网上冲浪指南

The problem

In a workflow-style data processing system, if all the data sources are granted to complete, how do we ensure that the execution of a processing task can reach its complete state?

Terms

Name	Description
Data Stream	An observable stream of async data emitted by something.
SourceNode (Abbr. S)	A node that creates a granted-to-complete Data Stream for the next node in the Flow, which is usually the start point of a data processing task.
OperatorNode (Abbr. O)	A node that observes the Data Stream from the previous node in the Flow, and creates a granted-to-complete Data Stream for the next node; which is usually used to transform data, fetch data from API using the incoming data, etc.
WatcherNode (Abbr. W)	A node that observes Data Stream from the previous node; which is usually used to inspect the data processing state, e.g. logging, and reporting progress.
Flow	A list consists of SourceNode, OperatorNode, and WatcherNode, it must start with SourceNode and end with WatcherNode (S, O….O, W) or completely consist of OperatorNode(O, O, …. O).
Diagram	A set of Flows

Define the complement

S

The S creates a granted-to-complete data stream, when the data stream completes, S itself also reaches its complete state.

The data stream S created is consumed by the next node (e.g., O, W) in the Flow list.

type S<T> = DataStream<T>

O

In the Flow list, every incoming data from the previous node, makes O create a new granted-to-complete data stream for the next node. Therefore, O is state less, it doesn’t have a complete state, it propagates the complete state from the previous node to the next node.

type O<A, B> = DataStream<A> => DataStream<B>

As you can see, the O is a function that takes data stream in and out.

W

In the Flow list, W consumes data from the previous node, when the previous node comes to the complete state, the W reaches to its complete state.

type W<T> = DataStream<T> => void

Flow

Stateful flow (S, O, …. O, W)

If flow starts with a S and ends with a W, it is a stateful flow.

When the last W, comes to a complete state, the Flow reaches to complete state.

The complete state is propagated from the beginning S to the last W via many Os.

type StatefullFlow<A, B> = {
  source: S<A>;
  operators: O[];
  watcher: W<B>;
}

Stateless flow (O, O, …. O, O)

The stateless flow behaves like an O, it creates a granted-to-complete data stream based on the incoming data.

type StatelessFlow = O[];

Diagram

When every flow in the Diagram completes, the diagram reaches a complete state, which means the current running data process task is completed.

To have a granted-to-complete Diagram, we need to make sure:

1. All S, O in the diagram are correctly implemented to create a granted-to-complete data stream, which can be verified by doing code reviews and writing unit tests for these nodes’ implementation
2. A method to prevent an Infinite Flow

The Infinite Flow

For the OperatorNodes, we can have a compose function, it connects the 2 OperatorNode and returns a newly composed OperatorNode.

function compose(op1: O<A, B>, op2: O<B,C>): O<A, C>;

This enables us to convert any Stateless Flow into an OperatorNode.

We can also have a delegate function for OperatorNode:

function delegate(operatorRef: ()=> O<A, B>): O<A, B>;

It creates a new O, which creates the data stream using the returned O of operatorRef when it receives incoming data from the previous node.

With the help of compose and delegate, it is possible to reuse stateless flow in the diagram:

Since the reuse is based on reference, it is possible to make reference circles in the diagram, which may lead to infinite processing.

Branch

To avoid this problem, we need to add the branching ability to OperatorNode.

function branch(selector: (data: T) => O<T,T>): O<T, T>

branch creates a new O, when data arrives at O, it sends the data to the O’ returned by selector(data), then pipe the output of O’ to the next node.

If there is no need to do branching for some data, the selector function can return a identity O:

function identity: O<T, T>

It just outputs what it received.

With the help of branch and identity, we can now add an exit branch for a circular flow, but the exit branch can not be verified until running the data processing task.

To avoid infinite processing at runtime, we need to implement a delegation depth detection at runtime.

Delegation Depth Detection

The delegation depth actually means “How many times does the data pass through a DeledateNode“, it simply comes to an idea that attaching a delegateCount property to the data inside the data stream. When data passed through a DelegateNode created by delegate, the delegateCount increases.

When the delegateCount reaches a limit, let’s say, 200k, the DelegateNode should drop the data and write a warning log.

Answer to the beginning question

1. Make sure all S, and O are correctly implemented to create a granted-to-complete data stream
2. Use compose, delegate to allow circular reference in the Diagram
3. Use branch to add an exit branch to the circular flow
4. Use delegate to avoid infinite-traveling data in the flow at runtime