Reduce Realm File Size - Flutter SDK
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The size of a realm is always larger than the total size of the objects stored within it. This architecture enables some of realm's performance, concurrency, and safety benefits.
Realm writes new data within unused space tracked inside a file. In some situations, unused space may comprise a significant portion of a realm file. Realm's default behavior is to automatically compact a realm to prevent it from growing too large. You can use manual compaction strategies when automatic compaction is not sufficient for your use case.
Automatic Compaction
New in version 0.9.0.
The SDK automatically compacts Realm files in the background by continuously reallocating data within the file and removing unused file space. Automatic compaction is sufficient for minimizing the Realm file size for most applications.
Automatic compaction begins when the size of unused space in the file is more than twice the size of user data in the file. Automatic compaction only takes place when the file is not being accessed.
Manual Compaction Strategies
If automatic compaction is deemed insufficient, manual compaction can be used for applications that require stricter management of file size to improve performance. A production application should implement manual compaction to periodically reduce the realm file size if it does not use automatic compaction.
Compacting a realm can be an expensive operation that can block the UI thread. Optimize compacting to balance frequency with performance gains. If your application runs in a resource-constrained environment, you may want to compact when you reach a certain file size or when the file size negatively impacts performance.
Use either of the following two strategies to compact a realm file manually:
Realm.compact() static method: Use this method to compact a realm. You can use this to compact a Flexible Sync realm.
Conditionally compact on open: use the
shouldCompactCallback()when you want to define one or more conditions to determine whether to compact the realm. You might check for a certain realm file size, a percentage of unused space, or other conditions that are relevant to your performance needs or runtime environment.
Realm.compact() Static Method
You can compact a realm file by calling Realm.compact(). This method takes a Configuration as an argument. When you use this method, the device must have enough free space to make a copy of the realm.
Realm.compact() obtains an instance of the realm, and opens it to
trigger any schema version upgrades, file format upgrades, migration and
initial data callbacks. Upon successfully opening the realm and performing
these operations, this method then compacts the realm.
If successful, a call to Realm.compact() returns true.
Do not call this method from inside a transaction. You also cannot compact an open realm.
final config = Configuration.local([Car.schema]); final compacted = Realm.compact(config); print( "Successfully compacted the realm: $compacted"); // On success, this prints "true" final realm = Realm(config);
Conditionally Compact on Open
You can define a shouldCompactCallback() as a property of a realm's configuration. You can use this with both local-only and synced realms with the Configuration.local() and Configuration.flexibleSync() methods, respectively.
This callback takes two int values representing the total number of
bytes and the used bytes of the realm file on disk. The callback returns
a bool. Compaction only occurs if the bool returns true and
another process is not currently accessing the realm file.
The most basic usage is to define a file size at which compaction should occur.
final config = Configuration.local([Car.schema], shouldCompactCallback: ((totalSize, usedSize) { // shouldCompactCallback sizes are in bytes. // For convenience, this example defines a const // representing a byte to MB conversion for compaction // at an arbitrary 10MB file size. const tenMB = 10 * 1048576; return totalSize > tenMB; })); final realm = Realm(config);
You can define more complex logic if you need to optimize performance for different use cases. For example, you could set a threshold for compaction when a certain percentage of the file size is used.
final config = Configuration.local([Car.schema], shouldCompactCallback: ((totalSize, usedSize) { // Compact if the file is over 10MB in size and less than 50% 'used' const tenMB = 10 * 1048576; return (totalSize > tenMB) && (usedSize.toDouble() / totalSize.toDouble()) < 0.5; })); final realm = Realm(config);