5

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.

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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:

0123456 7891011

timemachine pidinc

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.

總結

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