Add Peer forwarder to doc website repo.

_data-prepper/peer-forwarder.md

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+---
+layout: default
+title: Peer Forwarder
+nav_order: 12
+---
+
+# Peer Forwarder
+
+Peer Forwarder is an HTTP service that performs peer forwarding of an `event` between Data Prepper nodes for aggregation. Currently, Peer Forwarder is supported by the `aggregate`, `service_map_stateful`, and `otel_trace_raw` processors.
+
+Peer Forwarder groups events based on the identification keys provided by the processors. For `service_map_stateful` and `otel_trace_raw`, the identification key is `traceId` by default and cannot be configured. The `aggregate` processor is configured using the `identification_keys` configuration option. From here, you can specify which keys to use for Peer Forwarder. See [Aggregate Processor page](https://github.com/opensearch-project/data-prepper/tree/main/data-prepper-plugins/aggregate-processor#identification_keys) for more information about identification keys.
+
+Peer discovery is currently provided by a static list, a DNS record lookup, or AWS Cloud Map.  
+
+## Discovery modes
+
+The following sections provide information about discovery modes.
+
+### Static
+
+Static discovery mode allows a Data Prepper node to discover nodes using a list of IP addresses or domain names. See the following yaml file for an example of static discovery mode:
+
+```yaml
+peer_forwarder:4
+  discovery_mode: static
+  static_endpoints: ["data-prepper1", "data-prepper2"]
+```
+
+### DNS lookup
+
+DNS discovery is preferred over static discovery when scaling out a Data Prepper cluster. DNS discovery configures a DNS provider to return a list of Data Prepper hosts when given a single domain name. This list that is returned is a [DNS A record](https://www.cloudflare.com/learning/dns/dns-records/dns-a-record/), which indicates a list of IP addresses of a given domain. See the following yaml file example of DNS lookup:
+
+```yaml
+peer_forwarder:
+  discovery_mode: dns
+  domain_name: "data-prepper-cluster.my-domain.net"
+```
+
+### AWS Cloud Map
+
+[AWS Cloud Map](https://docs.aws.amazon.com/cloud-map/latest/dg/what-is-cloud-map.html) provides API-based service discovery as well as DNS-based service discovery.
+
+Peer Forwarder can use the API-based service discovery. To support this, you must have an existing namespace configured for API instance discovery. You can create a new one by following the instructions provided by the [AWS Cloud Map documentation](https://docs.aws.amazon.com/cloud-map/latest/dg/working-with-namespaces.html).
+
+Your Data Prepper configuration needs to include the following:
+* `aws_cloud_map_namespace_name` – Set to your AWS Cloud Map namespace name.
+* `aws_cloud_map_service_name` – Set to the service name within your specified namespace.
+* `aws_region` – Set to the AWS Region where your namespace exists.
+* `discovery_mode` – Set to `aws_cloud_map`.
+
+Your Data Prepper configuration can optionally include the following:
+* `aws_cloud_map_query_parameters` – Key-value pairs are used to filter the results based on the custom attributes attached to an instance. Only instances that match all the specified key-value pairs are returned.
+
+#### Example configuration
+
+See the following yaml file example of AWS Cloud Map configuration:
+
+```yaml
+peer_forwarder:
+  discovery_mode: aws_cloud_map
+  aws_cloud_map_namespace_name: "my-namespace"
+  aws_cloud_map_service_name: "data-prepper-cluster"
+  aws_cloud_map_query_parameters:
+    instance_type: "r5.xlarge"
+  aws_region: "us-east-1"
+```
+
+### IAM policy with necessary permissions
+
+Data Prepper must also be running with the necessary permissions. The following AWS Identity and Access Management (IAM) policy shows the necessary permissions:
+
+```json
+{
+    "Version": "2012-10-17",
+    "Statement": [
+        {
+            "Sid": "CloudMapPeerForwarder",
+            "Effect": "Allow",
+            "Action": "servicediscovery:DiscoverInstances",
+            "Resource": "*"
+        }
+    ]
+}
+```
+---
+## Configuration
+
+The following table provides optional configuration values.
+
+
+| Value | Description |
+| ----  | ----------- |
+| `port` | An `int` between 0 and 65535 represents the port that the Peer Forwarder server is running on. Default value is `4994`. |
+| `request_timeout` | An `int` representing the request timeout duration in milliseconds for the Peer Forwarder HTTP server. Default value is `10000`. |
+| `server_thread_count` | An `int` representing the number of threads used by the Peer Forwarder server. Default value is `200`.|
+| `client_thread_count` | An `int` representing the number of threads used by the Peer Forwarder client. Default value is `200`.|
+| `maxConnectionCount`  |  An `int` representing the maximum number of open connections for the Peer Forwarder server. Default value is `500`. |
+| `discovery_mode` | A `String` representing the peer discovery mode to be used. Allowable values are `local_node`, `static`, `dns`, and `aws_cloud_map`. Defaults to `local_node`, which processes events locally. |
+| `static_endpoints` |  A `list` containing the endpoints of all Data Prepper instances. Required if `discovery_mode` is set to `static`. |
+|  `domain_name` | A `String` representing the single domain name to query DNS against. Typically, used by creating multiple [DNS A records](https://www.cloudflare.com/learning/dns/dns-records/dns-a-record/) for the same domain. Required if `discovery_mode` is set to `dns`. |
+| `aws_cloud_map_namespace_name`  | A `String` representing the AWS Cloud Map namespace when using AWS Cloud Map service discovery. Required if `discovery_mode` is set to `aws_cloud_map`.  |
+| `aws_cloud_map_service_name` |  A `String` representing the AWS Cloud Map service when using AWS Cloud Map service discovery. Required if `discovery_mode` is set to `aws_cloud_map`. |
+| `aws_cloud_map_query_parameters`  |  A `Map` of key-value pairs used to filter the results based on the custom attributes attached to an instance. Only instances that match all the specified key-value pairs are returned. |
+| `buffer_size` |  An `int` representing the maximum number of unchecked records the buffer accepts (the number of unchecked records equals the number of records written into the buffer plus the number of in-flight records not yet checked by the Checkpointing API). Default is `512`. |
+| `batch_size` |  An `int` representing max number of records the buffer returns on read. Default is `48`. |
+|  `aws_region` |  A `String` represents the AWS Region to use `ACM`, `S3` or `AWS Cloud Map`. Required if `use_acm_certificate_for_ssl` is set to `true` or `ssl_certificate_file`,  and `ssl_key_file` is set to the `AWS S3` path, or if `discovery_mode` is set to `aws_cloud_map`. |
+| `drain_timeout`  | A `Duration` representing the wait time for the Peer Forwarder to complete processing data before shutdown. |
+
+## SSL configuration
+
+The following SSL configuration table provides optional SSL configuration values and allows you to set up trust manager for the peer forwarding client to connect to other Data Prepper instances.
+
+| Value | Description |
+| ----- | ----------- |
+| `ssl` | A `Boolean` that enables TLS/SSL. Default value is `true`. |
+| `ssl_certificate_file`| A `String` representing the SSL certificate chain file path or Amazon Simple Storage Service (Amazon S3) path. The following is an example of an Amazon S3 path: `s3://<bucketName>/<path>`. Defaults to the default certificate file,`config/default_certificate.pem`. See [Default Certificates](https://github.com/opensearch-project/data-prepper/tree/main/examples/certificates) for more information about how the certificate is generated. |
+| `ssl_key_file`| A `String` represents the SSL key file path or AWS S3 path. S3 path example `s3://<bucketName>/<path>`. Defaults to `config/default_private_key.pem` which is default private key file. Read more about how the private key file is generated at the [Default Certificates](https://github.com/opensearch-project/data-prepper/tree/main/examples/certificates) page. |
+| `ssl_insecure_disable_verification` | A `Boolean` that disables the verification of the server's TLS certificate chain. Default value is `false`. |
+| `ssl_fingerprint_verification_only` | A `Boolean` that disables the verification of the server's TLS certificate chain and instead verifies only the certificate fingerprint. Default value is `false`. |
+| `use_acm_certificate_for_ssl` | A `Boolean` that enables TLS/SSL using the certificate and private key from AWS Certificate Manager (ACM). Default value is `false`. |
+| `acm_certificate_arn`| A `String` representing the ACM certificate Amazon Resource Name (ARN). The ACM certificate takes precedence over S3 or the local file system certificate. Required if `use_acm_certificate_for_ssl` is set to `true`. |
+| `acm_private_key_password` | A `String` representing the ACM private key password that will be used to decrypt the private key. If it's not provided, a random password will be generated. |
+| `acm_certificate_timeout_millis` | An `Int` representing the timeout in milliseconds required for ACM to get certificates. Default value is `120000`. |
+| `aws_region` | A `String` representing the AWS Region that uses `ACM`, `S3` or `AWS Cloud Map`. Required if `use_acm_certificate_for_ssl` is set to `true` or `ssl_certificate_file`. Also required when the `ssl_key_file` is set to the `AWS S3` path, or if `discovery_mode` is set to `aws_cloud_map`. |
+
+#### Example configuration
+
+The following yaml file provides an example configuration:
+
+```yaml
+peer_forwarder:
+  ssl: true
+  ssl_certificate_file: "<cert-file-path>"
+  ssl_key_file: "<private-key-file-path>"
+```
+
+## Authentication
+
+This section describes optional authentication.
+
+`Authentication` is optional and is a `Map` that enables mTLS. It can either be `mutual_tls` or `unauthenticated`. The default value is `unauthenticated`. The following yaml file provides an exmaple for authentication:
+
+```yaml
+peer_forwarder:
+  authentication:
+    mutual_tls:
+```
+
+## Metrics
+
+This section describes Peer Forwarder's metrics capabilities, including timer, counter, and gauge.
+
+Core Peer Forwarder introduces the following custom metrics. All the metrics are prefixed by `core.peerForwarder`.
+
+### Timer
+
+Peer Forwarder's timer capability provides the following information:
+
+- `requestForwardingLatency`: Measures latency of requests forwarded by the Peer Forwarder client.
+- `requestProcessingLatency`: Measures latency of requests processed by the Peer Forwarder server.
+
+### Counter
+
+The following table provides counter metric options.
+
+| Value | Description |
+| ----- | ----------- |
+| `requests`| Measures the total number of forwarded requests. |
+| `requestsFailed`| Measures the total number of failed requests. Requests with HTTP response code other than `200`. |
+| `requestsSuccessful`|  Measures the total number of successful requests. Requests with HTTP response code `200`. |
+| `requestsTooLarge`| Measures the total number of requests that are too large to be written to the Peer Forwarder buffer. Requests with HTTP response code `413`. |
+| `requestTimeouts`| Measures the total number of requests that time out while writing content to the Peer Forwarder buffer. Requests with HTTP response code `408`. |
+| `requestsUnprocessable`| Measures the total number of requests that fail due to an unprocessable entity. Requests with HTTP response code `422`. |
+| `badRequests`| Measures the total number of requests with bad request format. Requests with HTTP response code `400`. |
+| `recordsSuccessfullyForwarded`| Measures the total number of successfully forwarded records. |
+| `recordsFailedForwarding`| Measures the total number of records fail to be forwarded. |
+| `recordsToBeForwarded` | Measures the total number of records to be forwarded. |
+| `recordsToBeProcessedLocally` | Measures the total number of records to be processed locally. |
+| `recordsActuallyProcessedLocally`| Measures the total number of records actually processed locally. This value is the sum of `recordsToBeProcessedLocally` and `recordsFailedForwarding`. |
+| `recordsReceivedFromPeers`| Measures the total number of records received from remote peers. |
+
+### Gauge
+
+`peerEndpoints` Measures the number of dynamically discovered peer Data Prepper endpoints. For `static` mode, the size is fixed.