Thursday, 1 March 2012

Recursively Building a Web Service using the same Web Service

Back during the later part of 2011 there was a common theme occurring in our retrospectives each week. How can we replicate our live environment as close as possible?

We took steps to achieve this goal by creating a single machine image to ensure all our machines were configured correctly. Another quick win was to ensure certain aspects of our live data was restored to our local development databases during the night. This enabled us to take stack traces from our logs, and quite literally paste them into our IDE and replicate the users problem instantly. Without the same data set we could have seen different results. Despite these positive steps, there was a missing link in our replication process. How do we simulate the traffic of our live environment? As an example, we average anywhere from four to five thousand calculations per minute with our current web services, with our local and demo environment no where near this figure.

During 2011 I found myself involved in many deployments in which despite heavy testing I was uneasy. On our demo environments we could throw the same amount of load against our services, yet sometime after deploying our service would fall over. We would quickly have to revert and go back to the drawing board. The problem we had despite our traffic being mimicked in terms of volume was the load was not real. Our customers however have many more variations of requests that we were simply not predicting. The other obvious issue was during local development, the service may well handle the same volume of traffic, yet once live and the process has been running for a few hours - things might go bump. Factors such as memory or timeouts being the culprits here.

Collectively we had a few ideas on how to solve this. We looked into low level solutions such as directing traffic from IIS/apache towards other servers. We examined other load testing tools, and we even contemplated creating our own load creator. This internal tool would go over our database and fire off a number of requests at our demo environment. I felt uneasy with all these solutions. They were not "real" enough. I wanted the real time traffic to be submitted to our demo services, only then could we have full confidence in our work.

My idea was rather radical in the sense it was so easy, yet dangerous enough that it might just work. I proposed we integrated our own service, into itself. In other words, just before our service returns the results of the calculation, it takes the users request and submits it again, against our demo environment. The same service would be recursively submitting into itself. In order to ensure we did not affect the speed of the service, the submission is performed via an async call, meaning if this second call was to die the live service would be unaffected. The obvious downside here was that in order to test this, we needed to deploy the changes to our live service. This was achieved via a feature toggle, meaning at any time we could turn the feature on or off without affecting any customers.

The end result of this was that when the feature is enabled, the traffic on our live service is sent to our demo service. This allows us to deploy experimental or new features and changes to the demo environment and check them under real load, with real time data. If all goes well after a period of time we can deploy to our live service, if not we roll back and no one is the wiser.