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real_newbie 2011-05-27 17:58:08 +08:00
UUID吗? 全宇宙.
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fzcs 2011-05-27 18:25:50 +08:00
UUID 能保证 在人类所能认知的范围里唯一.
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iiduce OP |
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tysx 2011-05-27 19:18:59 +08:00
全宇宙唯一
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mrkschan 2011-05-27 19:26:00 +08:00
@iiduce 你已提過了...
ObjectID = 32bit timestamp + md5(hostname) + process-id + sequence-no. (Full definition is quoted at bottom of this post) 在不同電腦內, process-id 跟 sequence-no 跟 timestamp 是有機會一樣的 跟據 wikipedia (http://is.gd/yZBK2B) md5 hash 不是 collision resistance. 即是說, 是有機會出現 md5(hostname_A) == md5(hostname_B). 所以, ObjectId 一定不是全宇宙唯一. ---- BSON ObjectID Specification (http://is.gd/XcorBb) A BSON ObjectID is a 12-byte value consisting of a 4-byte timestamp (seconds since epoch), a 3-byte machine id, a 2-byte process id, and a 3-byte counter. Note that the timestamp and counter fields must be stored big endian unlike the rest of BSON. This is because they are compared byte-by-byte and we want to ensure a mostly increasing order. Here's the schema: 0 1 2 3 4 5 6 7 8 9 10 11 time machine pid inc Here is a breakdown of the sections: TimeStamp This is a unix style timestamp. It is a signed int representing the number of seconds before or after January 1st 1970 (UTC). Machine This is the first three bytes of the (md5) hash of the machine host name, or of the mac/network address, or the virtual machine id. Pid This is 2 bytes of the process id (or thread id) of the process generating the object id. Increment This is an ever incrementing value, or a random number if a counter can't be used in the language/runtime. |
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fengluo 2011-05-27 19:34:44 +08:00
至少这个collection内啦~
前几天我一直在为24个字符这么长的id纠结⋯⋯ |
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iiduce OP |
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mrkschan 2011-05-27 20:18:41 +08:00
@iiduce Within the *same* mongodb process... I believe the BSON ObjectId is unique across all collections.
Note: On a single machine, you can have a master mongodb process and a slave mongodb process. Thus, I have to focus the phrase - *same* mongodb process. Anyway, with this master-slave topology, write operations only occur in master mongodb process and, as a result, the ObjectIds are unique. |
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mrkschan 2011-05-27 20:31:32 +08:00
@ratazzi http://is.gd/Gd3yDY mongodb official documentation (english version) ... 不知道要不要跳牆看 (我在牆外的世界)
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ratazzi 2011-05-27 21:17:45 +08:00
@mrkschan 很惭愧的说,用了一年的 mongodb 那个文档也打开过好几次,就是没看到那里,看来对英文敏感度有待提高啊
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mrkschan 2011-05-27 21:30:47 +08:00
@ratazzi 我只是先前搜過, 如何利用 BSON ObjectId 的 timestamp, 避免在 document 加一個 create_timestamp 的 property.
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est 2011-05-27 22:34:57 +08:00
全集群唯一。
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