Issue using d.listen

[motosota]

It looks like there may be a possible race condition in d.listen - or that I am misunderstanding how to use it (quite possible).

I have one function A which transacts data to a db using data from a websocket connection:

function listener(args, kwargs) {
console.log('** Listener called', kwargs[':tx-data']);
d.transact(conn, kwargs[':tx-data']);
db1 = d.db(conn);
}

I have a separate reporter function B which generates an event for react js to trigger a re-render.

d.listen(conn, function(report) {
console.log('**Reporter heard and declaring showtime here is my report', report);
document.dispatchEvent(new CustomEvent('SHOWTIME'));
});

Re-rendering triggers a query from React and isn't always working - it turns out the problem is the data which the reporter thinks should be in the DB (from the the report tx_data) isn't always - so as if the listener trigger isn't when the transaction actually finishes.

Confusing is that moving the 'showtime' event dispatch to the end of A works.
This would make sense if d.transact was blocking, but if that was the true then I wouldn't expect d.listen to run until the transaction was finished.
If d.transact is non-blocking then I would expect to see the same issue with the event dispatch in either place - and I don't.

Any thoughts / suggestions appreciated.

[tonsky]

Listeners are all called inside d.transact fn. First it applies a transaction, then swaps conn value, then call all listeners. By the end of d.transact all listeners will already be called. Maybe that’s your issue? How is render fn gets value of actual DB? What’s db1 = d.db(conn); there for?

[motosota]

So I'm probably doing this wrong..
conn and db1 are both defined as variables at the start of the application page.

var d = datascript;
var db1;
var conn;
...

I'm just using db1 as the single database used by the application

conn is set after retrieving schema and initialising:

 conn = d.create_conn(schema);

db1 is set after each transaction:

db1 = d.db(conn);

Render gets value of actual db (db1) directly:

  var data = d.q('query our data here', db1);
  React.render(
    <MyApp data={data} />,
    document.getElementById('content')
  );
}, false);

So in summary I'm viewing conn as something that doesn't change after creating the db, db1 is forced to change after each transaction - and db1 is then used directly for React queries

[tonsky]

Yep, you don’t need db1. Just use d.db(conn) where you need latest DB.