Connection Pools and Connection Parameters

This document describes how to configure connection pools and connection parameters when you use a driver or ORM framework to connect to TiDB.

If you are interested in more tips about Java application development, see Best Practices for Developing Java Applications with TiDB

Connection pool

Building TiDB (MySQL) connections is relatively expensive (for OLTP scenarios at least). Because in addition to building a TCP connection, connection authentication is also required. Therefore, the client usually saves the TiDB (MySQL) connections to the connection pool for reuse.

Java has many connection pool implementations such as HikariCP, tomcat-jdbc, druid, c3p0, and dbcp. TiDB does not limit which connection pool you use, so you can choose whichever you like for your application.

Configure the number of connections

It is a common practice that the connection pool size is well adjusted according to the application's own needs. Take HikariCP as an example:

maximumPoolSize: The maximum number of connections in the connection pool. If this value is too large, TiDB consumes resources to maintain useless connections. If this value is too small, the application gets slow connections. Therefore, you need to configure this value according to your application characteristics. For details, see About Pool Sizing.
minimumIdle: The minimum number of idle connections in the connection pool. It is mainly used to reserve some connections to respond to sudden requests when the application is idle. You also need to configure it according to your application characteristics.

The application needs to return the connection after finishing using it. It is recommended that the application uses the corresponding connection pool monitoring (such as metricRegistry) to locate connection pool issues in time.

Probe configuration

The connection pool maintains persistent connections from clients to TiDB as follows:

Before v5.4, TiDB does not proactively close client connections by default (unless an error is reported).
Starting from v5.4, TiDB automatically closes client connections after 28800 seconds (this is, 8 hours) of inactivity by default. You can control this timeout setting using the TiDB and MySQL compatible wait_timeout variable. For more information, see JDBC Query Timeout.

Moreover, there might be network proxies such as LVS or HAProxy between clients and TiDB. These proxies typically proactively clean up connections after a specific idle period (determined by the proxy's idle configuration). In addition to monitoring the proxy's idle configuration, connection pools also need to maintain or probe connections for keep-alive.

If you often see the following error in your Java application:

The last packet sent successfully to the server was 3600000 milliseconds ago. The driver has not received any packets from the server. com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failure

If n in n milliseconds ago is 0 or a very small value, it is usually because the executed SQL operation causes TiDB to exit abnormally. To find the cause, it is recommended to check the TiDB stderr log.

If n is a very large value (such as 3600000 in the above example), it is likely that this connection was idle for a long time and then closed by the proxy. The usual solution is to increase the value of the proxy's idle configuration and allow the connection pool to:

Check whether the connection is available before using the connection every time.
Regularly check whether the connection is available using a separate thread.
Send a test query regularly to keep alive connections.

Different connection pool implementations might support one or more of the above methods. You can check your connection pool documentation to find the corresponding configuration.

Formulas based on experience

According to the About Pool Sizing article of HikariCP, if you have no idea about how to set a proper size for the database connection pool, you can get started with a formula based on experience. Then, based on the performance result of the pool size calculated from the formula, you can further adjust the size to achieve the best performance.

The formula based on experience is as follows:

connections = ((core_count * 2) + effective_spindle_count)

The description of each parameter in the formula is as follows:

connections: the size of connections obtained.
core_count: the number of CPU cores.
effective_spindle_count: the number of hard drives (not SSD). Because each spinning hard disk can be called a spindle. For example, if you are using a server with a RAID of 16 disks, the effective_spindle_count should be 16. Because HDD usually can handle only one request at a time, the formula here is actually measuring how many concurrent I/O requests your server can manage.

In particular, pay attention to the following note below the formula.

A formula which has held up pretty well across a lot of benchmarks for years is
that for optimal throughput the number of active connections should be somewhere
near ((core_count * 2) + effective_spindle_count). Core count should not include
HT threads, even if hyperthreading is enabled. Effective spindle count is zero if
the active data set is fully cached, and approaches the actual number of spindles
as the cache hit rate falls. ... There hasn't been any analysis so far regarding
how well the formula works with SSDs.

This note indicates that:

core_count is the number of physical cores, regardless of whether you enable Hyper-Threading or not.
When data is fully cached, you need to set effective_spindle_count to 0. As the hit rate of cache decreases, the count is closer to the actual number of HDD.
Whether the formula works for SSD is not tested and unknown.

When using SSDs, it is recommended that you use the following formula based on experience instead:

connections = (number of cores * 4)

Therefore, you can set the maximum connection size of the initial connection pool to cores * 4 in the case of SSDs and further adjust the size to tune the performance.

Tuning direction

As you can see, a size calculated from the formulas based on experience is just a recommended base value. To get the optimal size on a specific machine, you need to try other values around the base value and test the performance.

Here are some basic rules to help you get the optimal size:

If your network or storage latency is high, increase your maximum number of connections to reduce the latency waiting time. Once a thread is blocked by latency, other threads can take over and continue processing.
If you have multiple services deployed on your server and each service has a separate connection pool, consider the sum of the maximum number of connections to all connection pools.

Connection parameters

Java applications can be encapsulated with various frameworks. In most of the frameworks, JDBC API is called on the bottommost level to interact with the database server. For JDBC, it is recommended that you focus on the following things:

JDBC API usage choice
API Implementer's parameter configuration

JDBC API

For JDBC API usage, see JDBC official tutorial. This section covers the usage of several important APIs.

Use Prepare API

For OLTP (Online Transactional Processing) scenarios, the SQL statements sent by the program to the database are several types that can be exhausted after removing parameter changes. Therefore, it is recommended to use Prepared Statements instead of regular execution from a text file and reuse Prepared Statements to execute directly. This avoids the overhead of repeatedly parsing and generating SQL execution plans in TiDB.

At present, most upper-level frameworks call the Prepare API for SQL execution. If you use the JDBC API directly for development, pay attention to choosing the Prepare API.

In addition, with the default implementation of MySQL Connector/J, only client-side statements are preprocessed, and the statements are sent to the server in a text file after ? is replaced on the client. Therefore, in addition to using the Prepare API, you also need to configure useServerPrepStmts = true in JDBC connection parameters before you perform statement preprocessing on the TiDB server. For detailed parameter configuration, see MySQL JDBC parameters.

Use Batch API

For batch inserts, you can use the addBatch/executeBatch API. The addBatch() method is used to cache multiple SQL statements first on the client, and then send them to the database server together when calling the executeBatch method.

Note

In the default MySQL Connector/J implementation, the sending time of the SQL statements that are added to batch with addBatch() is delayed to the time when executeBatch() is called, but the statements will still be sent one by one during the actual network transfer. Therefore, this method usually does not reduce the amount of communication overhead.

If you want to batch network transfer, you need to configure rewriteBatchedStatements = true in the JDBC connection parameters. For the detailed parameter configuration, see Batch-related parameters.

Use `StreamingResult` to get the execution result

In most scenarios, to improve execution efficiency, JDBC obtains query results in advance and saves them in client memory by default. But when the query returns a super large result set, the client often wants the database server to reduce the number of records returned at a time and waits until the client's memory is ready and it requests for the next batch.

Usually, there are two kinds of processing methods in JDBC:

Set FetchSize to Integer.MIN_VALUE to ensure that the client does not cache. The client will read the execution result from the network connection through StreamingResult.
When the client uses the streaming read method, it needs to finish reading or close resultset before continuing to use the statement to make a query. Otherwise, the error No statements may be issued when any streaming result sets are open and in use on a given connection. Ensure that you have called .close() on any active streaming result sets before attempting more queries. is returned.
To avoid such an error in queries before the client finishes reading or closes resultset, you can add the clobberStreamingResults=true parameter in the URL. Then, resultset is automatically closed but the result set to be read in the previous streaming query is lost.
To use Cursor Fetch, first set FetchSize as a positive integer and configure useCursorFetch=true in the JDBC URL.

TiDB supports both methods, but it is preferred that you use the first method, because it is a simpler implementation and has a better execution efficiency.

MySQL JDBC parameters

JDBC usually provides implementation-related configurations in the form of JDBC URL parameters. This section introduces MySQL Connector/J's parameter configurations (If you use MariaDB, see MariaDB's parameter configurations). Because this document cannot cover all configuration items, it mainly focuses on several parameters that might affect performance.

This section introduces parameters related to Prepare.

useServerPrepStmts
useServerPrepStmts is set to false by default, that is, even if you use the Prepare API, the "prepare" operation will be done only on the client. To avoid the parsing overhead of the server, if the same SQL statement uses the Prepare API multiple times, it is recommended to set this configuration to true.
To verify that this setting already takes effect, you can do:
- Go to TiDB monitoring dashboard and view the request command type through Query Summary > CPS By Instance.
- If COM_QUERY is replaced by COM_STMT_EXECUTE or COM_STMT_PREPARE in the request, it means this setting already takes effect.
cachePrepStmts
Although useServerPrepStmts=true allows the server to execute Prepared Statements, by default, the client closes the Prepared Statements after each execution and does not reuse them. This means that the "prepare" operation is not even as efficient as text file execution. To solve this, it is recommended that after setting useServerPrepStmts=true, you should also configure cachePrepStmts=true. This allows the client to cache Prepared Statements.
To verify that this setting already takes effect, you can do:
- Go to TiDB monitoring dashboard and view the request command type through Query Summary > CPS By Instance.
- If the number of COM_STMT_EXECUTE in the request is far more than the number of COM_STMT_PREPARE, it means this setting already takes effect.
  In addition, configuring useConfigs=maxPerformance will configure multiple parameters at the same time, including cachePrepStmts=true.
prepStmtCacheSqlLimit
After configuring cachePrepStmts, also pay attention to the prepStmtCacheSqlLimit configuration (the default value is 256). This configuration controls the maximum length of the Prepared Statements cached on the client.
The Prepared Statements that exceed this maximum length will not be cached, so they cannot be reused. In this case, you may consider increasing the value of this configuration depending on the actual SQL length of the application.
You need to check whether this setting is too small if you:
- Go to TiDB monitoring dashboard and view the request command type through Query Summary > CPS By Instance.
- And find that cachePrepStmts=true has been configured, but COM_STMT_PREPARE is still mostly equal to COM_STMT_EXECUTE and COM_STMT_CLOSE exists.
prepStmtCacheSize
prepStmtCacheSize controls the number of cached Prepared Statements (the default value is 25). If your application requires "preparing" many types of SQL statements and wants to reuse Prepared Statements, you can increase this value.
To verify that this setting already takes effect, you can do:
- Go to TiDB monitoring dashboard and view the request command type through Query Summary > CPS By Instance.
- If the number of COM_STMT_EXECUTE in the request is far more than the number of COM_STMT_PREPARE, it means this setting already takes effect.

While processing batch writes, it is recommended to configure rewriteBatchedStatements=true. After using addBatch() or executeBatch(), JDBC still sends SQL one by one by default, for example:

pstmt = prepare("INSERT INTO `t` (a) values(?)");
pstmt.setInt(1, 10);
pstmt.addBatch();
pstmt.setInt(1, 11);
pstmt.addBatch();
pstmt.setInt(1, 12);
pstmt.executeBatch();

Although Batch methods are used, the SQL statements sent to TiDB are still individual INSERT statements:

INSERT INTO `t` (`a`) VALUES(10);
INSERT INTO `t` (`a`) VALUES(11);
INSERT INTO `t` (`a`) VALUES(12);

But if you set rewriteBatchedStatements=true, the SQL statements sent to TiDB will be a single INSERT statement:

INSERT INTO `t` (`a`) values(10),(11),(12);

Note that the rewrite of the INSERT statements is to concatenate the values after multiple "values" keywords into a whole SQL statement. If the INSERT statements have other differences, they cannot be rewritten, for example:

INSERT INTO `t` (`a`) VALUES (10) ON DUPLICATE KEY UPDATE `a` = 10;
INSERT INTO `t` (`a`) VALUES (11) ON DUPLICATE KEY UPDATE `a` = 11;
INSERT INTO `t` (`a`) VALUES (12) ON DUPLICATE KEY UPDATE `a` = 12;

The above INSERT statements cannot be rewritten into one statement. But if you change the three statements into the following ones:

INSERT INTO `t` (`a`) VALUES (10) ON DUPLICATE KEY UPDATE `a` = VALUES(`a`);
INSERT INTO `t` (`a`) VALUES (11) ON DUPLICATE KEY UPDATE `a` = VALUES(`a`);
INSERT INTO `t` (`a`) VALUES (12) ON DUPLICATE KEY UPDATE `a` = VALUES(`a`);

Then they meet the rewrite requirement. The above INSERT statements will be rewritten into the following one statement:

INSERT INTO `t` (`a`) VALUES (10), (11), (12) ON DUPLICATE KEY UPDATE a = VALUES(`a`);

If there are three or more updates during the batch update, the SQL statements will be rewritten and sent as multiple queries. This effectively reduces the client-to-server request overhead, but the side effect is that a larger SQL statement is generated. For example:

UPDATE `t` SET `a` = 10 WHERE `id` = 1; UPDATE `t` SET `a` = 11 WHERE `id` = 2; UPDATE `t` SET `a` = 12 WHERE `id` = 3;

In addition, because of a client bug, if you want to configure rewriteBatchedStatements=true and useServerPrepStmts=true during batch update, it is recommended that you also configure the allowMultiQueries=true parameter to avoid this bug.

Integrate parameters

Through monitoring, you might notice that although the application only performs INSERT operations to the TiDB cluster, there are a lot of redundant SELECT statements. Usually this happens because JDBC sends some SQL statements to query the settings, for example, select @@session.transaction_read_only. These SQL statements are useless for TiDB, so it is recommended that you configure useConfigs=maxPerformance to avoid extra overhead.

useConfigs=maxPerformance includes a group of configurations. To get the detailed configurations in MySQL Connector/J 8.0 and those in MySQL Connector/J 5.1, see mysql-connector-j 8.0 and mysql-connector-j 5.1 respectively.

After it is configured, you can check the monitoring to see a decreased number of SELECT statements.

TiDB provides two MySQL-compatible parameters to control the timeout: wait_timeout and max_execution_time. These two parameters respectively control the connection idle timeout with the Java application and the timeout of the SQL execution in the connection; that is to say, these parameters control the longest idle time and the longest busy time for the connection between TiDB and the Java application. Since TiDB v5.4, the default value of wait_timeout is 28800 seconds, which is 8 hours. For TiDB versions earlier than v5.4, the default value is 0, which means the timeout is unlimited. The default value of max_execution_time is 0, which means the maximum execution time of a SQL statement is unlimited.

However, in an actual production environment, idle connections and SQL statements with excessively long execution time negatively affect databases and applications. To avoid idle connections and SQL statements that are executed for too long, you can configure these two parameters in your application's connection string. For example, set sessionVariables=wait_timeout=3600 (1 hour) and sessionVariables=max_execution_time=300000 (5 minutes).

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