public static Observable<String> generateJson(long id, int delay, int itemSize, int numItems) {
        return Observable.create((Subscriber<? super String> subscriber) -> {
            Worker worker = Schedulers.computation().createWorker();
            subscriber.add(worker);
            worker.schedule(() -> {
                try {
                    StringWriter jsonString = new StringWriter();
                    JsonGenerator json = jsonFactory.createJsonGenerator(jsonString);

                    json.writeStartObject();

                    // manipulate the ID such that we can know the response is from the server (client will know the logic)
                    long responseKey = getResponseKey(id);

                    json.writeNumberField("responseKey", responseKey);

                    json.writeNumberField("delay", delay);
                    if (itemSize > MAX_ITEM_LENGTH) {
                        throw new IllegalArgumentException("itemSize can not be larger than: " + MAX_ITEM_LENGTH);
                    }
                    json.writeNumberField("itemSize", itemSize);
                    json.writeNumberField("numItems", numItems);

                    json.writeArrayFieldStart("items");
                    for (int i = 0; i < numItems; i++) {
                        json.writeString(RAW_ITEM_LONG.substring(0, itemSize));
                    }
                    json.writeEndArray();
                    json.writeEndObject();
                    json.close();

                    subscriber.onNext(jsonString.toString());
                    subscriber.onCompleted();
                } catch (Exception e) {
                    subscriber.onError(e);
                }
            }, delay, TimeUnit.MILLISECONDS);
        });
    }