Important things to note on ATG Checkout

In checkout, if the order is incomplete OrderPriceInfo doesnt get stored by default. This is controlled by orderStateSaveModes property of SavePriceInfoObjects.properties.

Which order ATG picks up when the user is signed in or autologin is enabled
If the shopping cart is empty then the last saved abandoned order is merged into the empty cart when the user logs in.

How to enable the rollback logging in Transaction Manager

In the TranasactionManager component, set debugTracesForRollbacks to true. So that application throws the rollback details else you will never know which component actually set transaction to rollback.

How to configure ATG to replicate session
Before doing this make sure application servers in-memory replication is enabled. Then set the following in ATG layer to enable that
/atg/dynamo/Configuration.backinUpSessions=true

also
/atg/dynamo/Configuration.sessionBackupServerPropertyList set to the following
/SessionConfirmationNumberHolder.sessionConfirmationNumber,\
/atg/userprofiling/ProfileFailService.uidForFailover,\
/atg/userprofiling/ProfileFailService.dataSource,\
/atg/dynamo/security/UserFailService.savedPersonae,\
/atg/commerce/ShoppingCart.restorableOrders,\
/atg/userprofiling/ProfileFailService.activePromotions,\
/atg/commerce/catalog/comparison/ProductList.items,\
/atg/commerce/gifts/GiftlistFormHandler.eventName,\
/atg/userprofiling/ProfileFailService.failedOverSecurityStatus

then set
/atg/userprofiling/ProfileFailService.failedOverSecurityStatus=-1