GSoC 2019 – Simulating eventual consistency for

As I wrote in my prior blog post, I am working on for this GSoC. Specifically, I’m working on simulating various components of the system so that other parts can be tested and developed independently, reducing the overall coupling between various elements.

There is, however, some inevitable coupling. My original design called for three phases of implementation: quickly simulating a network of hosts, accurately portraying a network, and providing a HTTP API mirroring the real service, in that order. As it turns out, the API is not well enough defined at the moment to work from the ground up on the simulator, so I’ve focused my efforts onto the API itself and the second step of the process.


visn, meaning “knowledge” in Yiddish, is the testing framework I’ve created to bypass some of the initial effort of building a real simulation of the eventually-consistent real world network. Instead, visn uses synthetic event streaming to simulate incoming messages, which are translated into actual mutations of the state of a node through a resolution function.

A diagram showing synthetic events flowing through a resolution function to become real calls on the system under test, changing its state.

For example, we could define some synthetic events such as MessageReceived and FileUpdate. Our node under test, A, receives two FileUpdate events for some file (say,hello.txt), one creating the file and then one changing its contents, and a MessageReceived whose message refers to hello.txt.

It’s desirable to have the final displayed message show the updated contents of the file. To test this, we define the mapping from synthetic events to real state changes, and then write tests for each possible arrival order case.

Without visn, managing these changes becomes rather challenging. With a framework in place to manage the state and order of arrival, however, all synthetic event translation can be done in a single function, and each test is decoupled from the rest of the software, so true unit tests can be used.

The Big Picture

Now that visn is in a working state, I can use it to help build out the libqaul API and start testing its components. This will help make future development faster, as well as providing a target for more detailed network simulation in the future.

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