* Now all DB transactions use the "SERIALIZABLE" isolation level with explicit retries.
* Now DB transactions that includes only SELECT queries are marked as "read-only" so that
  the PostgreSQL engine could optimize concurrent access with the new isolation level.
  All future codes should use `beegin_readonly()` method from our own subclassed SQLAlchemy
  engine instance replacing all existing `db` context variables.
* Remove excessive database updates due to keypair API query counts and kernel API query counts.
  The keypair API query count is re-written to use Redis with one month retention. (#421)
  Now just calling an API does not trigger updates in the PostgreSQL database.
* Fix unnecessary database updates for agent heartbeats.
* Split many update-only DB transactions into smaller units, such as resource recalculation.
* Use PostgreSQL advisory locks to make the scheduling decision process as a critical section.
* Fix some of variable binding issues with nested functions inside loops.
* Apply event message coalescing to prevent event bursts (e.g., `DoScheduleEvent` fired after
  enqueueing new session requests) which hurts the database performance and potentially
  break the transaction isolation guarantees.

* This is to avoid excessive write transaction overheads to the database.
* Set the retention time since last update to one month.

Backported-From: main
Backported-To: 21.03

* This is to avoid excessive write transaction overheads to the database.
* Set the retention time since last update to one month.

Backported-From: main
Backported-To: 20.09

* Now all DB transactions use the "SERIALIZABLE" isolation level with explicit retries.
* Now DB transactions that includes only SELECT queries are marked as "read-only" so that
  the PostgreSQL engine could optimize concurrent access with the new isolation level.
  All future codes should use `beegin_readonly()` method from our own subclassed SQLAlchemy
  engine instance replacing all existing `db` context variables.
* Remove excessive database updates due to keypair API query counts and kernel API query counts.
  The keypair API query count is re-written to use Redis with one month retention. (#421)
  Now just calling an API does not trigger updates in the PostgreSQL database.
* Fix unnecessary database updates for agent heartbeats.
* Split many update-only DB transactions into smaller units, such as resource recalculation.
* Use PostgreSQL advisory locks to make the scheduling decision process as a critical section.
* Fix some of variable binding issues with nested functions inside loops.
* Apply event message coalescing to prevent event bursts (e.g., `DoScheduleEvent` fired after
  enqueueing new session requests) which hurts the database performance and potentially
  break the transaction isolation guarantees.

Backported-From: main
Backported-To: 21.03

* Now all DB transactions use the "SERIALIZABLE" isolation level with explicit retries.
* Now DB transactions that includes only SELECT queries are marked as "read-only" so that
  the PostgreSQL engine could optimize concurrent access with the new isolation level.
  All future codes should use `beegin_readonly()` method from our own subclassed SQLAlchemy
  engine instance replacing all existing `db` context variables.
* Remove excessive database updates due to keypair API query counts and kernel API query counts.
  The keypair API query count is re-written to use Redis with one month retention. (#421)
  Now just calling an API does not trigger updates in the PostgreSQL database.
* Fix unnecessary database updates for agent heartbeats.
* Split many update-only DB transactions into smaller units, such as resource recalculation.
* Use PostgreSQL advisory locks to make the scheduling decision process as a critical section.
* Fix some of variable binding issues with nested functions inside loops.
* Apply event message coalescing to prevent event bursts (e.g., `DoScheduleEvent` fired after
  enqueueing new session requests) which hurts the database performance and potentially
  break the transaction isolation guarantees.

Backported-From: main
Backported-To: 20.09