orleans

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### Orleans is a cross-platform framework for building robust, scalable distributed applications

Orleans builds on the developer productivity of .NET and brings it to the world of distributed applications, such as cloud services. Orleans scales from a single on-premises server to globally distributed, highly-available applications in the cloud.

Orleans takes familiar concepts like objects, interfaces, async/await, and try/catch and extends them to multi-server environments. As such, it helps developers experienced with single-server applications transition to building resilient, scalable cloud services and other distributed applications. For this reason, Orleans has often been referred to as "Distributed .NET".

It was created by [Microsoft Research](http://research.microsoft.com/projects/orleans/) and introduced the [Virtual Actor Model](https://www.microsoft.com/en-us/research/publication/orleans-distributed-virtual-actors-for-programmability-and-scalability/) as a novel approach to building a new generation of distributed systems for the Cloud era. The core contribution of Orleans is its programming model which tames the complexity inherent to highly-parallel distributed systems without restricting capabilities or imposing onerous constraints on the developer.

## Grains

![A grain is composed of a stable identity, behavior, and state](assets/grain_formulation.svg)

The fundamental building block in any Orleans application is a *grain*. Grains are entities comprising user-defined identity, behavior, and state. Grain identities are user-defined keys which make Grains always available for invocation. Grains can be invoked by other grains or by external clients such as Web frontends, via strongly-typed communication interfaces (contracts). Each grain is an instance of a class which implements one or more of these interfaces.

Grains can have volatile and/or persistent state that can be stored in any storage system. As such, grains implicitly partition application state, enabling automatic scalability and simplifying recovery from failures. Grain state is kept in memory while the grain is active, leading to lower latency and less load on data stores.

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  <image src="assets/managed_lifecycle.svg" alt="A diagram showing the managed lifecycle of a grain">
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Instantiation of grains is automatically performed on demand by the Orleans runtime. Grains which are not used for a while are automatically removed from memory to free up resour