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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 Realm Database validates each existing object
to ensure that an object schema was provided for its type and that it meets all of the
constraints specified in the schema. To determine the schema in iOS applications,
Realm automatically checks at runtime for any class that inherits from
Object and adds it to the schema.
- 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.