MongoDB ObjectID Generator - Create Unique BSON Identifiers

What Is a MongoDB ObjectID?

A MongoDB ObjectID is a 12-byte value that MongoDB assigns by default to the _id field of every document. It is written as 24 lowercase hexadecimal characters, for example 507f1f77bcf86cd799439011. This tool generates between 1 and 100 valid ObjectIDs at a time, directly in the browser, with no database connection needed.

MongoDB ObjectID Structure

An ObjectID is built from three parts, placed one after another:

• Timestamp (4 bytes). The number of seconds since January 1, 1970 (the Unix epoch), stored as an unsigned, big-endian number. It records when the ObjectID was created.
• Random value (5 bytes). A number chosen at random once, when a program starts. Every ObjectID that program creates afterward reuses the same random value, so different computers do not need to coordinate.
• Counter (3 bytes). A number that starts at a random point between 0 and 16,777,215 and rises by 1 for each new ObjectID. If it passes 16,777,215, it wraps back to 0.

Laid end to end, the parts look like this:

1timestamp (8 hex chars) + random value (10 hex chars) + counter (6 hex chars) = 24 hex chars
2

Older MongoDB drivers built the middle section from a machine identifier and a process ID rather than a random number. Current drivers, including the logic behind this tool, use one random 5-byte value per process instead.

MongoDB ObjectID Format and Formula

There is no arithmetic formula behind an ObjectID, unlike a percentage or an interest calculation. It is assembled by converting three numbers to hexadecimal text and joining them end to end:

1. Take the current time in seconds since the Unix epoch. Write it as an 8-digit hexadecimal number.
2. Take the process's fixed random value, a number from 0 up to 2^40 − 1. Write it as a 10-digit hexadecimal number.
3. Take the counter's current value, from 0 to 16,777,215. Write it as a 6-digit hexadecimal number, then raise the counter by 1 for the next ID.
4. Join the three hexadecimal strings, in that order, into one 24-character string.

Example

Suppose a program starts at Unix time 1,700,000,000 seconds, which is November 14, 2023, 22:13:20 UTC. It picks the random value 3f2a91c8d4 and starts its counter at 12,345.

• Timestamp in hex: 6553f100
• Random value in hex: 3f2a91c8d4
• Counter in hex: 003039

Joining these gives the first ObjectID:

6553f1003f2a91c8d4003039

If the program generates a second ObjectID in the same second, the timestamp and random value stay the same, but the counter rises to 12,346 (00303a):

6553f1003f2a91c8d400303a

Reading an ObjectID also works in reverse. Given 507f1f77bcf86cd799439011, its first 8 characters, 507f1f77, decode to 1,350,508,407 seconds since the epoch, which is October 17, 2012, 21:13:27 UTC.

Common Uses for MongoDB ObjectIDs

• Primary keys. MongoDB puts an ObjectID in a document's _id field automatically, unless a different value is supplied.
• Creation-time lookup. Because the timestamp is embedded, a program can read a document's approximate creation time without a separate date field.
• Sorting by insertion order. Sorting documents by _id roughly sorts them by creation time, since the timestamp comes first in the value.
• Test and mock data. Developers use generated ObjectIDs to build sample documents or automated tests without running a database.

MongoDB ObjectID vs. UUID

A UUID (Universally Unique Identifier) is a 16-byte value, four bytes larger than an ObjectID. A standard UUID is usually random throughout, so it does not reveal a creation time or sort in creation order. An ObjectID is shorter and roughly time-ordered, which is one reason MongoDB uses it by default, though it does not carry the same formal uniqueness guarantee that a properly generated UUID does.

Frequently Asked Questions

What is a MongoDB ObjectID used for? It is the default value MongoDB puts in a document's _id field, so every document gets a unique key without extra setup.

Are MongoDB ObjectIDs guaranteed to be unique? Not in a formal, mathematical sense, but collisions are extremely unlikely. Two ObjectIDs would need the same random value and the same counter position in the same second, which normally only happens within a single process.

Can the creation date be read from an ObjectID? Yes. The first 8 hexadecimal characters, 4 bytes, hold the number of seconds since the Unix epoch. Converting that number to a date gives the approximate creation time.

How many ObjectIDs can this generator make at once? Between 1 and 100 per batch. Every ID in a batch shares the same process-wide random value, and each has a counter one higher than the last.

Can MongoDB ObjectIDs be used outside of MongoDB? Yes. Since they are just 12-byte values encoded as hexadecimal text, they work as unique IDs in other systems too, though a database's own ID scheme may fit its needs better.

Is this generator connected to a real MongoDB database? No. It runs entirely in the browser and does not read from or write to any database.

References

1. "ObjectId." MongoDB Manual, https://docs.mongodb.com/manual/reference/method/ObjectId/.
2. "ObjectId Specification." MongoDB BSON Specifications, https://github.com/mongodb/specifications/blob/master/source/bson-objectid/objectid.md.
3. "BSON Types." BSON Specification, http://bsonspec.org/spec.html.