large string

I think there may be more to consider. Generally, you are correct. You should delete these as soon as possible. You should also consider, how many will be active at a time? What else is in the model expanding the footprint that can be optimized?

As far as I know, Once process instance is deleted it should free the memory consumed by all process variables, etc. If it does not free memory immediately ,it will free in next subsequent checkpoint script run on the process execution engines.

so it is safe to use process variables with large string if there is not many active instances at a time and delete the instances after completion.

@shaoyongw, You may also consider making the process variables hidden if they are going to contain very large strings. This way, it wont be captured in the process history - and it will avoid lot of processing that Appian has to do - extra CPU cycles and memory will be saved.
Of course this is not recommended it you want those process variables to be captured in the process history.

@chetany, what do you mean by making the process varibles hidden?

@shaoyongw, You can make a process variable as "Hidden" in process properties. Please find the attached screenshot. You can see a checkbox for "Hidden", which is checked. By making a PV as hidden, it will not be captured in process history and will not be available in task reports. This is fine if you don't want that PV for reporting/history purposes.

@chetany, I don't think this Hidden feature has anything to do with my question. My point is to minimize the impact to the server memory when using process variables with large string.

Go to this link - forum.appian.com/.../Creating_Memory_Efficient_Models_Best_Practices.html
Go to the section "Other Design Concerns"
And note the last point.
"•If operating on large data sets frequently or for an extended period of time and through many steps, consider designating the process variable as Hidden. This way, frequent changes to these variables are not going to be reflected in process history, which will have a smaller impact on the process instance memory footprint."
I think that makes it pretty clear

@chetany, thanks for explaining this. Dose Appian still keep caches for these PVs if the processes are deleted after completion.

No, I don't think the caches of the variables will be kept after the process instance is deleted. The reason is that, the process history can be fetched for only a process instance which exists in the environment.