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PatternScatter-gather broadcasts a message to multiple recipients and re-aggregates the responses back into a single message.
How Ballerina helps

Ballerina has a lightweight concurrency model with built-in syntax support. This helps to send messages parallelly and to wait on aggregation. Query expressions are convenient for transforming, ordering, and filtering the aggregated data.

Scatter-GatherAggregatorMessage ChannelMessage EndpointMessage RouterMessage
import ballerina/http;

type CarRental record {|
    string carType;
    string price;
    string resultLink;
    string vendor;

type AvisResponse record {
    record {
    }[] result;

final http:Client hotwireEP = check new ("");
final http:Client avisEP = check new ("");

service /api/v1 on new http:Listener(8080) {

    resource function get vehicles(string dropOffDateTime, string dropOffLocation, string pickupDateTime,
            string pickupLocation) returns map<CarRental[]>|error {

        worker hotwire returns CarRental[]|error {
            http:Response hotwireResponse = check hotwireEP->/v1/search/car.get(pickup = pickupLocation,
                dest = dropOffLocation, startDate = pickupDateTime, endDate = dropOffDateTime
            return transformHotwireResponse(check hotwireResponse.getXmlPayload());

        worker avis returns CarRental[]|error {
            AvisResponse avisResponse = check avisEP->/cars/catalog/v1/vehicles/rates.get(brand = "Avis",
                country_code = "US", dropOff_date = dropOffDateTime, dropoff_location = dropOffLocation,
                pickup_date = pickupDateTime, pickup_location = pickupLocation
            return transformAvisResponse(avisResponse);

        var responses = wait {hotwire, avis};
        return map from var [vendor, carRental] in responses.entries()
            where carRental !is error
            select [vendor, carRental];

function transformHotwireResponse(xml response) returns CarRental[] {
    return from xml item in response/<Response>/<Car>
        select {
            carType: (item/<CarType>).data(),
            price: (item/<Price>).data(),
            resultLink: (item/<ResultLink>).data(),
            vendor: "Hotwire"

function transformAvisResponse(AvisResponse response) returns CarRental[] {
    return from var {carType, price, resultLink, vendor} in response.result
        select {carType, price, resultLink, vendor};