-
Notifications
You must be signed in to change notification settings - Fork 2
/
Copy pathLecture 8 - CS50's Web Programming with Python and JavaScript.htm
1395 lines (1145 loc) · 72.1 KB
/
Lecture 8 - CS50's Web Programming with Python and JavaScript.htm
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
<!DOCTYPE html>
<html class="wf-ptsans-n4-active wf-ptsans-n7-active wf-active" lang="en-us"><head>
<meta http-equiv="content-type" content="text/html; charset=UTF-8">
<meta charset="utf-8">
<meta name="viewport" content="initial-scale=1, width=device-width"><meta property="og:description" content="This course picks up where Harvard University's CS50 leaves off, diving more deeply into the design and implementation of web apps with Python, JavaScript, and SQL using frameworks like Django, React, and Bootstrap. Topics include database design, scalability, security, and user experience. Through hands-on projects, students learn to write and use APIs, create interactive UIs, and leverage cloud services like GitHub and Heroku. By semester’s end, students emerge with knowledge and experience in principles, languages, and tools that empower them to design and deploy applications on the Internet.">
<meta property="og:image" content="https://img.youtube.com/vi/24Kf3v7kZyE/maxresdefault.jpg"><meta property="og:title" content="Lecture 8 - CS50's Web Programming with Python and JavaScript">
<link href="https://cs50.harvard.edu/web/2020/favicon.ico?1645555620" rel="icon">
<!-- https://fonts.google.com/specimen/PT+Sans?query=PT+Sans&selection.family=PT+Sans:ital,wght@0,400;0,700;1,400;1,700 -->
<script src="Lecture%208%20-%20CS50's%20Web%20Programming%20with%20Python%20and%20JavaScript_files/webfont.js"></script>
<!-- http://getbootstrap.com/docs/4.6/getting-started/introduction/ -->
<script src="Lecture%208%20-%20CS50's%20Web%20Programming%20with%20Python%20and%20JavaScript_files/jquery.js"></script>
<script src="Lecture%208%20-%20CS50's%20Web%20Programming%20with%20Python%20and%20JavaScript_files/bootstrap.js"></script>
<!-- https://bootstrap-table.com/docs/getting-started/introduction/ -->
<link href="Lecture%208%20-%20CS50's%20Web%20Programming%20with%20Python%20and%20JavaScript_files/bootstrap-table.css" rel="stylesheet">
<script src="Lecture%208%20-%20CS50's%20Web%20Programming%20with%20Python%20and%20JavaScript_files/bootstrap-table.js"></script>
<script src="Lecture%208%20-%20CS50's%20Web%20Programming%20with%20Python%20and%20JavaScript_files/bootstrap-table-mobile.js"></script>
<!-- https://fontawesome.com/how-to-use/on-the-web/referencing-icons/basic-use -->
<link href="Lecture%208%20-%20CS50's%20Web%20Programming%20with%20Python%20and%20JavaScript_files/all.css" rel="stylesheet">
<!-- https://moment.github.io/luxon/ -->
<script src="Lecture%208%20-%20CS50's%20Web%20Programming%20with%20Python%20and%20JavaScript_files/luxon.js"></script>
<!-- http://docs.mathjax.org/ -->
<!-- https://www.jsdelivr.com/package/npm/mathjax?path=es5 -->
<script crossorigin="anonymous" integrity="sha256-+nfSJ1LiSvANSK3wvi6FjM+oFwoIQvURU1frozp5z6o=" src="Lecture%208%20-%20CS50's%20Web%20Programming%20with%20Python%20and%20JavaScript_files/tex-chtml.js"></script><style type="text/css">.CtxtMenu_InfoClose { top:.2em; right:.2em;}
.CtxtMenu_InfoContent { overflow:auto; text-align:left; font-size:80%; padding:.4em .6em; border:1px inset; margin:1em 0px; max-height:20em; max-width:30em; background-color:#EEEEEE; white-space:normal;}
.CtxtMenu_Info.CtxtMenu_MousePost {outline:none;}
.CtxtMenu_Info { position:fixed; left:50%; width:auto; text-align:center; border:3px outset; padding:1em 2em; background-color:#DDDDDD; color:black; cursor:default; font-family:message-box; font-size:120%; font-style:normal; text-indent:0; text-transform:none; line-height:normal; letter-spacing:normal; word-spacing:normal; word-wrap:normal; white-space:nowrap; float:none; z-index:201; border-radius: 15px; /* Opera 10.5 and IE9 */ -webkit-border-radius:15px; /* Safari and Chrome */ -moz-border-radius:15px; /* Firefox */ -khtml-border-radius:15px; /* Konqueror */ box-shadow:0px 10px 20px #808080; /* Opera 10.5 and IE9 */ -webkit-box-shadow:0px 10px 20px #808080; /* Safari 3 & Chrome */ -moz-box-shadow:0px 10px 20px #808080; /* Forefox 3.5 */ -khtml-box-shadow:0px 10px 20px #808080; /* Konqueror */ filter:progid:DXImageTransform.Microsoft.dropshadow(OffX=2, OffY=2, Color="gray", Positive="true"); /* IE */}
</style><style type="text/css">.CtxtMenu_MenuClose { position:absolute; cursor:pointer; display:inline-block; border:2px solid #AAA; border-radius:18px; -webkit-border-radius: 18px; /* Safari and Chrome */ -moz-border-radius: 18px; /* Firefox */ -khtml-border-radius: 18px; /* Konqueror */ font-family: "Courier New", Courier; font-size:24px; color:#F0F0F0}
.CtxtMenu_MenuClose span { display:block; background-color:#AAA; border:1.5px solid; border-radius:18px; -webkit-border-radius: 18px; /* Safari and Chrome */ -moz-border-radius: 18px; /* Firefox */ -khtml-border-radius: 18px; /* Konqueror */ line-height:0; padding:8px 0 6px /* may need to be browser-specific */}
.CtxtMenu_MenuClose:hover { color:white!important; border:2px solid #CCC!important}
.CtxtMenu_MenuClose:hover span { background-color:#CCC!important}
.CtxtMenu_MenuClose:hover:focus { outline:none}
</style><style type="text/css">.CtxtMenu_Menu { position:absolute; background-color:white; color:black; width:auto; padding:5px 0px; border:1px solid #CCCCCC; margin:0; cursor:default; font: menu; text-align:left; text-indent:0; text-transform:none; line-height:normal; letter-spacing:normal; word-spacing:normal; word-wrap:normal; white-space:nowrap; float:none; z-index:201; border-radius: 5px; /* Opera 10.5 and IE9 */ -webkit-border-radius: 5px; /* Safari and Chrome */ -moz-border-radius: 5px; /* Firefox */ -khtml-border-radius: 5px; /* Konqueror */ box-shadow:0px 10px 20px #808080; /* Opera 10.5 and IE9 */ -webkit-box-shadow:0px 10px 20px #808080; /* Safari 3 & Chrome */ -moz-box-shadow:0px 10px 20px #808080; /* Forefox 3.5 */ -khtml-box-shadow:0px 10px 20px #808080; /* Konqueror */}
.CtxtMenu_MenuItem { padding: 1px 2em; background:transparent;}
.CtxtMenu_MenuArrow { position:absolute; right:.5em; padding-top:.25em; color:#666666; font-family: null; font-size: .75em}
.CtxtMenu_MenuActive .CtxtMenu_MenuArrow {color:white}
.CtxtMenu_MenuArrow.CtxtMenu_RTL {left:.5em; right:auto}
.CtxtMenu_MenuCheck { position:absolute; left:.7em; font-family: null}
.CtxtMenu_MenuCheck.CtxtMenu_RTL { right:.7em; left:auto }
.CtxtMenu_MenuRadioCheck { position:absolute; left: .7em;}
.CtxtMenu_MenuRadioCheck.CtxtMenu_RTL { right: .7em; left:auto}
.CtxtMenu_MenuInputBox { padding-left: 1em; right:.5em; color:#666666; font-family: null;}
.CtxtMenu_MenuInputBox.CtxtMenu_RTL { left: .1em;}
.CtxtMenu_MenuComboBox { left:.1em; padding-bottom:.5em;}
.CtxtMenu_MenuSlider { left: .1em;}
.CtxtMenu_SliderValue { position:absolute; right:.1em; padding-top:.25em; color:#333333; font-size: .75em}
.CtxtMenu_SliderBar { outline: none; background: #d3d3d3}
.CtxtMenu_MenuLabel { padding: 1px 2em 3px 1.33em; font-style:italic}
.CtxtMenu_MenuRule { border-top: 1px solid #DDDDDD; margin: 4px 3px;}
.CtxtMenu_MenuDisabled { color:GrayText}
.CtxtMenu_MenuActive { background-color: #606872; color: white;}
.CtxtMenu_MenuDisabled:focus { background-color: #E8E8E8}
.CtxtMenu_MenuLabel:focus { background-color: #E8E8E8}
.CtxtMenu_ContextMenu:focus { outline:none}
.CtxtMenu_ContextMenu .CtxtMenu_MenuItem:focus { outline:none}
.CtxtMenu_SelectionMenu { position:relative; float:left; border-bottom: none; -webkit-box-shadow:none; -webkit-border-radius:0px; }
.CtxtMenu_SelectionItem { padding-right: 1em;}
.CtxtMenu_Selection { right: 40%; width:50%; }
.CtxtMenu_SelectionBox { padding: 0em; max-height:20em; max-width: none; background-color:#FFFFFF;}
.CtxtMenu_SelectionDivider { clear: both; border-top: 2px solid #000000;}
.CtxtMenu_Menu .CtxtMenu_MenuClose { top:-10px; left:-10px}
</style>
<!-- https://github.com/verlok/vanilla-lazyload -->
<script src="Lecture%208%20-%20CS50's%20Web%20Programming%20with%20Python%20and%20JavaScript_files/intersection-observer.js"></script>
<script src="Lecture%208%20-%20CS50's%20Web%20Programming%20with%20Python%20and%20JavaScript_files/lazyload.js"></script>
<!-- https://github.com/davidjbradshaw/iframe-resizer -->
<!-- https://www.jsdelivr.com/package/npm/iframe-resizer?path=js -->
<script crossorigin="anonymous" integrity="sha256-S8f0Q/V9VcfrqYgWo9EFS9zuDMdPTBMC+CBW0RjxQbs=" src="Lecture%208%20-%20CS50's%20Web%20Programming%20with%20Python%20and%20JavaScript_files/iframeResizer.js"></script>
<!-- https://github.com/scratchblocks/scratchblocks/releases -->
<script src="Lecture%208%20-%20CS50's%20Web%20Programming%20with%20Python%20and%20JavaScript_files/scratchblocks.js"></script><style><![CDATA[ .sb-label {
font-family: Lucida Grande, Verdana, Arial, DejaVu Sans, sans-serif;
font-weight: bold;
fill: #fff;
font-size: 10px;
word-spacing: +1px;
}
.sb-obsolete { fill: #d42828; }
.sb-motion { fill: #4a6cd4; }
.sb-looks { fill: #8a55d7; }
.sb-sound { fill: #bb42c3; }
.sb-pen { fill: #0e9a6c; }
.sb-events { fill: #c88330; }
.sb-control { fill: #e1a91a; }
.sb-sensing { fill: #2ca5e2; }
.sb-operators { fill: #5cb712; }
.sb-variables { fill: #ee7d16; }
.sb-list { fill: #cc5b22 }
.sb-custom { fill: #632d99; }
.sb-custom-arg { fill: #5947b1; }
.sb-extension { fill: #4b4a60; }
.sb-grey { fill: #969696; }
.sb-bevel {
filter: url(#bevelFilter);
}
.sb-input {
filter: url(#inputBevelFilter);
}
.sb-input-number,
.sb-input-string,
.sb-input-number-dropdown {
fill: #fff;
}
.sb-literal-number,
.sb-literal-string,
.sb-literal-number-dropdown,
.sb-literal-dropdown {
font-weight: normal;
font-size: 9px;
word-spacing: 0;
}
.sb-literal-number,
.sb-literal-string,
.sb-literal-number-dropdown {
fill: #000;
}
.sb-darker {
filter: url(#inputDarkFilter);
}
.sb-outline {
stroke: #fff;
stroke-opacity: 0.2;
stroke-width: 2;
fill: none;
}
.sb-define-hat-cap {
stroke: #632d99;
stroke-width: 1;
fill: #8e2ec2;
}
.sb-comment {
fill: #ffffa5;
stroke: #d0d1d2;
stroke-width: 1;
}
.sb-comment-line {
fill: #ffff80;
}
.sb-comment-label {
font-family: Helevetica, Arial, DejaVu Sans, sans-serif;
font-weight: bold;
fill: #5c5d5f;
word-spacing: 0;
font-size: 12px;
}
.sb-diff {
fill: none;
stroke: #000;
}
.sb-diff-ins {
stroke-width: 2px;
}
.sb-diff-del {
stroke-width: 3px;
}
]]></style><style><![CDATA[ .sb3-label {
font: 500 12pt Helevetica Neue, Helvetica, sans-serif;
fill: #fff;
word-spacing: +1pt;
}
.sb3-motion { fill: #4c97ff; stroke: #3373cc; }
.sb3-motion-alt { fill: #4280d7; }
.sb3-motion-dark { fill: #4c97ff; }
.sb3-looks { fill: #9966ff; stroke: #774dcb; }
.sb3-looks-alt { fill: #855cd6; }
.sb3-looks-dark { fill: #bd42bd; }
.sb3-sound { fill: #cf63cf; stroke: #bd42bd; }
.sb3-sound-alt { fill: #c94fc9; }
.sb3-sound-dark { fill: #bd42bd; }
.sb3-control { fill: #ffab19; stroke: #cf8b17; }
.sb3-control-alt { fill: #ec9c13; }
.sb3-control-dark { fill: #cf8b17; }
.sb3-events { fill: #ffbf00; stroke: #cc9900; }
.sb3-events-alt { fill: #e6ac00; }
.sb3-events-dark { fill: #cc9900; }
.sb3-sensing { fill: #5cb1d6; stroke: #2e8eb8; }
.sb3-sensing-alt { fill: #47a8d1; }
.sb3-sensing-dark { fill: #2e8eb8; }
.sb3-operators { fill: #59c059; stroke: #389438; }
.sb3-operators-alt { fill: #46b946; }
.sb3-operators-dark { fill: #389438; }
.sb3-variables { fill: #ff8c1a; stroke: #db6e00; }
.sb3-variables-alt { fill: #ff8000; }
.sb3-variables-dark { fill: #db6e00; }
.sb3-list { fill: #ff661a; stroke: #e64d00; }
.sb3-list-alt { fill: #ff5500; }
.sb3-list-dark { fill: #e64d00; }
.sb3-custom { fill: #ff6680; stroke: #ff3355; }
.sb3-custom-alt { fill: #ff4d6a; }
.sb3-custom-dark { fill: #ff3355; }
.sb3-custom-arg { fill: #ff6680; stroke: #ff3355; }
/* extension blocks, e.g. pen */
.sb3-extension { fill: #0fbd8c; stroke: #0b8e69; }
.sb3-extension-alt { fill: #0da57a; }
.sb3-extension-line { stroke: #0da57a; }
.sb3-extension-dark { fill: #0b8e69; }
/* obsolete colors: chosen by hand, indicates invalid blocks */
.sb3-obsolete { fill: #ed4242; stroke: #ca2b2b; }
.sb3-obsolete-alt { fill: #db3333; }
.sb3-obsolete-dark { fill: #ca2b2b; }
/* grey: special color from the Scratch 3.0 design mockups */
.sb3-grey { fill: #bfbfbf; stroke: #909090; }
.sb3-grey-alt { fill: #b2b2b2; }
.sb3-grey-dark { fill: #909090; }
.sb3-input-color {
stroke: #fff;
}
.sb3-input-number,
.sb3-input-string {
fill: #fff;
}
.sb3-literal-number,
.sb3-literal-string,
.sb3-literal-number-dropdown,
.sb3-literal-dropdown {
word-spacing: 0;
}
.sb3-literal-number,
.sb3-literal-string {
fill: #575e75;
}
.sb3-comment {
fill: #ffffa5;
stroke: #d0d1d2;
stroke-width: 1;
}
.sb3-comment-line {
fill: #ffff80;
}
.sb3-comment-label {
font: 400 12pt Helevetica Neue, Helvetica, sans-serif;
fill: #000;
word-spacing: 0;
}
.sb3-diff {
fill: none;
stroke: #000;
}
.sb3-diff-ins {
stroke-width: 2px;
}
.sb3-diff-del {
stroke-width: 3px;
}
]]></style>
<link href="Lecture%208%20-%20CS50's%20Web%20Programming%20with%20Python%20and%20JavaScript_files/page.css" rel="stylesheet">
<link rel="stylesheet" href="Lecture%208%20-%20CS50's%20Web%20Programming%20with%20Python%20and%20JavaScript_files/css.css" media="all"><script src="Lecture%208%20-%20CS50's%20Web%20Programming%20with%20Python%20and%20JavaScript_files/jekyll-theme-cs50.js"></script>
<script>
window.CS50 = {
local: {"day":"numeric","hour":"numeric","minute":"numeric","month":"long","timeZoneName":"short","weekday":"long","year":"numeric"},
locale: "en",
tz: "America/New_York"
};
</script>
<title>Lecture 8 - CS50's Web Programming with Python and JavaScript</title>
<style id="MJX-CHTML-styles">
mjx-container[jax="CHTML"] {
line-height: 0;
}
mjx-container [space="1"] {
margin-left: .111em;
}
mjx-container [space="2"] {
margin-left: .167em;
}
mjx-container [space="3"] {
margin-left: .222em;
}
mjx-container [space="4"] {
margin-left: .278em;
}
mjx-container [space="5"] {
margin-left: .333em;
}
mjx-container [rspace="1"] {
margin-right: .111em;
}
mjx-container [rspace="2"] {
margin-right: .167em;
}
mjx-container [rspace="3"] {
margin-right: .222em;
}
mjx-container [rspace="4"] {
margin-right: .278em;
}
mjx-container [rspace="5"] {
margin-right: .333em;
}
mjx-container [size="s"] {
font-size: 70.7%;
}
mjx-container [size="ss"] {
font-size: 50%;
}
mjx-container [size="Tn"] {
font-size: 60%;
}
mjx-container [size="sm"] {
font-size: 85%;
}
mjx-container [size="lg"] {
font-size: 120%;
}
mjx-container [size="Lg"] {
font-size: 144%;
}
mjx-container [size="LG"] {
font-size: 173%;
}
mjx-container [size="hg"] {
font-size: 207%;
}
mjx-container [size="HG"] {
font-size: 249%;
}
mjx-container [width="full"] {
width: 100%;
}
mjx-box {
display: inline-block;
}
mjx-block {
display: block;
}
mjx-itable {
display: inline-table;
}
mjx-row {
display: table-row;
}
mjx-row > * {
display: table-cell;
}
mjx-mtext {
display: inline-block;
}
mjx-mstyle {
display: inline-block;
}
mjx-merror {
display: inline-block;
color: red;
background-color: yellow;
}
mjx-mphantom {
visibility: hidden;
}
mjx-assistive-mml {
position: absolute !important;
top: 0px;
left: 0px;
clip: rect(1px, 1px, 1px, 1px);
padding: 1px 0px 0px 0px !important;
border: 0px !important;
display: block !important;
width: auto !important;
overflow: hidden !important;
-webkit-touch-callout: none;
-webkit-user-select: none;
-khtml-user-select: none;
-moz-user-select: none;
-ms-user-select: none;
user-select: none;
}
mjx-assistive-mml[display="block"] {
width: 100% !important;
}
mjx-c::before {
display: block;
width: 0;
}
.MJX-TEX {
font-family: MJXZERO, MJXTEX;
}
.TEX-B {
font-family: MJXZERO, MJXTEX-B;
}
.TEX-I {
font-family: MJXZERO, MJXTEX-I;
}
.TEX-MI {
font-family: MJXZERO, MJXTEX-MI;
}
.TEX-BI {
font-family: MJXZERO, MJXTEX-BI;
}
.TEX-S1 {
font-family: MJXZERO, MJXTEX-S1;
}
.TEX-S2 {
font-family: MJXZERO, MJXTEX-S2;
}
.TEX-S3 {
font-family: MJXZERO, MJXTEX-S3;
}
.TEX-S4 {
font-family: MJXZERO, MJXTEX-S4;
}
.TEX-A {
font-family: MJXZERO, MJXTEX-A;
}
.TEX-C {
font-family: MJXZERO, MJXTEX-C;
}
.TEX-CB {
font-family: MJXZERO, MJXTEX-CB;
}
.TEX-FR {
font-family: MJXZERO, MJXTEX-FR;
}
.TEX-FRB {
font-family: MJXZERO, MJXTEX-FRB;
}
.TEX-SS {
font-family: MJXZERO, MJXTEX-SS;
}
.TEX-SSB {
font-family: MJXZERO, MJXTEX-SSB;
}
.TEX-SSI {
font-family: MJXZERO, MJXTEX-SSI;
}
.TEX-SC {
font-family: MJXZERO, MJXTEX-SC;
}
.TEX-T {
font-family: MJXZERO, MJXTEX-T;
}
.TEX-V {
font-family: MJXZERO, MJXTEX-V;
}
.TEX-VB {
font-family: MJXZERO, MJXTEX-VB;
}
mjx-stretchy-v mjx-c, mjx-stretchy-h mjx-c {
font-family: MJXZERO, MJXTEX-S1, MJXTEX-S4, MJXTEX, MJXTEX-A ! important;
}
@font-face /* 0 */ {
font-family: MJXZERO;
src: url("https://cdn.jsdelivr.net/npm/[email protected]/es5/output/chtml/fonts/woff-v2/MathJax_Zero.woff") format("woff");
}
@font-face /* 1 */ {
font-family: MJXTEX;
src: url("https://cdn.jsdelivr.net/npm/[email protected]/es5/output/chtml/fonts/woff-v2/MathJax_Main-Regular.woff") format("woff");
}
@font-face /* 2 */ {
font-family: MJXTEX-B;
src: url("https://cdn.jsdelivr.net/npm/[email protected]/es5/output/chtml/fonts/woff-v2/MathJax_Main-Bold.woff") format("woff");
}
@font-face /* 3 */ {
font-family: MJXTEX-I;
src: url("https://cdn.jsdelivr.net/npm/[email protected]/es5/output/chtml/fonts/woff-v2/MathJax_Math-Italic.woff") format("woff");
}
@font-face /* 4 */ {
font-family: MJXTEX-MI;
src: url("https://cdn.jsdelivr.net/npm/[email protected]/es5/output/chtml/fonts/woff-v2/MathJax_Main-Italic.woff") format("woff");
}
@font-face /* 5 */ {
font-family: MJXTEX-BI;
src: url("https://cdn.jsdelivr.net/npm/[email protected]/es5/output/chtml/fonts/woff-v2/MathJax_Math-BoldItalic.woff") format("woff");
}
@font-face /* 6 */ {
font-family: MJXTEX-S1;
src: url("https://cdn.jsdelivr.net/npm/[email protected]/es5/output/chtml/fonts/woff-v2/MathJax_Size1-Regular.woff") format("woff");
}
@font-face /* 7 */ {
font-family: MJXTEX-S2;
src: url("https://cdn.jsdelivr.net/npm/[email protected]/es5/output/chtml/fonts/woff-v2/MathJax_Size2-Regular.woff") format("woff");
}
@font-face /* 8 */ {
font-family: MJXTEX-S3;
src: url("https://cdn.jsdelivr.net/npm/[email protected]/es5/output/chtml/fonts/woff-v2/MathJax_Size3-Regular.woff") format("woff");
}
@font-face /* 9 */ {
font-family: MJXTEX-S4;
src: url("https://cdn.jsdelivr.net/npm/[email protected]/es5/output/chtml/fonts/woff-v2/MathJax_Size4-Regular.woff") format("woff");
}
@font-face /* 10 */ {
font-family: MJXTEX-A;
src: url("https://cdn.jsdelivr.net/npm/[email protected]/es5/output/chtml/fonts/woff-v2/MathJax_AMS-Regular.woff") format("woff");
}
@font-face /* 11 */ {
font-family: MJXTEX-C;
src: url("https://cdn.jsdelivr.net/npm/[email protected]/es5/output/chtml/fonts/woff-v2/MathJax_Calligraphic-Regular.woff") format("woff");
}
@font-face /* 12 */ {
font-family: MJXTEX-CB;
src: url("https://cdn.jsdelivr.net/npm/[email protected]/es5/output/chtml/fonts/woff-v2/MathJax_Calligraphic-Bold.woff") format("woff");
}
@font-face /* 13 */ {
font-family: MJXTEX-FR;
src: url("https://cdn.jsdelivr.net/npm/[email protected]/es5/output/chtml/fonts/woff-v2/MathJax_Fraktur-Regular.woff") format("woff");
}
@font-face /* 14 */ {
font-family: MJXTEX-FRB;
src: url("https://cdn.jsdelivr.net/npm/[email protected]/es5/output/chtml/fonts/woff-v2/MathJax_Fraktur-Bold.woff") format("woff");
}
@font-face /* 15 */ {
font-family: MJXTEX-SS;
src: url("https://cdn.jsdelivr.net/npm/[email protected]/es5/output/chtml/fonts/woff-v2/MathJax_SansSerif-Regular.woff") format("woff");
}
@font-face /* 16 */ {
font-family: MJXTEX-SSB;
src: url("https://cdn.jsdelivr.net/npm/[email protected]/es5/output/chtml/fonts/woff-v2/MathJax_SansSerif-Bold.woff") format("woff");
}
@font-face /* 17 */ {
font-family: MJXTEX-SSI;
src: url("https://cdn.jsdelivr.net/npm/[email protected]/es5/output/chtml/fonts/woff-v2/MathJax_SansSerif-Italic.woff") format("woff");
}
@font-face /* 18 */ {
font-family: MJXTEX-SC;
src: url("https://cdn.jsdelivr.net/npm/[email protected]/es5/output/chtml/fonts/woff-v2/MathJax_Script-Regular.woff") format("woff");
}
@font-face /* 19 */ {
font-family: MJXTEX-T;
src: url("https://cdn.jsdelivr.net/npm/[email protected]/es5/output/chtml/fonts/woff-v2/MathJax_Typewriter-Regular.woff") format("woff");
}
@font-face /* 20 */ {
font-family: MJXTEX-V;
src: url("https://cdn.jsdelivr.net/npm/[email protected]/es5/output/chtml/fonts/woff-v2/MathJax_Vector-Regular.woff") format("woff");
}
@font-face /* 21 */ {
font-family: MJXTEX-VB;
src: url("https://cdn.jsdelivr.net/npm/[email protected]/es5/output/chtml/fonts/woff-v2/MathJax_Vector-Bold.woff") format("woff");
}
</style></head>
<body class="">
<div class="alert fixed-top mb-0 rounded-0 shadow alert-success alert-dismissible" data-alert="success dismissible" data-hash="edc24fffed7a3a9ad8c189ef5e2aca41" id="alert" role="alert">
<button aria-label="Close" class="btn-close" data-bs-dismiss="alert" type="button"></button><p>Interested in <a href="https://cs50.harvard.edu/web/2020/#how-to-take-this-course" class="alert-link">a verified certificate, a professional certificate, or transfer credit and accreditation</a>?
And get vaccinated (as soon as it’s available to you). 💉 <a href="https://www.who.int/news-room/feature-stories/detail/getting-the-covid-19-vaccine" class="alert-link">Here’s why</a>. <a href="https://www.google.com/search?q=covid-19+vaccine+finder" class="alert-link">Here’s how</a>.</p></div>
<div class="container-fluid">
<div class="row">
<aside class="col-md" style="height: 513.667px; top: 81.3333px;">
<header><h1 data-id="cs50s-web-programming-with-python-and-javascript"><a href="https://cs50.harvard.edu/web/2020/">CS50’s Web Programming with Python and JavaScript</a></h1>
<p>OpenCourseWare</p>
<p><a class="pr-1 small" href="https://cs50.harvard.edu/donate">Donate<i aria-hidden="true" class="fas fa-external-link-alt ps-2"></i></a></p>
<p><a href="https://brianyu.me/">Brian Yu</a><br>
<a href="mailto:[email protected]">[email protected]</a></p>
<p><a href="https://cs.harvard.edu/malan/">David J. Malan</a>
<br>
<a href="mailto:[email protected]">[email protected]</a>
<br>
<a class="mr-1" href="https://www.facebook.com/dmalan"><i aria-hidden="true" class="fab fa-facebook-f" title="Facebook"></i><span class="sr-only">Facebook</span></a>
<a class="mr-1" href="https://github.com/dmalan"><i aria-hidden="true" class="fab fa-github" title="GitHub"></i><span class="sr-only">GitHub</span></a>
<a class="mr-1" href="https://www.instagram.com/davidjmalan/"><i aria-hidden="true" class="fab fa-instagram" title="Instagram"></i><span class="sr-only">Instagram</span></a>
<a class="mr-1" href="https://www.linkedin.com/in/malan/"><i aria-hidden="true" class="fab fa-linkedin" title="LinkedIn"></i><span class="sr-only">LinkedIn</span></a>
<a class="mr-1" href="https://orcid.org/0000-0001-5338-2522"><i aria-hidden="true" class="fab fa-orcid" title="ORCID"></i><span class="sr-only">ORCID</span></a>
<a class="mr-1" href="https://www.quora.com/profile/David-J-Malan"><i aria-hidden="true" class="fab fa-quora" title="Quora"></i><span class="sr-only">Quora</span></a>
<a class="mr-1" href="https://www.reddit.com/user/davidjmalan"><i aria-hidden="true" class="fab fa-reddit-alien" title="Reddit"></i><span class="sr-only">Reddit</span></a>
<a class="mr-1" href="https://www.tiktok.com/@davidjmalan"><i aria-hidden="true" class="fab fa-tiktok" title="TikTok"></i><span class="sr-only">Twitter</span></a>
<a class="mr-1" href="https://twitter.com/davidjmalan"><i aria-hidden="true" class="fab fa-twitter" title="Twitter"></i><span class="sr-only">Twitter</span></a></p></header>
<button aria-controls="nav" aria-expanded="false" class="btn btn-sm collapsed d-md-none" data-bs-target="aside > nav" data-bs-toggle="collapse">
Menu
</button>
<nav class="collapse d-md-block" id="nav"><hr>
<ul class="fa-ul">
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://cs50.harvard.edu/x/movie/">🍿 CS50x Movie Night 2022</a></li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://cs50.harvard.edu/x/puzzles/"><i class="fas fa-puzzle-piece pe-2"></i>CS50x Puzzle Day 2022</a></li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://cs50.harvard.edu/x/prepare/"><i class="fas fa-book pe-2"></i>How to Prepare for Technical Interviews</a></li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://cs50.ly/zoom"><i class="fas fa-video pe-2"></i>Zoom Meetings</a></li>
</ul>
<hr>
<ol start="0">
<li><a href="https://cs50.harvard.edu/web/2020/weeks/0/">HTML, CSS</a></li>
<li><a href="https://cs50.harvard.edu/web/2020/weeks/1/">Git</a></li>
<li><a href="https://cs50.harvard.edu/web/2020/weeks/2/">Python</a></li>
<li><a href="https://cs50.harvard.edu/web/2020/weeks/3/">Django</a></li>
<li><a href="https://cs50.harvard.edu/web/2020/weeks/4/">SQL, Models, and Migrations</a></li>
<li><a href="https://cs50.harvard.edu/web/2020/weeks/5/">JavaScript</a></li>
<li><a href="https://cs50.harvard.edu/web/2020/weeks/6/">User Interfaces</a></li>
<li><a href="https://cs50.harvard.edu/web/2020/weeks/7/">Testing, CI/CD</a></li>
<li><a href="https://cs50.harvard.edu/web/2020/weeks/8/">Scalability and Security</a></li>
</ol>
<hr>
<ul class="fa-ul">
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://cs50.harvard.edu/web/2020/honesty/">Academic Honesty</a></li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://cs50.harvard.edu/web/2020/certificate/">CS50 Certificate</a></li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://cs50.harvard.edu/web/2020/faqs/">FAQs</a></li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://cs50.me/cs50w">Gradebook</a></li>
</ul>
<hr>
<ul class="fa-ul">
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://cs50.edx.org/ed">Ed Discussion</a> for Q&A</li>
<li data-marker="*" class="small"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://us.edstem.org/quickstart/ed-discussion.pdf">Quick Start Guide</a></li>
</ul>
<hr>
<ul class="small fa-ul">
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://cs50.edx.org/web">edX</a></li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://itunes.apple.com/us/course/id1505432709">iTunes U</a></li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://www.youtube.com/playlist?list=PLhQjrBD2T380xvFSUmToMMzERZ3qB5Ueu">YouTube</a></li>
</ul>
<hr>
<ul class="small fa-ul">
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://cs50.noticeable.news/">Changelog</a></li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://cs50.statuspage.io/">Status Page</a></li>
</ul>
<hr>
<ul class="fa-ul">
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://cs50.harvard.edu/web/2020/communities/"><strong>Communities</strong></a></li>
<li data-marker="*" class="small"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://www.clubhouse.com/club/cs50">Clubhouse</a></li>
<li data-marker="*" class="small"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://discord.gg/cs50">Discord</a> <span class="badge bg-light ms-1 py-1 rounded-pill text-dark">Q&A</span></li>
<li data-marker="*" class="small"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://cs50.edx.org/ed">Ed</a> <span class="badge bg-light ms-1 py-1 rounded-pill text-dark">Q&A</span></li>
<li data-marker="*" class="small"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://www.facebook.com/groups/cs50/">Facebook Group</a> <span class="badge bg-light ms-1 py-1 rounded-pill text-dark">Q&A</span></li>
<li data-marker="*" class="small"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://www.facebook.com/cs50/">Facebook Page</a></li>
<li data-marker="*" class="small"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://github.com/cs50">GitHub</a></li>
<li data-marker="*" class="small"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://gitter.im/cs50/x">Gitter</a> <span class="badge bg-light ms-1 py-1 rounded-pill text-dark">Q&A</span></li>
<li data-marker="*" class="small"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://instagram.com/cs50">Instagram</a></li>
<li data-marker="*" class="small"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://www.linkedin.com/groups/7437240/">LinkedIn Group</a></li>
<li data-marker="*" class="small"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://www.linkedin.com/school/CS50/">LinkedIn Page</a></li>
<li data-marker="*" class="small"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://cs50.medium.com/">Medium</a></li>
<li data-marker="*" class="small"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://www.quora.com/topic/CS50">Quora</a></li>
<li data-marker="*" class="small"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://www.reddit.com/r/cs50/">Reddit</a> <span class="badge bg-light ms-1 py-1 rounded-pill text-dark">Q&A</span></li>
<li data-marker="*" class="small"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://cs50.edx.org/slack">Slack</a> <span class="badge bg-light ms-1 py-1 rounded-pill text-dark">Q&A</span></li>
<li data-marker="*" class="small"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://www.snapchat.com/add/cs50">Snapchat</a></li>
<li data-marker="*" class="small"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://soundcloud.com/cs50">SoundCloud</a></li>
<li data-marker="*" class="small"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://cs50.stackexchange.com/">Stack Exchange</a> <span class="badge bg-light ms-1 py-1 rounded-pill text-dark">Q&A</span></li>
<li data-marker="*" class="small"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://www.tiktok.com/@cs50">TikTok</a></li>
<li data-marker="*" class="small"><span class="fa-li"><i class="fas fa-square"></i></span><a href="https://twitter.com/cs50">Twitter</a></li>
<li data-marker="*" class="small"><span class="fa-li"><i class="fas fa-square"></i></span><a href="http://www.youtube.com/subscription_center?add_user=cs50tv">YouTube</a></li>
</ul>
<hr>
<p><a href="https://cs50.harvardshop.com/"><img src="Lecture%208%20-%20CS50's%20Web%20Programming%20with%20Python%20and%20JavaScript_files/GGMdcKt.png" alt="Harvard Shop"></a></p>
<hr>
<p><a href="https://cs50.harvard.edu/web/2020/license/" class="small"><i class="fab fa-creative-commons me-1"></i>License</a></p></nav>
<footer></footer>
</aside>
<main class="col-md" style="margin-bottom: 0px; margin-top: 81.3333px;">
<a data-id="" id="lecture-8" style="top: -81.3333px;"></a><h1 class="no_toc"><a data-id="" href="#lecture-8">Lecture 8</a></h1>
<ul id="markdown-toc">
<li><a href="#introduction" id="markdown-toc-introduction">Introduction</a></li>
<li><a href="#scalability" id="markdown-toc-scalability">Scalability</a></li>
<li><a href="#scaling" id="markdown-toc-scaling">Scaling</a></li>
<li><a href="#load-balancing" id="markdown-toc-load-balancing">Load Balancing</a></li>
<li><a href="#autoscaling" id="markdown-toc-autoscaling">Autoscaling</a> <ul>
<li><a href="#server-failure" id="markdown-toc-server-failure">Server Failure</a></li>
</ul>
</li>
<li><a href="#scaling-databases" id="markdown-toc-scaling-databases">Scaling Databases</a> <ul>
<li><a href="#database-replication" id="markdown-toc-database-replication">Database Replication</a></li>
</ul>
</li>
<li><a href="#caching" id="markdown-toc-caching">Caching</a></li>
<li><a href="#security" id="markdown-toc-security">Security</a> <ul>
<li><a href="#git-and-github" id="markdown-toc-git-and-github">Git and GitHub</a></li>
</ul>
</li>
<li><a href="#html" id="markdown-toc-html">HTML</a></li>
<li><a href="#https" id="markdown-toc-https">HTTPS</a> <ul>
<li><a href="#secret-key-cryptography" id="markdown-toc-secret-key-cryptography">Secret-Key Cryptography</a></li>
<li><a href="#public-key-cryptography" id="markdown-toc-public-key-cryptography">Public-Key Cryptography</a></li>
</ul>
</li>
<li><a href="#databases" id="markdown-toc-databases">Databases</a> <ul>
<li><a href="#apis" id="markdown-toc-apis">APIs</a></li>
<li><a href="#environment-variables" id="markdown-toc-environment-variables">Environment Variables</a></li>
</ul>
</li>
<li><a href="#javascript" id="markdown-toc-javascript">JavaScript</a> <ul>
<li><a href="#cross-site-request-forgery" id="markdown-toc-cross-site-request-forgery">Cross-Site Request Forgery</a></li>
</ul>
</li>
<li><a href="#whats-next" id="markdown-toc-whats-next">What’s next?</a></li>
</ul>
<a data-id="" id="introduction" style="top: -81.3333px;"></a><h2><a data-id="" href="#introduction">Introduction</a></h2>
<ul class="fa-ul">
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span>So
far, we’ve discussed how to build simple web pages using HTML and CSS,
and how to use Git and GitHub in order to keep track of changes to our
code and collaborate with others. We also familiarized ourselves with
the Python programming language, started using Django to create web
applications, and learned how to use Django models to store information
in our sites. We then introduced JavaScript and learned how to use it to
make web pages more interactive, and talked about using animation and
React to further improve our User Interfaces. We then talked about some
best practices in software development and some technologies commonly
used to achieve those best practces.</li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span>Today, in our final lecture, we’ll discuss the issues of scaling up and securing our web applications.</li>
</ul>
<a data-id="" id="scalability" style="top: -81.3333px;"></a><h2><a data-id="" href="#scalability">Scalability</a></h2>
<p>So far in this course, we’ve built applications that are run only
locally on our computers, but eventually, we’ll want to launch our sites
so they canbe accessed by anyone on the internet. In order to do this,
we run our sites on <strong>servers</strong>, which are physical pieces
of hardware dedicated to running applications. Servers can either be
on-premise (We own and maintain physical servers where our application
is hosted) or on the cloud (servers are owned by a different company
such as Amazon or Google, and we pay to rent server space where our
application is hosted). There are benefits and drawbacks to both
options:</p>
<ul class="fa-ul">
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><strong>Customization</strong>:
Hosting your own servers gives you the ability to decide exactly how
they work, allowing for more flexibility than cloud-based hosting.</li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><strong>Expertise</strong>: It is much simpler to host an application on the cloud than it is to maintain your own servers.</li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><strong>Cost</strong>:
Since server-hosting sites need to make a profit, they will charge you
more than it costs them to maintain their on-premise servers, making
cloud-based servers more expensive. However, the startup costs of
running on-premise servers can be high, as you need to purchase physical
servers and potentially hire someone with the expertise to set them up.</li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><strong>Scalability</strong>:
Scaling is typically easier when hosting on the cloud. For example, if
we host a site on premise that gets 500 visits per day, and then it
starts getting 500,000 visits per day, we would have to order and setup
more physical servers to handle the requests, and in the mean time many
of our users will not be able to access the site. Most cloud hosting
sites will allow you to rent server space flexibly, paying based on how
much action your site sees.</li>
</ul>
<p>When a user sends an HTTP request to this server, the server should
send back a response. However, in reality, most servers get far more
than one request at a time, as depicted below:</p>
<p><img src="Lecture%208%20-%20CS50's%20Web%20Programming%20with%20Python%20and%20JavaScript_files/server0.png" alt="server many inputs"></p>
<p>This is where we run into the issue of scalability. A single server
can handle only so many requests at once, forcing us to make plans about
what to do when our one server is overworked. Whether we decide to host
on premise or on the cloud, we have to determine how many requests a
server can handle without crashing, which can be done using any number
of <strong>benchmarking</strong> tools including Apache Bench.</p>
<a data-id="" id="scaling" style="top: -81.3333px;"></a><h2><a data-id="" href="#scaling">Scaling</a></h2>
<p>Once we have some upper limit on how many requests our server can
handling, we can begin thinking about how we want to handle the scaling
of our application. Two different approaches to scaling include:</p>
<ol>
<li><strong>Vertical Scaling</strong>: In vertical scaling, when our
server is overwhelmed we simply buy or build a larger server. This
strategy is limited however, as there is an upper limit on how powerful a
single server can be.</li>
<li><strong>Horizontal Scaling</strong>: In horizontal scaling, when
our server is overwhelmed we buy or build more servers, and then split
the requests among our multiple servers.</li>
</ol>
<a data-id="" id="load-balancing" style="top: -81.3333px;"></a><h2><a data-id="" href="#load-balancing">Load Balancing</a></h2>
<p>When we use horizontal scaling, we are faced with the additional
problem of how we decide which servers are assigned to which requests.
We answer that question by employing a <strong>load balancer</strong>,
which is another piece of hardware that intercepts incoming requests,
and then assigns those requests to one of our servers. There are a
number of different methods for deciding which server receives which
request, but here are a few:</p>
<ul class="fa-ul">
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><strong>Random</strong>: In this simple method, the load balancer will decide randomly which server it should assign a request to.</li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><strong>Round-Robin</strong>:
In this method, the load balancer will alternate which server receives
an incoming request. If we have three servers, the first request might
go to server A, the second to server B, the third to server C, and the
fourth back to server A.</li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><strong>Fewest Connections</strong>:
In this method, the load balancer looks for the server that is
currently handling the fewest requests, and assigns the incoming request
to that server. This allows us to make sure we’re not overworking one
particular server, but it also takes longer for the load balancer to
calculate the number of requests each server is currently handling than
it dows for it to simply choose a random server.</li>
</ul>
<p>There is no method of load balancing that is strictly better than all
other methods, and there are many different methods used in practice.
One problem that can arise when scaling horizontally is that we might
have sessions that are stored on one server but not another, and we
don’t want users to have to re-enter information just because the load
balancer pushes their request to a new server. Like many problems of
scalability, there are multiple approaches to solving the problem of
sessions:</p>
<ul class="fa-ul">
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><strong>Sticky Sessions</strong>:
Once a user visits a site, the load balancer remembers which server
they were sent to first, and makes sure to send them to the same one.
One big concern with this method is that we could end up having a large
number of users sticking to one server, causing it to crash.</li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><strong>Database Sessions</strong>:
All sessions are stored in a database that all servers have access to.
This way, a user’s information will be available no matter which server
they are assigned to. The drawback here is that it takes additional time
and computing power to read from and write to a database.</li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><strong>Client-Side Sessions</strong>:
Rather than storing information on our servers, we can choose to store
them locally on the user’s web browser as cookies. The drawbacks to this
method include the security concern of users creating false cookies
that allow them to log in as another user, and the computational concern
of sending cookie information back and forth with every request.</li>
</ul>
<p>Like with load balancing, there is no best answer to the sessions
problem, and the method you choose will often depend on your specific
circumstances.</p>
<a data-id="" id="autoscaling" style="top: -81.3333px;"></a><h2><a data-id="" href="#autoscaling">Autoscaling</a></h2>
<p>Another problem that we could run into is that many websites are
visited much more frequently at certain times. For example, if we decide
to launch our “Is it New Year’s?” app from earlier, we might expect it
to get a lot more traffic in late December to early January than any
other time of year. If we buy enough servers for the site to stay active
during the winter, those servers would be sitting idle for the rest of
the year, wasting space and energy. This scenario has brought about the
idea of <strong>autoscaling</strong> which has become common in cloud
computing, where the number of servers being used by your site can grow
and shrink based on the number of requests it gets. Autoscaling is not a
perfect solution though, as it takes time to determine that a new
server is needed and to launch that server. Another potential problem is
that the more servers you have running, the more opportunity there is
for one to fail.</p>
<a data-id="" id="server-failure" style="top: -81.3333px;"></a><h3><a data-id="" href="#server-failure">Server Failure</a></h3>
<p>Having multiple servers though, can help to avoid what is known as a <strong>Single Point of Failure</strong>,
which is a piece of hardware that, after failing, will cause the entire
site to crash. When scaling horizontally, the load balancer can detect
which servers have crashed by sending periodic <strong>heartbeat</strong>
requests to each server, and then stop assigning new requests to
servers that have crashed. At this point, it seems we have simply moved
our single point of failure from a server to the load balancer, but we
can account for this by having backup load balancers available if our
original happens to crash.</p>
<a data-id="" id="scaling-databases" style="top: -81.3333px;"></a><h2><a data-id="" href="#scaling-databases">Scaling Databases</a></h2>
<p>In addition to scaling our servers that process requests, we’ll also
want to think of ways to scale our Databases. In this course we use
SQLite which stores data inside a file on the server, but as we store
more and more data, it sometimes makes more sense to store data in a
number of different files, and maybe even on a separate server. This
brings up the problem then of what to do when our database server can no
longer handle all of the requests coming in. Like in other issues of
scalability, there are a number of methods we can use to mitigate this
problem:</p>
<ul class="fa-ul">
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><strong>Vertical Partitioning</strong>:
This is a method similar to the one we used when first discussing SQL,
where we split our data into multiple different tables rather than
having redundant information in one table. (Feel free to look back on
lecture 4 where we split the <code class="language-plaintext highlighter-rouge">flights</code> table into a <code class="language-plaintext highlighter-rouge">flights</code> table and an <code class="language-plaintext highlighter-rouge">airports</code> table).</li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><strong>Horizontal Partitioning</strong>: This method involves storing multiple tables with the same format, but different information. For example, we could split a <code class="language-plaintext highlighter-rouge">flights</code> table into a <code class="language-plaintext highlighter-rouge">domestic_flights</code> table and an <code class="language-plaintext highlighter-rouge">international_flights</code>
table. This way, when we wish to search for a flight from JFK to LHR,
we don’t have to waste time searching through a table full of domestic
flights. One drawback to this method is that it can be expensive to join
multiple tables once they have been split.</li>
</ul>
<a data-id="" id="database-replication" style="top: -81.3333px;"></a><h3><a data-id="" href="#database-replication">Database Replication</a></h3>