceeds the current Spot Price, which varies in real-time based on supply and demand.” –

How much can you save? Well, the c3.large instances which we use across the board for our application tier in on-demand pricing cost $0.12 per hour. When we use the same instance type with spot pricing we get them most of the time for around $0.02. That’s 6x cheaper compared to on-demand.

So what’s the trade-off? Well, if for some reason the spot-instance price exceeds your bid price, your spot reservations will get cancelled and your spot-instances will be killed. In short — your instances can and will die at random times and it’s not 100% guaranteed that you’ll get them when you want them.

That’s not good. Even if you use CloudFormations and auto-scaling as you could end up without instances when the spot price becomes too high – that could be almost the same as an AZ failure if you’re not prepared for it.

However, there’s a way to overcome that risk. In a single CloudFormation, you can create two launch configurations — one for on-demand instances and another one for spot-instances. With carefully tweaked scaling thresholds, you can make your spot-instances be preferred over on-demand instances, but still ensure on-demand takes over should spot-instances no longer be available at your bid price.

This way, if you can get spot-instances, your stack will be pretty much fully built using spot-instances. If (and when) the price goes over your bid price, spot instances will start getting killed and your on-demand instances will start booting up instead to cover the increased price. When the spot-price return beneath your bid price, spot instances will start booting up, slowly phasing out on-demand instances.

After few weeks of tests we managed to come up with a set of thresholds which work pretty well for us and keeps our stacks stable around the clock.

With on-demand, we always have a single instance running by setting the minimum to 1. Scale-up event happens when our average CPU usage exceeds 80% for a 5 minute period and we increase the on-demand autoscale group by 2 instances. We then scale down 1 instance at a time if the average CPU usage is less then 65% for a period of 5 minutes, and we ensure that a scale-down event only happens once in a 15 minute period.

With spot-instances, we also request a minimum of 1 instances but we set ourselves a bid price of $0.12 – remember, the bid price is not the price you pay, it’s the maximum you are willing to pay. Most of the time we have a spot-price cost of just $0.02!

As with on-demand we scale on average CPU in the spot-price autoscale group. However, we scale-up whenever we reach 50% (instead of 80%), and we also add 2 instances. We scale down and cancel our spot instances when we dip below 30% CPU usage.

The result is probably best as a picture from Ice (Ice is a great tool from Netflix that helps manage AWS costs). Below is the hourly cost of one of our app tiers before and after we started utilising spot instances.

Screen Shot 2014-10-15 at 11.45.23

For us, in the case of this specific stack, spot instances gave us savings up to 60%. Bear in mind the size of this specific stack is quite small (up to 10-12 instances at peak); so the bigger the stack, the more savings you’ll see!

To wrap up, I just wanted to share few tips and tricks we picked up along the way, that should help you:

  • bake AMIs; tools like Packer will greatly help you do this; this will let you minimise time required to boot up a new instances; it’ll give you much more, but the time is crucial when it comes to scale-up events, especially when spot-instances are being killed and you want on-demand instances to fill out the empty spaces ASAP. We managed to get time required to boot up a new instances down to around 75 seconds
  • use EBS based instances; they cost a fraction more (and yeah, EBS can be painful) but they’re boot time is significantly faster then the ephemeral-storage based instances
  • bid price = on-demand instance price; this way in worst case you’ll pay what you’d normally pay for on-demand instance
  • utilise reserved instances for the “base” on-demand instances in on-demand stacks
  • did I mention Ice from Netflix? Use it!

<shamelessplug> Obviously, the most important requirement is having an awesome application that is cloud-friendly. If you’re interested in building cloud-native applications and awesome infrastructures, we’d love to hear from you! ;-) </ shamelessplug>

That’ll be all folks! Happy spot-instancing!

Mindcandy is looking for an exceptional Android Engineer..

Mind Candy is looking for an exceptional Android Engineer to join the team building the worlds most exciting kids social networking app! The team moves rapidly, especially during the experimental phase of the project, and so will you. You’ll work closely with frontend designers and server engineers and members of the senior management team to design, implement and A/B test new features to make it fun for kids to connect with other kids. Culture is important to us – you’ll be humble but have huge aspirations. You’ll have a great work ethic, will thrive in a fast-paced environment and you’ll enjoy both autonomy and responsibility.


Design and implement high quality, visually rich Android applications that work on a wide range of devices

Integrate with 1st and 3rd party online services for in-app purchases, analytics, a/b testing and contacts

Collaborate with other mobile engineers to identify best practices

Requirements BS degree in Computer Science or equivalent experience Professional experience building Android applications


Experience working in an Agile environment.

Experience with static analysis tools.

Understanding of Gradle build system.

Apply Now

A Puppet module for Dynamic DynamoDB

As my colleagues have said in other posts, we make an extensive use of Amazon Web Services at Mind Candy. Recently we decided to use the AWS NoSQL offering DynamoDB for a specific use case in one of our products.

Whilst DynamoDB provides us with a highly distributed NoSQL solution, it works based on telling Amazon what read and write capacity you require via their API. If you find that you go over either of these value you begin to, potentially at least, lose queries if you have not factored in some sort of caching layer using, for example, Amazon SQS.

In the ideal world, Amazon would offer auto scaling features for DynamoDB, however at time of writing they don’t. Instead they advise people to use an independently developed tool called Dynamic DynamoDB written by Sebastian Dahlgren.

Dynamic DynamoDB is a tool written in Python that allows us to effectively auto scale our provisioned reads and writes. It use CloudWatch metrics to establish current usage and then based on the configuration option either scales up or down your provisioned capacity on a per table basis.

As I’ve posted before here, we use Puppet at Mind Candy, so the first point of call whenever a new tool comes along is to see if anyone has written, or started to write, a Puppet module for it. Sadly it didn’t look like anyone had, so we quickly wrote up our own, which is available on Github here.

Advanced Scala Meetup at Mind Candy

We’re very happy to announce the Advanced Scala Meetup in collaboration with the London Scala Users’ Group. This is a new regular meet up for proficient Scala developers to share out their problems, solutions and experience to make for better code.

At Mind Candy, all our new games and products use Scala, so we’re very interested in Scala and happy to host the meetup!

For more details and to sign up see the event page:

If you’d like to learn more about joining us at Mind Candy to work on highly scalable servers for our new wave of games, check out our Careers page for current vacancies. We’re hiring!

How I Learned to Stop Worrying and Love AWS CloudFormation

We love using AWS CloudFormation, here, at Mind Candy. Last year we moved all our cloud-based products application stacks to CloudFormations. We have learned, sometimes the hard way, how to design and use them in the best possible way for us. In this post I’m trying to summarize how we build and operate CloudFormations and what are the DOs and DON’Ts when using this technology. Throughout this post I will refer to CloudFormation as CF, to save some precious typing time.

First of all, you need to get to know cloud formation templates. This are just blocks of JSON, and as such are not really nice to edit (remember – no comments allowed in JSON). Because of that we use a helper technology – a templating tool to build CF templates. We decided to use tuxpiper’s cloudcast library (we are a python shop). You can take a peek or download it here If your primary language is different than python you can easily find or write your own templating tool – it was pointed to me by a former colleague that CFNDSL is a good starting point for rubyists ( So lesson one is – don’t use plain JSON to write your CF templates. You will save yourself a lot of tedious time.

Once you have your first stack up and running you’ll realise how easy it is to modify and use it. But wait, what about testing the changes? That’s one of the biggest flaws of the CF technology. There is no other way to test your template than to apply it. CF does not give you a second chance – you can easily terminate/recreate your whole stack by changing of single line in your template. The good practice we try to adhere to is to test every single change in the template using different AWS account (we use separate AWS accounts for our development, integration, staging and production environments) or region, i.e. launch identical stack first in another AWS location and then perform the change on it to test if we end up in the desired state.

To make it possible to launch identical stacks in different accounts or regions one can leverage CF mappings and parameters. We don’t use parameters yet, but we use mapping heavily. That allows us to use a single CF template file to create multiple stacks in different environments. All you have to do is to define environment-specific properties within a global mapping on top of our template and then use CF’s “Fn::FindInMap” intrinsic function (actually, cloudcast does it for you). Also, use CF Outputs – they will allow you to programmatically access the resources created in your CF.

Next one is a set of more generic hints for those who work with AWS, still 100% valid for CF. First, use IAM roles to launch your stacks/instances. Let me quote AWS IAM official documentation here:

A role is an entity that has its own set of permissions, but that isn’t a user or group. Roles also don’t have their own permanent set of credentials the way IAM users do. Instead, a role is assumed by other entities. Credentials are then either associated with the assuming identity, or IAM dynamically provides temporary credentials (in the case of Amazon EC2)“.

That will make your environment more secure and save you misery of maintaining IAM users and keys. Bear in mind that once the instance is created you cannot assign it to an IAM role, so if you’re not using IAM roles yet you should create IAM role with an “empty” policy now and use it for all your resources until you’re ready to benefit from full-fat IAM roles.

Secondly, use a minimalistic user data – make it identical for your whole estate. Delegate environment/application specific settings to your configuration management system. This will just make your life easier. Get familiar with and start using auto-scaling groups, even if you’re launching a single instance (in that case you can have an auto-scaling group with minimum and maximum number of instances equal to 1). You’ll benefit from that approach later, once your service starts to scale up.

Finally, use AWS tags to tag your AWS resources. Tags allow you to do a lot of funky stuff with your AWS resources (let me only mention grouping, accounting, monitoring and reporting here).

Now, a few DON’Ts for your CF:

  • Don’t mix VPC and non-VPC regions in your mappings – CF uses different set of properties for EC2-VPC resources than for EC2-classic resources
  • Don’t ever specify resource name properties in your CF template. Using auto-generated names makes your stack easily portable. Thus, you can copy your existing stack to another environment or launch a completely new stack (say your canary stack) using the same template. Also some of AWS resource names need to be globally/regionally unique, so defining a name in your stack is not such a good idea. Finally, virtually any resource which allows you to set its name will require replacement on update – just imagine your whole stack relaunching from scratch when someone comes with a clever idea to rename resources in line with a new naming convention or a new product name?
  • Don’t use existing (non-CF built) AWS objects in your stack, if you can. Using existing resources also makes your stack non-portable. A lot here depends on the use case (i.e. we have a couple of security groups which we use in our stacks, but even then we provide their names/ids in the mappings or parameters, rather than using them directly in resource declaration).

Know your limits – CF is great orchestration tool, but it has its limits. You cannot create or update some AWS resources (e.g. EC2 keypairs). You cannot self-reference security groups in their definitions, which sucks (how do I open all my cassandra nodes for inter-node communication on port 7001 within the CF?). Stacks are difficult to maintain, as there are no incremental changes. For the above and other, obvious, reasons – don’t forget to source control your CF stacks (we have a dedicated git repository for that).

Finally, the last, and maybe most important, point – separate your applications into multiple CF stacks. One can easily get excited about CF and create a single stack for the whole application (network, databases, application servers, load balancers, caches, queues and so one). That’s not a good idea – you don’t want your database servers to relaunch when you decide to modify the properties of the auto-scaling group for you application layer. The solution is simple – create multiple CF stacks for your four application stack. Make your database layer a separate CF stack, then your distribution (app server auto-scaling groups and ELBs) a second CF stack and so on. This will give you the flexibility of CF without taking a risk of unwanted service disruption, due to CF stack update (been there, done that…). It’s very tempting to create very sophisticated CF stack, with many inter-dependent components, but I cannot stress enough how important is not to do it.

What’s next?

We are all the time looking to improve our stacks and processes, so definitely we are only at the beginning of our CF journey. One of my colleagues is looking at another CF templating library ( to help us automate our processes of CF creation even more. We will very likely start to protect our CF resources in production using stack policies soon. We will start working with CF parameters and dependencies more to make our templates 100% independent of our account/regional settings. Finally, we need to research if Custom Resources are fit for our purposes.

Great British Summer Game Jam

Ready… Steady… JAM!
Autodesk and Mind Candy are combining forces to host an epic GAME JAM at Mindcandy’s famous central London HQ.
Keep a very British stiff upper lip whilst you get to grips with the best Game Developer technology in the business, such as Autodesk MAYA LT and Scaleform for Unity, and be ready to get your next game discovered as you put your skills to the test in this year’s most exciting game jam!

Tell your friends, form your team, learn the tools- then compete to win!
The full agenda of speakers is still to be announced, but make sure you save the date and stay tuned to Twitter (@GBSGameJam) and right here on Facebook, as the #GBSGameJam is not something to be missed!

Working from home in AWS (with access to everything)

Ever since we started moving parts of our services into EC2, we’ve been faced with a growing problem. It’s important that our team can access nodes directly in a troubleshooting situation, even at 3am from Poland if necessary. With resources in AWS, this can mean that before you log into a node, you first have to log into the console (with two-factor auth), find a relevant security group, find your public IP, and then give yourself access via SSH. This can make troubleshooting among nodes in Amazon take much longer than it should. So, we’ve been trying out a different approach. We already have a mechanism for authenticating our employees when working from home – our VPN. We can be reasonably confident that Cisco have done a good job at making this secure, and we’re able to assume that anyone who successfully logs in is probably worthy of access to other resources off-site, in EC2. So, we developed a tool which checks the VPN session database every minute, by connecting to one of our Cisco ASAs and extracting the username, tunnel-group and IP address of all logged-in users. In this way, it’s possible to manage security-groups in EC2, such that you can automatically give users access to the resources they need, based upon their tunnel-group on the Cisco. Essentially, we now no longer need to think about remote access; it just works. There are two components to this system; the first polls the ASA for information every minute, and creates a hash containing user information. It then sends this to an HTTP endpoint, where the correct security groups are updated. The latter part of this is embedded in our ‘mission control’ system, but is really quite basic, in that it simply uses boto to create security groups based on tunnel-group names, and keeps track of when the user was last seen on the VPN in a simple database, so that inactive users can be removed from the groups. The Cisco part is perhaps a little trickier, so we’ve put this on GitHub, in case it comes in handy. You can find it here:

Scaling Puppet for Donuts

In the last year we’ve had a fair number of challenges within NetOps, especially with our config management of choice which is Puppet. Not only did we have a big piece of work that involved taking the giant leap from Puppet 2.x to 3.x, we also faced some architectural and performance challenges.

Whilst the upgrade was successful, we continued to have an architecture that was vertically scaled, and worse still we had CA signing authority host that had become a snowflake due to manual intervention during our upgrade. The architecture issues then really started to become apparent when we started hitting around 600 client nodes.

Now, as the old saying goes, if you have a problem, and no one else can help, maybe you should open…. a JIRA! So we did and it included the promise of donuts to the person willing to do the following:

1: Puppetise all our puppet infrastructure – inception FTW.
2: Add a level of redundancy and resilience to our Puppet CA signing authority.
3: Get us moved to the latest version of PuppetDB.
4: Make Puppet Dashboard better somehow.
5: Do it all on Debian Wheezy because Debian Squeeze support expires soon.
6: Seamlessly move to the new infrastructure.

What happened next was three weeks of work creating new modules in our Puppet git repo that could sit alongside our current configuration and be ready for the proverbial flip of the switch at the appropriate moment.

After a quick bit of research it became clear that the best approach to take was to separate out our CA signing authority host from our Puppet masters that would serve the vast majority of requests. This would allow us to make the CA resilient, which we achieved through bi-directional syncing of the signed certificates between our primary and failover CA.

Separation also meant that our “worker” cluster could be horizontally scaled on demand, and we estimate we can easily accommodate 2000 client nodes with our new set-up, which looks like this:


You may be thinking at this point that PuppetDB is an anomaly because it’s not redundant. However, we took the view that as the reporting data was transient and could potentially change with every puppet run on our agent nodes, we could quite happily take the hit on losing it (temporarily).

Yes we would have to rebuild it, but the data would repopulate once back online. In order to cope with a PuppetDB failure we enabled the “soft_write_failure” option on our Puppet masters, i.e. CA and Worker hosts. This meant that they would still serve in the event of a PuppetDB outage.

Finally, we decided to move away from using the official Puppet Dashboard – which relied on reports and local sql storage – and used Puppetboard Github project instead as it talks directly to PuppetDB. Puppetboard was written using the Flask (Python) web framework and we run it internally fronted with Facebook’s Tornado web server

How bloated is your PostgreSQL database?

When dealing with databases (or, in fact, any data that you need to read from disk), we all know how important it’s to have a lot of memory.  When we’ve a lot of memory, a good portion of data gets nicely cached due to smart operating system caching and most of the data, when requested comes from memory rather then disk which is much, much faster.  Hence trying to keep your dataset size possibly small becomes quite important maintenance task.

One of the things that take quite a bit of space in PostgreSQL which we use across most of the systems here at Mind Candy, are indexes.  And it’s good because they speed up access to data vastly, however they easily get “bloated”, especially if data you store in your tables gets modified often.

However, before we even are able to tackle the actual problem of bloated indexes, first we need to figure out which indexes are bloated.  There’re some tricky SQL queries that you can run against the database to see the index bloat, but in our experience, results we got were not always accurate (and actually quite far off).

Beside having happy databases we also care a lot about the actual data we store so we back it up very often (nightly backups + PITR backups) and once a day we do a fully automatic database restore to make sure backups we take, work.

Now, a restore operation includes building indexes from scratch, what means, those indexes are fresh and free of bloat.

Now, if we only could compare the sizes of indexes from our production databases to the ones from restored backups, we could easily say, very precisely, how much bloat we’ve got in our production database.  To help with that, we wrote a simple python script.

$ ./ -ps csva.csv csvb.csv
Index idx3 size compare to clean import: 117 % (14.49G vs. 12.35G)
Index idx2 size compare to clean import: 279 % (14.49G vs. 5.18G)
Ough!  idx4 index is missing in the csvb.csv file.  Likely a problem with backup!
Total index bloat: 11.46G

The whole process works as following:

  1. At the time of backup, we run a long SQL query which prints all indexes in production database alongside with their sizes in CSV format
  2. After the backup is restored, we run the same SQL query that prints all indexes in “fresh” database alongside with their sizes in CSV format
  3. We then run the aforementioned Python script which parses both CSV files and prints out human-friendly information show exactly how much bloat we’ve got in our indexes

We also added a percent thresold option so it’ll print out only indexes with bloat more then X %.  This is so we won’t get bothered by little differencies.

The aforementioned script, called pgindexbloat, can be found on Mind Candy Github account.  It’s very easy to be run from cronjob or wrapped around into a check script and used to feed Nagios / Sensu.

As a interesting note, I’ll just add that the first go of the script uncovered we had nearly 40GB worth of bloat on our production database.  Much more then we anticipated and getting rid of that bloat, definitelly made our database much happier.

Musings on BDD

This month at the London Python Dojo I gave a talk on Behaviour Driven Development. People were a lot more interested than I expected, so I thought I’d share my thoughts more widely on here. The main motivation behind this talk was to dispel some of the myths around BDD and make it more accessible. You can view the slides on google drive.

What is BDD?
The main points I made could be summarised as:

  • BDD is a methodology, not a tool or a process
  • As a methodology it’s a collection of best practices
  • You don’t have to take the whole thing for it to be useful
  • As the name should make clear – it’s about focussing on behaviour not implementation.

For a more thorough introduction I recommend that you read the slides with the speaker notes.

What I found more interesting though was the Q&A, which lasted longer than the slides themselves. There appeared to be a lot of misconceptions about testing in general. This came out because BDD encourages you to look at behaviour at the user level. This is important as it helps focus your tests and give them tangible meaning. The leap of comprehension though is that when you test at different levels you are concerned with different users.

At a unit level your user is a software developer who will be using that unit of code. The most important thing at this level is the behaviour to this developer, who could be you, or could be someone else.

At an integration level your user is another service or unit of code. Here you are concerned with the interface that is provided to these other services and code units. When this second piece of code interfaces with the first, how does it expect it to behave?

At a system level your user is probably a person, or possibly a script. At this level you should really be using the user interface, be it a GUI or text. This is the level at which there was least confusion, as it should be fairly obvious that the behaviour your interested in here is that of the system as a whole when the user attempts to interact with it.

BDD in Practice

One of the questions I was asked was one about speed – as behaviour is only the first word, it is important to be able to use this to drive your development. Indeed system level tests shall rarely be as fast to execute as your unit tests. This does not however destroy the value of having these tests, and with a little work, in most cases, system and integration tests do not have to be as slow and unpredictable as you might have experienced. This comes down to the proper application of mocking.

The most useful way I have heard of explaining this is from a talk by Simon Stewart where he talks about having small, medium and large tests. A small test lives entirely in memory, is single threaded, doesn’t sleep, and doesn’t access system properties. They should ideally be entirely self contained and able to be run in parallel without any restrictions on ordering, and each one should take less than a minute to run.

A medium test is given much more free reign. Its only restriction is that it cannot touch anything not running on localhost, i.e. any network operations should run on loopback. It should take no more than five minutes. A large test has none of these restrictions.

From this it should be clear that your small tests are almost certainly unit tests of some description. They will act entirely on one unit of code, and any other units of code they interact with should be mocked out with the behaviour they expect to consume. Because they run so fast, you can afford to set these up to run on every commit, and wait for the results before you continue to write code that corrects any errors.

Your large tests are going to be full end-to-end system tests, but might also include integrations with third party services, such as AWS. These are going to be slow. You might choose to use your continuous integration server to run these, or you might run them nightly. You might also choose to include your manual tests in this category, and I find it a perfectly acceptable, or in fact desirable, first step into BDD to have your behaviour specifications executed by a manual tester.

Your medium tests therefore might be assumed to be integration tests, and in some cases they will be, however by mocking out everything non-local which your user interface interacts with, you can make these medium tests too, and if you have a behaviour specification for the units this interacts with, you should have no less confidence in the results of this test. It is true that there is value in having end-to-end tests, but this can give you a much earlier indication of any issues your changes may have introduced or resolved on the system level. With up to a five minute execution time you’re probably not going to wait for them to run before you carry on coding, but you should definitely wait before merging your work to somewhere it will affect others.