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.
* Collect the money.
* Send a notification back to the API user (the external developer).
    * This will be done by sending a POST request (from *your API*) to some *external API* provided by that external developer (this is the "callback").

    if (allMustSucceed) {
      // We need fail fast, so we have to keep track of which future failed so we can propagate
      // the exception immediately

      // Register a listener on each Future in the list to update the state of this future.
      // Note that if all the futures on the list are done prior to completing this loop, the last

	var skipLength int64
	var cumulativeActualSize int64
	var firstPartIdx int
	for i, part := range oi.Parts {
		cumulativeActualSize += part.ActualSize
		if cumulativeActualSize <= offset {
			compressedOffset += part.Size
		} else {
			firstPartIdx = i
			skipLength = cumulativeActualSize - part.ActualSize
			break
		}
	}
	partSkip = offset - skipLength

	defer t.Stop()
	for {
		select {
		case <-m.ctx.Done():
			return
		case <-t.C:
			last := time.Since(time.Unix(atomic.LoadInt64(&m.LastPing), 0))
			if last > 4*m.clientPingInterval {
				gridLogIf(m.ctx, fmt.Errorf("canceling remote connection %s not seen for %v", m.parent, last))
				m.close()
				return
			}
		}
	}
}

// Copyright (c) 2015-2021 MinIO, Inc.
//
// This file is part of MinIO Object Storage stack
//
// This program is free software: you can redistribute it and/or modify
// it under the terms of the GNU Affero General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// This program is distributed in the hope that it will be useful

// Copyright (c) 2015-2021 MinIO, Inc.
//
// This file is part of MinIO Object Storage stack
//
// This program is free software: you can redistribute it and/or modify
// it under the terms of the GNU Affero General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// This program is distributed in the hope that it will be useful

			return nil, err
		}

		// Pass should be an integer.
		if _, err := strconv.Atoi(fields[5]); err != nil {
			return nil, err
		}

		mounts = append(mounts, mountInfo{
			Device:  fields[0],
			Path:    fields[1],
			FSType:  fields[2],
			Options: strings.Split(fields[3], ","),
			Freq:    fields[4],
			Pass:    fields[5],
		})
	}
	return mounts, nil

  - apiserver_flowcontrol_current_r: R(the time of the last change in state of the queues)
  - apiserver_flowcontrol_dispatch_r: R(the time of the latest request dispatch)
  - apiserver_flowcontrol_latest_s: S(the request last dispatched) = R(when that request starts executing in the virtual world)

      byte[] msg = new byte[i];
      for (int j = 0; j < i; ++j) {
        msg[j] = (byte) j;
      }
      assertSip(msg, EXPECTED[i]);
    }
  }

  // This test data comes from "SipHash: a fast short-input PRF", "Appendix A: Test values".
  // It can be downloaded here: https://131002.net/siphash/siphash.pdf
  public void test15ByteStringFromSipHashPaper() {
    byte[] message =
        new byte[] {

      byte[] msg = new byte[i];
      for (int j = 0; j < i; ++j) {
        msg[j] = (byte) j;
      }
      assertSip(msg, EXPECTED[i]);
    }
  }

  // This test data comes from "SipHash: a fast short-input PRF", "Appendix A: Test values".
  // It can be downloaded here: https://131002.net/siphash/siphash.pdf
  public void test15ByteStringFromSipHashPaper() {
    byte[] message =
        new byte[] {