Timestamp data type

This is not the normal Date datatype. This is a special type for internal MongoDB use.

BSON includes a timestamp data type with special semantics in MongoDB.

Timestamps are stored as 64 bit values which, on a single mongod, are guaranteed unique. The first 32 bits are a time_t value (seconds since the UTC epoch). The second 32 bits are an incrementing ordinal for operations within a given second.

MongoDB uses the Timestamp datatype as "OpTimes" in the replication oplog's ts field.

Timestamps have special semantics when null. If null, and the timestamp is one of the first two fields of the object, the timestamp will automatically be converted to a unique value. (It must be one of the first two top level fields of the document for performance reasons; the entire document is not scanned for timestamps.)

An example from the mongo shell follows (example works with shells v1.7.5 and higher).

> // not one of the first 2 fields
> db.foo.insert( { x : 1, y : new Timestamp() } )
> db.foo.find()
{ "_id" : ObjectId("4d1d4ce78b1a04eeb294c098"), "x" : 1, "y" : { "t" : 0, "i" : 0 } }

> // in first 2 fields, auto fill of value works
> db.foo.drop()
> db.foo.insert( { y : new Timestamp(), x : 3 } )
> // the shell displays timestamps as { t : ..., i : ... } where t is the time 
> // component and i is the ordinal component
> db.foo.find()
{ "_id" : ObjectId("4d1d4cfd8b1a04eeb294c099"), "y" : { "t" : 1293765885000, "i" : 1 }, "x" : 3 }

> db.foo.drop()
> for( var i = 0; i < 10; i++ ) db.foo.insert({y:new Timestamp(), x : i})
> db.foo.find()
{ "_id" : ObjectId("4d1d4d178b1a04eeb294c09a"), "y" : { "t" : 1293765911000, "i" : 1 }, "x" : 0 }
{ "_id" : ObjectId("4d1d4d178b1a04eeb294c09b"), "y" : { "t" : 1293765911000, "i" : 2 }, "x" : 1 }
{ "_id" : ObjectId("4d1d4d178b1a04eeb294c09c"), "y" : { "t" : 1293765911000, "i" : 3 }, "x" : 2 }
{ "_id" : ObjectId("4d1d4d178b1a04eeb294c09d"), "y" : { "t" : 1293765911000, "i" : 4 }, "x" : 3 }
{ "_id" : ObjectId("4d1d4d178b1a04eeb294c09e"), "y" : { "t" : 1293765911000, "i" : 5 }, "x" : 4 }
{ "_id" : ObjectId("4d1d4d178b1a04eeb294c09f"), "y" : { "t" : 1293765911000, "i" : 6 }, "x" : 5 }
{ "_id" : ObjectId("4d1d4d178b1a04eeb294c0a0"), "y" : { "t" : 1293765911000, "i" : 7 }, "x" : 6 }
{ "_id" : ObjectId("4d1d4d178b1a04eeb294c0a1"), "y" : { "t" : 1293765911000, "i" : 8 }, "x" : 7 }
{ "_id" : ObjectId("4d1d4d178b1a04eeb294c0a2"), "y" : { "t" : 1293765911000, "i" : 9 }, "x" : 8 }
{ "_id" : ObjectId("4d1d4d178b1a04eeb294c0a3"), "y" : { "t" : 1293765911000, "i" : 10 }, "x" : 9 }
>

From MongoDB 2.1.0+, timestamps are displayed in the shell with a wrapper, as Timestamp(1293765911000, 1).

Timestamp data type

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