Move Serialization/Deserialization (serde) to task SDK

[amoghrajesh]

closes: #58887
closes: #58885

Motivation

This change is part of the broader effort to achieve client-server separation. The serialization and deserialization utilities (serde) are execution time utilities used primarily during task execution for:

• Serializing/deserializing XComs when communicating with the API server
• Processing deadline alerts

Since these utilities are only needed at execution time (in workers, DAG processors, and triggerers), they belong in the task-sdk rather than airflow-core. This allows the core server components (Scheduler, API Server) to operate without requiring the task SDK, and also enabled task SDK to not having to import serde from airflow core, enabling independent deployments and upgrades.

Blockers and Solutions

Several blockers were identified and resolved:

1. Migration File: The migration 0092_3_2_0_replace_deadline_inline_callback_with_fkey.py uses serde.deserialize()
   Effort tracked by @ramitkataria in Do not use serde library for database migrations

2. XCom API Stringification: The core API's xcom endpoint (/api/v2/dags/{dag_id}/dagRuns/{run_id}/taskInstances/{task_id}/xcomEntries) used deserialize(full=False) to stringify XCom values for UI display.
   The solution was to create a dedicated stringify.py module in airflow-core that provides stringification without any SDK dependencies. This module matches the behavior of deserialize(full=False) exactly but is self-contained and independent of any external libraries.

3. XComEncoder/XComDecoder: These JSON encoder/decoder classes in airflow-core directly imported serde for full serialization/deserialization of XCom values.
   The fix here was to implement lazy imports that attempt to load serde from the SDK, making the dependency optional. If the SDK is not available, appropriate error messages are shown. The consumers of this anyways were mostly fixed / handled by: Decouple xcom public API from using XcomEncoder #58900, which discovered that the same effect can be achieved without using those encoders. In a follow-up, I might move it to task SDK as well.

High Level Changes

1. Serde module is moved to task sdk now

   • All serializers moved to task-sdk/src/airflow/sdk/serialization/serializers/

2. Stringification Separation

   • Created airflow-core/src/airflow/serialization/stringify.py for UI stringification
   • This module is self-contained and does not import from SDK
   • Matches deserialize(full=False) behavior exactly

3. Deprecation Path

   • Old imports (from airflow.serialization.serde import ) continue to work
   • Redirection to SDK variant with deprecation warnings
   • Ensures backward compatibility till we remove it

4. Tests

   • Serialize/deserialize tests moved to task-sdk/tests/task_sdk/serialization/test_serde.py
   • Stringify tests created in airflow-core/tests/unit/serialization/test_stringify.py

5. Docs

   • Updated serializer documentation to reference `airflow.sdk.serialization.serializers

What does this mean for new serializers? (Debatable, check open questions below)

Important: New serializers should now be added to the SDK namespace:

The old airflow-core/src/airflow/serialization/serializers/ directory still exists for backward compatibility but is deprecated. New serializers should not be added there.

Open Questions

1. When should we delete airflow-core/src/airflow/serialization/serializers/? Currently kept for backward compatibility, but should be removed in a future release, no code reads off of here, shall we just delete it? What for people who have added their serializers in here?

2. Should XcomEncoder + Decoder stay in airflow core or task sdk?

Testing

• All serialize/deserialize tests moved to SDK and passing
• New stringify tests created to test all possible situations

---

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[potiuk]

This is actually splitting serialization properly as it should rightfully be and it separates things rightly towards the future direction. So, serialization is in two parts (somewhat messy, but let me still explain):

Makes sense. I never liked the idea of having "one serialization to rule them all". Thanks for explanation.

[potiuk]

Makes sense. I never liked the idea of having "one serialization to rule them all". Thanks for explanation.

Also it makes it far better from the security standpoint. And opens the door of having serde implementation connected with providers installed (and likely discovered by Providers Manager)

[amoghrajesh]

This is actually splitting serialization properly as it should rightfully be and it separates things rightly towards the future direction. So, serialization is in two parts (somewhat messy, but let me still explain):

Makes sense. I never liked the idea of having "one serialization to rule them all". Thanks for explanation.

Absolutely me too!

[pierrejeambrun]

API side LGTM.

[amoghrajesh]

Thanks, I will merge it once I am back from holidays (16 Dec), just because the scope of this PR

[amoghrajesh]

Alright, let's merge this one today

[amoghrajesh]

Broken tests are unrelated. Same failure on main too: https://github.com/apache/airflow/actions/runs/20298266352/job/58300350219

[amoghrajesh]

Merging this PR

[github-actions]

Backport failed to create: v3-1-test. View the failure log Run details

Status	Branch	Result
❌	v3-1-test

You can attempt to backport this manually by running:

cherry_picker 3af4d28 v3-1-test

This should apply the commit to the v3-1-test branch and leave the commit in conflict state marking
the files that need manual conflict resolution.

After you have resolved the conflicts, you can continue the backport process by running:

cherry_picker --continue

[amoghrajesh]

No backport needed, this is targetted for 3.2.0

[potiuk]

#protm

[amoghrajesh]

cc: @atul-astronomer try with various xcoms in our test suite to see if something breaks, it can be handled