Guide Flow-Based Programming: A New Approach To Application Development (2nd Edition)

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Typically, device manufacturers add value to their connected devices by supplying an actor with support for the device, enabling it to participate in a Calvin application. Similarly, a service provider can provide actors representing services of varying size and complexity. A useful feature of Calvin is that any 3rd party can provide an actor for a service by wrapping e. This lowers the barrier for Calvin uptake in the market, since there is no need to have a Calvin runtime on every constituent part in an application.

To create an application for the Calvin platform, we form a directed graph by connecting the ports of a number of actors.

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In the description of an actor, no assumption is made on how, or to what, it is connected; it simply processes data on its ports, unconcerned with anything beyond them. In the connect stage, information about how actors are connected is supplied in a straightforward fashion, best illustrated in a simple example.

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Here we use CalvinScript, a small, intuitive, declarative language to describe an application. A script, shown in Fig. The first three lines instantiate actors src, sum, and snk, and the last two lines connect the tick output port of src which reacts to some kind of event occurrence, be it radioactive decay or visitors to an exhibit to the input port in of the sum actor which will produce a cumulative sum of the occurrences , and the output port out of sum is connected to the snk actor, which will display the output in the given format, here plaintext.

While we now have a description of an application, we have neither specified anything regarding where the various actors should execute, nor how data should be transported between them. This is, in fact, handled dynamically during deployment of the application and continuously over its lifetime. These runtimes form a mesh network, wherein actors in a running application can migrate from one runtime to another, and how data is transported depends on the locations of actors.

Since actor migration in most cases is cheap and almost instantaneous, a simple way of deploying an application is to pass the application script to the closest accessible runtime. Once the runtime has instantiated and connected the actors locally, the distributed execution environment can move actors to any accessible runtime based on e. The trivial deployment algorithm - specifying for each actor where it should execute - is impractical, as a range of factors influence the performance of an application. Local factors, such as the load on a runtime, either caused by more actors being deployed on the same runtime, or a change in the workload of existing actors, could trigger a migration of actors, as could external factors, such as network congestion or hardware changes.

Left: The distributed execution environment red formed by a mesh of runtimes orange is seen by the applications blue as a single platform. Applications, themselves transparently distributed across the network, consist of actors green representing resources such as devices, compute nodes, and storage nodes. Right: The Calvin software stack consists of a platform dependent layer handling data transport and presenting the platform specific features like sensors, actuators, and storage in a uniform manner to the platform independent layer that is responsible for runtime coordination, migration decisions, and the API exposed to actors.

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Many aspects of the runtime are extensible through a plug-in mechanism. Once an application is running, it enters the managed phase. The distributed execution environment monitors the applications, handling e. Management also includes keeping track of resource usage, and allows for fine grained accounting, making it possible to, for example, charge customers for system usage with high resolution based on which actors are used, how frequently their actions fire, or how much data flows through the ports.

Deployment and management are key areas for future research. In the example in Fig.

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In the first application, a thermometer, with limited computational and storage capacity, comprises an actor that continuously reports the current temperature to an actor on a nearby runtime with more resources, e. This actor sends a log of reported temperatures, either at set intervals or when sufficient bandwidth becomes available, to an actor on a company wide server that keeps track of the temperature of a whole fleet of vehicles.

In the second application, a different actor residing on the thermometer runtime, one which only sends an output when the temperature rises above or falls below a set value, is acting as a watchdog. This actor is connected to one on the laptop by the driver, which issues an alert, for example in the form of a beep or a flashing icon on the screen, and forwards the alarm to an actor on a server which can log the issue, and schedule maintenance of the vehicle.

The high level architecture of Calvin is shown in Fig.

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Starting from the bottom, we have the hardware, the physical device or data center, and the operating system OS it exposes. Together, the hardware and OS constitute a platform for the runtime, and on top of this resides the platform dependent part of Calvin.

Here we handle communication between runtimes, and through a plug-in mechanism we support any kind of transport layer WiFi, BT, i2c and protocol, including efficient implementation of data transport between actors sharing the same runtime. This is also where inter-runtime communication and coordination takes place.

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Again, the actual algorithms controlling the behavior use a plug-in mechanism for flexibility and extensibility. Above the platform dependent level of the runtime we have the platform independent runtime layer, providing the interface towards the actors. This is where e. Since it has impact on things like latency and throughput, it too is extensible. On top of this, in the uppermost layers, we find the actors and the applications built from actors.

Not shown in this picture are the connections between actors as prescribed by the application. This is because the connections are logical, and where the actors reside is decided by the runtimes, and data transport is provided by the runtimes. It is important for the migration functionality that actors are unaware of their actual location.

They only know that their location requirements, if any, are satisfied. A Calvin runtime is inherently multi-tenant. Once an application is deployed and actors are migrated according to requirements and constraints, they may and will share runtimes with actors from other applications. The runtime can be configured to grant different access to resources depending on which application an actor is part of or rather the access level of the user running the application. There are a number of possible ways of implementing this, but at its simplest, a runtime may be divided into a "private" part with full access, and a "public" part with limited access.

There can also be restrictions on which actors are allowed on a runtime; simple ones, such as counters or timers, may be allowed, but more computationally expensive ones, e. Calvin is meant to be portable across languages and platforms, and it is our goal to support as many different platforms as possible, ranging from small sensor devices to data centers. To enable this, we emphasize the isolation between actors.

Only the format of data passed between ports is standardized, and how data is processed inside an actor is irrelevant. Similarly, how a runtime is implemented does not matter, as long as it adheres to the inter-runtime communication protocol and accepts the common set of control commands available to users and operators.

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  • To provide a basis for device manufacturers and 3rd party developers to port the Calvin runtime to various platforms, we will release a reference implementation of Calvin, written in python, under an Open Source license during the first half of The python implementation will run on a wide range of platforms, but to cover the tiniest devices a porting effort will be needed. Our intention is to form a community around Calvin in order to grow the number of supported platforms and build a library of reusable actors.

    In its basic form, an actor consists of ports, actions, and preconditions under which actions can fire. All communication with other actors is handled by unidirectional ports — in effect first in-first out queues. Apart from this there is also an initialization function and a priority list for the actions; should multiple actions be applicable at a given time, the priority list determines which one is actually applied. An action always has a condition, stating when the action can be applied in terms of the ports involved. A common situation is that there should be one or more tokens available on the in-ports, and space available for the result on the out-ports.

    Additionally, it is possible associate a guard with the action. A guard can be more complex than a condition; for efficiency, a condition is limited to superficial inspection of tokens - for example determining the number of tokens available - whereas a guard can inspect the data in the tokens, and prevent the action from firing depending on this data. In this section we outline the control software of a simple vending machine in order to demonstrate how easy it is to construct and reuse components.

    In this context, a component is a hierarchical construct of actors and their connections that can be used as if it was an actor, thereby hiding complexity. The controller, shown in Fig. The functionality of each of these components can, of course, be arbitrarily complex; it is non-trivial to build the electronics and software for determining which coins and bills to accept and which to reject, but this problem has already been solved for us, and we can write an actor, as a component, to handle this, and then use it in a component for handling money.

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    Left: A vending machine controller described in CalvinScript. Right: A CalvinScript component for handling money. Similarly, a card reader, either off-the-shelf with a "homemade" actor running on a separate runtime as interface, or one outfitted with a Calvin runtime and a choice of actors, can be included in the component. Figure 2 left depicts a sample money handling component.

    Although we have been diving into FBP in our work we have only scratched the surface. We don't have all of the answers as to how this work in practice in different contexts. We're inviting you to join us in exploring the possibilities. We're fully ready for curmudgeonly trolls to point out the ways that visual programming has come up short in the past. We are confident that our implementation addresses novel programming challenges in a well-designed way.

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