<!--

    Licensed to the Apache Software Foundation (ASF) under one
    or more contributor license agreements.  See the NOTICE file
    distributed with this work for additional information
    regarding copyright ownership.  The ASF licenses this file
    to you under the Apache License, Version 2.0 (the
    "License"); you may not use this file except in compliance
    with the License.  You may obtain a copy of the License at
    
        http://www.apache.org/licenses/LICENSE-2.0
    
    Unless required by applicable law or agreed to in writing,
    software distributed under the License is distributed on an
    "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
    KIND, either express or implied.  See the License for the
    specific language governing permissions and limitations
    under the License.

-->

# Maintenance Command
## FLUSH

Persist all the data points in the memory table of the database to the disk, and seal the data file. In cluster mode, we provide commands to persist the specified database cache of local node and persist the specified database cache of the cluster.

Note: This command does not need to be invoked manually by the client. IoTDB has WAL to ensure data security
and IoTDB will flush when appropriate.
Frequently call flush can result in small data files that degrade query performance.

```sql
IoTDB> FLUSH 
IoTDB> FLUSH ON LOCAL
IoTDB> FLUSH ON CLUSTER
IoTDB> FLUSH root.ln
IoTDB> FLUSH root.sg1,root.sg2 ON LOCAL
IoTDB> FLUSH root.sg1,root.sg2 ON CLUSTER
```

## CLEAR CACHE

Clear the cache of chunk, chunk metadata and timeseries metadata to release the memory footprint. In cluster mode, we provide commands to clear local node cache and clear the cluster cache.

```sql
IoTDB> CLEAR CACHE
IoTDB> CLEAR CACHE ON LOCAL
IoTDB> CLEAR CACHE ON CLUSTER
```


## SET SYSTEM TO READONLY / RUNNING

Manually set IoTDB system to running, read-only mode. In cluster mode, we provide commands to set the local node status and set the cluster status, valid for the entire cluster by default.

```sql
IoTDB> SET SYSTEM TO RUNNING
IoTDB> SET SYSTEM TO READONLY ON LOCAL
IoTDB> SET SYSTEM TO READONLY ON CLUSTER
```


## Kill Query

IoTDB supports setting session connection timeouts and query timeouts, and also allows to stop the executing query manually.

### Session timeout

Session timeout controls when idle sessions are closed. An idle session is one that had not initiated any query or non-query operations for a period of time.

Session timeout is disabled by default and can be set using the `dn_session_timeout_threshold` parameter in IoTDB configuration file.

### Query timeout

For queries that take too long to execute, IoTDB will forcibly interrupt the query and throw a timeout exception, as shown in the figure: 

```sql
IoTDB> select * from root;
Msg: 701 Current query is time out, please check your statement or modify timeout parameter.
```

The default timeout of a query is 60000 ms，which can be customized in the configuration file through the `query_timeout_threshold` parameter.

If you use JDBC or Session, we also support setting a timeout for a single query（Unit: ms）：

```java
((IoTDBStatement) statement).executeQuery(String sql, long timeoutInMS)
session.executeQueryStatement(String sql, long timeout)
```


> If the timeout parameter is not configured or with a negative number, the default timeout time will be used. 
> If value 0 is used, timeout function will be disabled.

### Query abort

In addition to waiting for the query to time out passively, IoTDB also supports stopping the query actively:

```sql
KILL QUERY <queryId>
```

You can abort the specified query by specifying `queryId`. If `queryId` is not specified, all executing queries will be killed.

To get the executing `queryId`，you can use the `show query processlist` command，which will show the list of all executing queries，with the following result set：

| Time | queryId | statement |
|------|---------|-----------|
|      |         |           |

The maximum display length of statement is 64 characters. For statements with more than 64 characters, the intercepted part will be displayed.