心得体会：kafka是如何保证消息不被重复消费_使用多个消费组消费kafka不重复

一、kafka自带的消费机制

　　kafka有个offset的概念，当每个消息被写进去后，都有一个offset，代表他的序号，然后consumer消费该数据之后，隔一段时间，会把自己消费过的消息的offset提交一下，代表我已经消费过了。下次我要是重启，就会继续从上次消费到的offset来继续消费。

　　但是当我们直接kill进程了，再重启。这会导致consumer有些消息处理了，但是没来得及提交offset。等重启之后，少数消息就会再次消费一次。

　　其他MQ也会有这种重复消费的问题，那么针对这种问题，我们需要从业务角度，考虑它的幂等性。

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KafkaCons
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offset = 299, key = 99, value = 99
offset = 300, key = 0, value = 0
offset = 301, key = 1, value = 1
offset = 302, key = 2, value = 2
offset = 303, key = 3, value = 3
offset = 304, key = 4, value = 4
offset = 305, key = 5, value = 5
offset = 306, key = 6, value = 6
offset = 307, key = 7, value = 7
offset = 308, key = 8, value = 8
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offset = 321, key = 21, value = 21
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offset = 324, key = 24, value = 24
offset = 325, key = 25, value = 25
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offset = 327, key = 27, value = 27
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offset = 330, key = 30, value = 30
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offset = 333, key = 33, value = 33
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offset = 335, key = 35, value = 35
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offset = 337, key = 37, value = 37
offset = 338, key = 38, value = 38
offset = 339, key = 39, value = 39
offset = 340, key = 40, value = 40
offset = 341, key = 41, value = 41
offset = 342, key = 42, value = 42
offset = 343, key = 43, value = 43
offset = 344, key = 44, value = 44
offset = 345, key = 45, value = 45
offset = 346, key = 46, value = 46
offset = 347, key = 47, value = 47
offset = 348, key = 48, value = 48
offset = 349, key = 49, value = 49
offset = 350, key = 50, value = 50
offset = 351, key = 51, value = 51
offset = 352, key = 52, value = 52
offset = 353, key = 53, value = 53
offset = 354, key = 54, value = 54
offset = 355, key = 55, value = 55
offset = 356, key = 56, value = 56
offset = 357, key = 57, value = 57
offset = 358, key = 58, value = 58
offset = 359, key = 59, value = 59
offset = 360, key = 60, value = 60
offset = 361, key = 61, value = 61
offset = 362, key = 62, value = 62
offset = 363, key = 63, value = 63
offset = 364, key = 64, value = 64
offset = 365, key = 65, value = 65
offset = 366, key = 66, value = 66
offset = 367, key = 67, value = 67
offset = 368, key = 68, value = 68
offset = 369, key = 69, value = 69
offset = 370, key = 70, value = 70
offset = 371, key = 71, value = 71
offset = 372, key = 72, value = 72
offset = 373, key = 73, value = 73
offset = 374, key = 74, value = 74
offset = 375, key = 75, value = 75
offset = 376, key = 76, value = 76
offset = 377, key = 77, value = 77
offset = 378, key = 78, value = 78
offset = 379, key = 79, value = 79
offset = 380, key = 80, value = 80
offset = 381, key = 81, value = 81
offset = 382, key = 82, value = 82
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offset = 385, key = 85, value = 85
offset = 386, key = 86, value = 86
offset = 387, key = 87, value = 87
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offset = 390, key = 90, value = 90
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offset = 394, key = 94, value = 94
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offset = 397, key = 97, value = 97
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offset = 399, key = 99, value = 99
offset = 400, key = 0, value = 0
offset = 401, key = 1, value = 1
offset = 402, key = 2, value = 2
offset = 403, key = 3, value = 3
offset = 404, key = 4, value = 4
offset = 405, key = 5, value = 5
offset = 406, key = 6, value = 6
offset = 407, key = 7, value = 7
offset = 408, key = 8, value = 8
offset = 409, key = 9, value = 9
offset = 410, key = 10, value = 10
offset = 411, key = 11, value = 11
offset = 412, key = 12, value = 12
offset = 413, key = 13, value = 13
offset = 414, key = 14, value = 14
offset = 415, key = 15, value = 15
offset = 416, key = 16, value = 16
offset = 417, key = 17, value = 17
offset = 418, key = 18, value = 18
offset = 419, key = 19, value = 19
offset = 420, key = 20, value = 20
offset = 421, key = 21, value = 21
offset = 422, key = 22, value = 22
offset = 423, key = 23, value = 23
offset = 424, key = 24, value = 24
offset = 425, key = 25, value = 25
offset = 426, key = 26, value = 26
offset = 427, key = 27, value = 27
offset = 428, key = 28, value = 28
offset = 429, key = 29, value = 29
offset = 430, key = 30, value = 30
offset = 431, key = 31, value = 31
offset = 432, key = 32, value = 32
offset = 433, key = 33, value = 33
offset = 434, key = 34, value = 34
offset = 435, key = 35, value = 35
offset = 436, key = 36, value = 36
offset = 437, key = 37, value = 37
offset = 438, key = 38, value = 38
offset = 439, key = 39, value = 39
offset = 440, key = 40, value = 40
offset = 441, key = 41, value = 41
offset = 442, key = 42, value = 42
offset = 443, key = 43, value = 43
offset = 444, key = 44, value = 44
offset = 445, key = 45, value = 45
offset = 446, key = 46, value = 46
offset = 447, key = 47, value = 47
offset = 448, key = 48, value = 48
offset = 449, key = 49, value = 49
offset = 450, key = 50, value = 50
offset = 451, key = 51, value = 51
offset = 452, key = 52, value = 52
offset = 453, key = 53, value = 53
offset = 454, key = 54, value = 54
offset = 455, key = 55, value = 55
offset = 456, key = 56, value = 56
offset = 457, key = 57, value = 57
offset = 458, key = 58, value = 58
offset = 459, key = 59, value = 59
offset = 460, key = 60, value = 60
offset = 461, key = 61, value = 61
offset = 462, key = 62, value = 62
offset = 463, key = 63, value = 63
offset = 464, key = 64, value = 64
offset = 465, key = 65, value = 65
offset = 466, key = 66, value = 66
offset = 467, key = 67, value = 67
offset = 468, key = 68, value = 68
offset = 469, key = 69, value = 69
offset = 470, key = 70, value = 70
offset = 471, key = 71, value = 71
offset = 472, key = 72, value = 72
offset = 473, key = 73, value = 73
offset = 474, key = 74, value = 74
offset = 475, key = 75, value = 75
offset = 476, key = 76, value = 76
offset = 477, key = 77, value = 77
offset = 478, key = 78, value = 78
offset = 479, key = 79, value = 79
offset = 480, key = 80, value = 80
offset = 481, key = 81, value = 81
offset = 482, key = 82, value = 82
offset = 483, key = 83, value = 83
offset = 484, key = 84, value = 84
offset = 485, key = 85, value = 85
offset = 486, key = 86, value = 86
offset = 487, key = 87, value = 87
offset = 488, key = 88, value = 88
offset = 489, key = 89, value = 89
offset = 490, key = 90, value = 90
offset = 491, key = 91, value = 91
offset = 492, key = 92, value = 92
offset = 493, key = 93, value = 93
offset = 494, key = 94, value = 94
offset = 495, key = 95, value = 95
offset = 496, key = 96, value = 96
offset = 497, key = 97, value = 97
offset = 498, key = 98, value = 98
offset = 499, key = 99, value = 99
offset = 500, key = null, value = hello world
offset = 501, key = null, value = caijunjie 思密达
offset = 502, key = null, value = caijunjie 思密达
offset = 503, key = null, value = caijunjie world

二、通过保证消息队列消费的幂等性来保证

　　举个例子，当消费一条消息时就往数据库插入一条数据。如何保证重复消费也插入一条数据呢？

　　那么我们就需要从幂等性角度考虑了。幂等性，通俗点说，就一个数据，或者一个请求，无论来多次，对应的数据都不会改变的，不能出错。

怎么保证消息队列消费的幂等性？

需要结合业务来思考，比如下面的例子：

　　1.比如某个数据要写库，你先根据主键查一下，如果数据有了，就别插入了，update一下好吧

　　2.比如你是写redis，那没问题了，反正每次都是set，天然幂等性

　　3.对于消息，我们可以建个表（专门存储消息消费记录）

　　　　生产者，发送消息前判断库中是否有记录（有记录说明已发送），没有记录，先入库，状态为待消费，然后发送消息并把主键id带上。

　　　　消费者，接收消息，通过主键ID查询记录表，判断消息状态是否已消费。若没消费过，则处理消息，处理完后，更新消息记录的状态为已消费。