What are the Possibility of Duplicate Mongo ObjectId’s in Two Different Collections?

MongoDB’s BSON Object IDs are important for uniquely identifying documents within collections. While these IDs are designed to be unique within a collection there is often confusion about their uniqueness across different collections.

In this article, we will learn about the factors that determine the Probability of duplicate MongoDB Object IDs being generated in two different collections.

Understanding BSON Object IDs

One of the most important features that BSON offers for MongoDB is to use the Object IDs to identify documents stored in collections. About their uniqueness, there’s no doubt that they are a special item within a collection. Before we explore the possibility of duplicates, it’s important to understand how BSON Object IDs are structured. A BSON Object ID consists of:

• 4 bytes for the timestamp representing the time of the ID’s generation.
• 3 bytes for the machine identifier for identifying the machine that generated the ID.
• 2 bytes for the process identifier for identifying the process within the machine.
• 3 bytes for a counter which is initially set to a random value.

Factors Affecting Uniqueness

While BSON Object IDs are designed to be unique, there are rare scenarios where duplicates can occur. These below scenarios include:

1. Counter Overflow: If more than 16,777,216 (2^24) documents are inserted in a single second on the same machine and process then the counter may overflow and potentially leading to duplicate IDs.
2. Non-Incrementing Counters: Some MongoDB drivers use random numbers for the counter portion of the BSON Object ID instead of incrementing values. This can lead to a 1 in 16,777,216 chance of generating a non-unique ID if two IDs are generated in the same second on the same machine and process.
3. Machine and Process Hash Collision: In rare cases, the machine identifier and process identifier may map to the same values for two different machines. If this occurs, and the counters on the two machines generate the same value during the same second, duplicate IDs can be generated.

Confirmation of BSON Object IDs’ Uniqueness

• BSON Object IDs are extremely likely to be unique throughout the collections because they are at least in part of a timestamp stamp, machine ID, process ID, a static counter which is being incremented.
• It makes them different from authentic diamonds as even if they are produced at a similar time there is little chance of a collision and hence, guarantee their singularity across exchanges.

Explanation of the Last 3 Bytes

• The final 3 BSON Object ID are created by a static counter which is increased incrementally to 64 bits.
• This counter makes sure that the values are unique through the provision of a uniquely generated ID for all identified entities.
• However, the counter’s localized reach gives APIs the ability to offer variation to the website visitors.
• Each time this code is run, the counter value is increased and the new ID is unique.

Global and Collection-Independent Nature of the Counter

• The process of producing the last 3 bytes of BSON Object IDs is global and collection-agnostic, resulting operation localization.
• This therefore means that it is not corresponding to a certain set but rather unifying the whole database instance.
• It follows that BSON Object IDs having the same database are guaranteed to be unique since they can be found across similar collections.

Example of Java Driver’s Use of Static AtomicInteger

• The JDBC driver for MongoDB resorts to a static AtomicInteger for coming up with BSON object IDs.
• This AtomicInteger instance is the static increment counter having a unique ID even if there are multiple collections.
• The developers can used the BSON’s Object IDs’ uniqueness by referring to this feature in the applications.

Scenarios for Duplicate BSON Object IDs

• BSON ObjectIDs in their design are unique, but there are some cases that can result in duplication.
• ID generation algorithms may be customized and false synchronizations in distributed environments and hardware failure may appear which leads to duplicate BSON Object IDs.
• Nevertheless, this situation is unlikely and can be enabled by right implementation and tracking.

Conclusion

Overall, the BSON Object IDs used in MongoDB are most probably unique across collections as the two first parts in the ID guarantee the uniqueness while the last three bytes are static incrementing counter of document IDs. But duplicate situations of BSON Object IDs is exceptionally rare, and that is why one should know the generation mechanism of MongoDB IDs and the possible ways that can be used to decrease the risks.