Add non-deterministic qa blog 1 (#176)

* Add non-deterministic qa blog 1

* Add more questions

src/blog/_posts/2024-02-15-cadence-non-deterministic-common-qa.md

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+---
+title: Cadence non-derministic errors common question Q&A (part 1)
+
+date: 2023-08-27
+author: Chris Qin
+authorlink: https://www.linkedin.com/in/chrisqin0610/
+---
+
+#### If I change code logic inside an Cadence activity (for example, my activity is calling database A but now I want it to call database B),  will it trigger an non-deterministic error?
+
+<b>NO</b>. This change will not trigger non-deterministic error.
+
+An Activity is the smallest unit of execution for Cadence and what happens inside activities are not recorded as historical events and therefore will not be replayed. In short, this change is deterministic and it is fine to modify logic inside activities.
+
+#### Does changing the workflow definition trigger non-determinstic errors?
+
+<b>YES</b>. This is a very typical non-deterministic error.
+
+When a new workflow code change is deployed, Cadence will find if it is compatible with
+Cadence history. Changes to workflow definition will fail the replay process of Cadence
+as it finds the new workflow definition imcompatible with previous historical events.
+
+Here is a list of common workflow definition changes.
+- Changing workflow parameter counts
+- Changing workflow parameter types
+- Changing workflow return types
+
+The following changes are not categorized as definition changes and therefore will not
+trigger non-deterministic errors.
+- Changes of workflow return values
+- Changing workflow parameter names as they are just positional
+
+#### Does changing activity definitions trigger non-determinstic errors?
+
+<b>YES</b>. Similar to workflow definition change, this is also a very typical non-deterministic error.
+
+Activities are also recorded and replayed by Cadence. Therefore, changes to activity must also be compatible with Cadence history. The following changes are common ones that trigger non-deterministic errors.
+- Changing activity parameter counts
+- Changing activity parameter types
+- Changing activity return types
+
+As activity paremeters are also positional, these two changes will NOT trigger non-deterministic errors.
+- Changes of activity return values
+- Changing activity parameter names
+
+Activity return values inside workflows are not recorded and replayed.
+
+#### What changes inside workflows may potentially trigger non-deterministic errors?
+
+Cadence records each execution of a workflow and activity execution inside each of them.Therefore, new changes must be compatible with execution orders inside the workflow. The following changes will fail the non-deterministic check.
+
+- Append another activity
+- Delete an existing activity
+- Reordering activities
+
+If you really need to change the activity implementation based on new business requirements, you may consider using versioning your workflow.