--- title: REGISTER slug: data-types/register docTags: createdAt: 2023-10-05T15:09:43.650Z --- The `REGISTER` type is the core building block of every app built on Ditto. This article provides an overview of the `REGISTER` type, including its merge strategy and the various scalar subtypes you can use to encode single values within a `REGISTER`. # Supported Scalar Subtypes A `REGISTER` stores a single scalar value. You can encode the value using any JSON‑compatible scalar subtype, such as a `string`, `boolean`, JSON object to represent two or more fields as a single object, and so on. The following table provides an overview of the different scalar types you can use to store a single value in a `REGISTER`: | **Data Type** | **Corresponding Values** | | -------------- | -------------------------------------------------------------------------------------------------------------------------------------------------- | | `array` | An ordered list of values, where each value represents any primitive type, as well as nested collection types such as other `arrays` or `MAP type` | | `boolean` | `true` or `false` | | `float` | 64-bit floating-point | | `int` | Signed 64-bit integer | | `JSON-object` | Embedded JSON blob | | `null` | Represents an absence of a value | | `string` | UTF-8 encodable | | `unsigned int` | Unsigned 64-bit integer | ## Example Updating a REGISTER For example, the following snippet shows a basic `UPDATE` statement query setting the values of two separate `REGISTERS`: - A `REGISTER` storing a `string` set to the value `'blue'`. - A `REGISTER` storing an `integer` set to the value `3001`. :::CodeblockTabs DQL ```sql UPDATE cars SET name = 'blue' mileage = 3001 WHERE _id = '123' ``` ::: ## Limitations to Arrays Unlike typical `arrays`, the multiple elements within an `array` scalar subtype function collectively as a single `REGISTER` with last-write-wins merge behavior. An `array` scalar subtype is an ordered collection of items represented using any primitive JSON‑compatible data type enclosed within square brackets (\[ ]).  The `array` scalar subtype, while great at managing unordered operations, such as a GPS coordinate where both latitude and longitude values change together, as follows, maintaining a strict order could potentially lead to data inconsistencies at merge. ```json "location": { "coordinates": [-122.0308, 37.3318], "address": "123 Main St, San Francisco, CA 94105" } ``` Considering this, when managing collections of data that have concurrent editors, opt for a `MAP` instead, which effectively functions as a set, more commonly known as an *associative array. *For more information, see [MAP](docId:5abei_YfBmm3Jn_ysEdZF). # Last-Write-Wins Merge Strategy The `REGISTER` type adheres to the *last-write-wins* principle when handling concurrency conflicts. This means that when changes occur, all peers observing the change will sequence these changes in the same order. This ensures that only a consistent single value syncs across the entire peer-to-peer mesh. For example, one flight attendant updates a customer's seat number to `6` and another to seat `9`. When the two conflicting versions merge, the edit with the highest timestamp wins. Put another way, by enforcing the last-write-wins merge strategy, for events A and B, where B is a result of A, event A always occurs before B. # Syncing with REGISTER Each `REGISTER` in a document syncs independently. When the value that the `REGISTER` holds changes, the entire `REGISTER` value syncs across the mesh. For example, the following code, once executed, results in the field `color` being changed and the value `'blue'` being synced across peers: :::CodeblockTabs DQL ```sql UPDATE cars SET color = 'black' WHERE _id = '123abc' ``` ::: Since a `REGISTER` acts as a single object, to update just the `color`, you'd need to update the entire object by providing the entire set of nested key-value pairs, including the modified `color` , as follows: :::CodeblockTabs ```swift let newProperties = [ "color": "blue", "sunroof": true, "door_count": 4 ] await ditto.store.execute( "UPDATE cars SET properties = :newProperties WHERE _id = '123'", ["newProperties": newProperties]); ``` ```kotlin var result = ditto.store.execute( "INSERT INTO cars DOCUMENTS (:newCar)", mapOf("newProperties" to mapOf("color" to "blue", "sunroof" to true, "door_count" to 4))) ``` ```javascript const newProperties = { color: 'blue', sunroof: true, door_count: 4 } await ditto.store.execute( "UPDATE cars SET properties = :newProperties WHERE _id = '123'", {newProperties}); ``` ```java Map newProperties = new HashMap<>(); newCar.put("color", "blue"); newCar.put("sunroof", true); newCar.put("door_count", 4); DittoQueryResult result = (DittoQueryResult) ditto.store.execute( "UPDATE cars SET properties = :newProperties WHERE _id = '123'", Collections.singletonMap("newProperties", newProperties), new Continuation<>() { @NonNull @Override public CoroutineContext getContext() { return EmptyCoroutineContext.INSTANCE; } @Override public void resumeWith(@NonNull Object o) { if (o instanceof Result.Failure) { // Handle failure } } } ); ``` ```csharp var args = new Dictionary(); args.Add("newProperties", new { color="blue", sunroof=true, doorCount=4}); await ditto.Store.ExecuteAsync( "UPDATE cars SET properties = :newProperties WHERE _id = '123'", args); ``` ```cpp struct Properties { std::string color; bool sunroof; int door_count; }; // ... std::map args; args["newProperties"] = {"blue", true, 4}; ditto.get_store().execute( "UPDATE cars SET properties = :newProperties WHERE _id = '123'", args); ``` ```rust struct Properties { color: String, sunroof: bool, door_count: i32 } struct Args { newProperties: Properties, } // ... let args = Args { newProperties: Properties { color: "blue".to_string(), sunroof: true, door_count: 4 } }; await ditto.store().execute( "UPDATE cars SET properties = :newProperties WHERE _id = '123'", args); ``` ```dart final newProperties = { "color": "blue", "sunroof": true, "door_count": 4, }; await ditto.store.execute( "UPDATE cars SET properties = :newProperties WHERE _id = '123'", arguments: {"newProperties": newProperties}, ); ``` :::