db-stream.js

class DbStream extends stream.Writable {

    constructor() {
        super( {objectMode: true} );
    }

    _write(data, encoding, cb) {

        let specificFields = ["name", "pid", "hostname", "time", "level", "msg", "req_id", "v"];
        let fieldData = [];

        specificFields.forEach( (field) => {
            fieldData.push(data[field]);
            delete data[field];
        } );

        // Manually add the data field, as we didn't want to iterate over it earlier

        specificFields.push("data");
        fieldData.push(data);

        // The query needs to be formatted as $1, $2, $3 etc. So create an array for that.
        let variableNumbers = specificFields.map((item, i) => "$" + (i + 1));

        let query = "INSERT INTO log_entries (" + 
            specificFields.join(",") +
            ") VALUES (" +
            variableNumbers.join(",")
            + ":: jsonb)";

        client.query(query, fieldData, (err, result) => {
            if (err) {
                console.error(err);
            }
            cb()
        });

    }
}

@alastaircoote how scalable have you found this to be? and where does client come from?