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Author microservices without thinking about faults or servers. Then compile and deploy anywhere.
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Applications January overhaul (#13) Jan 29, 2019
Hosts January overhaul (#13) Jan 29, 2019
Telemetry January overhaul (#13) Jan 29, 2019
LICENSE.txt Update Dec 21, 2018
ReactiveMachine.sln January overhaul (#13) Jan 29, 2019

Reactive Machine

Reactive Machine is a serverless programming model for writing reliable elastic micro-services. The documentation is at

The idea is to express the application logic at a level of abstraction that hides platform failures (machines or connections going down) and configuration choices (e.g. the number of servers). This high-level representation is then compiled and deployed on a back-end host of choice. Importantly, the host can recover from failures transparently and consistently, because our compiler produces a deterministic state machine that makes it possible to reliably track the application state by logging messages and/or persisting snapshots to durable storage.

Programming Model

Reactive machine programs are built on task abstraction that we call an operation. The model offers a number of different operations:

  • Orchestrations describe operations that are composed of one or more other operations. They are written in async/await style, and execute deterministically and reliably. When executing an operation, orchestrations can either perform it (meaning they wait for its completion) or fork it (meaning it executes completely independently of its parent).
  • Activities are operations that can be unreliable or nondeterministic, such as calls to external services.
  • States represent a small piece of information (cf. key-value pair, or a grain, or virtual actor) that can be atomically accessed via a specified set of read and update operations.
  • Affinities define locality, by specifying keys that can be used to place state, orchestrations, and activities. These keys are also used for synchronization (locking).
  • Events provide reliable, consistent pub-sub. When an event is raised by an orchestration, all the states that subscribe to it are modified. Events appear to be globally ordered and virtually synchronous.


At the moment, both the programming model and the hosts are written in C#. Support for other languages is conceivable, but not on our immediate Radar.


Because reactive machine applications are compiled into an intermediate representation (specifically, deterministically replayable state machines), it is easy to build and experiment with multiple hosting back-ends. Different hosts always provide the same application semantics, and are therefore easily interchangeable. But they can exhibit large differences in non-semantic characteristics, such as cost, scalability, latency, throughput, and recovery speed.

Currently, the repository contains two hosts:

  • A local emulator, meant for debugging and local profiling. It emulates the reactive machine processes in a single process, using either a single thread or multiple threads.
  • A functions host, built on top of Azure Functions, Azure EventHubs, and Azure Blobs. It uses EventHubs to launch the reactive machine processes inside Azure functions, and to implement reliable communication between the processes. State snapshots are stored in Azure Blobs.

Also, we are currently working on releasing a third host that executes on top of the Ambrosia system. Ambrosia is the ideal back-end for Reactive Machine, because it provides virtual resiliency with very good performance.

Status and Plan

We are currently at 1.0.0-alpha, meaning that this is preview which should give you an idea of what this is all about once finished. What we have is:

  • A C# implementation of the reactive machine programming model and compiler
  • Two host implementations (emulator and functions)
  • Application examples to demonstrate the features of the programming model
  • A Hello World sample to demonstrate how to use the 2 hosts

Before we can release 1.0.0-beta, we still need to do some significant work:

  • Add an Ambrosia Host
  • Plug the many gaping holes in the documentation
  • Implement support for code updates, placement updates, and changing the number of processes
  • Fix known bugs in existing tests
  • Build and test on Linux

Further in the future, we will consider moving from 1.0.0-beta to 1.0.0 as a matter of stability, i.e. we will remove the 'beta' tag once we feel comfortable with users placing trust on the stability of the code in a production environment.


This project welcomes contributions and suggestions. Most contributions require you to agree to a Contributor License Agreement (CLA) declaring that you have the right to, and actually do, grant us the rights to use your contribution. For details, visit

When you submit a pull request, a CLA-bot will automatically determine whether you need to provide a CLA and decorate the PR appropriately (e.g., label, comment). Simply follow the instructions provided by the bot. You will only need to do this once across all repos using our CLA.

This project has adopted the Microsoft Open Source Code of Conduct. For more information see the Code of Conduct FAQ or contact with any additional questions or comments.

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