Control structures

When a client connects to the database server, the database server creates a session structure, which is called a session control block, to hold information about the connection and the user.

A session begins when a client connects to the database server, and it ends when the connection terminates.

Next, the database server creates a thread structure, which is called a thread-control block (TCB) for the session, and initiates a primary thread (sqlexec) to process the client request. When a thread yields—that is, when it pauses and allows another thread to run—the virtual processor saves information about the state of the thread in the thread-control block. This information includes the content of the process system registers, the program counter (address of the next instruction to execute), and the stack pointer. This information constitutes the context of the thread.

In most cases, the database server runs one primary thread per session. In cases where it performs parallel processing, however, it creates multiple session threads for a single client, and, likewise, multiple corresponding thread-control blocks.


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