Add _flush API (opensearch-project#7217)

* Add _flush API Signed-off-by: Archer <[email protected]> * Apply suggestions from code review Signed-off-by: Naarcha-AWS <[email protected]> * Apply suggestions from code review Signed-off-by: Naarcha-AWS <[email protected]> * Update _api-reference/index-apis/flush.md Signed-off-by: Naarcha-AWS <[email protected]> * Apply suggestions from code review Signed-off-by: Naarcha-AWS <[email protected]> * Update flush.md Signed-off-by: Naarcha-AWS <[email protected]> * Apply suggestions from code review Co-authored-by: Melissa Vagi <[email protected]> Signed-off-by: Naarcha-AWS <[email protected]> * Apply suggestions from code review Co-authored-by: Nathan Bower <[email protected]> Signed-off-by: Naarcha-AWS <[email protected]> --------- Signed-off-by: Archer <[email protected]> Signed-off-by: Naarcha-AWS <[email protected]> Co-authored-by: Melissa Vagi <[email protected]> Co-authored-by: Nathan Bower <[email protected]> Signed-off-by: RasonJ <[email protected]>
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+---
+layout: default
+title: Flush
+parent: Index APIs
+nav_order: 36
+---
+
+# Flush
+
+**Introduced 1.0**
+{: .label .label-purple }
+
+The Flush API stores all in-memory operations to segments on disk. Operations flushed to an index segment are no longer needed in transaction logs during a cluster restart because these operations are now stored in the Lucene index. 
+
+OpenSearch automatically performs flushes in the background based on conditions like transaction log size, which is controlled by the `index.translog.flush_threshold_size` setting. Use the Flush API sparingly, for example, for manual restarts or to free up memory.
+
+## Path and HTTP methods
+
+The Flush API supports the following paths:
+
+```
+GET /_flush
+POST /_flush
+GET /{index}/_flush
+POST /{index}/_flush
+```
+
+## Path parameters
+
+The following table lists the available path parameters. All path parameters are optional.
+
+| Parameter | Data type | Description |
+| :--- | :--- | :--- |
+| `<index>` | String | A comma-separated list of indexes, data streams, or index aliases to which the operation is applied. Supports wildcard expressions (`*`). Use `_all` or `*` to specify all indexes and data streams in a cluster. |
+
+## Query parameters
+
+The Flush API supports the following query parameters.
+
+| Parameter | Data type | Description |
+| :--- | :--- | :--- |
+| `allow_no_indices` | Boolean | When `false`, the request returns an error if any wildcard expression or index alias targets any closed or missing indexes. Default is `true`. |
+| `expand_wildcards` | String | Specifies the types of indexes to which wildcard expressions can expand. Supports comma-separated values. Valid values are: <br> - `all`: Expand to all open and closed indexes, including hidden indexes. <br> - `open`: Expand to open indexes. <br> - `closed`: Expand to closed indexes. <br> - `hidden`: Include hidden indexes when expanding. Must be combined with `open`, `closed`, or both. <br> - `none`: Do not accept wildcard expressions. <br> Default is `open`. |
+| `force` | Boolean | When `true`, forces a flush to occur even when no changes to the index exist in-memory. Default is `true`. |
+| `ignore_unavailable` | Boolean | When `true`, OpenSearch ignores missing or closed indexes. If `false`, OpenSearch returns an error if the force merge operation encounters missing or closed indexes. Default is `false`. |
+| `wait_if_ongoing` | Boolean | When `true`, the Flush API does not run while another flush request is active. When `false`, OpenSearch returns an error if another flush request is active. Default is `true`. |
+
+## Example request: Flush a specific index
+
+The following example flushes an index named `shakespeare`:
+
+```
+POST /shakespeare/_flush
+```
+
+## Example request: Flush all indexes
+
+The following example flushes all indexes in a cluster:
+
+```
+POST /_flush
+```
+
+## Example response
+
+OpenSearch responds with the number of shards that acknowledged the flush request, the number of shards that completed the request, and the number of shards that failed:
+
+```
+{
+  "_shards": {
+    "total": 10,
+    "successful": 10,
+    "failed": 0
+  }
+}
+```
+
+