BSON Types
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BSON is a binary serialization format used to store documents and make remote procedure calls in MongoDB. The BSON specification is located at bsonspec.org.
Each BSON type has both integer and string identifiers as listed in the following table:
Type | Number | Alias | Notes |
|---|---|---|---|
Double | 1 | "double" | |
String | 2 | "string" | |
Object | 3 | "object" | |
Array | 4 | "array" | |
Binary data | 5 | "binData" | |
Undefined | 6 | "undefined" | Deprecated. |
ObjectId | 7 | "objectId" | |
Boolean | 8 | "bool" | |
Date | 9 | "date" | |
Null | 10 | "null" | |
Regular Expression | 11 | "regex" | |
DBPointer | 12 | "dbPointer" | Deprecated. |
JavaScript | 13 | "javascript" | |
Symbol | 14 | "symbol" | Deprecated. |
32-bit integer | 16 | "int" | |
Timestamp | 17 | "timestamp" | |
64-bit integer | 18 | "long" | |
Decimal128 | 19 | "decimal" | |
Min key | -1 | "minKey" | |
Max key | 127 | "maxKey" |
The
$typeoperator supports using these values to query fields by their BSON type.$typealso supports thenumberalias, which matches the integer, decimal, double, and long BSON types.The
$typeaggregation operator returns the BSON type of its argument.The
$isNumberaggregation operator returnstrueif its argument is a BSON integer, decimal, double, or long.
To determine a field's type, see Type Checking.
If you convert BSON to JSON, see the Extended JSON reference.
The following sections describe special considerations for particular BSON types.
Binary Data
A BSON binary binData value is a byte array. A binData value
has a subtype that indicates how to interpret the binary data. The
following table shows the subtypes:
Number | Description |
|---|---|
0 | Generic binary subtype |
1 | Function data |
2 | Binary (old) |
3 | UUID (old) |
4 | UUID |
5 | MD5 |
6 | Encrypted BSON value |
7 | Compressed time series data New in version 5.2. |
8 | Sensitive data, such as a key or secret. MongoDB does not log
literal values for binary data with subtype 8. Instead, MongoDB
logs a placeholder value of ###. |
9 | Vector data, which is densely packed arrays of numbers of the
same type. |
128 | Custom data |
ObjectId
ObjectIds are small, likely unique, fast to generate, and ordered. ObjectId values are 12 bytes in length, consisting of:
A 4-byte timestamp, representing the ObjectId's creation, measured in seconds since the Unix epoch.
A 5-byte random value generated once per process. This random value is unique to the machine and process.
A 3-byte incrementing counter, initialized to a random value.
For timestamp and counter values, the most significant bytes appear first in the byte sequence (big-endian). This is unlike other BSON values, where the least significant bytes appear first (little-endian).
If an integer value is used to create an ObjectId, the integer replaces the timestamp.
In MongoDB, each document stored in a collection requires a unique
_id field that acts as a primary key. If an inserted
document omits the _id field, the MongoDB driver automatically
generates an ObjectId for the _id field.
This also applies to documents inserted through update operations with upsert: true.
MongoDB clients should add an _id field with a unique ObjectId.
Using ObjectIds for the _id field provides the following additional
benefits:
You can access
ObjectIdcreation time inmongoshusing theObjectId.getTimestamp()method.ObjectIds are approximately ordered by creation time, but are not perfectly ordered. Sorting a collection on an
_idfield containingObjectIdvalues is roughly equivalent to sorting by creation time.Important
While ObjectId values should increase over time, they are not necessarily monotonic. This is because they:
Only contain one second of temporal resolution, so ObjectId values created within the same second do not have a guaranteed ordering, and
Are generated by clients, which may have differing system clocks.
Use the ObjectId() methods to set and retrieve ObjectId
values.
Starting in MongoDB 5.0, mongosh replaces the legacy mongo
shell. The ObjectId() methods work differently in mongosh than
in the legacy mongo shell. For more information on the legacy
methods, see Legacy mongo Shell.
String
BSON strings are UTF-8. In general, drivers for each programming
language convert from the language's string format to UTF-8 when
serializing and deserializing BSON. This makes it possible to store
most international characters in BSON strings with ease.
[1] In addition, MongoDB
$regex queries support UTF-8 in the regex string.
| [1] | Given strings using UTF-8
character sets, using sort() on strings
will be reasonably correct. However, because internally
sort() uses the C++ strcmp api, the
sort order may handle some characters incorrectly. |
Timestamps
BSON has a special timestamp type for internal MongoDB use and is not associated with the regular Date type. This internal timestamp type is a 64 bit value where:
the most significant 32 bits are a
time_tvalue (seconds since the Unix epoch)the least significant 32 bits are an incrementing
ordinalfor operations within a given second.
While the BSON format is little-endian, and therefore stores the least
significant bits first, the mongod instance
always compares the time_t value before
the ordinal value on all platforms, regardless of
endianness.
Within a single mongod instance, timestamp values are
always unique.
In replication, the oplog has a ts field. The values in
this field reflect the operation time, which uses a BSON timestamp
value.
Note
The BSON timestamp type is for internal MongoDB use. For most cases, in application development, you will want to use the BSON date type. See Date for more information.
When inserting a document that contains top-level fields with empty
timestamp values, MongoDB replaces the empty timestamp values with the
current timestamp value, with the following exception. If the _id
field itself contains an empty timestamp value, it will always be
inserted as is and not replaced.
Example
Insert a document with an empty timestamp value:
db.test.insertOne( { ts: new Timestamp() } );
Running db.test.find() would then
return a document which resembles the following:
{ "_id" : ObjectId("542c2b97bac0595474108b48"), "ts" : Timestamp(1412180887, 1) }
The server has replaced the empty timestamp value for ts with the
timestamp value at time of insert.
Date
BSON Date is a 64-bit integer that represents the number of milliseconds since the Unix epoch (Jan 1, 1970). This results in a representable date range of about 290 million years into the past and future.
The official BSON specification refers to the BSON Date type as the UTC datetime.
BSON Date type is signed. [2] Negative values represent dates before 1970.
Example
Construct a Date using the new Date() constructor in
mongosh:
var mydate1 = new Date()
Example
Construct a Date using the ISODate() constructor in
mongosh:
var mydate2 = ISODate()
Example
Return the Date value as string:
mydate1.toString()
Example
Return the month portion of the Date value; months are zero-indexed,
so that January is month 0:
mydate1.getMonth()
| [2] | Prior to version 2.0, Date values were
incorrectly interpreted as unsigned integers, which affected
sorts, range queries, and indexes on Date fields. Because
indexes are not recreated when upgrading, please re-index if you
created an index on Date values with an earlier version, and
dates before 1970 are relevant to your application. |