1
00:00:02,120 --> 00:00:05,300
We learned about ordered and unordered inserts,

2
00:00:05,330 --> 00:00:12,920
now there is a second option we can specify on insertOone and insertMany and that is the write concern

3
00:00:13,070 --> 00:00:13,810
option,

4
00:00:13,820 --> 00:00:17,150
now let me explain what the write concern is all about.

5
00:00:17,150 --> 00:00:20,990
We have a client and we have our mongodb server, the client would be the shell

6
00:00:21,050 --> 00:00:25,820
or your application using a mongodb driver, the mongodb server is what you start

7
00:00:25,850 --> 00:00:28,320
with the mongod executable.

8
00:00:28,670 --> 00:00:34,370
Now let's say you want to insert one document in there and it would also be true for insert many and

9
00:00:34,370 --> 00:00:35,360
for update and so on,

10
00:00:35,360 --> 00:00:39,950
so for all the write operations because it's the write concern option we're talking about.

11
00:00:40,190 --> 00:00:45,530
So we're trying to insert one document, now as you learned in the first module of this course, on the mongodb

12
00:00:45,530 --> 00:00:48,470
server, we have a so-called storage engine,

13
00:00:48,470 --> 00:00:53,420
this is the engine being responsible for really writing our data onto the disk 

14
00:00:53,600 --> 00:00:56,720
and also for managing it in memory.

15
00:00:56,720 --> 00:01:03,260
So your write might first end up in memory here and there, it manages the data which it needs to

16
00:01:03,260 --> 00:01:07,920
access with high frequency because that is faster than working with the disk.

17
00:01:08,030 --> 00:01:11,830
Of course your write is also scheduled to then end up on the disk,

18
00:01:11,900 --> 00:01:12,800
that is not the thing,

19
00:01:12,800 --> 00:01:16,940
so it will also eventually store your data on the disk.

20
00:01:17,000 --> 00:01:23,030
Now you can configure a so-called write concern on all the write operations like insert one with an

21
00:01:23,030 --> 00:01:30,230
additional argument, the write concern argument which is in turn a document where you can set settings

22
00:01:30,230 --> 00:01:30,910
like that,

23
00:01:30,920 --> 00:01:32,340
now what does this mean?

24
00:01:32,730 --> 00:01:34,400
The w simply means

25
00:01:34,400 --> 00:01:34,890
write

26
00:01:34,910 --> 00:01:43,070
and the number here indicates to how many instances, in case you're using multiple instances on one server,

27
00:01:43,160 --> 00:01:45,740
that is also something we'll cover towards the end of the course so

28
00:01:45,800 --> 00:01:47,240
let's ignore it for now,

29
00:01:47,240 --> 00:01:54,350
so on how many instances you want this write to be acknowledged. With W1 which is the default, you're

30
00:01:54,370 --> 00:01:58,730
basically saying hey my mongodb server should have accepted that write,

31
00:01:58,760 --> 00:02:05,170
so the storage engine is aware of it and will eventually write it to the disk. The J stands for the

32
00:02:05,180 --> 00:02:06,680
journal,

33
00:02:06,680 --> 00:02:12,850
the journal is an additional file which the storage engine manages which is like a To-Do file.

34
00:02:13,040 --> 00:02:20,500
The journal can be kept to then for example save operations that the storage engine needs to-do that have

35
00:02:20,540 --> 00:02:22,200
not been completed yet,

36
00:02:22,230 --> 00:02:29,510
like the write. Now it is aware of the write and that it needs to store that data on disk just by the write

37
00:02:29,540 --> 00:02:32,240
being acknowledged and being there in memory,

38
00:02:32,240 --> 00:02:34,530
it doesn't need to keep a journal for that.

39
00:02:34,640 --> 00:02:41,540
The idea of that journal file which is a real file on the disk is just that it is aware of this

40
00:02:41,630 --> 00:02:48,320
and if the server should go down for some reason or anything else happens, that file is still there

41
00:02:48,410 --> 00:02:54,380
and if you restart the server or if it recovers basically, it can look into that file and see what it

42
00:02:54,380 --> 00:02:55,260
needs to-do

43
00:02:55,460 --> 00:03:00,440
and that is of course a nice back up because the memory might have been wiped by then.

44
00:03:00,440 --> 00:03:06,140
So your write could be lost if it's not written to the journal, if it hasn't been written to the real

45
00:03:06,230 --> 00:03:12,120
data files yet, that is the idea of the journal, it's like a back up to-do list.

46
00:03:12,140 --> 00:03:16,820
Now the question is why do we write it in the journal and not directly into the database files? Because

47
00:03:16,820 --> 00:03:20,440
writing into the database files simply is more performance heavy,

48
00:03:20,480 --> 00:03:26,300
the journal is like a single line which describes the write operation, writing into the database files

49
00:03:26,300 --> 00:03:31,270
is of course a more complex task because there you need to find the right position to insert it,

50
00:03:31,400 --> 00:03:34,310
if you have indexes, something we'll cover later in this course,

51
00:03:34,310 --> 00:03:35,880
you also need to update these,

52
00:03:35,990 --> 00:03:39,840
so it simply takes longer, adding a to-do in a journal is faster.

53
00:03:39,980 --> 00:03:43,280
Still that also takes longer than not using the journal

54
00:03:43,430 --> 00:03:46,940
and the default is that the journal is not getting used with j undefined

55
00:03:47,120 --> 00:03:52,000
and that does simply mean that the storage engine will eventually handle this write

56
00:03:52,010 --> 00:03:56,870
and also write it to the journal but you don't have that information yet, you don't know if it has

57
00:03:56,870 --> 00:04:01,400
been stored in the journal yet, if the write succeeded yet, if the write has been done on the disk, you don't

58
00:04:01,400 --> 00:04:02,690
know any of that,

59
00:04:02,720 --> 00:04:05,110
you just know that the server is aware of your write.

60
00:04:05,120 --> 00:04:10,280
So if the server should go down in that exact moment, it might indeed not have done the write because

61
00:04:10,280 --> 00:04:15,320
it hasn't been added to the journal yet, it hasn't been saved to the database files yet.

62
00:04:15,320 --> 00:04:21,860
Now you can set a different option and set J to true, what you're now saying is hey please only report

63
00:04:21,930 --> 00:04:28,090
a success for this write to me after it has been both acknowledged and been saved to the journal,

64
00:04:28,190 --> 00:04:34,190
so now I have a greater security that this will happen and succeed even if the server should face issues

65
00:04:34,190 --> 00:04:35,810
right now.

66
00:04:35,810 --> 00:04:37,120
Now there also is a third option

67
00:04:37,130 --> 00:04:41,160
not directly related to the journal but it's a W timeout option,

68
00:04:41,210 --> 00:04:48,440
now this simply means which timeframe do you give your server to report a success for this write before

69
00:04:48,440 --> 00:04:49,420
you cancel it.

70
00:04:49,430 --> 00:04:55,330
So if you have some issues with the network connection or anything like that, you might simply timeout

71
00:04:55,330 --> 00:05:01,060
here, obviously setting this too low might timeout even though it would have perfectly succeeded

72
00:05:01,110 --> 00:05:07,260
normally and therefore you should know what you do when you set this timeout value because if you

73
00:05:07,260 --> 00:05:09,860
set it to a very low number, you might fail a lot

74
00:05:09,870 --> 00:05:13,750
even though there is no actual problem, just some small latency.

75
00:05:14,160 --> 00:05:20,310
So this is the write concern and how you can control this, obviously enabling the journal confirmation

76
00:05:20,400 --> 00:05:26,070
means that your writes will take longer because you don't just tell the server about them but you also need

77
00:05:26,070 --> 00:05:29,820
to wait for the server to store that write operation in the journal

78
00:05:29,820 --> 00:05:33,950
but you get higher security that the write also succeeded.

79
00:05:33,990 --> 00:05:38,280
Again this is a decision you have to make depending on your application needs,

80
00:05:38,370 --> 00:05:39,340
what you need.

81
00:05:39,450 --> 00:05:41,610
Now let's see this in practice in the next lecture.