## An app with callbacks { #an-app-with-callbacks }

Let's see all this with an example.

Imagine you develop an app that allows creating invoices.

These invoices will have an `id`, `title` (optional), `customer`, and `total`.

The user of your API (an external developer) will create an invoice in your API with a POST request.

Then your API will (let's imagine):

* Send the invoice to some customer of the external developer.

  /** The bytes consumed and acknowledged by the application. */
  val readBytes: WindowCounter = WindowCounter(streamId = 0)

  /** The total number of bytes produced by the application. */
  var writeBytesTotal = 0L
    private set

  /** The total number of bytes permitted to be produced according to `WINDOW_UPDATE` frames. */
  var writeBytesMaximum: Long = peerSettings.initialWindowSize.toLong()
    private set

        return state == ChannelState.ACTIVE || state == ChannelState.ESTABLISHED;
    }
    
    public double getErrorRate() {
        long total = requestsSent.get();
        if (total == 0) return 0.0;
        return (double) errors.get() / total;
    }
    
    public long getThroughput() {
        long duration = System.currentTimeMillis() - establishedTime;
        if (duration == 0) return 0;

  )

  constructor() : this(5, 5, TimeUnit.MINUTES)

  /** Returns the number of idle connections in the pool. */
  fun idleConnectionCount(): Int = delegate.idleConnectionCount()

  /** Returns total number of connections in the pool. */
  fun connectionCount(): Int = delegate.connectionCount()

  internal val connectionListener: ConnectionListener
    get() = delegate.connectionListener

import jcifs.internal.util.SMBUtil;

/**
 * Represents the FILE_FS_SIZE_INFORMATION structure used to query file system size information.
 * This structure provides details about the total allocation units, free allocation units,
 * sectors per allocation unit, and bytes per sector for a file system volume.
 */
public class FileFsSizeInformation implements AllocInfo {

public class EncryptContext extends LookupContext {
    public EncryptContext(InvokerRequest invokerRequest, EncryptOptions encryptOptions) {
        super(invokerRequest, true, encryptOptions);
    }

    public Map<String, Goal> goals;

    public List<AttributedString> header;
    public AttributedStyle style;
    public LineReader reader;

    public void addInHeader(String text) {
        addInHeader(AttributedStyle.DEFAULT, text);
    }

	ilmTransitionMissedImmediateTasksMD = NewCounterMD(transitionMissedImmediateTasks, "Number of missed immediate ILM transition tasks")
	ilmVersionsScannedMD                = NewCounterMD(versionsScanned, "Total number of object versions checked for ILM actions since server start")
)

// loadILMMetrics - `MetricsLoaderFn` for ILM metrics.
func loadILMMetrics(_ context.Context, m MetricValues, _ *metricsCache) error {

    }

    /**
     * Concatenates multiple byte arrays into a single array.
     */
    private static byte[] concat(byte[]... arrays) {
        int total = 0;
        for (byte[] a : arrays) {
            total += a.length;
        }
        byte[] res = new byte[total];
        int off = 0;
        for (byte[] a : arrays) {
            System.arraycopy(a, 0, res, off, a.length);
            off += a.length;
        }

// disk usage per path.
// The above means that any added/deleted fields are incompatible.
//
// The above means that any added/deleted fields are incompatible.
//
//msgp:tuple DiskInfo
type DiskInfo struct {
	Total      uint64
	Free       uint64
	Used       uint64
	UsedInodes uint64
	FreeInodes uint64
	Major      uint32
	Minor      uint32
	NRRequests uint64
	FSType     string
	RootDisk   bool
	Healing    bool

  // performed while the lock is held.

  /** The bytes consumed and acknowledged by the stream. */
  val readBytes: WindowCounter = WindowCounter(id)

  /** The total number of bytes produced by the application. */
  var writeBytesTotal = 0L
    internal set

  /** The total number of bytes permitted to be produced by incoming `WINDOW_UPDATE` frame. */
  var writeBytesMaximum: Long = connection.peerSettings.initialWindowSize.toLong()