restful-stream

RESTful-stream.ts

This repository contains a small library function that allows to convert paginated RESTful data into a AsyncIterableIterator which chunks the pages into memory as soon as they are requested (and caches them) and serves the elements on the page one by one in a stream.

This is particularly useful if you want to consume paginated REST apis in e.g. a nodejs program. Given two methods:

function queryInitial(): Promise<PageType> {
    // use your asynchronous rest library of choice here
}
 
function querySubsequent(link: string): Promise<PageType> {
    // use your asynchronous rest library of choice here
}
 
function parseFn(obj: PageType): Promise<DataType[]> {
    // convert a JSON page into 
}

instead of writing

let page = await queryInitial();
while(page) {
    // do stuff with page
    const parsed = await parseFn(page);
 
    for(const elem of parsed) {
        // do stuff with element
    }
 
    if(page.nextLink && page.nextLink != null) {
        page = await querySubsequent(page.nextLink);
    } else {
        page = null;
    }
}

we can now write:

type PageType = ...;
type DataType = ...;
 
const initialLink: string = // url to the initial link
const ctrl: Control<PageType, DataType> = {
    hasNext(page: PageType) {
        return page.nextLink && page.nextLink != null;
    },
    next(page: PageType) {
        return querySubsequent(page.nextLink);
    },
    parse(page: PageType) {
        return parseFn(initialLink);
    }
};
 
for await(const elem of iterate(parse(ctrl, queryInitial()))) {
    // do stuff with element
}

We can even go further and define a utility method ctrlGen specific to our REST API:

type PageType = // generic PageType for our API
function ctrlGen<DataType>(initialLink: string, parseFn: (page: PageType) => Promise<DataType>) {
    const ctrl: Control<PageType, DataType> = {
        hasNext(page: PageType) {
            return page.nextLink && page.nextLink != null;
        },
        next(page: PageType) {
            return querySubsequent(page.initialLink);
        },
        parse(page: PageType) {
            return parseFn(initialLink);
        }
    };
};

This allows us to define the parse function for each Entity type, and can then use it:

function parse1(page: PageType): Promise<Type1> {
    const ret = ...;
    return ret;
}
 
function parse2(page: PageType): Promise<Type1> {
    const ret = ...;
    return ret;
}
 
for await(const elem1 of iterate(parse(ctrlGen('https://url.to.rest.api/type1', parse1), queryInitial1()))) {
    for await(const elem2 of iterate(parse(ctrlGen(`https://url.to.rest.api/${type1.id}/type2`, parse2), queryInitial2()))) {
        // do stuff
    }
}

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