.net中的线程池ThreadPool

还有和数据库交互的连接池，比如SQL Server Connection Pooling (ADO.NET) https://msdn.microsoft.com/en-us/library/8xx3tyca.aspx

SQL Server Connection Pooling (ADO.NET)

Connecting to a database server typically consists of several time-consuming steps. A physical channel such as a socket or a named pipe must be established, the initial handshake with the server must occur, the connection string information must be parsed, the connection must be authenticated by the server, checks must be run for enlisting in the current transaction, and so on.

handshake  握手

enlist 支持；从军；应募；赞助

transaction  交易；事务；办理；会报，学报

In practice, most applications use only one or a few different configurations for connections. This means that during application execution, many identical connections will be repeatedly opened and closed. To minimize the cost of opening connections, ADO.NET uses an optimization technique called connection pooling.

in practice  在实践中；实际上，事实上

identical   同一的；完全相同的

optimization  最佳化，最优化

Connection pooling reduces the number of times that new connections must be opened. The pooler maintains ownership of the physical connection. It manages connections by keeping alive a set of active connections for each given connection configuration. Whenever a user calls Open on a connection, the pooler looks for an available connection in the pool. If a pooled connection is available, it returns it to the caller instead of opening a new connection. When the application calls Close on the connection, the pooler returns it to the pooled set of active connections instead of closing it. Once the connection is returned to the pool, it is ready to be reused on the nextOpen call.

ownership   所有权；物主身份

Only connections with the same configuration can be pooled. ADO.NET keeps several pools at the same time, one for each configuration. Connections are separated into pools by connection string, and by Windows identity when integrated security is used. Connections are also pooled based on whether they are enlisted in a transaction. When using ChangePassword, the SqlCredential instance affects the connection pool. Different instances ofSqlCredential will use different connection pools, even if the user ID and password are the same.

credential   证书；凭据；国书

Pooling connections can significantly enhance the performance and scalability of your application. By default, connection pooling is enabled in ADO.NET. Unless you explicitly disable it, the pooler optimizes the connections as they are opened and closed in your application. You can also supply several connection string modifiers to control connection pooling behavior. For more information, see "Controlling Connection Pooling with Connection String Keywords" later in this topic.

scalability  可扩展性；可伸缩性；可量测性

Note:

When connection pooling is enabled, and if a timeout error or other login error occurs, an exception will be thrown and subsequent connection attempts will fail for the next five seconds, the "blocking period". If the application attempts to connect within the blocking period, the first exception will be thrown again. Subsequent failures after a blocking period ends will result in a new blocking periods that is twice as long as the previous blocking period, up to a maximum of one minute.

subsequent   后来的，随后的

Pool Creation and Assignment

When a connection is first opened, a connection pool is created based on an exact matching algorithm that associates the pool with the connection string in the connection. Each connection pool is associated with a distinct connection string. When a new connection is opened, if the connection string is not an exact match to an existing pool, a new pool is created. Connections are pooled per process, per application domain, per connection string and when integrated security is used, per Windows identity. Connection strings must also be an exact match; keywords supplied in a different order for the same connection will be pooled separately.

In the following C# example, three new SqlConnection objects are created, but only two connection pools are required to manage them. Note that the first and second connection strings differ by the value assigned for Initial Catalog.