Introduction to the MongoDB Realm Sync
Welcome back! We really appreciate you coming back and showing your interest in Realm. This is a follow-up article to Introduction to Realm for Android. If you haven’t read that yet, we recommend you go through it first.
This is a beginner-level article, where we introduce you to MongoDB Realm Sync. As always, we demonstrate its usage by building an Android app using the MVVM architecture.
Prerequisites
You have created at least one app using Android Studio.
What Are We Trying to Solve?
In the previous article, we learned that the Realm SDK is easy to use when working with a local database. But in the world of the internet we want to share our data, so how do we do that with the Realm Database?
MongoDB Realm Sync
Realm Sync is the solution to our problem. It’s one of the many features provided by MongoDB Realm. It synchronizes the data between client-side Realms and the server-side cloud, MongoDB Atlas, without worrying about conflict resolution and error handling.
The illustration below demonstrates how MongoDB Realm has simplified the complex architecture:
To demonstrate how to use MongoDB Realm Sync, we will extend our previous application, which tracks app views, to use Realm Sync.
Step 1: Get the Base Code
Clone the original repo and rename it “HelloRealmSync.”
Step 2: Enable Realm Sync
Update the syncEnabled state as shown below in the Gradle file (at the module level):
Also, add the buildConfigField to buildTypes in the same file:
You can ignore the value of App Key for now, as it will be covered in a later step.
Step 3: Set Up Your Free MongoDB Atlas Cloud Database
Once this is done, we have a cloud database where all our mobile app data can be saved, i.e., MongoDB Atlas. Now we are left with linking our cloud database (in Atlas) with the mobile app.
Step 4: Create a Realm App
In layman’s terms, Realm apps on the MongoDB cloud are just links between the data flowing between the mobile apps (Realm SDK) and Atlas.
Step 5: Add the Realm ID to the Android Project
Copy the Realm app ID and use it to replace App Key in the build.gradle file, which we added in Step 2.
With this done, MongoDB Realm and your Android App are connected.
Step 6: Enable Realm Sync and Authentication
MongoDB Realm is a very powerful tool and has a bunch of cool features from data security to its manipulation. This is more than sufficient for one application. Let’s enable authentication and sync.
But Why Authentication?
MongoDB Realm is designed to make apps secure by default, by not allowing an unknown user to access data.
We don’t have to force a user to sign up for them to become a known user. We can enable anonymous authentication, which is a win-win for everyone.
So let’s enable both of them:
Let's quickly recap what we have done so far.
In the Android app:
- Added Realm ID to the Gradle file.
- Enabled Realm Sync.
In MongoDB Realm:
- Set up an account.
- Created a free cluster for MongoDB Atlas.
- Created a Realm app.
- Enabled anonymous authentication.
- Enabled sync.
Now, the final piece is to make the necessary modifications to our Android app.
Step 7: Update the Android App Code
The only code change is to get an instance of the Realm database from the Realm app instance.
a. Get a Realm app instance from which the Realm instance can be derived:
b. Update the View Model constructor to accept the Realm app instance:
c. Update the creation of the View Model:
d. Update the updateData method in HomeViewModel:
In the above snippet, we are doing two primary things
a. Getting a user instance by signing in anonymously.
b. Getting a Realm instance using SyncConfiguration.Builder.
SyncConfiguration.Builder(user, user.id).build()Where user.id is the partition key we defined in our MongoDB Realm Sync configuration (Step 6). In simple terms, the partition key is an identifier that helps you to get the exact data as per client needs. For more details, please refer to the article on Realm partitioning strategies.
Step 8: View Your Results in MongoDB Atlas
You can find the complete working code in our GitHub repo.
