interface SqlGatewayService {
        
    // -------------------------------------------------------------------------------------------
    // Session Management
    // -------------------------------------------------------------------------------------------

    SessionHandle openSession(SessionEnvironment environment) throws SqlGatewayException;
    
    void closeSession(SessionHandle sessionHandle) throws SqlGatewayException;
    
    Map<String, String> getSessionConfig(SessionHandle sessionHandle) throws SqlGatewayException;
    
    // -------------------------------------------------------------------------------------------
    // Operation Management
    // -------------------------------------------------------------------------------------------
        
    /**
     * Get operation info to describe the Operation.
     */
    OperationInfo getOperationInfo(SessionHandle sessionHandle, OperationHandle operationHandle) throws SqlGatewayException;
    
    /** Get the result schema for the specified Operation. */
    ResolvedSchema getOperationResultSchema(SessionHandle sessionHandle, OperationHandle opreationHandle) throws SqlGatewayException;
    
    void cancelOperation(SessionHandle sessionHandle, OperationHandle operationHandle) throws SqlGatewayException;
    
    void closeOperation(SessionHandle sessionHandle, OperationHandle operationHandle) throws SqlGatewayException;
    
    // -------------------------------------------------------------------------------------------
    // Statements 
    // -------------------------------------------------------------------------------------------
    
    /** 
     * Using the statement to initialize the Session. It's only allowed to 
     * execute SET/RESET/CREATE/DROP/USE/ALTER/LOAD MODULE/UNLOAD MODULE/ADD JAR/REMOVE JAR.
     */
    void configureSession(SessionHandle sessionHandle, String statement, long executionTimeoutMs) throws SqlGatewayException;
    
    /** Execute the statement with the specified Session. It allows to execute with Operation-level configuration.*/
    OperationHandle executeStatement(
        SessionHandle sessionHandle, 
        String statement, 
        long executionTimeoutMs, 
        Configuration executionConfig) throws SqlGatewayException;
   
	/**
	 * Fetch the results with token id. 
     */
    ResultSet fetchResults(SessionHandle sessionHandle, OperationHandle operationHandle, int token, int maxRows) throws SqlGatewayException;

  	/**
     * Fetch the Operation-level log from the GatewayService. For some endpoint, it allows to fetch the log at the operation level.
     */
    ResultSet fetchLog(SessionHandle sessionHandle, OperationHandle operationHandle, FetchOrientation orientation, int maxRows) throws SqlGatewayException;
     
   /**
    * Only supports to fetch results in FORWARD/BACKWARD orientation.
    * - Users can only BACKWARD from the current offset once.
    * - The Gateway doesn't materialize the changelog.
    */
    ResultSet fetchResult(SessionHandle sessionHandle, OperationHandle operationHandle, FetchOrientation orientation, int maxRows) throws SqlGatewayException;          

	/**
     * For the same functionality, every endpoint has its result schema. Therefore, 
     * the endpoint submit the callable executor to the OperationManager that manages
     * lifecycle of the Operaiton. The callable executor organizes the results
     * as the Endpoint requires.
     */
    OperationHandle submitOperation(OperationType type, Callable<ResultSet> executor, ResolvedSchema resultSchema) throws SqlGatewayException;
    
    // -------------------------------------------------------------------------------------------
    // Utils 
    // -------------------------------------------------------------------------------------------
    
    /**
     * Describe the cluster info.
     */
    Map<String, String> getGatewayInfo();
    
    void heartbeat(SessionHandle sessionHandle) throws SqlGatewayException;
    
    /**
     * Endpoint is status-less. All the session configs are memorized in the GatewayService side.
     */     
	EndpointVersion getSessionEndpointVersion(SessionHandle sessionHandle) throws SqlGatewayException;
    
    /** Returns a list of completion hints for the given statement at the given position. */
    List<String> completeStatement(String sessionId, String statement, int position) throws SqlGatewayException;

   
    // -------------------------------------------------------------------------------------------
    // Catalog API 
    // -------------------------------------------------------------------------------------------
    
    String getCurrentCatalog(SessionHandle sessionHandle) throws SqlGatewayException;

    String getCurrentDatabase(SessionHandle sessionHandle) throws SqlGatewayException;
    
    List<String> listCatalogs(SessionHandle sessionHandle) throws SqlGatewayException;
    
    List<String> listDatabases(SessionHandle sessionHandle, String catalogName) throws SqlGatewayException;   
    
    List<TableInfo> listTables(SessionHandle sessionHandle, String catalogName, String databaseName, TableKind tableKind) throws SqlGatewayException;
    
    List<FunctionInfo> listFunctions(SessionHandle sessionHandle, String catalogName, String databaseName, FunctionScope scope) throws SqlGatewayException;
    
    ContextResolvedTable getTable(SessionHandle sessionHandle, ObjectIdentifier tableIdentifier) throws SqlGatewayException;
    
    ContextResolvedFunction getFunction(SessionHandle sessionHandle, ObjectIdentifier functionIdentifier) throws SqlGatewayException;
    
}

class TableInfo {
    boolean isTemporary;
    ObjectIdentifier identifier;
    TableKind tableKind;
}

class UserDefinedFunctionInfoFunctionInfo {
    boolean isTemporary;
    ObjectIdentifier identifier;
    FunctionKind kind;
}

class SessionEnvironment {
    private String sessionName;
    private EndpointVersion sessionEndpointVersion;
    private List<URL> libs;
    private List<URL> jars;
    private Map<String, String> sessionConfig
}

public class OperationInfo {
    OperationStatus status;
    OperationType type;
    boolean hasResult;
}

public class ResultSet {
    
    int nextToken;
    
    ResultType resultType; 

    ResolvedSchema resultSchema;
    
    List<RowData> results;
    
    Exception exception;
}

public enum ResultType {
    PAYLOAD,
    EMPTY,
    EOS,
    ERROR
}

{
	"result_type": "PAYLOAD",
	"results": {
		"columns": [
			{
				"name": "id",
				"type": {"type":  "BIGINT", "nullable": false}
			},
   			{
				"name": "name",
				"type": {"type":  "VARCHAR", "nullable": true, "length": 300}
			},
		 	{
				"name": "birthday",
				"type": {"type":  "TIMESTAMP", "nullable": true, "precision": 3}
			}
		],
		"data": [
			{
				"kind": "INSERT",
				"fields": [101, "Jay", "1990-01-12T12:00.12"], # a row data
			},
			{
				"kind": "DELETE",
				"fields": [102, "Jimmy", null]
			}
		]
	},
	  "next_result_uri": /v1/sessions/:session_id/jobs/:job_id/result/:{token+1} # if not empty, uses this uri to fetch next part of result, else there is no more result.
	"exception": {
    	"root_cause": "....",
    	"exception_stack": "..." 
	}
}