feat: es 7-8 compat with Plugin and API version 9

.github/workflows/linux.yml

@@ -1,6 +1,5 @@
 name: Testing on Ubuntu
 on:
-  - push
   - pull_request
 permissions:
   contents: read

README.compat.md

@@ -0,0 +1,77 @@
+# Compat
+
+## Mixed Elasticsearch Version Environments
+
+### Compatibility Matrix
+
+| Elasticsearch Gem | ES 7.x Server | ES 8.x Server | ES 9.x Server |
+|-------------------|---------------|---------------|---------------|
+| gem 7.17.x        | ✅ Works      | ✅ Works      | ❌ No         |
+| gem 8.15.x        | ✅ Works      | ✅ Works      | ❌ No         |
+| gem 9.1.x + patch | ✅ With config| ✅ With config| ✅ Works      |
+
+### Configuration Options
+
+#### force_content_type
+
+Type: String
+Default: nil (auto-detect)
+Valid values: "application/json", "application/x-ndjson"
+Manually override the Content-Type header. Required when using elasticsearch gem 9.x with ES 7.x or 8.x servers.
+
+```ruby
+<match **>
+  @type elasticsearch
+  force_content_type application/json
+</match>
+```
+
+#### ignore_version_content_type_mismatch
+
+Type: Bool
+Default: false
+Automatically fallback to application/json if Content-Type version mismatch occurs. Enables seamless operation across mixed ES 7/8/9 environments.
+
+Example:
+
+```ruby
+<match **>
+  @type elasticsearch
+  force_content_type application/json
+  ignore_version_content_type_mismath true
+</match>
+```
+
+### Recommended Configuration
+
+#### For ES 7/8 environments (Recommended)
+
+Use elasticsearch gem 8.x - works with both versions, no configuration needed:
+
+```ruby
+# Gemfile
+gem 'elasticsearch', '~> 8.15.0'
+
+# fluent.conf
+<match **>
+  @type elasticsearch
+  hosts es7:9200,es8:9200
+  # No special config needed!
+</match>
+```
+
+#### For gem 9.x with ES 7/8 (Not recommended, but supported)
+
+```ruby
+# Gemfile
+gem 'elasticsearch', '~> 9.1.0'
+
+# fluent.conf
+<match **>
+  @type elasticsearch
+  hosts es7:9200,es8:9200
+
+  # REQUIRED: Force compatible content type
+  force_content_type application/json
+</match>
+```

lib/fluent/plugin/elasticsearch_api_bugfix.rb

@@ -0,0 +1,42 @@
+# Elasticsearch API 9.x Compatibility Patch
+# 
+# Fixes crashes in elasticsearch-api gem 9.1.2 when connecting to ES 7.x/8.x servers.
+# 
+# Bug: The gem expects ES 9 headers and crashes with NoMethodError when they're nil
+#
+# This patch is only needed if using elasticsearch gem 9.x
+# Not needed if using elasticsearch gem 7.x or 8.x
+
+require 'elasticsearch/api'
+
+module Elasticsearch
+  module API
+    module Utils
+      class << self
+        if method_defined?(:update_ndjson_headers!)
+          alias_method :original_update_ndjson_headers!, :update_ndjson_headers!
+
+          def update_ndjson_headers!(headers, client_headers)
+            return headers unless client_headers.is_a?(Hash)
+
+            current_content = client_headers.keys.find { |c| c.to_s.match?(/content[-_]?type/i) }
+            return headers unless current_content
+
+            content_value = client_headers[current_content]
+            return headers unless content_value
+
+            # ES 7/8 compatibility: Only process ES9-specific headers
+            # If no "compatible-with" present, this is ES 7/8 format
+            return headers unless content_value.to_s.include?('compatible-with')
+
+            # ES 9 detected, safe to call original
+            original_update_ndjson_headers!(headers, client_headers)
+          rescue StandardError => e
+            warn "[elasticsearch-api-compat] Failed to update headers: #{e.class} - #{e.message}"
+            headers
+          end
+        end
+      end
+    end
+  end
+end

lib/fluent/plugin/out_elasticsearch.rb

@@ -27,6 +27,7 @@
 require_relative 'elasticsearch_index_lifecycle_management'
 require_relative 'elasticsearch_tls'
 require_relative 'elasticsearch_fallback_selector'
+require_relative 'elasticsearch_api_bugfix'
 begin
   require_relative 'oj_serializer'
 rescue LoadError
@@ -177,6 +178,12 @@ def initialize(retry_stream)
     config_param :use_legacy_template, :bool, :default => true
     config_param :catch_transport_exception_on_retry, :bool, :default => true
     config_param :target_index_affinity, :bool, :default => false
+    config_param :force_content_type, :string, :default => nil,
+                :desc => "Force specific Content-Type header (e.g., 'application/json' or 'application/x-ndjson'). " \
+                          "Overrides automatic version detection. Useful for mixed ES environments."
+    config_param :ignore_version_content_type_mismatch, :bool, :default => false,
+                :desc => "Automatically fallback to application/json if Content-Type version mismatch occurs. " \
+                          "Enables seamless operation across mixed ES 7/8/9 environments."
 
     config_section :metadata, param_name: :metainfo, multi: false do
       config_param :include_chunk_id, :bool, :default => false
@@ -361,14 +368,31 @@ class << self
             @type_name = nil
           end
           @accept_type = nil
-          if @content_type != ES9_CONTENT_TYPE
+
+          # Only set ES9 content type if not overridden and mismatch handling is not enabled
+          if @content_type.to_s != ES9_CONTENT_TYPE && !@ignore_version_content_type_mismatch
             log.trace "Detected ES 9.x or above: Content-Type will be adjusted."
             @content_type = ES9_CONTENT_TYPE
             @accept_type = ES9_CONTENT_TYPE
+          elsif @ignore_version_content_type_mismatch
+            log.info "Ignoring ES version for Content-Type, using application/json for compatibility"
+            @content_type = :'application/json'
+            @accept_type = nil
           end
         end
       end
 
+      if @content_type.nil?
+        log.warn "content_type was nil, defaulting to application/json"
+        @content_type = :'application/json'
+      end
+
+      if @force_content_type
+        log.info "Forcing Content-Type to: #{@force_content_type}"
+        @content_type = @force_content_type
+        @accept_type = nil
+      end
+
       if @validate_client_version && !dry_run?
         if @last_seen_major_version != client_library_version.to_i
           raise Fluent::ConfigError, <<-EOC
@@ -623,11 +647,17 @@ def client(host = nil, compress_connection = false)
                        else
                          {}
                        end
-        headers = { 'Content-Type' => @content_type.to_s }
+
+        content_type_value = @content_type ? @content_type.to_s : 'application/json'
+        accept_type_value = @accept_type ? @accept_type.to_s : nil
+        content_type_value = 'application/json' if content_type_value.strip.empty?
+
+        headers = { 'Content-Type' => content_type_value }
                     .merge(@custom_headers)
                     .merge(@api_key_header)
                     .merge(gzip_headers)
-        headers.merge!('Accept' => @accept_type) if @accept_type
+
+        headers.merge!('Accept' => accept_type_value) if accept_type_value && !accept_type_value.strip.empty?
 
         ssl_options = { verify: @ssl_verify, ca_file: @ca_file}.merge(@ssl_version_options)
 

test/es-compat/docker-compose.yaml

@@ -0,0 +1,40 @@
+services:
+  elasticsearch7:
+    image: docker.elastic.co/elasticsearch/elasticsearch:7.17.16
+    container_name: es7
+    environment:
+      - discovery.type=single-node
+      - "ES_JAVA_OPTS=-Xms512m -Xmx512m"
+      - xpack.security.enabled=false
+    ports:
+      - "9207:9200"
+    networks:
+      - elastic
+
+  elasticsearch8:
+    image: docker.elastic.co/elasticsearch/elasticsearch:8.12.0
+    container_name: es8
+    environment:
+      - discovery.type=single-node
+      - "ES_JAVA_OPTS=-Xms512m -Xmx512m"
+      - xpack.security.enabled=false
+    ports:
+      - "9208:9200"
+    networks:
+      - elastic
+
+  elasticsearch9:
+    image: docker.elastic.co/elasticsearch/elasticsearch:9.1.5
+    container_name: es9
+    environment:
+      - discovery.type=single-node
+      - "ES_JAVA_OPTS=-Xms512m -Xmx512m"
+      - xpack.security.enabled=false
+    ports:
+      - "9209:9200"
+    networks:
+      - elastic
+
+networks:
+  elastic:
+    driver: bridge