javascript

EventReduce JavaScript Implementation

This is the javascript version of the EventReduce algorithm.

Installation

npm run install event-reduce-js --save

Usage

In the following we will use EventReduce together with minimongo as an example. You can apply the code to any other database.

1. First you need some QueryParams that can be used by EventReduce to analyze result-event combinations.

import {
    getSortFieldsOfQuery,
    ChangeEvent,
    calculateActionName,
    StateResolveFunctionInput,
    runAction
} from 'event-reduce-js';

// some stuff must not be coded by hand but is already in the minimongo library
import {
    compileDocumentSelector,
    compileSort
} from 'minimongo/src/selector';

// create this helper function that can be used for all queries
export function getQueryParamsByMongoQuery(query: MongoQuery): QueryParams<any> {
    const sort = query.sort ? query.sort : ['_id'];
    return {
        // primary key of the documents
        primaryKey: '_id',
        // a string[] with all fields that are used in the sorting
        sortFields: getSortFieldsOfQuery(query),
        skip: query.skip ? query.skip : undefined,
        limit: query.limit ? query.limit : undefined,
        // a function that returns true if the given document matches the query's selector
        queryMatcher: compileDocumentSelector(query.selector),
        // a function that can be used as comparator in Array.sort() (returns 1 or -1)
        sortComparator: compileSort(sort)
    };
}

const exampleQuery: MongoQuery = {
    selector: {
        age: {
            $gt: 18
        },
        gender: 'm'
    },
    limit: 10,
    sort: ['name', '_id']
};

const queryParams = getQueryParamsByMongoQuery(exampleQuery);

2. Now lets say you have an changeEvent from whatever changestream or observable your database provides. You also have the currentResults of the query.

// build the input
const input: StateResolveFunctionInput<DocumentType> = {
    // the changeEvent
    changeEvent,
    // queryParams from above
    queryParams,
    // array with previous results documents
    previousResults: currentResults,
    // key->document map with previous results indexed by primary key
    // (optional) improves performance
    keyDocumentMap: currentDocMap
};

// calculate the correct action name
const action = calculateActionName(input);

if (action === 'runFullQueryAgain') {
    /**
     * when EventReduce could not optimize the input,
     * we get the actionName 'runFullQueryAgain'
     * and run the query over the database again
     */
    currentResults = await implementation.getRawResults(query);
    // also refresh the key-document map
    currentDocMap.clear();
    currentResults.forEach(doc => currentDocMap.set(doc._id, doc));
} else {
    // event-reduce could optimize the event, run the action function
    runAction(
        action,
        queryParams,
        changeEvent,
        currentResults,
        currentDocMap
    );
}

// show new results
// notice that for performance resons,
// the functions of event-reduce will mutate the input variables
console.dir(currentResults);