A realm is a set of related objects that conform to a pre-defined schema and share user-level access permissions. Realms may contain more than one type of data as long as a schema exists for each type.
A realm allows you to partition data according to who uses it and when they use it. Every realm stores data in a separate realm file that contains a binary encoding of each object in the realm. You can automatically synchronize a realm across multiple devices and set up reactive event handlers that call a function any time an object in a realm is created, modified, or deleted.
Comparison with Other Databases¶
The MongoDB Realm data model is similar to both relational and document databases but has distinct differences from both. To underscore these differences, it's helpful to highlight what a realm is not:
- A realm is not a single, application-wide database.
- Unlike other applications, which store all of their data in a single database, Realm apps often split data across multiple realms to organize data more efficiently and to enforce access controls.
- A realm is not a table.
- Tables typically only store one kind of information, such as street addresses or items in a store inventory, whereas a realm can contain any number of object types.
- A realm is not a collection of schemaless documents.
- Application objects are similar to documents, but every object in a realm conforms to a defined schema for its object type. An object cannot contain a field that is not described by its schema.
Realms are Partitions of Atlas Data¶
When using Realm Sync, each realm corresponds to a subset of the data in your Realm app's synced MongoDB Atlas cluster. You can customize the partitioning of data using your application's partition key. Unique values of the partition key, known as partition values, correspond to individual realms.
You can customize permissions for these partitions of data using Realm Rules.
A Realm Schema is a list of valid object schemas that each define an object type that a Realm app may persist. All objects in a realm must conform to the Realm Schema.
Client applications provide a Realm Object Model when they open a realm. If a realm already contains data, then it already has a schema, and when it is opened, Realm Database validates the schema on the client against the existing schema.
To define the schema, you can specify the ObjectClasses property on the RealmConfiguration. If you omit this property, then all classes that inherit from RealmObject or EmbeddedObject are added to the schema automatically.
A schema version identifies the state of a Realm Schema at some point in time. Realm Database tracks the schema version of each realm and uses it to map the objects in each realm to the correct schema.
Schema versions are ascending integers that you can optionally include
in the realm configuration when you open a realm. If a client
application does not specify a version number when it opens a realm then
the realm defaults to version
Migrations must update a realm to a higher schema version. Realm Database will throw an error if a client application opens a realm with a schema version that is lower than the realm's current version or if the specified schema version is the same as the realm's current version but includes different object schemas.
- A realm is a collection of objects that conform to a schema. It is not a single, application-wide database. There can and usually will be more than one realm per application.
- A partition value is a realm's unique identifier.
- Realm Rules define who can read or write the realm and apply to the realm as a whole.
- A Realm Schema is a specification of the object types used in a Realm app.