Is MongoDB _id (ObjectId) generated in an ascending order?

Question

I know how the _id column contains a representation of timestamp when the document has been inserted into the collection. here is an online utility to convert it to timestamp: http://steveridout.github.io/mongo-object-time/

What I'm wondering is if the object id string itself is guaranteed maintain the ascending order or not? i.e. does this comparison always return true?

"newest object id" > "second newest object id"

Answer 1

No, there is no guarantee whatsoever. From the official documentation:

The relationship between the order of ObjectId values and generation time is not strict within a single second. If multiple systems, or multiple processes or threads on a single system generate values, within a single second; ObjectId values do not represent a strict insertion order. Clock skew between clients can also result in non-strict ordering even for values, because client drivers generate ObjectId values, not the mongod process.

Answer 2

_id: ObjectId(4 bytes timestamp, 3 bytes machine id, 2 bytes process id, 3 bytes incrementer)

This is the id structure. So only last 3 bytes will increment uniquely. So the answer of your question is yes.

Answer 3

对于mongo version >= 3.4，Objectid的生成略有变化。 其结构为：

• 一个4字节的值，表示自Unix时代以来的秒数，
• 5个字节的随机值，并且
• 3字节计数器，以随机值开头。

所以前4个字节仍然是自Unix时代以来的秒数，它仍然几乎是递增的，但并不严格。

https://docs.mongodb.com/manual/reference/bson-types/#objectid