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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 Database 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 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.