QueryRunner stale/idle connections issue

[michaelndu]

Issue type:

[x] question
[ ] bug report
[ ] feature request
[ ] documentation issue

Database system/driver:

[ ] cordova
[ ] mongodb
[ ] mssql
[x] mysql / mariadb
[ ] oracle
[ ] postgres
[ ] cockroachdb
[ ] sqlite
[ ] sqljs
[ ] react-native
[ ] expo

TypeORM version:

[ ] latest
[ ] @next
[x] 0.2.11 (or put your version here)

Hello,

We're running a NestJS app using typeorm to read/write data from a mysql (Aurora) instance. We're seeing an issue with what appears to be idle connections being used by the QueryRunner, and we encounter the following error:

QueryFailedError: Connection lost: The server closed the connection.
    at new QueryFailedError (/usr/src/app/node_modules/typeorm/error/QueryFailedError.js:28:28)
    at Query.<anonymous> (/usr/src/app/node_modules/typeorm/driver/mysql/MysqlQueryRunner.js:227:45)
    at Query.<anonymous> (/usr/src/app/node_modules/mysql/lib/Connection.js:502:10)
    at Query._callback (/usr/src/app/node_modules/mysql/lib/Connection.js:468:16)
    at Query.Sequence.end (/usr/src/app/node_modules/mysql/lib/protocol/sequences/Sequence.js:83:24)
    at /usr/src/app/node_modules/mysql/lib/protocol/Protocol.js:396:18
    at Array.forEach (<anonymous>)
    at /usr/src/app/node_modules/mysql/lib/protocol/Protocol.js:395:13
    at process._tickCallback (internal/process/next_tick.js:61:11)

So far we haven't seen this issue when using repository.save or repository.update, we've only seen this from the calls that use a queryBuilder, such as

this.repo.createQueryBuilder('merchant')
      .leftJoinAndSelect('merchant.profile', 'profile')
      ...
      .leftJoinAndSelect('merchant.orderRules', 'orderRules')
      .addOrderBy('orderRules.index');
      .where('merchant.usageReportingScheduleBucket = :bucket', { bucket })
      .andWhere('merchant.billingId is not null')
      .getMany();

I'm wondering if this is potentially a bug where connections are being held open for too long, and only recycled when a connection error is encountered? From my understanding, the QueryRunner is supposed to create and use a dedicated connection, but that doesn't seem to be the case here? Are there any typeorm settings that we can configure differently to prevent encountering this error?