High Availability Design
High Availability Design
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If you have ever travelled in an Indian Railways you would have noticed that the capacity for which the train is supposed is handle holds no meaning because the number of people it will be carrying is just going to be way over. That’s how the passenger load and platforms all across the country are managed. The method mostly works fine, but from time to time there are breakdown and trains are delayed, sometime cancelled but the life goes on as people expect that this will happen with Indian Railways.
When we design and write those big platforms/software something similar happens but the biggest difference is that customers/clients who have paid for the software don’t like those downtimes (cancellations) and slowness (delays). Last 2 years or so I have had so many conversations where 2 key NFRs intersect – Performance and Availability. I have noticed and started to realize that while these two end up being joint at the hip and need to work closely together they still mean a world of difference and what it means when we speak about performance and availability and each of these need to be addressed differently.
Of course, eventually with all many fixes you will eradicate a lot of cases that led to failures but it would have taken you so long and the reputation that the brand holds do dear is already damaged.
Most project (almost all) in today’s world have some Non Functional Requirements and the 3 that take the top most priority are Performance, Availability and Security. Some numbers that get most often thrown around are:
- Pages should open in under 1 second and so forth
- There should be an uptime of 99.99% – which is actually 1.01 minutes per week
And that’s about it. Of course there are more, but 95% of the conversations revolve around the two here. then we go about designing solutions to meet those numbers.
Just before GO Live
Things are al good when we are under implementation and we do everything to make sure we meet those 2 or so numbers met with out design. We do performance modeling and then we execute those performance models and prove out that the trafic model/simulation as to what we understand as client use cases work fine. So that is not we say with confidence that our system will meet the performance needs. In this model, we do take a capacity increase of 40% or so; again based on anayltic and some future growth and we incubate those numbers into our calculations and then we are even sure that our system will be able to handle a bit more if that happens at times.
Now that we are so sure that performance is all good for the traffic we expect to get we believe that our software should continue to work fine because everything will work in the same constraints in which we have tested it. And because those constraints are well defined there should really be no problem with us meeting our availability numbers.
We are in flight Houston
Then the next cool thing happens – we go live and our system that we have build with so much pride and caution and we have tested so much is Live and so many people start to use it. It certainly is an exhilarating feeling seeing your sweat and hard work go live and people are seeing and interacting with what you have build.
Until a time comes when the system goes down. You would be sleeping in middle of the night and you will get a call and someone will be telling you to get up, switch on your laptop and get ready to debug as to why the system went down and get it up and running quick. It takes you a while to think – what the hell just happened. We did everything we had to do. There were even reviews that we did and everything was looking good. How can it go down.
Well there is something called Universe that has a different set of plans for you and those plans just went into motion.
So what happens?
Indian Railways happens :) You realize that there is a traffic pattern that has suddenly come and hit your servers which you did not know. A Chinese search engine has started to crawl all over your site and the site is not even in china. Well it’s a free ride this internet and anyone can get on. Why can some people sitting in a country not see a website that is not supposed to be targeted at them – but we did not have those specifications. We put in all checks but we never anticipated for all those search engines and bots and the crawling they will do.
What do we do?
We fix the problem and put in either a block to stop that traffic pattern or we throttle it or we add servers to handle it. And then we go about thinking okay now i am good; this is done and dusted and wont happen again.
Universe has other plans
Next time someone will add some bad servers into WIP and cluster will fail. and the next time someone will delete the database and it will crash again and the next and the next and the next…
In the whole software development process we miss this key step – to design for this game and how we will be setup to get the system back up and running in that time frame.
Of course we have to do something but what we do is to start looking at our solution and check why performance is bad. Performance – really? We do everything we can do to fix the performance problem but we spend no time on availability aspect.
Going back to the India Railways analogy I drew upfront; a train – engine and bogeys are built to handle certain load and they was agreed. It cant be more precise as there are seats and there are tickets that need to be bought to get into that train. As long as the number of people that get in there are within those constraints our problems will be much less. Everything around our software (our train) needs to work in tandem. But, it is difficult to control. Internet is much more wider than an Indian Railways and who comes, when they come and how many come is just not predictable. It becomes important to acknowledge that no matter how you do there will always be a model of traffic that will come and visit you that will take your system outside of the known boundaries and more often than not once our system is operating outside of those boundaries it’s bound to fail some point. This is where the Availability and Resilience perspectives need to be brought into the picture.
Next Time do something else too
Availability and Resilience
At their core this perspective asks you to set some designs, practice and most importantly an expectation with your clients as to what you are dealing with. We all know that in the last decade how we run our business and how we deal with internet hosted sites is very different than how we used to make systems in past. I paraphrase from article – “If your site is down, your business will suffer” and yet everyone will want a 24×7 uptime but yet we sold what NFRs – 99.99 (1.06 minutes) or maybe 99.9999 (0.605 seconds) thinking it’s okay. If the expectation is 24×7 why would we even start with something less?
We then need to look at the next 2 most important metrics which we miss all along and we never plan, design or test for. It’s like we take them for granted. It’s the Recovery Time Objective (RTO) and RPO (Recovery Point Objective). As we speak about uptime and outages, whenever we have an unplanned outage and we have promised a certain uptime, we need to have the Operational Ability to be do whatever is needed to get the system up and running. If we designed for 99.99% we need to have methods in place to get system back in 1.06 seconds – it feels like Minute to Win It. In the whole software development process we miss this key step – to design for this game and how we will be setup to get the system back up and running in that time frame.
The Operational ViewPoint
Operation Viewpoint is a key architectural principle that we omit to design for when we are building software systems and platform. How we run software now has been completely changed in the last decade with the cloud hosting. As cloud makes things so easy to provision and host (AWS) we believe that everything should be easy. So where in the past when we used to focus on Availability design a lot more we almost take it for granted. This Viewpoint is something that should become our bread ad butter during the implementation phase with a dedicated team who is going to look at operational processes and tools and provide methods to make the recovery possible in the time it’s expected to be.
Categorize and Prioritize
This is where it becomes critical to have a conversation with out clients and understand how various parts of the systems can be identified and broken down into services. A classification of sorts like “Platinum”, “Gold”, “Bronze” starts to make sense to get the business to prioritize as to what services should get the top most priority incase of an unplanned outage. The focus of the operational design and implementation team then needs to focus on how to look at the system and how to make those services up and running quickly. This is a key inputs for the implementation phase because unless those services are not known there won’t be a way those services are coded such.
Recover and not debug
When these unplanned outages happen, the team which is responsible for managing the system more often than not start with a different mind set. They are like Cops who have reached a crime scene after a crime has happened. They start by looking around for evidence and analyze the crime scene as to what happened. The idea is to look for evidence and then solve the crime and hopefully then find the criminal and put them behind bars. Well we all know it takes so long to get there. With cops, i can see the point – you can’t have cameras in all the home and everywhere, so you will have do post-mortems. But this is a software system and we need to be fire fighters. The idea has to be to put the fire out and do it quickly before it takes the block away. The idea has to be to realize some damage has been done – thats a lost cause; let’s see how we can save what’s left of it.
In softwares that we write and platforms that we host we need to have a something I refer to as “Last know good state” and when an outage happens what we need to do it so just ourselves back to that state. But, when do we do when the state or behavior is not under your control. Going to back to Indian Railways, what do you do when you can’t control the number of people who are coming in on the platform and onto your train – they just keep coming in; no matter if you find a way to replace the train, they will keep coming in. The other way is to of course start adding more trains on the platform. With cloud you can do that and keep adding servers until all the traffic is dealt with. This is where we move seamlessly into Performance and Scalability perspective.
This is where we lose sight of the problem and we try to fix something else in modern software.
So what should do if we can not control traffic. We need an effective mechanism on our train that wont allow everyone to get in. We need to have the ability to know who can get in. We have these ID cards and Turnstil on platform. So if our platforms do not give us those why can we not put those in in our trains. It may not stop all malicious traffic, but it will certainly stop a lot of it. Most importantly you can go back and authorize your Platinum users to get in while you block everyone else.
So in software world, you need to have a cutover switch that will stop all traffic and only allow what is key for business. Unless everything that is coming in is Platinum which is not the case most times, you will be able to recover your most important services easy. Of course there is a degradation of other services but that is something you would already set expectations with your clients and the business. They will be mad but less mad.
If you have not seen 300, you have got to see this and learn what it can do to your systems and ability to recover if you handle your enemy (the traffic) though a funnel. You will longer and you will get a lot more time to fight the enemy. On top of it, the pressure and stress that the business creates when their platinum services are not available will also be reduced. You can then go about debugging once you have contained.
Nothing is for Free
Of course we do so much more, but the more we do the more it will cost. Back to indian railways, we can either chose to save some money on building my train by not installing those ID cards turnstiles or we can invest that money and ensure we have continuity. The Turnstiles will be more and more complex and will need a lot more fine tuning to handle all scenarios. So if you need to also handle for use cases where you don’t want your turnstile to fail, then you need to install 2 of them on all bogeys which will cost more and the setup goes on.
The point I am trying to make is that when we go from 99% to 99.99% to 99.999% we dont look at the cost drivers and what it will do to the project. We may think – it would mean a few more rounds of performance testing and we should done. We we know now what’s going to happen.
If you fail to articulate to the clients what these numbers will mean to them in terms of cost, you wont ever get them accept the reality of internet and the universe. More often than not you will realize how business will realize that there are services they can live without. Of course, eventually with all many fixes you will eradicate a lot of cases that led to failures but it would have taken you so long and the reputation that the brand holds do dear is already damaged. If you think of this as “risk money” and how you invest in guard your reputation will justify the cost every time – that much i can assure you.
Published at DZone with permission of Kapil Viren Ahuja , DZone MVB. See the original article here.
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