Comparison

A · AGENTS.md · elastic/elasticsearch B · AGENTS.md · elastic/elasticsearch

What each file covers, counted
Dimension	Only in A	Only in B	Overlap
Sections	3	28	0%
Commands	0	9	0%
Section tags	0	10	0%

What each file covers

Sections

0 shared · 3 only in A · 28 only in B

− ES|QL Compute Engine
− Code generation
− Hand-written classes
+ Elasticsearch
+ Toolchain Snapshot
+ Build & Run Commands
+ Verification & Lint Tasks
+ Project Structure
+ Stateless Elasticsearch
+ Plugin `deploymentTarget`
+ Plugin locations
+ Key subsystems
+ Testing Cheatsheet
+ Test Types
+ Distribution selection for external-module tests
+ Dependency Hygiene
+ Entitlement Policy
+ Formatting & Imports
+ Types, Generics, and Suppressions
+ Naming Conventions
+ Logging & Error Handling
+ Javadoc & Comments
+ License Headers
+ Generated Files
+ Debugging Missing Tests
+ `No tests found for given includes: [**/*$*.class]`
+ Best Practices for Automation Agents
+ Methods with Required Javadoc Reading
+ ES|QL tests
+ Backwards compatibility
+ Documentation

Commands

0 shared · 0 only in A · 9 only in B

+ ./gradlew
+ ./gradlew spotlessJavaCheck
+ ./gradlew test
+ ./gradlew :server:test
+ ./gradlew :server:test --tests org.elasticsearch.package.ClassName
+ ./gradlew :server:test --tests 'org.elasticsearch.package.*'
+ ./gradlew :server:test --tests org.elasticsearch.package.ClassName.methodName -Dtests.iters=N
+ ./gradlew ":x-pack:plugin:esql:internalClusterTest" --tests "org.elasticsearch.xpack.esql.CsvIT.*<csv-file>*"
+ ./gradlew generateTransportVersion

Section tags

0 shared · 0 only in A · 10 only in B

+ build
+ test
+ lint-format
+ code-style
+ architecture
+ types
+ testing-strategy
+ git-pr
+ do-not
+ docs

Line diff

+170 added−15 removed10 unchanged5.6% identical

elastic/elasticsearch · x-pack/plugin/esql/compute/AGENTS.md

@@ −1 @@

1# ES|QL Compute Engine

3## Code generation

5Most classes in this module are generated via two mechanisms:

71. **StringTemplate** (`*.java.st` files)

82. **Annotation processors** (`compute/gen`) — classes annotated with `@Evaluator`,

9 `@Aggregator`, `@GroupingAggregator`, etc. are generated at compile time automatically.

11Do not hand-edit generated files. Edit the template or annotated source instead.

13## Hand-written classes

15Some classes are hand-written across multiple type variants instead of generated.

16Two rules apply:

18- When changing one variant, keep all variants in sync.

19- When changing annotation processors or generated output, update hand-written classes

20 to stay consistent with the new generated code.

22Hand-written families:

24- `Rate*GroupingAggregatorFunction` — `IncreaseExponentialHistogramGroupingAggregatorFunction` also belongs to this family.

elastic/elasticsearch · AGENTS.md

@@ +1 @@

1# Elasticsearch

3## Toolchain Snapshot

4- **Java**: JDK 25 via `JAVA_HOME`; use the bundled Gradle wrapper (`./gradlew`).

5- **Build tooling**: Gradle composite build with `build-conventions`, `build-tools`, and `build-tools-internal`; Docker is required for some packaging/tests.

6- **OS packages**: Packaging and QA jobs expect ephemeral hosts; do not run packaging suites on your workstation.

7- **Security**: Default dev clusters enable security; use `elastic-admin:elastic-password` or disable with `-Dtests.es.xpack.security.enabled=false`.

8- **Cursor/Copilot rules**: None provided in repo; follow this guide plus CONTRIBUTING.md.

10## Build & Run Commands

11- Refer to BUILDING.md, CONTRIBUTING.md & TESTING.asciidoc for comprehensive build/test instructions.

13## Verification & Lint Tasks

14- `./gradlew spotlessJavaCheck` / `spotlessApply` (or `:server:spotlessJavaCheck`): enforce formatter profile in `build-conventions/formatterConfig.xml`.

15- `spotlessApply` also prunes unused imports and reorders imports automatically. Run it instead of manually hunting for unused imports after refactoring.

17## Project Structure

18The repository is organized into several key directories:

19* `server`: The core Elasticsearch server. Few third-party dependencies (Lucene plus a handful of small libraries). Key `org.elasticsearch` sub-packages: `cluster` (cluster state machine), `index` (per-index logic), `search` (query execution), `action` (transport actions), `snapshots` (snapshot/restore), plus `indices`, `repositories`, `rest`, `ingest`, etc.

20* `modules`: Features shipped with Elasticsearch by default, but not considered "core" server code. Many modules provide a specific implementation of a pluggable interface defined in `server`, such as `transport-netty4` (the transport layer) or `repository-s3`/`repository-gcs`/`repository-azure` (snapshot repositories). Others integrate with external systems, such as `apm` (Application Performance Monitoring agent integration).

21* `plugins`: Optional, not bundled by default, but officially supported. Examples: `discovery-ec2`/`discovery-gce`/`discovery-azure-classic` (cloud-aware cluster discovery).

22* `libs`: Internal libraries used by multiple parts of the project. Examples: `logging`, `x-content` (JSON/CBOR/YAML/SMILE parsing abstraction).

23* `client`: The official Java REST client.

24* `test`: Test infrastructure used by the rest of the repo. `framework` holds `ESTestCase`/`ESIntegTestCase`/`ESSingleNodeTestCase`; also `test-clusters` and `yaml-rest-runner` (runner for YAML-based REST API tests).

25* `qa`: Integration and multi-version tests. Examples: `rolling-upgrade`, `mixed-cluster`.

26* `rest-api-spec`: JSON spec definitions for the public REST API endpoints.

27* `docs`: Project documentation.

28* `distribution`: Logic for building distribution packages.

29* `x-pack`: Modules, plugins, and commercial features under the Elastic License 2.0. Example sub-plugins: `security`, `ml` (machine learning), `ccr` (cross-cluster replication), `logsdb` (optimized index mode for log data), and `stateless`.

30* `build-conventions`, `build-tools`, `build-tools-internal`: Gradle build logic. Refer to BUILDING.md for details on how these are structured and used.

32## Stateless Elasticsearch

34Stateless Elasticsearch is a distribution where shard data is stored in an **object store** (e.g., S3, GCS, Azure) rather than local disk. Nodes carry no durable local state. The cluster distinguishes two node roles: **indexing nodes** (`index` role, write path + translog replication to object store) and **search nodes** (`search` role, read-only via shared blob cache). The `DiscoveryNode.STATELESS_ENABLED_SETTING` gates stateless behavior at runtime.

36### Plugin `deploymentTarget`

38Plugins can set `deploymentTarget` in `build.gradle`. That value tells the node **whether to load the plugin**: **`STATEFUL_ONLY`** (stateful clusters only), **`STATELESS_ONLY`** (stateless mode on only), or **`ALL`** (always loaded; this is the default when the property is omitted).

40### Plugin locations

42| Plugin | Gradle path | Purpose |

43|---|---|---|

44| `stateless` | `:x-pack:plugin:stateless` | Core stateless — engines, allocation, cache, object store, recovery |

45| `stateless-sigterm` | `:x-pack:plugin:stateless-sigterm` | Clean SIGTERM shutdown for Kubernetes |

46| `stateless-master-failover` | `:x-pack:plugin:stateless-master-failover` | Master failover behavior |

47| `stateless-no-wait-for-active-shards` | `:x-pack:plugin:stateless-no-wait-for-active-shards` | Suppresses wait-for-active-shards |

48| `stateless-health-shards-availability` | `:x-pack:plugin:stateless-health-shards-availability` | Shard availability health indicators |

50**Package**: `org.elasticsearch.xpack.stateless.*` throughout.

52### Key subsystems

54- **Object store** (`objectstore/`): `ObjectStoreService`, bucket config, GC tasks for stale indices and translogs.

55- **Commits** (`commits/`): `StatelessCommitService` manages shard commits to blob store; `HollowShardsService` manages hollow indexing shards.

56- **Cache & prewarming** (`cache/`): `StatelessSharedBlobCacheService`, online prewarming, `SearchCommitPrefetcher`.

57- **Engines** (`engine/`): `IndexEngine` (write path) and `SearchEngine` (read-only); `TranslogReplicator` replicates translog to object store.

58- **Allocation** (`allocation/`): `StatelessExistingShardsAllocator`, separate balancing weights per tier, heap-usage-aware allocation decisions.

59- **Recovery** (`recovery/`): custom primary relocation and unpromotable shard relocation protocols.

61## Testing Cheatsheet

62- Standard suite: `./gradlew test` (respects cached results; add `-Dtests.timestamp=$(date +%s)` to bypass caches when reusing seeds).

63- Single project: `./gradlew :server:test` (or other subproject path).

64- Single class: `./gradlew :server:test --tests org.elasticsearch.package.ClassName`.

65- Single package: `./gradlew :server:test --tests 'org.elasticsearch.package.*'`.

66- Single method / repeated runs: `./gradlew :server:test --tests org.elasticsearch.package.ClassName.methodName -Dtests.iters=N`.

67- Deterministic seed: append `-Dtests.seed=DEADBEEF` (each method uses derived seeds).

68- JVM tuning knobs: `-Dtests.jvms=8`, `-Dtests.heap.size=4G`, `-Dtests.jvm.argline="-verbose:gc"`, `-Dtests.output=always`, etc.

69- Debugging: append `--debug-jvm` to the Gradle test task and attach a debugger on port 5005.

70- CI reproductions: copy the `REPRODUCE WITH` line from CI logs; it includes project path, seed, and JVM flags.

71- Yaml REST tests: `./gradlew ":rest-api-spec:yamlRestTest" --tests "org.elasticsearch.test.rest.ClientYamlTestSuiteIT.test {yaml=<relative_test_file_path>}"`

72- ES|QL CSV tests: `./gradlew ":x-pack:plugin:esql:internalClusterTest" --tests "org.elasticsearch.xpack.esql.CsvIT.*<csv-file>*"` (e.g. `--tests "...CsvIT.*stats_first_last*"`); append `*<test-name>*` to target a single test within the file.

73- Use the Elasticsearch testing framework where possible for unit and yaml tests and be consistent in style with other elasticsearch tests.

74- Use real classes over mocks or stubs for unit tests, unless the real class is complex then either a simplified subclass should be created within the test or, as a last resort, a mock or stub can be used. Unit tests must be as close to real-world scenarios as possible.

75- Ensure mocks or stubs are well-documented and clearly indicate why they were necessary.

77### Test Types

78- Unit Tests: Preferred. Extend `ESTestCase`.

79- Single Node: Extend `ESSingleNodeTestCase` (lighter than full integ test).

80- Integration: Extend `ESIntegTestCase`.

81- REST API: Extend `ESRestTestCase` or `ESClientYamlSuiteTestCase`. **YAML based REST tests are preferred** for integration/API testing.

83### Distribution selection for external-module tests

84- Prefer the OSS/minimal distribution over `usesDefaultDistribution` whenever possible. `usesDefaultDistribution` packages the full default distribution, which is significantly more expensive to build and run.

85- Only use `usesDefaultDistribution` when the test genuinely requires a feature that is only available in the default distribution and cannot be replicated with a custom cluster configuration that includes just the needed plugins. Always document the reason in the `usesDefaultDistribution(...)` message.

87## Dependency Hygiene

88- Never add a dependency without checking for existing alternatives in the repo.

90## Entitlement Policy

91- Never add an entitlement speculatively. Each entry in `entitlement-policy.yaml` must have a specific justification — ideally a concrete `NotEntitledException` that was observed, or at minimum a clear explanation of why the library requires that capability. Entitlements are a least-privilege mechanism; granting one "just in case" defeats the purpose.

92- Every use of `ESTestCase.WithoutEntitlements` must be accompanied by a comment explaining why the entitlement failure is spurious in the test context and would not occur in production.

94## Formatting & Imports

95- Absolutely no wildcard imports; keep existing import order and avoid reordering untouched lines.

96- In `switch` statements, do not use `default` as a branch for valid or expected options. Enumerate those cases explicitly and reserve `default` for throwing an exception for unexpected values, or an assertion error if this code branch is unreachable.

98## Types, Generics, and Suppressions

99- Prefer type-safe constructs; avoid raw types and unchecked casts.

100- If suppressing warnings, scope `@SuppressWarnings` narrowly (ideally a single statement or method).

101- Document non-obvious casts or type assumptions via Javadoc/comments for reviewers.

102

103## Naming Conventions

104- REST handlers typically use the `Rest*Action` pattern; transport-layer handlers mirror them with `Transport*Action` classes.

105- REST classes expose routes via `RestHandler#routes`; when adding endpoints ensure naming matches existing REST/Transport patterns to aid discoverability.

106- Transport `ActionType` strings encode scope (`indices:data/read/...`, `cluster:admin/...`, etc.); align new names with these conventions to integrate with privilege resolution.

107

108## Logging & Error Handling

109- Elasticsearch should prefer its own logger `org.elasticsearch.logging.LogManager` & `org.elasticsearch.logging.Logger`; declare `private static final Logger logger = LogManager.getLogger(Class.class)`.

110- Always use parameterized logging (`logger.debug("operation [{}]", value)`); never build strings via concatenation.

111- Wrap expensive log-message construction in `() -> Strings.format(...)` suppliers when logging at `TRACE`/`DEBUG` to avoid unnecessary work.

112- Log levels:

113 - `TRACE`: highly verbose developer diagnostics; usually read alongside code.

114 - `DEBUG`: detailed production troubleshooting; ensure volume is bounded.

115 - `INFO`: default-enabled operational milestones; prefer factual language.

116 - `WARN`: actionable problems users must investigate; include context and, if needed, exception stack traces.

117 - `ERROR`: reserve for unrecoverable states (e.g., storage health failures); prefer `WARN` otherwise.

118- Only log client-caused exceptions when the cluster admin can act on them; otherwise rely on API responses.

119- Tests can assert logging via `MockLog` for complex flows.

120

121## Javadoc & Comments

122- New packages/classes/public or abstract methods require Javadoc explaining the "why" rather than the implementation details.

123- Avoid documenting trivial getters/setters; focus on behavior, preconditions, or surprises.

124- For tests, Javadoc can describe scenario setup/expectations to aid future contributors.

125- Do not remove existing comments from code unless the code is also being removed or the comment has become incorrect.

126

127## License Headers

128- Default header (outside `x-pack`): Elastic License 2.0, SSPL v1, or AGPL v3—they are already codified at the top of Java files; copy from existing sources.

129- Files under `x-pack` require the Elastic License 2.0-only header; IDEs configured per CONTRIBUTING.md can insert correct text automatically.

130

131## Generated Files

132- Never hand-edit generated files. Instead, edit the source they are generated from and regenerate.

133- ANTLR-generated files can be regenerated by running the `regen` task on the relevant subproject.

134- Other generated files are regenerated by compiling the project.

135

136## Debugging Missing Tests

137

138When expected test methods are absent from results (not failed, not skipped — simply not present in the XML or binary event stream), check `muted-tests.yml` first. The build translates every entry into a Gradle `TestFilter.excludePattern`, which silently drops matching tests before the randomized runner receives them. A muted test fires no `testStarted` event and leaves no trace in `results-generic.bin`.

139 ```bash

140 grep 'ClassName\|methodName' muted-tests.yml

141 ```

142

143### `No tests found for given includes: [**/*$*.class]`

144

145When a test task fails at execution with `No tests found for given includes: [**/*$*.class](exclude rules)`, it usually does **not** mean Gradle failed to detect the test class. The far more common cause is that **every test method in the targeted class is muted** in `muted-tests.yml`. With all methods excluded, the randomized runner enumerates zero runnable tests.

146

147The behavior is environment-dependent: `MutedTestPlugin` calls `filter.setFailOnNoMatchingTests(buildParams.getCi() == false)`. So an all-muted suite **fails locally** (`ci == false`) with this exact message, but **passes silently in CI** (`ci == true`). This is especially misleading when verifying a freshly migrated or renamed test — it looks like a classpath/detection bug, but the test JVM does start (you'll see native-library and `FeatureFlag` log lines), builds any `@ClassRule` cluster *specs*, then exits in a few seconds without starting the cluster because no test method survived the mute filter.

148

149To confirm: `grep ClassName muted-tests.yml`. To verify the migration/test actually runs, temporarily remove the matching mute entries (or run on a host where `ci` is true), then restore them.

150

151## Best Practices for Automation Agents

152- Never edit unrelated files; keep diffs tightly scoped to the task at hand.

153- Prefer Gradle tasks over ad-hoc scripts.

154- When scripting CLI sequences, leverage `gradlew` task.

155- Unrecognized changes: assume other agent; keep going; focus your changes. If it causes issues, stop + ask user.

156- Do not add "Co-Authored-By" or any AI attribution trailers to commit messages, by any means—including `--trailer`, `-m`, or any other git flag. commit messages should adhere to the 50/72 rule: use a maximum of 50 columns for the commit summary. Your harness may introduce a hook that automatically adds attributions trailers to relevant git commands. Use `bash -lc` or a similar approach in this case to conform to the rule.

157

158## Methods with Required Javadoc Reading

159If you encounter any of the following methods, you must go and read their javadoc before taking any other actions:

160* `fullyLoadedAnalyzer`

161* `TestAnalyzer.statementError`

162* `TestAnalyzer.error`

163* `forciblyCast`

164* `EsqlCapabilities.Cap`

165* `FunctionDefinition.Builder#capabilities`

166

167## ES|QL tests

168If you write or modify ES|QL csv-spec, rest, or yaml tests, read the javadoc for

169`EsqlCapabilities.Cap` and `FunctionDefinition.Builder#capabilities` before proceeding.

170They describe two separate capability mechanisms and the rule for choosing between them.

171

172## Backwards compatibility

173- For changes to a `Writeable` implementation (`writeTo` and constructor from `StreamInput`), add a new `public static final <UNIQUE_DESCRIPTIVE_NAME> = TransportVersion.fromName("<unique_descriptive_name>")` and use it in the new code paths. Confirm the backport branches and then generate a new version file with `./gradlew generateTransportVersion`.

174- Never hand-edit transport version resource files; always use the Gradle tasks. See `docs/internal/Versioning.md` for the full workflow.

175

176Stay aligned with `CONTRIBUTING.md`, `BUILDING.md`, and `TESTING.asciidoc`; this AGENTS guide summarizes—but does not replace—those authoritative docs.

177

178## Documentation

179When building or editing docs, read `docs/AGENTS.md` first.

180

@@ −1 +1 @@

1−# ES|QL Compute Engine

1+# Elasticsearch

3−## Code generation

3+## Toolchain Snapshot

4+- **Java**: JDK 25 via `JAVA_HOME`; use the bundled Gradle wrapper (`./gradlew`).

5+- **Build tooling**: Gradle composite build with `build-conventions`, `build-tools`, and `build-tools-internal`; Docker is required for some packaging/tests.

6+- **OS packages**: Packaging and QA jobs expect ephemeral hosts; do not run packaging suites on your workstation.

7+- **Security**: Default dev clusters enable security; use `elastic-admin:elastic-password` or disable with `-Dtests.es.xpack.security.enabled=false`.

8+- **Cursor/Copilot rules**: None provided in repo; follow this guide plus CONTRIBUTING.md.

5−Most classes in this module are generated via two mechanisms:

10+## Build & Run Commands

11+- Refer to BUILDING.md, CONTRIBUTING.md & TESTING.asciidoc for comprehensive build/test instructions.

612

7−1. **StringTemplate** (`*.java.st` files)

8−2. **Annotation processors** (`compute/gen`) — classes annotated with `@Evaluator`,

9− `@Aggregator`, `@GroupingAggregator`, etc. are generated at compile time automatically.

13+## Verification & Lint Tasks

14+- `./gradlew spotlessJavaCheck` / `spotlessApply` (or `:server:spotlessJavaCheck`): enforce formatter profile in `build-conventions/formatterConfig.xml`.

15+- `spotlessApply` also prunes unused imports and reorders imports automatically. Run it instead of manually hunting for unused imports after refactoring.

1016

11−Do not hand-edit generated files. Edit the template or annotated source instead.

17+## Project Structure

18+The repository is organized into several key directories:

19+* `server`: The core Elasticsearch server. Few third-party dependencies (Lucene plus a handful of small libraries). Key `org.elasticsearch` sub-packages: `cluster` (cluster state machine), `index` (per-index logic), `search` (query execution), `action` (transport actions), `snapshots` (snapshot/restore), plus `indices`, `repositories`, `rest`, `ingest`, etc.

20+* `modules`: Features shipped with Elasticsearch by default, but not considered "core" server code. Many modules provide a specific implementation of a pluggable interface defined in `server`, such as `transport-netty4` (the transport layer) or `repository-s3`/`repository-gcs`/`repository-azure` (snapshot repositories). Others integrate with external systems, such as `apm` (Application Performance Monitoring agent integration).

21+* `plugins`: Optional, not bundled by default, but officially supported. Examples: `discovery-ec2`/`discovery-gce`/`discovery-azure-classic` (cloud-aware cluster discovery).

22+* `libs`: Internal libraries used by multiple parts of the project. Examples: `logging`, `x-content` (JSON/CBOR/YAML/SMILE parsing abstraction).

23+* `client`: The official Java REST client.

24+* `test`: Test infrastructure used by the rest of the repo. `framework` holds `ESTestCase`/`ESIntegTestCase`/`ESSingleNodeTestCase`; also `test-clusters` and `yaml-rest-runner` (runner for YAML-based REST API tests).

25+* `qa`: Integration and multi-version tests. Examples: `rolling-upgrade`, `mixed-cluster`.

26+* `rest-api-spec`: JSON spec definitions for the public REST API endpoints.

27+* `docs`: Project documentation.

28+* `distribution`: Logic for building distribution packages.

29+* `x-pack`: Modules, plugins, and commercial features under the Elastic License 2.0. Example sub-plugins: `security`, `ml` (machine learning), `ccr` (cross-cluster replication), `logsdb` (optimized index mode for log data), and `stateless`.

30+* `build-conventions`, `build-tools`, `build-tools-internal`: Gradle build logic. Refer to BUILDING.md for details on how these are structured and used.

1231

13−## Hand-written classes

32+## Stateless Elasticsearch

1433

15−Some classes are hand-written across multiple type variants instead of generated.

16−Two rules apply:

34+Stateless Elasticsearch is a distribution where shard data is stored in an **object store** (e.g., S3, GCS, Azure) rather than local disk. Nodes carry no durable local state. The cluster distinguishes two node roles: **indexing nodes** (`index` role, write path + translog replication to object store) and **search nodes** (`search` role, read-only via shared blob cache). The `DiscoveryNode.STATELESS_ENABLED_SETTING` gates stateless behavior at runtime.

1735

18−- When changing one variant, keep all variants in sync.

19−- When changing annotation processors or generated output, update hand-written classes

20− to stay consistent with the new generated code.

36+### Plugin `deploymentTarget`

2137

22−Hand-written families:

38+Plugins can set `deploymentTarget` in `build.gradle`. That value tells the node **whether to load the plugin**: **`STATEFUL_ONLY`** (stateful clusters only), **`STATELESS_ONLY`** (stateless mode on only), or **`ALL`** (always loaded; this is the default when the property is omitted).

2339

24−- `Rate*GroupingAggregatorFunction` — `IncreaseExponentialHistogramGroupingAggregatorFunction` also belongs to this family.

40+### Plugin locations

41+

42+| Plugin | Gradle path | Purpose |

43+|---|---|---|

44+| `stateless` | `:x-pack:plugin:stateless` | Core stateless — engines, allocation, cache, object store, recovery |

45+| `stateless-sigterm` | `:x-pack:plugin:stateless-sigterm` | Clean SIGTERM shutdown for Kubernetes |

46+| `stateless-master-failover` | `:x-pack:plugin:stateless-master-failover` | Master failover behavior |

47+| `stateless-no-wait-for-active-shards` | `:x-pack:plugin:stateless-no-wait-for-active-shards` | Suppresses wait-for-active-shards |

48+| `stateless-health-shards-availability` | `:x-pack:plugin:stateless-health-shards-availability` | Shard availability health indicators |

49+

50+**Package**: `org.elasticsearch.xpack.stateless.*` throughout.

51+

52+### Key subsystems

53+

54+- **Object store** (`objectstore/`): `ObjectStoreService`, bucket config, GC tasks for stale indices and translogs.

55+- **Commits** (`commits/`): `StatelessCommitService` manages shard commits to blob store; `HollowShardsService` manages hollow indexing shards.

56+- **Cache & prewarming** (`cache/`): `StatelessSharedBlobCacheService`, online prewarming, `SearchCommitPrefetcher`.

57+- **Engines** (`engine/`): `IndexEngine` (write path) and `SearchEngine` (read-only); `TranslogReplicator` replicates translog to object store.

58+- **Allocation** (`allocation/`): `StatelessExistingShardsAllocator`, separate balancing weights per tier, heap-usage-aware allocation decisions.

59+- **Recovery** (`recovery/`): custom primary relocation and unpromotable shard relocation protocols.

60+

61+## Testing Cheatsheet

62+- Standard suite: `./gradlew test` (respects cached results; add `-Dtests.timestamp=$(date +%s)` to bypass caches when reusing seeds).

63+- Single project: `./gradlew :server:test` (or other subproject path).

64+- Single class: `./gradlew :server:test --tests org.elasticsearch.package.ClassName`.

65+- Single package: `./gradlew :server:test --tests 'org.elasticsearch.package.*'`.

66+- Single method / repeated runs: `./gradlew :server:test --tests org.elasticsearch.package.ClassName.methodName -Dtests.iters=N`.

67+- Deterministic seed: append `-Dtests.seed=DEADBEEF` (each method uses derived seeds).

68+- JVM tuning knobs: `-Dtests.jvms=8`, `-Dtests.heap.size=4G`, `-Dtests.jvm.argline="-verbose:gc"`, `-Dtests.output=always`, etc.

69+- Debugging: append `--debug-jvm` to the Gradle test task and attach a debugger on port 5005.

70+- CI reproductions: copy the `REPRODUCE WITH` line from CI logs; it includes project path, seed, and JVM flags.

71+- Yaml REST tests: `./gradlew ":rest-api-spec:yamlRestTest" --tests "org.elasticsearch.test.rest.ClientYamlTestSuiteIT.test {yaml=<relative_test_file_path>}"`

72+- ES|QL CSV tests: `./gradlew ":x-pack:plugin:esql:internalClusterTest" --tests "org.elasticsearch.xpack.esql.CsvIT.*<csv-file>*"` (e.g. `--tests "...CsvIT.*stats_first_last*"`); append `*<test-name>*` to target a single test within the file.

73+- Use the Elasticsearch testing framework where possible for unit and yaml tests and be consistent in style with other elasticsearch tests.

74+- Use real classes over mocks or stubs for unit tests, unless the real class is complex then either a simplified subclass should be created within the test or, as a last resort, a mock or stub can be used. Unit tests must be as close to real-world scenarios as possible.

75+- Ensure mocks or stubs are well-documented and clearly indicate why they were necessary.

76+

77+### Test Types

78+- Unit Tests: Preferred. Extend `ESTestCase`.

79+- Single Node: Extend `ESSingleNodeTestCase` (lighter than full integ test).

80+- Integration: Extend `ESIntegTestCase`.

81+- REST API: Extend `ESRestTestCase` or `ESClientYamlSuiteTestCase`. **YAML based REST tests are preferred** for integration/API testing.

82+

83+### Distribution selection for external-module tests

84+- Prefer the OSS/minimal distribution over `usesDefaultDistribution` whenever possible. `usesDefaultDistribution` packages the full default distribution, which is significantly more expensive to build and run.

85+- Only use `usesDefaultDistribution` when the test genuinely requires a feature that is only available in the default distribution and cannot be replicated with a custom cluster configuration that includes just the needed plugins. Always document the reason in the `usesDefaultDistribution(...)` message.

86+

87+## Dependency Hygiene

88+- Never add a dependency without checking for existing alternatives in the repo.

89+

90+## Entitlement Policy

91+- Never add an entitlement speculatively. Each entry in `entitlement-policy.yaml` must have a specific justification — ideally a concrete `NotEntitledException` that was observed, or at minimum a clear explanation of why the library requires that capability. Entitlements are a least-privilege mechanism; granting one "just in case" defeats the purpose.

92+- Every use of `ESTestCase.WithoutEntitlements` must be accompanied by a comment explaining why the entitlement failure is spurious in the test context and would not occur in production.

93+

94+## Formatting & Imports

95+- Absolutely no wildcard imports; keep existing import order and avoid reordering untouched lines.

96+- In `switch` statements, do not use `default` as a branch for valid or expected options. Enumerate those cases explicitly and reserve `default` for throwing an exception for unexpected values, or an assertion error if this code branch is unreachable.

97+

98+## Types, Generics, and Suppressions

99+- Prefer type-safe constructs; avoid raw types and unchecked casts.

100+- If suppressing warnings, scope `@SuppressWarnings` narrowly (ideally a single statement or method).

101+- Document non-obvious casts or type assumptions via Javadoc/comments for reviewers.

102+

103+## Naming Conventions

104+- REST handlers typically use the `Rest*Action` pattern; transport-layer handlers mirror them with `Transport*Action` classes.

105+- REST classes expose routes via `RestHandler#routes`; when adding endpoints ensure naming matches existing REST/Transport patterns to aid discoverability.

106+- Transport `ActionType` strings encode scope (`indices:data/read/...`, `cluster:admin/...`, etc.); align new names with these conventions to integrate with privilege resolution.

107+

108+## Logging & Error Handling

109+- Elasticsearch should prefer its own logger `org.elasticsearch.logging.LogManager` & `org.elasticsearch.logging.Logger`; declare `private static final Logger logger = LogManager.getLogger(Class.class)`.

110+- Always use parameterized logging (`logger.debug("operation [{}]", value)`); never build strings via concatenation.

111+- Wrap expensive log-message construction in `() -> Strings.format(...)` suppliers when logging at `TRACE`/`DEBUG` to avoid unnecessary work.

112+- Log levels:

113+ - `TRACE`: highly verbose developer diagnostics; usually read alongside code.

114+ - `DEBUG`: detailed production troubleshooting; ensure volume is bounded.

115+ - `INFO`: default-enabled operational milestones; prefer factual language.

116+ - `WARN`: actionable problems users must investigate; include context and, if needed, exception stack traces.

117+ - `ERROR`: reserve for unrecoverable states (e.g., storage health failures); prefer `WARN` otherwise.

118+- Only log client-caused exceptions when the cluster admin can act on them; otherwise rely on API responses.

119+- Tests can assert logging via `MockLog` for complex flows.

120+

121+## Javadoc & Comments

122+- New packages/classes/public or abstract methods require Javadoc explaining the "why" rather than the implementation details.

123+- Avoid documenting trivial getters/setters; focus on behavior, preconditions, or surprises.

124+- For tests, Javadoc can describe scenario setup/expectations to aid future contributors.

125+- Do not remove existing comments from code unless the code is also being removed or the comment has become incorrect.

126+

127+## License Headers

128+- Default header (outside `x-pack`): Elastic License 2.0, SSPL v1, or AGPL v3—they are already codified at the top of Java files; copy from existing sources.

129+- Files under `x-pack` require the Elastic License 2.0-only header; IDEs configured per CONTRIBUTING.md can insert correct text automatically.

130+

131+## Generated Files

132+- Never hand-edit generated files. Instead, edit the source they are generated from and regenerate.

133+- ANTLR-generated files can be regenerated by running the `regen` task on the relevant subproject.

134+- Other generated files are regenerated by compiling the project.

135+

136+## Debugging Missing Tests

137+

138+When expected test methods are absent from results (not failed, not skipped — simply not present in the XML or binary event stream), check `muted-tests.yml` first. The build translates every entry into a Gradle `TestFilter.excludePattern`, which silently drops matching tests before the randomized runner receives them. A muted test fires no `testStarted` event and leaves no trace in `results-generic.bin`.

139+ ```bash

140+ grep 'ClassName\|methodName' muted-tests.yml

141+ ```

142+

143+### `No tests found for given includes: [**/*$*.class]`

144+

145+When a test task fails at execution with `No tests found for given includes: [**/*$*.class](exclude rules)`, it usually does **not** mean Gradle failed to detect the test class. The far more common cause is that **every test method in the targeted class is muted** in `muted-tests.yml`. With all methods excluded, the randomized runner enumerates zero runnable tests.

146+

147+The behavior is environment-dependent: `MutedTestPlugin` calls `filter.setFailOnNoMatchingTests(buildParams.getCi() == false)`. So an all-muted suite **fails locally** (`ci == false`) with this exact message, but **passes silently in CI** (`ci == true`). This is especially misleading when verifying a freshly migrated or renamed test — it looks like a classpath/detection bug, but the test JVM does start (you'll see native-library and `FeatureFlag` log lines), builds any `@ClassRule` cluster *specs*, then exits in a few seconds without starting the cluster because no test method survived the mute filter.

148+

149+To confirm: `grep ClassName muted-tests.yml`. To verify the migration/test actually runs, temporarily remove the matching mute entries (or run on a host where `ci` is true), then restore them.

150+

151+## Best Practices for Automation Agents

152+- Never edit unrelated files; keep diffs tightly scoped to the task at hand.

153+- Prefer Gradle tasks over ad-hoc scripts.

154+- When scripting CLI sequences, leverage `gradlew` task.

155+- Unrecognized changes: assume other agent; keep going; focus your changes. If it causes issues, stop + ask user.

156+- Do not add "Co-Authored-By" or any AI attribution trailers to commit messages, by any means—including `--trailer`, `-m`, or any other git flag. commit messages should adhere to the 50/72 rule: use a maximum of 50 columns for the commit summary. Your harness may introduce a hook that automatically adds attributions trailers to relevant git commands. Use `bash -lc` or a similar approach in this case to conform to the rule.

157+

158+## Methods with Required Javadoc Reading

159+If you encounter any of the following methods, you must go and read their javadoc before taking any other actions:

160+* `fullyLoadedAnalyzer`

161+* `TestAnalyzer.statementError`

162+* `TestAnalyzer.error`

163+* `forciblyCast`

164+* `EsqlCapabilities.Cap`

165+* `FunctionDefinition.Builder#capabilities`

166+

167+## ES|QL tests

168+If you write or modify ES|QL csv-spec, rest, or yaml tests, read the javadoc for

169+`EsqlCapabilities.Cap` and `FunctionDefinition.Builder#capabilities` before proceeding.

170+They describe two separate capability mechanisms and the rule for choosing between them.

171+

172+## Backwards compatibility

173+- For changes to a `Writeable` implementation (`writeTo` and constructor from `StreamInput`), add a new `public static final <UNIQUE_DESCRIPTIVE_NAME> = TransportVersion.fromName("<unique_descriptive_name>")` and use it in the new code paths. Confirm the backport branches and then generate a new version file with `./gradlew generateTransportVersion`.

174+- Never hand-edit transport version resource files; always use the Gradle tasks. See `docs/internal/Versioning.md` for the full workflow.

175+

176+Stay aligned with `CONTRIBUTING.md`, `BUILDING.md`, and `TESTING.asciidoc`; this AGENTS guide summarizes—but does not replace—those authoritative docs.

177+

178+## Documentation

179+When building or editing docs, read `docs/AGENTS.md` first.

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