Part 10 — partial sync

Off by default. Turning it on tells the relay something it can filter on, and everything you tell it is readable forever.

const engine = await SkmEngine.open({
  ...,
  metadata: {
    kind: true,                                   // send the record's kind
    shard: (r) => (r.data.channelTag as string),  // an app-chosen label
  },
});

await engine.openGraph('g:acme', { filter: { kinds: ['msg'] }, name: 'messages' });
engine.subscribe('g:acme', { filter: { kinds: ['msg'], shards: [tag] }, name: 'one-channel' });

Each subscription keeps its own cursor, because an unfiltered and a filtered view of one graph are at different places in the same sequence. name keeps them apart across restarts; it defaults to a stable hash of the filter, which is right unless you change a filter’s meaning without changing its shape.

What each field leaks:

• kind says “this is a message, that is a task”. Usually mild, occasionally not — a kind: 'crisis-plan' is a sentence.
• shard says as much as the label you choose. Use a derived tag, not a channel name: await engine.personalId('shard', channelId) gives you a stable opaque label. A shard must not change over a record’s life — the relay filters on it, so a record that moved shards would vanish from a subscription that had already passed it.

Sending neither is the default and costs only the ability to sync a subset.

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