Cross-server sync

Two servers sharing one database will happily serve stale data to each other. There are two problems in that sentence: noticing a row changed, and telling the other node about it.

Row sync

Polls a table for rows whose version column moved past a cursor, and hands them to a listener.

RowSyncConfig config = RowSyncConfig.builder("players", "uuid", "version", "updated_by", nodeId)
        .batchLimit(200)
        .startCursor(0)
        .build();

RowSyncPoller<Profile> poller = new RowSyncPoller<>(sql, config, Profile::from, changed -> apply(changed));

RowSyncService<Profile> service = new RowSyncService<>(poller);
service.start(scheduler, Duration.ofSeconds(5));
service.stop();
service.isRunning();

The updated-by column is what stops a node reacting to its own writes: rows stamped with your own node id are skipped.

poller.skipWhenDirty(key -> writeBehind.isDirty(key));
poller.cursor();
int applied = poller.pollOnce();

skipWhenDirty is the important one. A row you have unsaved local edits for should not be overwritten by a poll; skipping it lets your flush win rather than resurrecting stale data.

pollOnce runs a single pass, which is what a test drives and what an admin command can trigger.

The message bus

Row sync answers "what changed". A synchronizer answers "tell the others now".

DataSynchronizer sync = new LocalDataSynchronizer();
DataSynchronizer sync = new RedisDataSynchronizer("redis://localhost:6379");
DataSynchronizer sync = new RedisDataSynchronizer(uri, "myplugin");   // channel namespace

Subscription subscription = sync.subscribe("profiles", payload -> invalidate(payload));
sync.publish("profiles", uuid.toString());

subscription.close();
sync.close();

LocalDataSynchronizer is the single-node default and delivers in-process. RedisDataSynchronizer bridges the same contract across nodes. Code written against DataSynchronizer does not change when a server grows from one node to five.

The umbrella parameter namespaces channels, so two plugins on one Redis do not collide.

Message encoding

String frame = SyncMessage.encode(new SyncMessage("profiles", payload));
SyncMessage message = SyncMessage.decode(frame);

A tiny channel-plus-payload envelope, so the transport carries a string and the routing stays explicit.

Which to use

Row sync alone is often enough and needs nothing but the database you already have. Add a synchronizer when a five-second poll interval is too long to be correct.

For a lower-level binary bus with no relational dependency at all, see uxmlib-redis.