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Driver-Aware Connection Pooling in the ODBC Driver for SQL Server

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The ODBC Driver for SQL Server supports Driver-Aware Connection Pooling. This article describes the enhancements made to driver-aware connection pooling in the Microsoft ODBC Driver for SQL Server on Windows:

  • Whatever the connection properties, connections that use SQLDriverConnect go into a separate pool from connections that use SQLConnect.

  • When using SQL Server Authentication and driver-aware connection pooling, the driver doesn't use the Windows user's security context for the current thread to separate connections in the pool. That is, if connections are equivalent in their parameters for Windows impersonation scenarios with SQL Server Authentication, and they're using the same SQL Server Authentication credentials to connect to the backend, different Windows users can potentially use the same pool of connections. When using Windows Authentication and driver-aware connection pooling, the driver uses the current Windows user's security context to separate connections in the pool. That is, for Windows impersonation scenarios, different Windows users don't share connections even if the connections use the same parameters.

  • When using Microsoft Entra ID (formerly Azure Active Directory) and driver-aware connection pooling, the driver also uses the Authentication value to determine the membership in the connection pool.

  • Driver-aware connection pooling prevents a bad connection from being returned from the pool.

  • Driver-aware connection pooling recognizes driver-specific connection attributes. So, if a connection uses SQL_COPT_SS_APPLICATION_INTENT set to read only, that connection gets its own connection pool.

  • Setting the SQL_COPT_SS_ACCESS_TOKEN attribute causes a connection to be pooled separately

If one of the following connection-attribute IDs or connection string keywords is different between your connection string and the pooled connection string, the driver uses a pooled connection. However, performance is better if all connection attribute IDs or connection string keywords match. (To match a connection in the pool, the driver resets the attribute.) Performance degrades because resetting the following parameters requires an extra network call.

  • If two or more of the following connection attributes or connection keywords differ, a pooled connection isn't used.

    • Language
    • QuoteId
    • SQL_ATTR_TXN_ISOLATION
    • SQL_COPT_SS_QUOTED_IDENT

  • If there's a difference in any of the following connection keywords between your connection string and a pooled connection string, a pooled connection isn't used.

    Keyword ODBC Driver 13+ ODBC Driver 11
    Address Yes Yes
    AnsiNPW Yes Yes
    App Yes Yes
    ApplicationIntent Yes Yes
    Authentication Yes No
    ColumnEncryption Yes No
    Database Yes Yes
    Encrypt Yes Yes
    Failover_Partner Yes Yes
    FailoverPartnerSPN Yes Yes
    MARS_Connection Yes Yes
    Network Yes Yes
    PWD Yes Yes
    Server Yes Yes
    ServerSPN Yes Yes
    TransparentNetworkIPResolution Yes Yes
    Trusted_Connection Yes Yes
    TrustServerCertificate Yes Yes
    UID Yes Yes
    WSID Yes Yes

  • If there's a difference in any of the following connection attributes between your connection string and a pooled connection string, a pooled connection isn't used.

    Attribute ODBC Driver 13+ ODBC Driver 11
    SQL_ATTR_CURRENT_CATALOG Yes Yes
    SQL_ATTR_PACKET_SIZE Yes Yes
    SQL_COPT_SS_ANSI_NPW Yes Yes
    SQL_COPT_SS_ACCESS_TOKEN Yes No
    SQL_COPT_SS_AUTHENTICATION Yes No
    SQL_COPT_SS_ATTACHDBFILENAME Yes Yes
    SQL_COPT_SS_BCP Yes Yes
    SQL_COPT_SS_COLUMN_ENCRYPTION Yes No
    SQL_COPT_SS_CONCAT_NULL Yes Yes
    SQL_COPT_SS_ENCRYPT Yes Yes
    SQL_COPT_SS_FAILOVER_PARTNER Yes Yes
    SQL_COPT_SS_FAILOVER_PARTNER_SPN Yes Yes
    SQL_COPT_SS_INTEGRATED_SECURITY Yes Yes
    SQL_COPT_SS_MARS_ENABLED Yes Yes
    SQL_COPT_SS_OLDPWD Yes Yes
    SQL_COPT_SS_SERVER_SPN Yes Yes
    SQL_COPT_SS_TRUST_SERVER_CERTIFICATE Yes Yes
    SSPROP_AUTH_REPL_SERVER_NAME Yes Yes
    SQL_COPT_SS_TNIR Yes No

  • The driver can reset and adjust the following connection keywords and attributes without making an extra network call. The driver resets these parameters to ensure that the connection doesn't contain incorrect information.

    These connection keywords aren't considered when the Driver Manager tries to match your connection with a connection in the pool. (Even if you change one of these parameters, an existing connection can be reused. The driver will reset the options, as needed.) These attributes can be reset in the client side without making an extra network call.

    Keyword ODBC Driver 13+ ODBC Driver 11
    AutoTranslate Yes Yes
    Description Yes Yes
    MultisubnetFailover Yes Yes
    QueryLog_On Yes Yes
    QueryLogFile Yes Yes
    QueryLogTime Yes Yes
    Regional Yes Yes
    StatsLog_On Yes Yes
    StatsLogFile Yes Yes

    If you change one of the following connection attributes, an existing connection can be reused. The driver will reset the value, as needed. The driver can reset these attributes in the client without making an extra network call.

    Attribute ODBC Driver 13+ ODBC Driver 11
    All statement attributes Yes Yes
    SQL_ATTR_AUTOCOMMIT Yes Yes
    SQL_ATTR_CONNECTION_TIMEOUT Yes Yes
    SQL_ATTR_DISCONNECT_BEHAVIOR SQL_ATTR_CONNECTION_TIMEOUT Yes Yes
    SQL_ATTR_LOGIN_TIMEOUT Yes Yes
    SQL_ATTR_ODBC_CURSORS Yes Yes
    SQL_COPT_SS_PERF_DATA Yes Yes
    SQL_COPT_SS_PERF_DATA_LOG Yes Yes
    SQL_COPT_SS_PERF_DATA_LOG_NOW Yes Yes
    SQL_COPT_SS_PERF_QUERY Yes Yes
    SQL_COPT_SS_PERF_QUERY_INTERVAL Yes Yes
    SQL_COPT_SS_PERF_QUERY_LOG Yes Yes
    SQL_COPT_SS_PRESERVE_CURSORS Yes Yes
    SQL_COPT_SS_TRANSLATE Yes Yes
    SQL_COPT_SS_USER_DATA Yes Yes
    SQL_COPT_SS_WARN_ON_CP_ERROR Yes Yes

See Also

Microsoft ODBC Driver for SQL Server on Windows