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To make your Android apps fast and responsive, you must balance the computing time needed to lay out the visuals and handle user interactions with the time needed to process your data and run your business logic. Typically, app developers spread this work across multiple threads: the main or UI thread for all of the user interface-related work, and one or more background threads to compute heavier workloads before sending it to the UI thread for presentation. By offloading heavy work to background threads, the UI thread can remain highly responsive regardless of the size of the workload.
MongoDB Realm enables simple and safe multithreaded code when you follow these three rules:
- Avoid writes on the UI thread if you write on a background thread:
- You can write to a realm from any thread, but there can be only one writer at a time. Consequently, write transactions block each other. A write on the UI thread may result in your app appearing unresponsive while it waits for a write on a background thread to complete. If you are using Realm Sync, avoid writing on the UI thread as Sync writes on a background thread.
- Don't pass live objects, collections, or realms to other threads:
- Live objects, collections, and realm instances are thread-confined: that is, they are only valid on the thread on which they were created. Practically speaking, this means you cannot pass live instances to other threads. However, Realm Database offers several mechanisms for sharing objects across threads.
- Don't lock to read:
- Realm Database's Multiversion Concurrency Control (MVCC) architecture eliminates the need to lock for read operations. The values you read will never be corrupted or in a partially-modified state. You can freely read from realms on any thread without the need for locks or mutexes. Unnecessarily locking would be a performance bottleneck since each thread might need to wait its turn before reading.
Live objects, collections, and realms are thread-confined. If you need to work with the same data across multiple threads, you should open the same realm on multiple threads as separate realm instances. The Java SDK consolidates underlying connections across threads where possible to make this pattern more efficient.
When you need to communicate across threads, you have several options depending on your use case:
- To modify the data on two threads, query for the object on both threads using a primary key.
- To send a fast, read-only view of an object to other threads, freeze the object.
- To keep and share many read-only views of the object in your app, copy the object from the realm.
- To react to changes made on any thread, use notifications.
- To see changes from other threads in the realm on the current thread, refresh your realm instance (event loop threads refresh automatically).
RealmObject instances are
not thread-safe or
Parcelable, so you cannot pass them between
activities or threads via an
Intent. Instead, you can pass an object
identifier, like a primary key,
Intent extras bundle, and then open a new realm instance
in the separate thread to query for that identifier. Alternatively, you
can freeze Realm objects.
Live, thread-confined objects work fine in most cases. However, some apps -- those based on reactive, event stream-based architectures, for example -- need to send immutable copies across threads. In this case, you can freeze objects, collections, and realms.
Freezing creates an immutable view of a specific object, collection, or realm that still exists on disk and does not need to be deeply copied when passed around to other threads. You can freely share a frozen object across threads without concern for thread issues.
Frozen objects are not live and do not automatically update. They are effectively snapshots of the object state at the time of freezing. When you freeze a realm all child objects and collections also become frozen. You can't modify frozen objects, but you can read the primary key from a frozen object, query a live realm for the underlying object, and then update that live object instance.
Frozen objects remain valid for as long as the realm that spawned them stays open. Avoid closing realms that contain frozen objects until all threads are done working with those frozen objects.
When working with frozen objects, an attempt to do any of the following throws an exception:
- Opening a write transaction on a frozen realm.
- Modifying a frozen object.
- Adding a change listener to a frozen realm, collection, or object.
Once frozen, you cannot unfreeze an object. You
isFrozen() to check if an object is frozen.
This method is always thread-safe.
To freeze an object, collection, or realm, use the freeze() method:
Frozen objects preserve an entire copy of the realm that contains them at the moment they were frozen. As a result, freezing a large number of objects can cause a realm to consume more memory and storage than it might have without frozen objects. If you need to separately freeze a large number of objects for long periods of time, consider copying what you need out of the realm instead.
When you open a realm, it reflects the most recent successful write commit and remains on that version until it is refreshed. This means that the realm will not see changes that happened on another thread until the next refresh. Realms on any event loop thread (including the UI thread) automatically refresh themselves at the beginning of that thread's loop. However, you must manually refresh realm instances that are tied to non-looping threads or that have auto-refresh disabled. To refresh a realm, call Realm.refresh():
Realms also automatically refresh after completing a write transaction.
Realm Database provides safe, fast, lock-free, and concurrent access across threads with its Multiversion Concurrency Control (MVCC) architecture.
If you are familiar with a distributed version control system like Git, you may already have an intuitive understanding of MVCC. Two fundamental elements of Git are:
- Commits, which are atomic writes.
- Branches, which are different versions of the commit history.
Similarly, Realm Database has atomically-committed writes in the form of transactions. Realm Database also has many different versions of the history at any given time, like branches.
Unlike Git, which actively supports distribution and divergence through forking, a realm only has one true latest version at any given time and always writes to the head of that latest version. Realm Database cannot write to a previous version. This makes sense: your data should converge on one latest version of the truth.
A realm is implemented using a B+ tree data structure. The top-level node represents a version of the realm; child nodes are objects in that version of the realm. The realm has a pointer to its latest version, much like how Git has a pointer to its HEAD commit.
Realm Database uses a copy-on-write technique to ensure isolation and durability. When you make changes, Realm Database copies the relevant part of the tree for writing, then commits the changes in two phases:
- Write changes to disk and verify success.
- Set the latest version pointer to point to the newly-written version.
This two-step commit process guarantees that even if the write failed partway, the original version is not corrupted in any way because the changes were made to a copy of the relevant part of the tree. Likewise, the realm's root pointer will point to the original version until the new version is guaranteed to be valid.
The following diagram illustrates the commit process:
- The realm is structured as a tree. The realm has a pointer to its latest version, V1.
- When writing, Realm Database creates a new version V2 based on V1. Realm Database makes copies of objects for modification (A 1, C 1), while links to unmodified objects continue to point to the original versions (B, D).
- After validating the commit, Realm Database updates the realm's pointer to the new latest version, V2. Realm Database then discards old nodes no longer connected to the tree.
Realm Database uses zero-copy techniques like memory mapping to handle data. When you read a value from the realm, you are virtually looking at the value on the actual disk, not a copy of it. This is the basis for live objects. This is also why a realm head pointer can be set to point to the new version after the write to disk has been validated.
MongoDB Realm enables simple and safe multithreaded code when you follow three rules:
- Avoid writes on the UI thread if you write on background threads or use Realm Sync.
- Don't pass live objects to other threads.
- Don't lock to read.
- In order to see changes made on other threads in your realm instance, you must manually refresh realm instances that do not exist on "loop" threads or that have auto-refresh disabled.
- For apps based on reactive, event-stream-based architectures, you can freeze objects, collections, and realms in order to pass copies around efficiently to different threads for processing.
- Realm Database's multiversion concurrency control (MVCC) architecture is similar to Git's. Unlike Git, Realm Database has only one true latest version for each realm.
- Realm Database commits in two stages to guarantee isolation and durability.