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Read & Write Data - Kotlin SDK

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  • Write Transactions
  • Run a Transaction

Realm Database uses a highly efficient storage engine to persist objects. You can:

Because the create, update, and delete 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, the SDK queues the change for synchronization with the backend.

  • 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 the realm's 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.


The following code shows how to run a transaction with write() or writeBlocking(). If the code in the callback throws an exception, Realm cancels the transaction. Otherwise, Realm commits the transaction.

realm.write {
// create a frog object in the realm
val frog = this.copyToRealm(Frog().apply {
name = "Kermit"
age = 45
species = "Green"
owner = "Jim"
// update the frog
frog.age = frog.age + 1
// get all frogs that belong to Jim
val jimsFrogs = this.query<Frog>("owner == 'Jim'").find()
// give all of Jim's frogs to Brian
jimsFrogs.forEach {
it.owner = "Brian"
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