Meteor - using snychronised non-persistent / in-memory MongoDB on the server

Question

in a Meteor app, having real-time reactive updates between all connected clients is achieved with writing in collections, publishing and subscribing the right data. In normal case this means also database writes.

But what if I would like to sync particular data which does not need to be persistent and I would like to save the overhead of writing in the database ? Is it possible to use mini-mongo or other in-memory caching on the server by still preserving DDP synchronisation to all clients ?

Example

In my app I have a multiple collapsed threads and I want to show, which users currently expanded particular thread

Viewed by: Mike, Johny, Steven ...

I can store the information in the threads collection or make make a separate viewers collection and publish the information to the clients. But there is actually no meaning in making this information persistent an having the overhead of database writes.

I am confused by the collections documentation. which states:

OPTIONS connection Object The server connection that will manage this collection. Uses the default connection if not specified. Pass the return value of calling DDP.connect to specify a different server. Pass null to specify no connection.

and

... when you pass a name, here’s what happens:

... On the client (and on the server if you specify a connection), a Minimongo instance is created.

But If I create a new collection and pass the option object with conneciton: null

// Creates a new Mongo collections and exports it
export const Presentations = new Mongo.Collection('presentations', {connection: null});

/**
 * Publications
 */
if (Meteor.isServer) {
    // This code only runs on the server
    Meteor.publish(PRESENTATION_BY_MAP_ID, (mapId) => {
        check(mapId, nonEmptyString);
        return Presentations.find({ matchingMapId: mapId });
    });
}

no data is being published to the clients.

Answer 1

TLDR: it's not possible.

There is no magic in Meteor that allow data being synced between clients while the data doesn't transit by the MongoDB database. The whole sync process through publications and subscriptions is triggered by MongoDB writes. Hence, if you don't write to database, you cannot sync data between clients (using the native pub/sub system available in Meteor).

Answer 2

After countless hours of trying everything possible I found a way to what I wanted:

export const Presentations = new Mongo.Collection('presentations', Meteor.isServer ? {connection: null} : {});

I checked the MongoDb and no presentations collection is being created. Also, n every server-restart the collection is empty. There is a small downside on the client, even the collectionHanlde.ready() is truthy the findOne() first returns undefined and is being synced afterwards.

I don't know if this is the right/preferable way, but it was the only one working for me so far. I tried to leave {connection: null} in the client code, but wasn't able to achieve any sync even though I implemented the added/changed/removed methods.

Sadly, I wasn't able to get any further help even in the meteor forum here and here