I get this is a backend library, which is great, but like does it use postgres replication slots? Per the inherited queries, do they all live on 1 machine, and we just assume that machine needs to be sufficiently beefy to serve all currently-live queries?

Do all of my (backend) live-queries live/run on that one beefy machine? What's the life cycle for live-queries? Like how can I deploy new ones / kill old ones / as I'm making deployments / business logic changes that might change the queries?

This is all really hard ofc, so apologies for all the questions, just trying to understand -- thanks!

Let me begin by answering: what exactly is this engine? It's simply a computation + cache layer that lives in the same process as the calling code, not a server on its own.

Think of a LinkedQL instance (new PGClient()) and its concept of a "Live Query" engine as simply a query client (e.g. new pg.Client()) with an in-memory compute + cache layer.

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1. Deployment model (current state)

The Live Query engine runs as part of your application process — the same place you’d normally run a Postgres/MySQL client.

For Postgres, yes: it uses one logical replication slot per LinkedQL engine instance. The live query engine instantiates on top of that slot and uses internal "windows" to dedupe overlapping queries, so 500 queries that are only variations of "SELECT * FROM users" still map to one main window; and 500 of such "windows" still run over the same replication slot.

The concept of query windows and the LinkedQL inheritance model is fully covered here: https://linked-ql.netlify.app/engineering/realtime-engine

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2. Do all live queries “live” on one machine?

As hinted at above, yes; each LinkedQL instance (new PGClient()) runs on the same machine as the running app (just as you'd have it with new pg.Client()) – and maps to a single Live Query engine under the hood.

  That engine uses a single replication slot. You specify the slot name like:

  new PGClient({ ..., walSlotName: 'custom_slot_name' }); // default is: "linkedql_default_slot" – as per https://linked-ql.netlify.app/docs/setup#postgresql

  A second LinkedQL instance would require another slot name:
  
  new PGClient({ ..., walSlotName: 'custom_slot_name_2' });

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3. Lifecycle of live queries

The Live Query engine runs on-demand and not indefinitely. It begins to exist when at least one client subscribes ({ live: true }) and effectively cleans up and disappears the moment the last subscriber disconnects (result.abort()). Calling client.disconnect() also ends all subscriptions and does clean up.

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4. Deployments / code changes

Deploying new code doesn’t require “migrating” live queries.

When you restart the application:

• the Live Query starts on a clean slate with the first subscribing query (client.query('...', { live: true })).

• if you have provided a persistent replication slot name (the default being ephemeral), LinkedQL moves the position to the slot's current position and runs from there.

In other words: nothing persists across deploys; everything starts clean as your app starts.

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5. Diagram / docs

A deployment diagram is a good idea — I’ll add one to the docs.

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Well, I hope that helps — and no worries about the questions. This space is hard, and happy to explain anything in more detail.

LinkedQL definitely optimizes at multiple levels between a change happening on your database and the live result your application sees. The most significant of these being its concept of query windows and query inheritance which ensure multiple overlapping queries converge on a single "actual" query window under the hood.

You want to see the engineering paper for the full details: https://linked-ql.netlify.app/engineering/realtime-engine

- How does authz work? Can I use Postgres RLS? If not, how would you address row or column-level permissions in a system that uses this? - If you're using logical replication to sync with PG, is there a limit to the number of clients you can have connected? I see there is a lot of work around de-duping live queries, but how well does that work in practice? - Any thought to making an extension for Postgres? My main hesitation right now is that I have to go through an NPM package to use this but a lot of our tooling expects a plain Postgres connection. - REALLY looking forward to seeing how the schema migration story looks.

Overall, it seems to address most of the use-cases where I'd reach for an ORM or API server so I'm really interested to see where this could go.

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Auth / RLS

Yes — LinkedQL works with Postgres Row-Level Security. Each LinkedQL connection is equivalent to a regular DB connection (e.g., new LinkedQLClient(connectionInfo) is like new pg.Client(connectionInfo)). There’s no new permission model to maintain — the DB remains the enforcement point.

Live queries always execute under the same authenticated role you provided, so RLS policies apply on every refresh or incremental update. LinkedQL never uses a “superuser” backend that could widen visibility.

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Replication limits & scaling

Right now, each database connection supports one logical replication slot. LinkedQL dedupes overlapping live queries on top of it — so 1,000 clients watching the same underlying SELECT only cost the DB one change stream.

We plan to support a distributed architecture as well — multiple instances of the live query engine coordinating load for high-traffic deployments.

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Why an npm package (and future extension)

Right now LinkedQL plugs directly into JavaScript apps, matching how many teams already query Postgres from frontend or backend code.

We definitely have a Postgres extension in the roadmap for your exact use case – tighter operational integration.

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Schema migration story

This is also one I’m personally excited about. We previously had an automatic schema versioning layer in the earlier LinkedQL prototype:

https://github.com/linked-db/linked-ql/wiki/Automatic-Schema...

https://github.com/linked-db/linked-ql/wiki/Migrations

The goal in the current version is a cleaner rewrite of that whole feature. So, migration support is returning – with everything we learned in the previous baked in.

For example, while the previous implementation of the diff-based migration feature spoke JSON for schema declarations, we plan to let that be pure SQL – yet, diff-based.

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Thanks again for the thoughtful look! We can zoom into any other area of your choice.

Happy to dig into internals if anyone’s curious — how live updates propagate, how JOINs and complex queries resolve, consistency expectations, worst-case scaling, etc.

To keep the main post short, here are deep-dive links if you want to explore:

• Live update mechanics https://linked-ql.netlify.app/capabilities/live-queries

• Engineering paper (replication pipelines, differential projection, query inheritance) https://linked-ql.netlify.app/engineering/realtime-engine

Totally open to questions — I’m hanging around the thread to learn what concerns matter most.

MySQL/MariaDB support is in progress (binlog-based) and is why the docs say “coming soon.”

The post wasn’t meant to imply that MySQL/MariaDB are already live; the intention was to describe the overall design rather than claim full parity. I’ll update the wording to avoid that confusion.

Short answer: the core LinkedQL live query engine runs on the backend today, and there’s an embeddable variant (FlashQL) that runs directly in the frontend with the same LinkedQL capabilities – live queries, DeepRefs, etc.

1. Pure frontend / local data

For data that can live entirely on the client, you can spin up an in-browser FlashQL instance:

const client = new FlashQL(); // runs in the page / worker

await client.query(` CREATE TABLE users ( id UUID PRIMARY KEY, name TEXT ) `);

// Live query works the same way as on the backend: const result = await client.query( 'SELECT * FROM users', { live: true } );

From there, result is a live result set: inserts/updates/deletes that match the query will show up in the rows, and all the same features (live queries, DeepRefs, etc.) behave as they do on a backend instance.

At the moment FlashQL is in-memory only; persistence backends like IndexedDB / LocalStorage are on the roadmap.

2. Remote database from the frontend

If your source of truth is a remote Postgres/MySQL instance, the model we’re building is:

a LinkedQL engine next to the database, and

a FlashQL instance in the frontend that federates/syncs with that backend engine.

That federation/sync path is in alpha right now (early docs here: https://linked-ql.netlify.app/flashql/foreign-io ), so today the “stable” story is:

run LinkedQL on the backend against Postgres/MySQL,

expose whatever API you like to the frontend,

and use FlashQL locally where a client-side store makes sense.

The goal is that the frontend doesn’t need a special framework — just a LinkedQL/FlashQL client wherever JavaScript runs.

The commit history is legitimately insane though: https://github.com/linked-db/linked-ql/commits/master/

[0] https://news.ycombinator.com/item?id=46194311