Concurrency error

[JaredBoone]

If you run pg-live-select in multiple processes concurrently, postgres throws "ERROR: tuple concurrently updated". This happens because CREATE FUNCTION (from triggers.sql.tpl) can't be used concurrently.

[JaredBoone]

I have worked around this by wrapping the SQL in trigger.sql.tpl in a transaction that includes a transaction-level advisory lock as follows:

BEGIN;
SELECT pg_advisory_xact_lock(510127006994284723); --64 bit unsigned int
CREATE OR REPLACE FUNCTION ... 
$$ LANGUAGE plpgsql;
COMMIT;

See https://stackoverflow.com/questions/40525684/tuple-concurrently-updated-when-creating-functions-in-postgresql-pl-pgsql for more details.

[JaredBoone]

In case anyone else runs into this, the package owner informed me that the channel string must be unique for each child process. Here's an example of how to do this:

parent.js:

const { fork } = require('child_process');
const numForks = 2;
for (var i = 0; i < numForks; i++) {
    var child = fork(require.resolve('./child.js'), [], { env: {
      CONN: 'postgres://meteor:meteor@127.0.0.1/meteor',
      CHANNEL: 'chan_'+i } });
}

child.js:

const LivePG = require('pg-live-select');
var liveDb;
process.on('exit', () => {
   if (liveDb) {
        liveDb.cleanup(() => console.log('cleaned up'));
    }
});
liveDb = new LivePG(process.env.CONN, process.env.CHANNEL)
liveDb.select('SELECT * FROM assignments')
    .on('update', function (diff, data) {
        console.log(process.env.CHANNEL + ': ', data);
    });