2. Programming Model¶
2.1. Intent-based programming model¶
Currently the following two programing models are provided by FogFlow to support different types of workload patterns.
2.1.1. Service Topology¶
The first workload pattern is to trigger necessary processing flows to produce some output data only when the output data are requested by consumers. To define an IoT service based on this pattern, the service provider needs to define a service topology, which consists of a set of linked operators and each operator is annotated with a specific granularity. The granularity of an operator will be taken into account by FogFlow to decide how many task instances of such an operator should be instantiated based on the available data.
A service topology must be triggered explicitly by an intent object issued by a consumer or any application. The intent object defines when (based on priority) a service topology needs to be triggered and it can also optionally define a specific geo-scope to filter out data sources for applying the triggered processing logic. The Part of service topology that meets the intent conditions and has input data available, will be triggered.
2.1.2. Fog Function¶
The second workload pattern is designed for the scenario in which service designers do not a-priori know the exact sequence of stream processing steps. Instead they can define a fog function to include a specific operator for handling a given type of information. FogFlow can then create the graph of processing flows based on this description of all fog functions. Different from service topology, a fog function is a very simple topology with only one operator and it is triggered when its input data become available. As FogFlow can automatically chain different fog functions as well as allow more than one fog functions to handle a new data item, a constantly changing execution graph can be automatically triggered and managed by the FogFlow runtime as data arrive and disappear. From the design perspective, fog function is more flexible than service topology, because the overall processing logic of an IoT service can be easily changed over time by adding or removing fog functions when the service processing logic needs to modify for new business requirements. With the fog function programming model, FogFlow can support serverless computing for a cloud-edge based environment.
2.2. Context-driven service orchestration¶
In FogFlow each worker is an agent to perform the following two type of deployment actions assigned by Topology Master.
- start/terminate a task instance
- add/remove an input stream to an existing running task instance
For example, each worker takes the following steps to start a task instance.
- the worker asks its local docker engine to create a new container for running the new task instance with a specified listening port number
- the task instance starts to run and listening on the given port number to receive input data streams
- the worker sends a configuration object to the running task instance via its listening port
- the worker issues NGSI10 subscriptions on behalf of the running task instance
- the required input data streams flow into the running task instance for further processing via its listening port
- the running task instance processes the received input stream data and then generate its result
- the running task instance publishes the generated result as its output by sending NGSI10 updates