mongodb中的objectid是collection中唯一，还是全db唯一，还是全宇宙唯一？

UUID吗? 全宇宙.

UUID 能保证 在人类所能认知的范围里唯一.

@fzcs @real_newbie
不是uuid，是mongodb自己构造的一种数据类型，用来做document的默认主键。

全宇宙唯一

@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.

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⋯⋯

@fengluo @mrkschan

其实我很想确认 objectid在一个db中是不是唯一的。
相关一些知识我不太完备，所以不能确切推断出来，又没有找到相关的文档说明这个问题的。

@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.

@mrkschan 我想知道 md5(hostname) 是哪里来的，文档中没有吧，还是在代码里看到的

@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)

@mrkschan 谢谢，终于找到了

@mrkschan
清楚一些了，谢谢你的回答。

@ratazzi @iiduce 只是 copy 了一下 document 而已.

@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

@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.

全集群唯一。