## Una aplicación con callbacks

Veamos todo esto con un ejemplo.

Imagina que desarrollas una aplicación que permite crear facturas.

Estas facturas tendrán un `id`, `title` (opcional), `customer`, y `total`.

El usuario de tu API (un desarrollador externo) creará una factura en tu API con un request POST.

Luego tu API (imaginemos):

* Enviará la factura a algún cliente del desarrollador externo.
* Recogerá el dinero.

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 {

		b.uint32(0)                      // number of the disk with the start of the central directory
		b.uint64(records)                // total number of entries in the central directory on this disk
		b.uint64(records)                // total number of entries in the central directory
		b.uint64(size)                   // size of the central directory

    }

    /**
     * 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;
        }

    const val MAX_FRAME_SIZE = 5

    /** HTTP/2: Advisory only. Size in bytes of the largest header list the sender will accept. */
    const val MAX_HEADER_LIST_SIZE = 6

    /** Total number of settings. */
    const val COUNT = 10
  }

  )

  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

  /** True if this web socket failed and the listener has been notified. */
  private var failed = false

  /** Total number of pings sent by this web socket. */
  private var sentPingCount = 0

  /** Total number of pings received by this web socket. */
  private var receivedPingCount = 0

  /** Total number of pongs received by this web socket. */
  private var receivedPongCount = 0

    
    public double getCacheHitRatio() {
        long hits = cacheHits.get();
        long misses = cacheMisses.get();
        long total = hits + misses;
        
        if (total == 0) return 0.0;
        return (double) hits / total;
    }
    
    public void recordCacheHit() { cacheHits.incrementAndGet(); }
    public void recordCacheMiss() { cacheMisses.incrementAndGet(); }

        operationErrors.incrementAndGet();
    }
    
    public double getErrorRate() {
        long total = rdmaReads.get() + rdmaWrites.get() + rdmaSends.get() + rdmaReceives.get();
        if (total == 0) return 0.0;
        return (double) operationErrors.get() / total;
    }
    
    // Getters for all statistics...
}
```

## 9. Error Handling and Fallback

    // GET /api/admin/log/files
    @Execute
    public JsonResponse<ApiResult> files() {
        final List<Map<String, Object>> list = getLogFileItems();
        return asJson(new ApiResult.ApiLogFilesResponse().files(list).total(list.size()).status(ApiResult.Status.OK).result());
    }

    /**
     * Downloads a specific log file by ID.
     *
     * @param id the base64-encoded filename of the log file to download