Deprecate REST client (finally)

[reta]

Description

Deprecate REST client ( RestClient). The internal HTTP client (see please #20634) has been introduced and designed for OpenSearch internal use only. The RestHighLevelClient is still going to be supported for now but migrated away from RestClient.

Related Issues

Closes #5424

Check List

• Functionality includes testing.
• API changes companion pull request created, if applicable.
• Public documentation issue/PR created, if applicable.

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[github-actions]

PR Reviewer Guide 🔍

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🧪 No relevant tests

🔒 No security concerns identified

✅ No TODO sections

🔀 No multiple PR themes

⚡ Recommended focus areas for review Typo in deprecation message The deprecation message contains a grammatical error: "is deprecated is going to be removed" should be "is deprecated and is going to be removed" or "is deprecated and will be removed". This typo appears in user-facing documentation that will be visible in IDE warnings and generated API docs. * @deprecated the {@code RestClient} is deprecated is going to be removed in future releases, please consider using official * OpenSearch Java Client or {@code RestHighLevelClient}

[github-actions]

PR Code Suggestions ✨

Explore these optional code suggestions:

Category	Suggestion	Impact
General	Fix grammatical error in deprecation message The deprecation message contains a grammatical error ("is deprecated is going to be"). Remove the duplicate "is" to make the message grammatically correct and professional. client/rest/src/main/java/org/opensearch/client/RestClient.java [140-141] -@deprecated the {@code RestClient} is deprecated is going to be removed in future releases, please consider using official +@deprecated the {@code RestClient} is deprecated and is going to be removed in future releases, please consider using official OpenSearch Java Client or {@code RestHighLevelClient} Suggestion importance[1-10]: 7 __ Why: The suggestion correctly identifies a grammatical error in the deprecation message where "is deprecated is going to be" should be "is deprecated and is going to be". This improves the professionalism and clarity of the documentation, though it's a minor issue that doesn't affect functionality.	Medium

[github-actions]

✅ Gradle check result for c11e9b3: SUCCESS

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.34%. Comparing base (b32e6fa) to head (c11e9b3).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #22116      +/-   ##
============================================
- Coverage     73.36%   73.34%   -0.03%     
+ Complexity    75847    75837      -10     
============================================
  Files          6064     6064              
  Lines        344500   344500              
  Branches      49575    49575              
============================================
- Hits         252733   252659      -74     
- Misses        71599    71686      +87     
+ Partials      20168    20155      -13     

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[marko-bekhta]

hey @reta 👋🏻
I noticed this deprecation during the upgrade and am trying to understand: what is an actual alternative to it?

we have an OpenSearch-based client in Hibernate Search (https://github.com/hibernate/hibernate-search/tree/main/backend/elasticsearch-client/opensearch-rest-client) that users can use when connecting to OpenSearch clusters. Neither high-level (my understanding is that you plan to deprecate it anyway, and it's not suitable in the first place for the use case 🙂) nor the opensearch-Java clients, which are suggested, look like a good alternative here ...

Why the current RestClient was a good fit -- we need a client that can talk to any OpenSearch version without bringing version specifics with it. Hibernate Search can handle versions internally, but it needs something to handle the node management + sending requests.