Apex heap limit differences between synchronous and asynchronous transactions

Question

Heap sizing for bulk ApexA design is being evaluated for bulk processing that keeps data in memory during an Apex transaction. The documented capability is the Apex heap size governor limit, which caps in-memory data per transaction and is enforced separately for synchronous and asynchronous contexts. Heap usage is measured per transaction and includes variables, collections and deserialized sObjects, with memory reclaimed only at transaction end and no developer-exposed compaction or garbage collection control.There is uncertainty about exact current limits and how heap is accounted for with large collections and JSON deserialization. Platform Cache and external data access do not directly increase Apex heap, but data retrieved into Apex variables does count toward the limit. Static variables are scoped per transaction and do not persist heap across invocations.What is the documented heap size limit for synchronous Apex versus asynchronous Apex such as Queueable and Batch execute in the current release? How is heap accounted for when a large JSON payload is deserialized into a List of sObjects held for the duration of the transaction? Does retrieving data from Platform Cache increase Apex heap only when the data is copied into Apex variables?

Tasadduq Burney · Answer

Apex heap limits
Confirmed facts (from Salesforce Governor Limits):

Synchronous Apex (triggers, Visualforce controllers, anonymous blocks) – 6 MB heap limit.
Asynchronous Apex (future, Queueable, Batch execute/finish, Scheduled) – 12 MB heap limit.

How heap is accounted for
When a JSON payload is deserialized into a List<sObject>, the memory used by the list elements, their fields, and the internal overhead of the list and objects all count toward the heap limit. The limit is checked after the deserialization completes; if the total exceeds the transaction’s heap limit, a System.LimitException is thrown.
Platform Cache interaction
Retrieving a value from Platform Cache does not consume heap until the cached value is assigned to an Apex variable (e.g., String cached = Cache.Org.get('key');). At that point the variable’s contents are added to the heap usage.
Verification steps

Run anonymous Apex in Developer Console:
System.debug('Heap used: '+ Limits.getHeapSize() +' bytes');
System.debug('Heap limit: '+ Limits.getLimitHeapSize() +' bytes');

For asynchronous code, add the same debug statements inside a future method, Queueable execute, or Batch execute method and view the debug log.
In a Batch Apex class, place the debug statements at the start and end of the execute method to see the per‑chunk limit reset.

If you encounter unexpected limit exceptions, please share the heap size logged via Limits.getHeapSize() at the point of failure so we can confirm whether the limit was exceeded.

Apex heap limit differences between synchronous and asynchronous transactions

Heap sizing for bulk Apex

1 answer

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Apex heap limits

How heap is accounted for

Platform Cache interaction

Verification steps

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