Add CAT segment replication API

_api-reference/cat/cat-repositories.md

@@ -3,7 +3,7 @@ layout: default
 title: CAT repositories
 parent: CAT API
 
-nav_order: 55
+nav_order: 52
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_api-reference/cat/cat-segment-replication.md

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+---
+layout: default
+title: CAT segment replication
+parent: CAT API
+nav_order: 53
+has_children: false
+---
+
+# CAT segment replication
+Introduced 2.7
+{: .label .label-purple }
+
+The CAT segment replication operation returns information about active and last completed [segment replication]({{site.url}}{{site.baseurl}}/opensearch/segment-replication/index) events on each replica shard, including related shard-level metrics. These metrics provide information about how far behind the replicas are lagging with respect to the primary shard. 
+
+Call the CAT Segment Replication API only on indexes with segment replication enabled.
+{: .note}
+
+## Path and HTTP methods
+
+```json
+GET /_cat/segment_replication
+GET /_cat/segment_replication/<index>
+```
+
+## Path parameters
+
+The following table lists the available optional path parameter.
+
+Parameter | Type | Description
+:--- | :--- | :---
+`index` | String | The name of the index, or a comma-separated list or a wildcard expression of index names used to filter results. If this parameter is not provided, the response contains information about all indexes in the cluster.
+
+## Query parameters
+
+The CAT segment replication API operation supports the following optional query parameters.
+
+Parameter | Data type | Description
+:--- |:---| :---
+`active_only` | Boolean | If `true`, the response only includes active segment replications. Defaults to `false`. 
+[`detailed`](#additional-metrics-a-detailed-response-returns) | String | If `true`, the response includes additional metrics of each stage of a segment replication event. Defaults to `false`.
+`shards` | String | A comma-separated list of shards to display.
+`format` | String | A short version of the HTTP accept header. Valid values include `JSON` and `YAML`.  
+`h` | String | A comma-separated list of column names to display. 
+`help` | Boolean | If `true`, the response includes help information. Defaults to `false`.
+`time` | Time value | [Units]({{site.url}}{{site.baseurl}}/opensearch/units) used to display time values. Defaults to `ms` (milliseconds).
+`v` | Boolean | If `true`, the response includes column headings. Defaults to `false`.
+`s` | String | Specifies to sort the results. For example, `s=shardId:desc` sorts by shardId in descending order.
+
+## Examples 
+
+The following examples illustrate various segment replication responses.
+
+#### Example 1: No active segment replication events
+
+The following query requests segment replication metrics with column headings for all indexes:
+
+```json
+GET /_cat/segment_replication?v=true
+```
+{% include copy-curl.html %}
+
+The response contains the metrics for the preceding request:
+
+```bash
+shardId target_node target_host checkpoints_behind bytes_behind current_lag last_completed_lag rejected_requests
+[index-1][0] runTask-1 127.0.0.1 0 0b 0s 7ms 0
+```
+
+#### Example 2: Shard ID specified
+
+The following query requests segment replication metrics with column headings for shards with the ID `0` from indexes `index1` and `index2`:
+
+```json
+GET /_cat/segment_replication/index1,index2?v=true&shards=0
+```
+{% include copy-curl.html %}
+
+The response contains the metrics for the preceding request. The column headings correspond to the metric names:
+
+```bash
+shardId target_node target_host checkpoints_behind bytes_behind current_lag last_completed_lag rejected_requests
+[index-1][0] runTask-1 127.0.0.1 0 0b 0s 3ms 0
+[index-2][0] runTask-1 127.0.0.1 0 0b 0s 5ms 0
+```
+
+#### Example 3: Detailed response
+
+The following query requests detailed segment replication metrics with column headings for all indexes:
+
+```json
+GET /_cat/segment_replication?v=true&detailed=true
+```
+{% include copy-curl.html %}
+
+The response contains additional metrics about the files and stages of a segment replication event:
+
+```bash
+shardId target_node target_host checkpoints_behind bytes_behind current_lag last_completed_lag rejected_requests stage time files_fetched files_percent bytes_fetched bytes_percent start_time stop_time files files_total bytes bytes_total replicating_stage_time_taken get_checkpoint_info_stage_time_taken file_diff_stage_time_taken get_files_stage_time_taken finalize_replication_stage_time_taken
+[index-1][0] runTask-1 127.0.0.1 0 0b 0s 3ms 0 done 10ms 6 100.0% 4753 100.0% 2023-03-16T13:46:16.802Z 2023-03-16T13:46:16.812Z 6 6 4.6kb 4.6kb 0s 2ms 0s 3ms 3ms
+[index-2][0] runTask-1 127.0.0.1 0 0b 0s 5ms 0 done 7ms 3 100.0% 3664 100.0% 2023-03-16T13:53:33.466Z 2023-03-16T13:53:33.474Z 3 3 3.5kb 3.5kb 0s 1ms 0s 2ms 2ms
+```
+
+#### Example 4: Sorting the results
+
+The following query requests segment replication metrics with column headings for all indexes, sorted by shard ID in descending order:
+
+```json
+GET /_cat/segment_replication?v&s=shardId:desc
+```
+{% include copy-curl.html %}
+
+The response contains the sorted results:
+
+```bash
+shardId    target_node  target_host checkpoints_behind bytes_behind current_lag last_completed_lag rejected_requests
+[test6][1] runTask-2   127.0.0.1   0                  0b           0s          5ms                0
+[test6][0] runTask-2   127.0.0.1   0                  0b           0s          4ms                0
+```
+
+#### Example 5: Using a metric alias 
+
+You can either use the metric's full name or one of its aliases in a request. The following query is the same as the preceding query, but it uses the alias `s` instead of `shardID` for sorting:
+
+```json
+GET /_cat/segment_replication?v&s=s:desc
+```
+{% include copy-curl.html %}
+
+## Response metrics
+
+The following table lists the response metrics that are returned for all requests. When referring to a metric in a query parameter, you can provide either the metric's full name or any of its aliases, as shown in the previous [example](#example-5-using-a-metric-alias).
+
+Metric | Alias | Description
+:--- | :--- | :---
+`shardId` | `s` | The ID of a specific shard.
+`target_host` | `thost` | The target host IP address.
+`target_node` | `tnode` | The target node name.
+`checkpoints_behind` | `cpb` | The number of checkpoints by which the replica shard is behind the primary shard.
+`bytes_behind` | `bb` | The number of bytes by which the replica shard is behind the primary shard.
+`current_lag` | `clag` | The time elapsed while waiting for a replica shard to catch up to the primary shard.
+`last_completed_lag` | `lcl` | The time it took for a replica shard to catch up to the latest primary shard refresh.
+`rejected_requests` | `rr` | The number of rejected requests for the replication group.
+
+### Additional metrics a detailed response returns
+
+The following table lists the additional response fields returned if `detailed` is set to `true`.
+
+Metric | Alias | Description
+:--- |:--- |:---
+`stage` | `st` | The current stage of a segment replication event.
+`time` | `t`, `ti` | The time a segment replication event took to complete, in milliseconds.
+`files_fetched` | `ff` | The number of files fetched so far for a segment replication event.
+`files_percent` | `fp` | The percentage of files fetched so far for a segment replication event.
+`bytes_fetched` | `bf` | The number of bytes fetched so far for a segment replication event.
+`bytes_percent` | `bp` | The number of bytes fetched so far for a segment replication event as a percentage.
+`start_time` | `start` | The segment replication start time.
+`stop_time` | `stop` | The segment replication stop time.
+`files` | `f` | The number of files that needs to be fetched for a segment replication event.
+`files_total` | `tf` | The total number of files that are part of this recovery, including both reused and recovered files.
+`bytes` | `b` | The number of bytes that needs to be fetched for a segment replication event.
+`bytes_total` | `tb` | The total number of bytes in the shard.
+`replicating_stage_time_taken` | `rstt` | The time the `replicating` stage of a segment replication event took to complete. 
+`get_checkpoint_info_stage_time_taken` | `gcistt` | The time the `get checkpoint info` stage of a segment replication event took to complete. 
+`file_diff_stage_time_taken` | `fdstt` | The time the `file diff` stage of a segment replication event took to complete. 
+`get_files_stage_time_taken` | `gfstt` | The time the `get files` stage of a segment replication event took to complete. 
+`finalize_replication_stage_time_taken` | `frstt` | The time the `finalize replication` stage of a segment replication event took to complete.