Create follower API

Creates a follower index and makes that index follow a leader index.

PutFollowRequest putFollowRequest = new PutFollowRequest(
    "local", 
    "leader", 
    "follower", 
    ActiveShardCount.ONE 
);
Settings settings =
    Settings.builder().put("index.number_of_replicas", 0L).build();
putFollowRequest.setSettings(settings); 

The PutFollowResponse indicates if the request was received.

boolean isFollowIndexCreated =
    putFollowResponse.isFollowIndexCreated(); 
boolean isFollowIndexShardsAcked =
    putFollowResponse.isFollowIndexShardsAcked(); 
boolean isIndexFollowingStarted =
    putFollowResponse.isIndexFollowingStarted(); 

When executing a PutFollowRequest in the following manner, the client waits for the PutFollowResponse to be returned before continuing with code execution:

PutFollowResponse putFollowResponse =
    client.ccr().putFollow(putFollowRequest, RequestOptions.DEFAULT);

Synchronous calls may throw an IOException in case of either failing to parse the REST response in the high-level REST client, the request times out or similar cases where there is no response coming back from the server.

In cases where the server returns a 4xx or 5xx error code, the high-level client tries to parse the response body error details instead and then throws a generic ElasticsearchException and adds the original ResponseException as a suppressed exception to it.

Executing a PutFollowRequest can also be done in an asynchronous fashion so that the client can return directly. Users need to specify how the response or potential failures will be handled by passing the request and a listener to the asynchronous ccr-put-follow method:

client.ccr().putFollowAsync(putFollowRequest,
    RequestOptions.DEFAULT, listener); 

The asynchronous method does not block and returns immediately. Once it is completed the ActionListener is called back using the onResponse method if the execution successfully completed or using the onFailure method if it failed. Failure scenarios and expected exceptions are the same as in the synchronous execution case.

A typical listener for ccr-put-follow looks like:

ActionListener<PutFollowResponse> listener =
    new ActionListener<PutFollowResponse>() {
        @Override
        public void onResponse(PutFollowResponse response) { 
            boolean isFollowIndexCreated =
                putFollowResponse.isFollowIndexCreated();
            boolean isFollowIndexShardsAcked =
                putFollowResponse.isFollowIndexShardsAcked();
            boolean isIndexFollowingStarted =
                putFollowResponse.isIndexFollowingStarted();
        }

        @Override
        public void onFailure(Exception e) {
            
        }
    };