深入源码和内核，一篇文章彻底理解数据库的各种超时参数

- The loginTimeout specifies how long the whole process of logging into the database is allowed to take. It governs the operation of connecting and authenticating to the dbms server, this involves establishing a TCP connection followed by one or more exchanges of packets for the handshake and authentication to the dbms server;
- The connectTimeout specifies how long to wait for a TCP network connection to get established, it governs the time needed to establish a TCP socket connection, and as establishing a TCP connection is part of establishing a database connection and doesn't guarantee a login, so loginTimeout >= connectTimeout；
- A connection timeout occurs only upon starting the TCP connection. This usually happens if the remote machine does not answer. If you get an ConnectException, possible reasons are: the server has been shut down, you used the wrong IP/DNS name, wrong port or the network connection to the server is down.
- A connection timeout is the maximum amount of time that the program is willing to wait to setup a connection to another process. You aren't getting or posting any application data at this point, just establishing the connection, itself.
- The socketTimeout specifies how long the client will wait for a response to a command from the server before throwing an error, it governs the time a socket can be blocked waiting to read from a socket, this involves all reads from the server, not just during connect, but also during subsequent interaction with the server (eg executing queries)，so you may want to set it higher (eg for other operations that take a long time to get a response back) than you are willing to wait for the login to complete；
- A socket timeout is dedicated to monitor the continuous incoming data flow. If the data flow is interrupted for the specified time the connection is considered as stalled/broken. Of course this only works with connections where data is received all the time and there are no delays longer than the configured socket timeout.
- A socket timeout is the timeout when waiting for individual packets. It's a common misconception that a socket timeout is the timeout to receive the full response. So if you have a socket timeout of 1 second, and a response comprised of 3 IP packets, where each response packet takes 0.9 seconds to arrive, for a total response time of 2.7 seconds, then there will be no timeout.
- By setting socket timeout to 1000 (ms) this would require that every second new data is received (assuming that you read the data block wise and the block is large enough).If only the incoming stream stalls for more than a second you are running into a socket timeout.This is especially important when HTTP servers process a complex request that requires some time on server side before the HTTP response data is available. If you configure socket timeout to 10000 (10 seconds) but the HTTP server requires 15 seconds after receiving the HTTP request, then you will never get the response as after 10 seconds you will get an SocketTimeoutException (no data is transmitted between reception of the HTTP request until the HTTP response is ready).
- A socketTimeout can be used as both a brute force global query timeout and a method of detecting network problems;
- the loginTimeout and connectTimeout are related to establishing a connection, while socketTimeout is relevant for the whole database session;
- connectTimeout and socketTimeout are timeouts on low-level socket operations, while loginTimeout is on a high level - the database level;
- Generally, the application hangs from network issues when the application is calling Socket.read(). However, depending on the network composition or the error type, it can rarely be in waiting status while running Socket.write(). When the application calls Socket.write(), the data is recorded to the OS kernel buffer and then the right to control is returned to the application immediately. Thus, as long as a valid value is recorded to the kernel buffer, Socket.write() is always successful. However, if the OS kernel buffer is full due to a special network error, even Socket.write() can be put into waiting status;

- The higher level timeout is dependent on the lower level timeout. The higher level timeout will operate normally only if the lower level timeout operates normally as well. If the JDBC driver socket timeout does not work properly, then higher level timeouts such as statement timeout and transaction timeout will not work properly either.
- The statement timeout does not handle the timeouts at the time of network failure, it does only one thing: restricts the operation time of 1 statement,and handling timeout to prevent network failure must be done by JDBC Driver；
- Socket timeout value for JDBC driver is necessary when the DBMS is terminated abruptly or an network error has occured (equipment malfunction, etc.). 
- Because of the structure of TCP/IP, there are no means for the socket to detect network errors. Therefore, the application cannot detect any disconnection with the DBMS. If the socket timeout is not configured, then the application may wait for the results from the DBMS indefinitely. (This connection is also called a "dead connection."),to prevent dead connections, a timeout must be configured for the socket.
- Socket timeout can be configured via JDBC driver. By setting up the socket timeout, you can prevent the infinite waiting situation when there is a network error and shorten the failure time.
- It is not recommended to use the socket timeout value to limit the statement execution time. So the socket timeout value must be higher than the statement timeout value.
- If the socket timeout value is smaller than the statement timeout value, as the socket timeout will be executed first, and the statement timeout value becomes meaningless and will not be executed.

# 配置参数
finalstatic String url= "jdbc:oracle:thin:@myhost:1521/myorcldbservicename";
finalstatic String user = "hr";
finalstatic String password = "hr";
finalstatic String CONNECT_TIMEOUT = "20000";
finalstatic String READ_TIMEOUT = "50000";
# 使用 DataSource 获取连接
Properties connectionProperties = new Properties(); 
connectionProperties.put(“oracle.net.CONNECT_TIMEOUT”, CONNECT_TIMEOUT);
connectionProperties.put(“oracle.jdbc.ReadTimeout”, READ_TIMEOUT);
OracleDataSource ods = new OracleDataSource();
ods.setURL(url); 
ods.setUser(user); 
ods.setPassword(password);    
ods.setConnectionProperties(connectionProperties);
# 使用 DriverManager 获取连接
Class<?> oracleDriverClass = Class.forName("oracle.jdbc.driver.OracleDriver");
Properties connectionProperties = new Properties(); 
connectionProperties.put(“oracle.net.CONNECT_TIMEOUT”, CONNECT_TIMEOUT);
connectionProperties.put(“oracle.jdbc.ReadTimeout”, READ_TIMEOUT);  
//也可以通过环境变量/系统参数设置，注意需要在 connection 连接之前设置
//System.setProperty("oracle.net.CONNECT_TIMEOUT", connectTimeout);
//System.setProperty("oracle.jdbc.ReadTimeout", readTimeout); 
connectionProperties.put(“user”, user);
connectionProperties.put(“password”, password);  
Connection cnotallow=DriverManager.getConnection(url, props);

import java.net.Socket;
import jdk.net.ExtendedSocketOptions;
Socket socket = new Socket();
socket.setOption(ExtendedSocketOptions.TCP_KEEPIDLE, 10);
socket.setOption(ExtendedSocketOptions.TCP_KEEPCOUNT, 2);
socket.setOption(ExtendedSocketOptions.TCP_KEEPINTERVAL, 3);

# 查询内核参数
- sysctl -a  //显示当前所有可用的内核参数
- sysctl net.ipv4.tcp_keepalive_time //查询某个内核参数
- cat /proc/sys/net/ipv4/tcp_keepalive_time //查询某个内核参数
#修改内核参数
- sysctl net.ipv4.tcp_keepalive_time=3600//修改某个内核参数
- vim /etc/sysctl.conf//在配置文件中修改内核参数
- sysctl  -p  //从配置文件 sysctl.conf 中重新加载内核参数

- If the socket timeout or the connect timeout is not configured, most of the time, applications cannot detect network errors. So, until the applications are connected or are able to read data, they will wait indefinitely. 
- To prevent this, we can configure a socket timeout time at the OS level, so the Linux servers can check the network connection at the OS level.
- If you set the KeepAlive checking cycle for the Linux servers to 30 minutes, then even if someone set the JDBC driver‘s socket timeout to 0, which means no timeout, the DBMS network connection problems caused by network issues do not surpass 30 minutes/The JDBC connection hang recovers 30 minutes after the network connection failure, that is to say, the JDBC driver's socket timeout is affected by the OS's socket timeout configuration. 
- Generally, the application hangs from network issues when the application is calling Socket.read(). However, depending on the network composition or the error type, it can rarely be in waiting status while running Socket.write(). When the application calls Socket.write(), the data is recorded to the OS kernel buffer and then the right to control is returned to the application immediately. Thus, as long as a valid value is recorded to the kernel buffer, Socket.write() is always successful. However, if the OS kernel buffer is full due to a special network error, even Socket.write() can be put into waiting status. In thiscase, the OS tries to resend the packet for a certain amount of time, and generates an error when it reaches the limit.

# JDBC API 相关类与方法
java.sql.DriverManager#setLoginTimeout
javax.sql.CommonDataSource#setLoginTimeout
java.sql.Connection#getNetworkTimeout
java.sql.Connection#setNetworkTimeout
java.sql.Statement#setQueryTimeout
# oracle JDBC driver 相关类与方法
oracle.jdbc.OracleDriver
oracle.jdbc.pool.OracleDataSource#setLoginTimeout
oracle.jdbc.OracleConnection
oracle.jdbc.OracleConnection#CONNECTION_PROPERTY_THIN_READ_TIMEOUT
oracle.jdbc.OracleConnection#CONNECTION_PROPERTY_THIN_NET_CONNECT_TIMEOUT。
oracle.jdbc.OracleConnectionWrapper#setNetworkTimeout
oracle.jdbc.driver.PhysicalConnection#setNetworkTimeout
oracle.jdbc.driver.OracleStatement#setQueryTimeout
oracle.jdbc.driver.OracleStatement#doExecuteWithTimeout
oracle.jdbc.driver.OraclePreparedStatement#executeForRowsWithTimeout
oracle.jdbc.driver.OracleTimeoutPollingThread
# mysql JDBC driver 相关类与方法
com.mysql.cj.jdbc.Driver
com.mysql.cj.jdbc.MysqlDataSource#setLoginTimeout
com.mysql.cj.jdbc.ConnectionImpl#setNetworkTimeout
com.mysql.cj.jdbc.ConnectionWrapper#setNetworkTimeout
com.mysql.cj.jdbc.StatementImpl#setQueryTimeout
com.mysql.cj.jdbc.StatementWrapper#setQueryTimeout
#参考链接
- https://www.cubrid.org/blog/3826470
- https://prashantatridba.wordpress.com/tag/tcp_keepidle/
- https://bugs.openjdk.org/browse/JDK-8194298