You can create objects in a realm, update objects in a realm, and eventually delete objects from a realm. Because these operations modify the state of the realm, we call them writes.
Realm handles writes in terms of transactions. A transaction is a list of read and write operations that Realm treats as a single indivisible operation. In other words, a transaction is all or nothing: either all of the operations in the transaction succeed or none of the operations in the transaction take effect.
All writes must happen in a transaction.
A realm allows only one open write transaction at a time. Realm blocks other writes on other threads until the open transaction is complete. Consequently, there is no race condition when reading values from the realm within a transaction.
When you are done with your transaction, Realm either commits it or cancels it:
When Realm commits a transaction, Realm writes all changes to disk. For synced realms, Realm queues the change for synchronization with Atlas App Services.
When Realm cancels a write transaction or an operation in the transaction causes an error, all changes are discarded (or "rolled back").
Realm Object Writes are File Writes
Whenever you create, update, or delete a Realm object, your changes update the representation of that object in Realm and emit notifications to any subscribed listeners. As a result, you should only write to Realm objects when necessary to persist data.
Synchronous Reads and Writes on the UI Thread
By default, you can only read or write to a realm in your
application's UI thread using
asynchronous transactions. That is,
you can only use
Realm methods whose name ends with the word
Async in the main thread of your Android application unless you
explicitly allow the use of synchronous methods.
This restriction exists for the benefit of your application users:
performing read and write operations on the UI thread can lead to
unresponsive or slow UI interactions, so it's usually best to handle
these operations either asynchronously or in a background thread.
However, if your application requires the use of synchronous
realm reads or writes on the UI thread, you can explicitly allow
the use of synchronous methods with the following
Managed objects are live Realm objects that update based on changes to underlying data in Realm. Managed objects can only come from an open realm, and receive updates as long as that realm remains open. Managed objects cannot be passed between threads.
Unmanaged objects are instances of Realm objects that are not live. You can get an unmanaged object by manually constructing a Realm object yourself, or by calling Realm.copyFromRealm(). Unmanaged objects can be passed between threads.
Realm represents each transaction as a callback function
that contains zero or more read and write operations. To run
a transaction, define a transaction callback and pass it to
write method. Within this callback, you are
free to create, read, update, and delete on the realm. If
the code in the callback throws an exception when Realm runs
it, Realm cancels the transaction. Otherwise, Realm commits
the transaction immediately after the callback.