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    /**
     * Buffer to hold request entries that should be persisted into the
     * destination.
     * <p>
     * A request entry contain all relevant details to make a call to the
     * destination. Eg, for Kinesis Data Streams a request entry contains the
     * payload and partition key.
     * <p>
     * It seems more natural to buffer InputT, ie, the events that should be
     * persisted, rather than RequestEntryT. However, in practice, the response
     * of a failed request call can make it very hard, if not impossible, to
     * reconstruct the original event. It is much easier, to just construct a
     * new (retry) request entry from the response and add that back to the
     * queue for later retry.
     */
    private final 
    Deque<RequestEntryT> bufferedRequests = new ConcurrentLinkedDeque<>();


    /**
     * Tracks all pending async calls that have been executed since the last
     * checkpoint. Calls that already completed (successfully or unsuccessfully)
     * are automatically removed from the queue. Any request entry that was not
     * successfully persisted need to be handled and retried by the logic in
     * {@code submitRequestsToApi}.
     * <p>
     * There is a limit on the number of concurrent (async) requests that can be
     * handled by the client library. This limit must be checked before issunin
     * <p>
     * To complete a checkpoint, we need to make sure that no requests are in
     * flight, as they may fail, which could then lead to data loss.
     */
    private 
    Queue<CompletableFuture<?>> inFlightRequests = new ConcurrentLinkedQueue<>();


    /**
     * Signals if enough RequestEntryTs have been buffered according to the user
     * specified buffering hints to make a request against the destination. This
     * functionality will be added to the sink interface by means of an
     * additional FLIP.
     *
     * @return a future that will be completed once there is are record available   
     * to make a request against the destination
     */
    public CompletableFuture<Void> isAvailable() {
        ...
    }


    @Override
    public void write(InputT element, Context context) throws IOException {
        bufferedRequests.offerLast(elementConverter.apply(element, context));
    }


    /**
     * A request or single request entries of a request may fail, eg, because of
     * network issues or service side throttling. All request entries that
     * failed with transient failures need to be re-queued with this method so
     * that aren't lost and can be retried later.
     * <p>
     * Request entries that are causing the same error in a reproducible manner,
     * eg, ill-formed request entries, must not be re-queued but the error needs
     * to be handled in the logic of {@code submitRequestEntries}. Otherwise
     * these request entries will be retried indefinitely, always causing the
     * same error.
     */
    protected void requeueFailedRequestEntry(RequestEntryT requestEntry) {
        bufferedRequests.offerFirst(requestEntry);
    }


    /**
     * In-flight requests may fail, but they will be retried if the sink is
     * still healthy.
     * <p>
     * To not lose any requests, there cannot be any outstanding in-flight
     * requests when a commit is initialized. To this end, all in-flight
     * requests need to be completed as part of the pre commit.
     */
    @Override
    public List<Collection<CompletableFuture<?>>> prepareCommit(boolean flush) throws IOException {

        // reuse current inFlightRequests as commitable and create an empty queue 
        // to avoid copy and clearing
        List<Collection<CompletableFuture<?>>> committable = Collections.singletonList(inFlightRequests);

        // all in-flight requests are handled by the AsyncSinkCommiter and new 
        // elements cannot be added to the queue during a commit, so it's save to 
        // create a new queue
        inFlightRequests = new ConcurrentLinkedQueue<>();

        return committable;
    }



    /**
     * All in-flight requests have been completed, but there may still be
     * request entries in the internal buffer that are yet to be sent to the
     * endpoint. These request entries are stored in the snapshot state so that
     * they don't get lost in case of a failure/restart of the application.
     */
    @Override
    public List<Collection<RequestEntryT>> snapshotState() throws IOException {
        return Collections.singletonList(bufferedRequests);
    }