10 MongoDB _id Best Practices

The MongoDB _id field is a vital part of the document structure. In this article, we'll go over 10 best practices for working with the MongoDB _id field.

The MongoDB _id is a 12-byte BSON type, which is comprised of a 4-byte timestamp, a 3-byte machine identifier, a 2-byte process identifier, and a 3-byte counter. The _id field is used as a primary key for collections in MongoDB.

In this article, we will discuss 10 best practices for working with the MongoDB _id field. These best practices will help you to avoid common mistakes and optimize your application for performance.

1. Use a Unique _id for Each Document

The _id is the primary key for a MongoDB document, and it must be unique within a collection. The value of the _id field must be unique across all documents in a collection, so using the same _id for two different documents in the same collection will result in an error.

If you don’t specify an _id when inserting a document, MongoDB will generate one for you. However, it’s best to always explicitly specify an _id when you insert a document, so that you can control the value.

There are a few different ways to generate a unique _id:

– Use a UUID
– Use a monotonically increasing number
– Use a hash of the document contents

2. Don’t Reuse an _id Value

When you insert a document into MongoDB, the database automatically assigns it an _id field if one is not specified. The value of the _id field must be unique across the collection because it acts as a primary key. If you try to insert a document with an _id that already exists in the collection, MongoDB will return an error.

While it’s tempting to reuse an _id value (for example, when you’re inserting a large number of documents), doing so can lead to problems down the road. For instance, let’s say you have a collection of users, and each user has an _id field with a value of 1. If you try to update a specific user’s document, you might accidentally update all of the documents in the collection because they all have the same _id value.

To avoid these types of errors, make sure you always generate a new _id value for each document you insert into MongoDB.

3. Avoid Using the _id Field as a Primary Key

The _id field is automatically generated by MongoDB to uniquely identify each document in a collection. It’s indexed by default, so it’s fast to query. However, the _id field is not immutable, which means it can be changed. If you use the _id field as a primary key, and the value of the _id field changes, then you will no longer be able to query for that document using the original _id value.

It’s better to use a separate field as your primary key, such as an email address or a username. You can index this field to make queries faster. And, if you need to change the value of the field, you can do so without affecting your ability to query for the document.

4. Choose a Meaningful _id Value

The _id field is the primary key for a MongoDB document, and as such, it must be unique. The value of the _id field must be unique across the collection, so you cannot have two documents with the same _id value.

If you don’t specify an _id value when inserting a document, MongoDB will generate an _id value for you. However, it’s best to specify your own _id value so that you can control what the value is.

There are a few things to keep in mind when choosing an _id value:

– The _id value must be unique across the collection.
– The _id value must be immutable (i.e. you cannot change it once it’s been set).
– The _id value must be a 12-byte hexadecimal string (24 characters in total).

5. Consider Using UUIDs or Hash-Based Values

The default MongoDB _id value is a 12-byte ObjectID. While this offers some benefits in terms of space efficiency and performance, there are also some drawbacks. For example, ObjectIDs are sequential, which means they can be guessed. They’re also not very user-friendly, which can make them difficult to work with.

UUIDs, on the other hand, are 16-byte values that are generated using a random or pseudo-random number generator. This makes them much more difficult to guess, and they’re also more user-friendly. Hash-based values offer similar benefits, but they may be less user-friendly.

Ultimately, the decision of whether to use ObjectIDs, UUIDs, or hash-based values will depend on your specific needs and requirements. However, if security or usability is a concern, it’s worth considering one of the alternatives.

6. Generate _id Values on the Client Side

The MongoDB _id field is a primary key, and as such, it must be unique. If you allow the database to generate _id values, then there’s a chance (however small) that two clients could end up with the same _id value. This would cause problems, so it’s best to avoid it altogether by generating _id values on the client side.

To generate an _id value on the client side, you can use the ObjectId() function. This function takes a 12-byte string as an argument, and returns an _id value. The 12-byte string can be generated using a variety of methods, but the most common is to use a cryptographically secure random number generator.

7. Do Not Modify the _id Field

The _id field is used to uniquely identify a document in a MongoDB collection. It is also used to index the documents for faster query performance. If you were to modify the _id field, then it would no longer be unique, and the index would be broken.

It is possible to change the value of the _id field, but it is not recommended. If you absolutely must change the _id field, then you should first drop the index on the _id field, change the _id field, and then recreate the index.

8. Do Not Create Your Own _id Indexes

The _id index is created automatically when a collection is created, and it is always the first index in the list of indexes for a collection. The _id index is special because:

– It is guaranteed to be unique
– It is immutable (cannot be changed)
– It is indexed by default

Creating your own _id index will not improve performance, and it can actually degrade performance because now there are two _id indexes that must be maintained.

9. Be Careful with Sharding and _id Fields

When you shard a collection, the _id field must be included in the shard key. This is because the _id field is used to uniquely identify documents in a collection, and so it must be used to determine which shard a document belongs to.

However, if you use the _id field as your shard key, then all writes to the collection will go to the same shard. This can lead to performance problems, as the shard will become a bottleneck.

Therefore, it’s important to choose a shard key that will distribute write traffic evenly across all shards. A good way to do this is to use a compound shard key, where the _id field is combined with another field that has a high cardinality (i.e. a large number of unique values).

10. Understand How _id Works in Replica Sets

When a document is inserted into a MongoDB collection, the database will automatically generate an _id field if one is not provided. The value of the _id field must be unique across the collection, but it does not have to be globally unique. This means that two documents in different collections can have the same _id value.

The _id field is also used as the default shard key for a collection. In a replica set, each document must have a unique _id value so that the replica set can sync correctly. If two documents in a replica set have the same _id value, then one of the documents will be overwritten by the other and data will be lost.

To avoid this, it’s important to understand how the _id field works in replica sets and make sure that all documents in a replica set have a unique _id value.


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