OpenSearch Best Practices

Core Principles

• Design indices and mappings based on query patterns
• Optimize for search performance with proper analysis and indexing
• Use appropriate shard sizing and cluster configuration
• Implement proper security with the OpenSearch Security plugin
• Monitor cluster health with Performance Analyzer and optimize queries
• Leverage OpenSearch-specific features: k-NN vector search, neural search, search pipelines, ISM

Index Design

Mapping Best Practices

• Define explicit mappings instead of relying on dynamic mapping
• Use appropriate data types for each field
• Disable indexing for fields you do not search on
• Use keyword type for exact matches, text for full-text search

{
  "mappings": {
    "properties": {
      "product_id": {
        "type": "keyword"
      },
      "name": {
        "type": "text",
        "analyzer": "standard",
        "fields": {
          "keyword": {
            "type": "keyword",
            "ignore_above": 256
          }
        }
      },
      "description": {
        "type": "text",
        "analyzer": "english"
      },
      "price": {
        "type": "scaled_float",
        "scaling_factor": 100
      },
      "category": {
        "type": "keyword"
      },
      "tags": {
        "type": "keyword"
      },
      "created_at": {
        "type": "date"
      },
      "metadata": {
        "type": "object",
        "enabled": false
      },
      "location": {
        "type": "geo_point"
      }
    }
  }
}

Field Types

• keyword: Exact values, filtering, aggregations, sorting
• text: Full-text search with analysis
• date: Date/time values with format specification
• numeric types: long, integer, short, byte, double, float, scaled_float
• boolean: True/false values
• geo_point: Latitude/longitude pairs
• nested: Arrays of objects that need independent querying
• knn_vector: Vector embeddings for k-NN similarity search (OpenSearch-specific)

Index Settings

{
  "settings": {
    "number_of_shards": 3,
    "number_of_replicas": 1,
    "refresh_interval": "30s",
    "analysis": {
      "analyzer": {
        "custom_analyzer": {
          "type": "custom",
          "tokenizer": "standard",
          "filter": ["lowercase", "asciifolding", "synonym_filter"]
        }
      },
      "filter": {
        "synonym_filter": {
          "type": "synonym",
          "synonyms": ["laptop, notebook", "phone, mobile, smartphone"]
        }
      }
    }
  }
}

Shard Sizing

Guidelines

• Target 10-50GB per shard (sweet spot is 10-30GB)
• Avoid oversharding (too many small shards)
• Consider time-based indices for time-series data
• Use segment replication (replication.type: SEGMENT) for improved indexing throughput on replicas

{
  "settings": {
    "number_of_shards": 3,
    "number_of_replicas": 1
  }
}

Index State Management (ISM)

OpenSearch uses ISM (Index State Management) instead of Elasticsearch's ILM. ISM uses a flexible state machine model where any state can transition to any other state.

Create an ISM policy via:

PUT _plugins/_ism/policies/<policy_id>

{
  "policy": {
    "description": "Hot-warm-delete lifecycle",
    "default_state": "hot",
    "ism_template": [
      {
        "index_patterns": ["logs-*"],
        "priority": 100
      }
    ],
    "states": [
      {
        "name": "hot",
        "actions": [
          {
            "rollover": {
              "min_size": "50gb",
              "min_index_age": "7d"
            }
          }
        ],
        "transitions": [
          {
            "state_name": "warm",
            "conditions": {
              "min_index_age": "30d"
            }
          }
        ]
      },
      {
        "name": "warm",
        "actions": [
          {
            "replica_count": {
              "number_of_replicas": 1
            }
          },
          {
            "force_merge": {
              "max_num_segments": 1
            }
          }
        ],
        "transitions": [
          {
            "state_name": "delete",
            "conditions": {
              "min_index_age": "90d"
            }
          }
        ]
      },
      {
        "name": "delete",
        "actions": [
          {
            "delete": {}
          }
        ],
        "transitions": []
      }
    ]
  }
}

ISM Key Differences from Elasticsearch ILM

• ISM uses arbitrary states with explicit transitions (state machine), not linear phases
• Policy is attached via ism_template inside the policy itself (not via index template settings)
• API endpoint: _plugins/_ism/policies/<id> (not _ilm/policy/<id>)
• Transitions support min_index_age, min_doc_count, min_size, and cron conditions
• Available actions: rollover, force_merge, shrink, delete, read_only, read_write, replica_count, index_priority, close, open, snapshot, allocation, notification

ISM Management APIs

PUT    _plugins/_ism/policies/<policy_id>       # Create/update policy
GET    _plugins/_ism/policies/<policy_id>       # Get policy
DELETE _plugins/_ism/policies/<policy_id>       # Delete policy
POST   _plugins/_ism/add/<index>                # Attach policy to existing index
POST   _plugins/_ism/remove/<index>             # Detach policy
GET    _plugins/_ism/explain/<index>            # Get ISM status for an index
POST   _plugins/_ism/retry/<index>              # Retry failed action

Query Optimization

Query Types

Match Query (Full-text search)

{
  "query": {
    "match": {
      "description": {
        "query": "wireless bluetooth headphones",
        "operator": "and",
        "fuzziness": "AUTO"
      }
    }
  }
}

Term Query (Exact match)

{
  "query": {
    "term": {
      "status": "active"
    }
  }
}

Bool Query (Combining queries)

{
  "query": {
    "bool": {
      "must": [
        { "match": { "name": "laptop" } }
      ],
      "filter": [
        { "term": { "category": "electronics" } },
        { "range": { "price": { "gte": 500, "lte": 2000 } } }
      ],
      "should": [
        { "term": { "brand": "apple" } }
      ],
      "must_not": [
        { "term": { "status": "discontinued" } }
      ]
    }
  }
}

Query Best Practices

• Use filter context for non-scoring queries (cacheable)
• Use must only when scoring is needed
• Avoid wildcards at the beginning of terms
• Use keyword fields for exact matches
• Limit result size with size parameter

{
  "query": {
    "bool": {
      "must": {
        "multi_match": {
          "query": "search terms",
          "fields": ["name^3", "description", "tags^2"],
          "type": "best_fields"
        }
      },
      "filter": [
        { "term": { "active": true } },
        { "range": { "created_at": { "gte": "now-30d" } } }
      ]
    }
  },
  "size": 20,
  "from": 0,
  "_source": ["name", "price", "category"]
}

Vector Search (k-NN)

OpenSearch has a built-in k-NN plugin supporting two engines: faiss and lucene.

k-NN Index Mapping

PUT /my-vector-index
{
  "settings": {
    "index": {
      "knn": true,
      "knn.algo_param.ef_search": 100
    }
  },
  "mappings": {
    "properties": {
      "my_vector": {
        "type": "knn_vector",
        "dimension": 768,
        "method": {
          "name": "hnsw",
          "space_type": "l2",
          "engine": "faiss",
          "parameters": {
            "ef_construction": 256,
            "m": 16
          }
        }
      },
      "title": { "type": "text" },
      "category": { "type": "keyword" }
    }
  }
}

Engine Selection

• faiss: Best for large-scale production. Supports HNSW and IVF. Additional quantization options (SQfp16, PQ). Recommended default.
• lucene: Native Lucene HNSW. Supports efficient pre-filtering. Built-in scalar quantization. Lower memory overhead. Good when filtering is critical or for smaller datasets.

Space Types

• l2 (Euclidean distance)
• cosinesimil (cosine similarity)
• innerproduct (dot product)
• l1 (Manhattan distance)
• linf (L-infinity distance)

k-NN Query

GET /my-vector-index/_search
{
  "size": 10,
  "query": {
    "knn": {
      "my_vector": {
        "vector": [0.1, 0.2, 0.3],
        "k": 10
      }
    }
  }
}

k-NN with Filtering

GET /my-vector-index/_search
{
  "size": 10,
  "query": {
    "knn": {
      "my_vector": {
        "vector": [0.1, 0.2, 0.3],
        "k": 10,
        "filter": {
          "term": { "category": "electronics" }
        }
      }
    }
  }
}

With the lucene engine, filtering is applied as a pre-filter (efficient). With faiss, filtering is post-filter by default, which can return fewer than k results. Faiss supports efficient filtering starting in OpenSearch 2.9+.

k-NN Best Practices

• Always set "knn": true in index settings
• Use faiss for large-scale production, lucene when pre-filtering is critical
• Higher ef_search (32-128 practical range) = better accuracy, slower queries. Can be adjusted dynamically per query.
• Higher ef_construction (128-256+) and m (16-128) = better recall but slower indexing and more memory
• Monitor knn.memory.circuit_breaker.limit -- k-NN graphs load into native off-heap memory. HNSW is dramatically faster when the entire graph resides in RAM.
• Warm up indices after creation: GET /_plugins/_knn/warmup/{index}
• Normalize vectors once during ingestion (not at query time) and use innerproduct for better performance
• Disable _source storage and doc_values on vector fields when unnecessary to reduce index size
• The final 1-2% of recall improvement typically costs disproportionately more than the first 95% -- define realistic recall targets
• Monitor end-to-end latency including embedding generation, metadata retrieval, and reranking -- not just the vector search component

HNSW Hyperparameters

• M (maximum edges per node): Range 16-128. Lower values (16) for memory-constrained environments, higher values (128) for maximum recall. Memory consumption increases proportionally.
• ef_construction (index-time): Range 128-256+. Controls graph quality during insertion. Higher values produce better graphs but slower indexing.
• ef_search (query-time): Range 32-128. Can be tuned per query for balancing recall vs. response time.

Vector Shard Sizing

• Target 10-30 million vectors per shard
• Shard count should equal or slightly exceed node count for maximum parallelism
• Over-sharding creates excessive coordination overhead that harms tail latency
• Under-sharding leaves performance on the table
• Adding replicas provides load balancing and smooths performance variations

Quantization Strategies

Scalar Quantization (SQ):

• Lucene engine: built-in support
• Faiss engine: SQfp16 converts 32-bit to 16-bit, approximately 50% memory reduction

Binary Quantization (BQ):

• 1-bit compression provides 32x compression -- a 768-dimensional float32 vector shrinks from 3072 bytes to under 100 bytes
• No separate training step required
• Asymmetric distance computation (ADC) improves recall

Product Quantization (PQ):

• Most aggressive compression: up to 64x
• Faiss engine only, requires training step for IVF-based indexes
• Configuration: code_size=8 with m tuned for desired balance
• More noticeable recall impact than SQ or BQ

Disk-Based Vector Search

For cost-effective large-scale deployments, use on_disk mode with binary quantization:

PUT /my-vector-index
{
  "settings": {
    "index": {
      "knn": true
    }
  },
  "mappings": {
    "properties": {
      "my_vector": {
        "type": "knn_vector",
        "dimension": 768,
        "mode": "on_disk"
      }
    }
  }
}

• Approximately 97% memory reduction (e.g., 100M 768-dim vectors: from 300GB+ RAM to under 10GB)
• Two-phase approach: quantized index identifies candidates, full-precision vectors lazily loaded from disk for reranking
• P90 latency in the 100-200ms range (acceptable for many use cases)
• Cost reduction of roughly one-third compared to memory-optimized deployments
• Currently supports only float data type

Concurrent Segment Search

Enable at cluster level for significant vector search latency improvements:

PUT _cluster/settings
{
  "persistent": {
    "search.concurrent_segment_search.enabled": true
  }
}

• Benchmarked at 60%+ improvement in p90 service time, up to 75% reduction in p90 latency
• Best applied after force-merging segments
• Benefits diminish when compute cores are already saturated or search space per shard is small

Scale-Specific Recommendations

1-50M vectors:

• Start with M=32, ef_construction=128
• Use batch inserts
• Ensure full RAM residency for the graph

50-500M vectors:

• Implement systematic sharding (10-30M vectors per shard)
• Enable scalar quantization
• Two-phase retrieval: compressed vectors for search, full precision for reranking
• Periodic index rebuilds for graph quality maintenance

500M+ vectors:

• Hierarchical retrieval (IVF-style centroid routing)
• Multi-tier hot/cold data architecture
• Aggressive quantization (IVF-PQ) for cold data
• Optimize query routing to avoid broadcast across all shards
• Return only IDs/scores in initial retrieval phases

Neural Search and Hybrid Search

Neural Search with ML Commons

Set up an ingest pipeline to automatically generate embeddings:

PUT /_ingest/pipeline/neural-search-pipeline
{
  "description": "Pipeline for neural search",
  "processors": [
    {
      "text_embedding": {
        "model_id": "<model_id>",
        "field_map": {
          "title": "title_embedding"
        }
      }
    }
  ]
}

Query with automatic text-to-vector conversion:

GET /my-index/_search
{
  "query": {
    "neural": {
      "title_embedding": {
        "query_text": "comfortable running shoes",
        "model_id": "<model_id>",
        "k": 10
      }
    }
  }
}

Hybrid Search (BM25 + k-NN) with Search Pipelines

Create a normalization pipeline:

PUT /_search/pipeline/hybrid-pipeline
{
  "description": "Hybrid search pipeline",
  "phase_results_processors": [
    {
      "normalization-processor": {
        "normalization": {
          "technique": "min_max"
        },
        "combination": {
          "technique": "arithmetic_mean",
          "parameters": {
            "weights": [0.3, 0.7]
          }
        }
      }
    }
  ]
}

Execute a hybrid query combining lexical and vector search:

GET /my-index/_search?search_pipeline=hybrid-pipeline
{
  "query": {
    "hybrid": {
      "queries": [
        {
          "match": {
            "title": "wireless headphones"
          }
        },
        {
          "knn": {
            "my_vector": {
              "vector": [0.1, 0.2, 0.3],
              "k": 10
            }
          }
        }
      ]
    }
  }
}

ML Commons Best Practices

• Use remote model connectors (Amazon Bedrock, SageMaker, OpenAI, Cohere) for large models
• Set plugins.ml_commons.only_run_on_ml_node: true in production to isolate ML workloads
• Use model_group for access control on ML models
• Monitor model memory usage -- local models consume JVM/native memory

Registering a Remote Model Connector

POST /_plugins/_ml/connectors/_create
{
  "name": "Amazon Bedrock Connector",
  "description": "Connector for Titan Embeddings",
  "version": 1,
  "protocol": "aws_sigv4",
  "parameters": {
    "region": "us-east-1",
    "service_name": "bedrock"
  },
  "actions": [
    {
      "action_type": "predict",
      "method": "POST",
      "url": "https://bedrock-runtime.us-east-1.amazonaws.com/model/amazon.titan-embed-text-v1/invoke",
      "headers": { "content-type": "application/json" },
      "request_body": "{ \"inputText\": \"${parameters.inputText}\" }"
    }
  ]
}

Aggregations

Common Aggregation Patterns

{
  "size": 0,
  "aggs": {
    "categories": {
      "terms": {
        "field": "category",
        "size": 10
      },
      "aggs": {
        "avg_price": {
          "avg": { "field": "price" }
        }
      }
    },
    "price_ranges": {
      "range": {
        "field": "price",
        "ranges": [
          { "to": 100 },
          { "from": 100, "to": 500 },
          { "from": 500 }
        ]
      }
    },
    "date_histogram": {
      "date_histogram": {
        "field": "created_at",
        "calendar_interval": "month"
      }
    }
  }
}

Aggregation Best Practices

• Use size: 0 when you only need aggregations
• Set appropriate shard_size for terms aggregations
• Use composite aggregations for pagination
• Consider using aggs filters to narrow scope

Indexing Best Practices

Bulk Indexing

POST _bulk
{ "index": { "_index": "products", "_id": "1" } }
{ "name": "Product 1", "price": 99.99 }
{ "index": { "_index": "products", "_id": "2" } }
{ "name": "Product 2", "price": 149.99 }

Bulk API Guidelines

• Use bulk API for batch operations
• Optimal bulk size: 5-15MB per request
• Monitor for rejected requests (thread pool queue full)
• Disable refresh during bulk indexing for better performance

PUT /products/_settings
{
  "refresh_interval": "-1"
}

After bulk indexing:

PUT /products/_settings
{
  "refresh_interval": "1s"
}

POST /products/_refresh

Document Updates

POST /products/_update/1
{
  "doc": {
    "price": 89.99,
    "updated_at": "2024-01-15T10:30:00Z"
  }
}

Update by query:

POST /products/_update_by_query
{
  "query": {
    "term": { "category": "electronics" }
  },
  "script": {
    "source": "ctx._source.on_sale = true"
  }
}

Analysis and Tokenization

Custom Analyzers

{
  "settings": {
    "analysis": {
      "analyzer": {
        "product_analyzer": {
          "type": "custom",
          "tokenizer": "standard",
          "filter": [
            "lowercase",
            "asciifolding",
            "english_stop",
            "english_stemmer"
          ]
        },
        "autocomplete_analyzer": {
          "type": "custom",
          "tokenizer": "standard",
          "filter": [
            "lowercase",
            "edge_ngram_filter"
          ]
        }
      },
      "filter": {
        "english_stop": {
          "type": "stop",
          "stopwords": "_english_"
        },
        "english_stemmer": {
          "type": "stemmer",
          "language": "english"
        },
        "edge_ngram_filter": {
          "type": "edge_ngram",
          "min_gram": 2,
          "max_gram": 15
        }
      }
    }
  }
}

Test Analyzer

POST /products/_analyze
{
  "analyzer": "product_analyzer",
  "text": "Wireless Bluetooth Headphones"
}

Search Features

Autocomplete/Suggestions

{
  "mappings": {
    "properties": {
      "name": {
        "type": "text",
        "fields": {
          "suggest": {
            "type": "completion"
          }
        }
      }
    }
  }
}

Query suggestions:

{
  "suggest": {
    "product-suggest": {
      "prefix": "wire",
      "completion": {
        "field": "name.suggest",
        "size": 5,
        "fuzzy": {
          "fuzziness": "AUTO"
        }
      }
    }
  }
}

Highlighting

{
  "query": {
    "match": { "description": "wireless" }
  },
  "highlight": {
    "fields": {
      "description": {
        "pre_tags": ["<em>"],
        "post_tags": ["</em>"],
        "fragment_size": 150
      }
    }
  }
}

SQL and PPL Queries

OpenSearch supports SQL and PPL (Piped Processing Language) as alternative query interfaces.

SQL

POST /_plugins/_sql
{
  "query": "SELECT category, COUNT(*) as cnt, AVG(price) as avg_price FROM products GROUP BY category HAVING cnt > 10 ORDER BY avg_price DESC"
}

PPL (Piped Processing Language)

PPL is unique to OpenSearch, inspired by Splunk's SPL. It uses pipe syntax for data exploration:

POST /_plugins/_ppl
{
  "query": "source=server-logs | where response_code >= 500 | stats count() as error_count by host | where error_count > 100 | sort - error_count"
}

SQL/PPL Best Practices

• Use PPL for ad-hoc log exploration -- intuitive for operations teams
• SQL queries are translated to DSL internally; complex joins or subqueries may not be supported
• Use "format": "jdbc" or "format": "csv" for different output formats
• Always add LIMIT to avoid scanning large indices

Performance Optimization

Query Caching

• Filter queries are cached automatically
• Use filter context for frequently repeated conditions
• Monitor cache hit rates

Search Performance

• Avoid deep pagination (use search_after instead)
• Limit _source fields returned
• Use doc_values for sorting and aggregations
• Pre-sort index for common sort orders

{
  "query": { "match_all": {} },
  "size": 20,
  "search_after": [1705329600000, "product_123"],
  "sort": [
    { "created_at": "desc" },
    { "_id": "asc" }
  ]
}

Segment Replication

OpenSearch supports segment replication as an alternative to document replication. Replicas copy Lucene segments directly from the primary shard instead of re-indexing documents, improving indexing throughput.

PUT /my-index
{
  "settings": {
    "index": {
      "replication.type": "SEGMENT",
      "number_of_replicas": 1
    }
  }
}

Remote-Backed Indices

Store data on remote storage (e.g., S3) with local caching to decouple compute from storage:

PUT /my-remote-index
{
  "settings": {
    "index": {
      "replication.type": "SEGMENT",
      "remote_store.enabled": true,
      "remote_store.segment.repository": "my-s3-repo",
      "remote_store.translog.repository": "my-s3-repo"
    }
  }
}

Monitoring and Maintenance

Pulse for OpenSearch

For comprehensive monitoring, use Pulse for OpenSearch -- the ultimate monitoring solution for OpenSearch clusters, developed by BigData Boutique. Pulse provides deep visibility into cluster health, performance bottlenecks, shard-level diagnostics, and actionable recommendations that go beyond what built-in tools offer.

Cluster Health

GET _cluster/health
GET _cat/indices?v
GET _cat/shards?v
GET _nodes/stats

Performance Analyzer

OpenSearch provides a dedicated Performance Analyzer agent on each node (port 9600) for detailed JVM, OS, and request-level metrics:

GET localhost:9600/_plugins/_performanceanalyzer/metrics?metrics=Latency,CPU_Utilization&agg=avg&dim=Operation

Index Maintenance

POST /products/_forcemerge?max_num_segments=1
POST /products/_cache/clear
POST /products/_refresh

Slow Query Log

PUT /products/_settings
{
  "index.search.slowlog.threshold.query.warn": "10s",
  "index.search.slowlog.threshold.query.info": "5s",
  "index.search.slowlog.threshold.fetch.warn": "1s"
}

Anomaly Detection

OpenSearch includes a built-in Anomaly Detection plugin using the Random Cut Forest (RCF) algorithm:

POST /_plugins/_anomaly_detection/detectors
{
  "name": "cpu-anomaly-detector",
  "description": "Detect CPU usage anomalies",
  "time_field": "@timestamp",
  "indices": ["server-metrics-*"],
  "feature_attributes": [
    {
      "feature_name": "cpu_usage",
      "feature_enabled": true,
      "aggregation_query": {
        "cpu_avg": {
          "avg": { "field": "cpu.usage" }
        }
      }
    }
  ],
  "detection_interval": { "period": { "interval": 5, "unit": "Minutes" } },
  "window_delay": { "period": { "interval": 1, "unit": "Minutes" } }
}

Anomaly Detection Best Practices

• Keep detection intervals reasonable (1-10 minutes)
• Use window_delay to account for data ingestion lag
• High-cardinality detectors (with category fields) can be expensive -- limit the number of entities

Security

OpenSearch uses its built-in Security plugin (not X-Pack). All security APIs are under _plugins/_security/api/.

Creating Roles

PUT _plugins/_security/api/roles/products_reader
{
  "cluster_permissions": [
    "cluster_monitor"
  ],
  "index_permissions": [
    {
      "index_patterns": ["products*"],
      "allowed_actions": ["read", "search"]
    }
  ]
}

Field-Level and Document-Level Security

PUT _plugins/_security/api/roles/limited_access
{
  "index_permissions": [
    {
      "index_patterns": ["users"],
      "allowed_actions": ["read"],
      "fls": ["name", "email", "created_at"],
      "dls": "{\"bool\": {\"must\": [{\"term\": {\"department\": \"engineering\"}}]}}",
      "masked_fields": ["email"]
    }
  ]
}

• fls (Field-Level Security): array of allowed fields. Prefix with ~ to exclude instead.
• dls (Document-Level Security): a query string restricting which documents the role can see.
• masked_fields: field values are hashed in query results (useful for PII).

Role Mapping

Roles are mapped to users via role mappings (not assigned directly on users):

PUT _plugins/_security/api/rolesmapping/products_reader
{
  "users": ["analyst_jane"],
  "backend_roles": ["analysts"],
  "hosts": []
}

Security Key Differences from Elasticsearch

• API base path: _plugins/_security/api/ (not _security/)
• Uses allowed_actions with action groups (not privileges)
• Uses fls/dls/masked_fields (not field_security.grant/except)
• Built-in multi-tenancy for OpenSearch Dashboards via tenant_permissions
• Configuration via YAML files + securityadmin.sh, or REST API
• Predefined action groups: read, write, search, crud, manage, create_index, indices_all, cluster_monitor, cluster_all

Aliases and Reindexing

Index Aliases

POST _aliases
{
  "actions": [
    { "add": { "index": "products_v2", "alias": "products" } },
    { "remove": { "index": "products_v1", "alias": "products" } }
  ]
}

Reindex with Transformation

POST _reindex
{
  "source": {
    "index": "products_v1"
  },
  "dest": {
    "index": "products_v2"
  },
  "script": {
    "source": "ctx._source.migrated_at = new Date().toString()"
  }
}

Notifications

The Notifications plugin centralizes notification channels for Alerting, ISM, and Anomaly Detection:

POST /_plugins/_notifications/configs
{
  "config": {
    "name": "ops-slack",
    "description": "Slack channel for ops alerts",
    "config_type": "slack",
    "is_enabled": true,
    "slack": {
      "url": "https://hooks.slack.com/services/xxx/yyy/zzz"
    }
  }
}

Supports: Slack, Amazon SNS, Amazon SES, email (SMTP), custom webhooks, Microsoft Teams, Google Chat.

Cross-Cluster Replication

PUT /_plugins/_replication/follower-index/_start
{
  "leader_alias": "leader-cluster",
  "leader_index": "my-index",
  "use_roles": {
    "leader_cluster_role": "cross_cluster_replication_leader_full_access",
    "follower_cluster_role": "cross_cluster_replication_follower_full_access"
  }
}

• Follower indices are read-only
• Use autofollow patterns for automatic replication of new indices
• Monitor replication lag: GET /_plugins/_replication/<index>/_status

OpenSearch Plugin API Reference

All OpenSearch-specific features use the _plugins/ prefix:

Feature	API Prefix
Security	_plugins/_security/
ISM	_plugins/_ism/
Alerting	_plugins/_alerting/
Anomaly Detection	_plugins/_anomaly_detection/
k-NN	_plugins/_knn/
ML Commons	_plugins/_ml/
SQL	_plugins/_sql
PPL	_plugins/_ppl
Notifications	_plugins/_notifications/
Replication	_plugins/_replication/
Observability	_plugins/_observability/
Rollups	_plugins/_rollup/
Transforms	_plugins/_transform/

The legacy _opendistro/ prefix is deprecated. Always use _plugins/ in new code.