@RahulBilove

You can’t directly “turn on” pessimistic locking only for DX API – DX API follows the same locking model as your case type and assignments, so first make sure pessimistic locking is enabled on the case type settings and that your flow uses assignments (not only data transforms or utilities). When using DX API, always open the case via an assignment (GET assignment) so Pega acquires the lock, and then pass the etag / lock token from that response into your subsequent DX API calls (PATCH/POST) so the lock is honored. Make sure all updates go through DX API (not a mix of DX API and classic UI on the same case at the same time), otherwise you will see “lost updates” that look like optimistic locking. Also confirm that your service REST rules and access groups used by Cognito are the same ones that see the pessimistic-lock configuration; if they use a different ruleset stack, they may be hitting an older version of the case type with optimistic locking. Finally, turn on Pega logs for locking (and tracer with Locking events) to verify that locks are actually acquired and released when DX API calls are made, and adjust your design so every edit starts from an assignment that obtains a lock.