Accessing indexedDB in ServiceWorker. Race condition

Question

There aren't many examples demonstrating indexedDB in a ServiceWorker yet, but the ones I saw were all structured like this:

const request = indexedDB.open( 'myDB', 1 );
var db;

request.onupgradeneeded = ...

request.onsuccess = function() {
    db = this.result; // Average 8ms
};


self.onfetch = function(e)
{
    const requestURL = new URL( e.request.url ),
    path = requestURL.pathname;

    if( path === '/test' )
    {
        const response = new Promise( function( resolve )
        {
            console.log( performance.now(), typeof db ); // Average 15ms

            db.transaction( 'cache' ).objectStore( 'cache' ).get( 'test' ).onsuccess = function()
            {
                resolve( new Response( this.result, { headers: { 'content-type':'text/plain' } } ) );
            }
        });

        e.respondWith( response );
    }
}

Is this likely to fail when the ServiceWorker starts up, and if so what is a robust way of accessing indexedDB in a ServiceWorker?

Answer 1

Opening the IDB every time the ServiceWorker starts up is unlikely to be optimal, you'll end up opening it even when it isn't used. Instead, open the db when you need it. A singleton is really useful here (see https://github.com/jakearchibald/svgomg/blob/master/src/js/utils/storage.js#L5), so you don't need to open IDB twice if it's used twice in its lifetime.

The "activate" event is a great place to open IDB and let any "onupdateneeded" events run, as the old version of ServiceWorker is out of the way.