Change tracking is the process of determining what has changed in managed entities since the last time they were synchronized with the database.
Doctrine provides 2 different change tracking policies, each having its particular advantages and disadvantages. The change tracking policy can be defined on a per-class basis (or more precisely, per-hierarchy).
The deferred implicit policy is the default change tracking policy and the most convenient one. With this policy, Doctrine detects the changes by a property-by-property comparison at commit time and also detects changes to entities or new entities that are referenced by other managed entities (persistence by reachability). Although the most convenient policy, it can have negative effects on performance if you are dealing with large units of work (see Understanding the Unit of Work). Since Doctrine can't know what has changed, it needs to check all managed entities for changes every time you invoke EntityManager#flush(), making this operation rather costly.
The deferred explicit policy is similar to the deferred implicit policy in that it detects changes through a property-by-property comparison at commit time. The difference is that Doctrine ORM only considers entities that have been explicitly marked for change detection through a call to EntityManager#persist(entity) or through a save cascade. All other entities are skipped. This policy therefore gives improved performance for larger units of work while sacrificing the behavior of automatic dirty checking.
Therefore, flush() operations are potentially cheaper with this policy. The negative aspect this has is that if you have a rather large application and you pass your objects through several layers for processing purposes and business tasks you may need to track yourself which entities have changed on the way so you can pass them to EntityManager#persist().
This policy can be configured as follows: