# Sync Configuration > Source-anchored research for Sync Configuration. Run status: COMPLETED. Parent: [MongoDB Realm Mobile Sync](https://llms-explorer.com/tree/mongodb-realm-mobile-sync/) · 5 facets · 18 facts · page: https://llms-explorer.com/tree/sync-configuration/ ## Structure and components - The 2022 vendor Flexible Sync tutorial configures the backend cluster, allowed query fields, and permissions, then adds client subscriptions. Its initial User query contains no synced documents before a subscription is registered. (low confidence) — [source](https://medium.com/realm/using-realm-flexible-sync-in-your-app-da6b4abb0288) ## How it works - The Java SyncConfiguration.Builder's waitForInitialRemoteData setting delays the first Realm open until known server changes download, so the Realm will not open offline. The docs recommend opening it on a background thread or using getInstanceAsync, and a timeout overload bounds the wait. The check runs only when a Realm is first created; later freshness checks use downloadAllServerChanges(). (low confidence) — [source](https://realm.netlify.app/docs/java/latest/api/io/realm/syncconfiguration.builder) ## Problems, failure modes and limitations - Java issue #7818 reports offline startup failure with SDK 10.10.1, waitForInitialRemoteData(500, TimeUnit.MILLISECONDS), and initial subscriptions. A maintainer requested the sync error handler and subscription error details. The issue was closed for lack of information, so it does not establish a confirmed SDK defect or a verified fix. (low confidence) — [source](https://github.com/realm/realm-java/issues/7818) ## Comparisons and alternatives - MongoDB announced that Atlas Device Sync would be phased out by September 30, 2025; Couchbase's post on that date reports the end of support. Configuration examples for that managed service must be presented as historical, rather than as a currently supported deployment recipe. (medium confidence) (verified as of 2025-09-30) — [source](https://www.mongodb.com/company/blog/innovation/future-proof-your-apps-with-mongodb-wekan) - A Tecnocampus educational Android application project documents synchronous versus asynchronous Realm SDK access and describes opting into synchronous transactions through SyncConfiguration. This is evidence from a specific Java application implementation; it does not establish identical defaults across Realm SDKs. (low confidence) — [source](https://repositori.tecnocampus.cat/bitstream/handle/20.500.12367/2175/TFG_CelsSantiago_Memoria.pdf?sequence=1) - The vendor tutorial describes a subscription tradeoff: retaining subscriptions for previously visited chat rooms enables offline revisits but keeps more data synchronized, consuming storage and bandwidth. (low confidence) — [source](https://medium.com/realm/using-realm-flexible-sync-in-your-app-da6b4abb0288) ## Facts and statements - Historical Realm Object Server SDKs used sync configuration to bind a Realm to an authenticated user and a server Realm URI. The cited Java builder and Swift 2.9.1 API document this pattern; it should not be treated as the Atlas-era constructor signature. (high confidence) — [source](https://realm.netlify.app/docs/java/latest/api/io/realm/syncconfiguration.builder) - Synced files carry user identity. A .NET maintainer explained that different users need separate Realm files and that copying another user's prefilled synced file does not remove its user association. The Java SyncConfiguration.Builder also places synced files under user-specific paths derived from the Realm URI by default. (high confidence) — [source](https://github.com/realm/realm-dotnet/issues/3205) - The historical Java and Swift APIs enable server certificate validation by default or provide an explicit disabling option, and both warn against disabling verification in production. (high confidence) — [source](https://realm.netlify.app/docs/java/latest/api/io/realm/syncconfiguration.builder) - The Flutter workshop opens a synced Realm with Configuration.flexibleSync(user, [Item.schema]) and then adds a subscription. This illustrates two separate steps: constructing the client configuration and defining the data subscription. (low confidence) — [source](https://markandeyp.github.io/atlas-app-services-codelab/) - A Swift user's logs show successful local writes while the sync session repeatedly reports that initial sync is incomplete. Realm Core separately defines initial_sync_not_completed as opening a session before initial sync finishes. A local write succeeding therefore does not demonstrate a working remote sync session. (medium confidence) — [source](https://stackoverflow.com/questions/76659835/error-client-tried-to-connect-using-flexible-sync-before-initial-sync-is-comple) - MongoDB's transition guidance includes integrating new SDKs and changing security configurations. ObjectBox's own connector documentation describes ObjectBox clients and an ObjectBox Sync Server between devices and MongoDB. These sources support planning a new integration, not assuming that an old Realm SyncConfiguration is portable to another provider. (medium confidence) — [source](https://www.mongodb.com/company/blog/innovation/future-proof-your-apps-with-mongodb-wekan) - Java SyncConfiguration extends RealmConfiguration and adds an authenticated user, server connection, and synchronization settings to local file configuration; historical and Atlas-era APIs expose local naming and encryption controls. (high confidence) — [source](https://github.com/realm/realm-java/blob/main/realm/realm-library/src/objectServer/java/io/realm/mongodb/sync/SyncConfiguration.java) - Configuration examples require SDK-generation labels: the Realm Object Server Java builder takes a SyncUser and a server Realm URI, whereas the Atlas-era Java source exposes Builder(User) for Flexible Sync. (high confidence) — [source](https://realm.netlify.app/docs/java/latest/api/io/realm/syncconfiguration.builder) - Local persistence does not establish successful synchronization. A November 2022 Android report created local data but failed to upload; the author identified a backend schema mismatch. The vendor tutorial separately requires backend schemas and explains development-mode schema generation. (medium confidence) — [source](https://stackoverflow.com/questions/74543074/mongodb-realm-flexible-sync-not-syncing-to-server-on-an-android-app) - A July 2024 React SDK question reports offline startup hanging despite OpenImmediately for both new and existing files. It specifies package ranges realm ^12.11.1 and @realm/react ^0.8.0, and shows network, location, and token-refresh errors. The question has no posted answer or confirmed resolution; it does not establish a universal configuration fix. (low confidence) — [source](https://stackoverflow.com/questions/78795709/realm-device-sync-hangs-on-app-launch-when-offline-react-sdk?r=31) - MongoDB's Realm JavaScript repository records the September 2024 Device Sync and SDK deprecation announcement and directs users seeking a version without sync to the community branch or community npm distribution. (low confidence) (verified as of 2026-10-23) — [source](https://github.com/realm/realm-js) - Couchbase's September 30, 2025 announcement says MongoDB no longer supports Atlas Device Sync and Device SDKs. It is competitor migration marketing and supports the service-lifecycle boundary, not the correctness of specific Realm configuration APIs. (low confidence) (verified as of 2025-09-30) — [source](https://www.couchbase.com/blog/realm-mongodb-eol-day-2025/)