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Vaadin Flow and Distributed Session Stores: State Synchronization Consistency
Lax Vale
0 reputation · 30 Dec 2025, 17:18 UTC
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0 reputation · 30 Dec 2025, 17:18 UTC
Vaadin Flow maintains a server-side component tree that mirrors the browser DOM. In high-availability environments, this state is often offloaded to a distributed session store to allow for seamless failover across multiple server nodes.
When the Java server state is serialized and stored externally, any discrepancy between the serialized state and the active Client-Side Engine in the browser can lead to synchronization failures. This is particularly critical when the communication protocol relies on specific component IDs and session identifiers to map JSON payloads to the correct DOM elements.
Given the reliance on a unique session ID and the Atmosphere-based Push mechanism, it is unclear how the framework ensures atomic state recovery when a session is migrated between nodes during an active WebSocket connection.
31775 reputation · 31 Dec 2025, 02:21 UTC
When a Vaadin Flow session moves to another cluster node while a WebSocket connection remains active, the server recovers the last-serialized UI state from the distributed session store. The client’s Push mechanism then supplies a delta of changes that occurred after that snapshot. The server compares the incoming delta against the recovered state using a monotonically increasing syncId; any changes newer than the snapshot are replayed, and a ComponentTreeSync message is sent to bring the client’s DOM tree into alignment.
Vaadin Flow serializes the component tree into the HttpSession. With a distributed session store, that session must be replicated across nodes. If the recovered state is stale, the server relies on the client‑sent push delta and syncId to identify and apply missing updates before synchronizing the tree.
\nAll session attributes that Vaadin Flow accesses must be serializable; non‑serializable objects or transient fields in UI components will cause replication to fail or produce incomplete state.
vaadintransient.Are you using Spring Session or the application server’s native clustering mechanism? This detail may alter the specific configuration steps.
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31,775 reputation · 31 Dec 2025, 05:18 UTC
When a browser sends an AJAX or Push request, the client includes the current syncId in the HTTP header X‑VAADIN‑SYNC‑ID. VaadinService reads this header, compares it to the syncId stored in the deserialized UI, and if the values differ it knows the client is ahead of the server.
In that case the server does not try to patch the tree; instead it issues a JSON payload with command":"redirect" and the current URL. The browser then performs a full page load, rebuilding the component tree from scratch on the new node.
Note that the redirect payload is sent as a normal 200 response, not a 302, so the browser keeps the session cookie and the UI instance is re‑hydrated with the latest state.