A Look Inside Realm's Core DB Engine MobileOptimized, July 2015 JP Simard, @simjp, realm.io 1 

Realm 2 

What is Realm? —Fast, zero-copy, embedded database —Powers top 10 apps, Fortune 500, 150M+ devices —Object & model-oriented —Full ACID transactions —Cross-platform file format & C++ core —Language bindings (Objective-C, Swift & Android) —Launched summer 2014 3 

Swift let company = Company() // Standalone Realm Object company.name = "Realm" // etc... let realm = Realm() // Default Realm realm.write { // Transactions realm.add(company) // Persisted Realm Object } // Queries let companies = realm.objects(Company) // Typesafe companies[0].name // => Realm (generics) // "Jack"s who work full time (lazily loaded & chainable) let ftJacks = realm.objects(Employee).filter("name = 'Jack'") .filter("fullTime = true") 4 

Objective-C // Standalone Realm Object Company *company = [[Company alloc] init]; company.name = @"Realm"; // etc... // Transactions RLMRealm *realm = [RLMRealm defaultRealm]; [realm transactionWithBlock:^{ [realm addObject:company]; }]; RLMResults *companies = [Company allObjects]; // "Jack"s who work full time (lazily loaded & chainable) RLMResults *ftJacks = [[Employee objectsWhere:@"name = 'Jack'"] objectsWhere:@"fullTime == YES"]; 5 

Java Realm realm = Realm.getInstance(this.getContext()); // Default Realm realm.beginTransaction(); // Transactions Company company = realm.createObject(Company.class); // Persisted dog.setName("Realm"); // etc... realm.commitTransaction(); // Queries Company company = realm.where(Company.class).findFirst(); company.getName; // => Realm // "Jack"s who work full time (lazily loaded & chainable) RealmResults ftJacks = realm.where(Employee.class) .equalTo("name", "Jack") .equalTo("fullTime", true) .findAll(); 6 

Why did Realm build its own db engine from scratch? 7 

NoORM —ORM stands for "Object Relational Mapper" —Realm aims to have the features & flexibility of ORMs, without the performance overhead or the leaky abstractions 8 

9 

MVCC Algorithm 10 

11 

MVCC Algorithm —Multiversion concurrency control —Each transaction has a snapshot of the database —Writes are performed as append-only —It means you can have an immutable view of data —Reads don't block reads or writes —Writes block writes, but not reads 12 

Native Links 13 

14 

Native Links —Realm is a giant B+ tree —To-one & to-many links are 1st class citizens —No expensive join operations for relationships —Just following pointers —Great for object graph traversals 15 

String & Int Optimizations 16 

String Optimization —Convert common strings to enums —Expensive-ish operation, but smaller file size & faster reads Integer Packing —Ints take as little space on disk as possible —Little to no performance overhead 17 

Crash Safety 18 

Crash Safety —Kernal panics & sudden power loss should never corrupt the database —File format is append-only —Top root node is switched only upon full transactional durability —Runs F_FULLFSYNC on every logical file size change 19 

Zero Copy 20 

Traditional ORMs Must Copy 1. Data on disk 2. Read from disk 3. Copy raw data into deserialized intermediate in- memory representation (allocates memory) 4. Copy intermediate representation into language- level in-memory object (allocates memory) 5. Return final object from property access 21 

Realm Skips The Copy Whole file is memory-mapped & same format on disk as in-memory. 1. Calculate offset of data to read 2. Read from mmapped file 3. Return raw value from property access 22 

True Lazy Loading 23 

True Lazy Loading —Impossible to read a single bit from a hard disk —Properties are grouped together —Avoids reading unused properties —Saves disk roundtrips 24 

Built-In Encryption 25 

Built-In Encryption —Encrypted at rest on disk —AES-256+SHA2 —All cryptography done in the virtual memory manager —Works by marking mmapped region as readwrite- protected, throwing access violations on access 26 

Multiprocess Support 27 

Multiprocess Support —MVCC algorithm makes this work mostly out-of- the-box —Plus a named pipe for change notifications 28 

Null Values 29 

Null Values class Conference: Object { dynamic var name: String? = nil } 30 

Compromises ! 31 

Long Development Time 32 

33 

Pre-1.0 —APIs are in flux —File format could change 34 

Fewer Features At First 35 

Moving Forward 36 

Moving Forward —KVO & fine-grain notifications: pr #2050 —Nullable values: pr #1798 —Query handover between threads: seg-handover 37 

Links —MVCC on Wikipedia —B+ Trees on Wikipedia —About fsync() —More about fsync() —Realm on GitHub —Realm.io 38 

MobileOptimized().questions.ask()! JP Simard, @simjp, realm.io 39