A session is a logical connection between the user and SQL Engine. A session permits a user to submit one transaction at a time and receive one response at a time, and the current session can have only one transaction outstanding at any time. A user may communicate through one or more active sessions concurrently.
A session is explicitly logged on and off from SQL Engine and is established when the SQL Engine server accepts the logon string of the user. When a session is logged off, the system discards the user-session identification and does not accept additional Teradata SQL statements from that session.
Request and Response Messages
When a session logs on, SQL Engine communicates with the host through request and response messages containing parcels of information. The maximum request message size is 7 MB. The maximum response message size is 16 MB.
How Request Parcels Are Handled
To access SQL Engine from a mainframe client, the user or application program logs on through a Teradata client interface product and submits a request. The request is processed by the interface and directed to a TDP.
Each request is handled as follows:
- In a mainframe environment, mainframe CLIv2 builds the request parcel and sends it to the TDP, which sends it through the mainframe connection to the PE associated with that TDP.
- In a network environment, the client interface product (such as CLIv2 or the Teradata JDBC Driver) builds the request parcel and sends it over the network to the gateway. The gateway sends the parcel to the PE for the session, which may or may not reside on the same node as the gateway.
Response Parcels
The result of a query or a status becomes a returned answer set. The PE turns the answer sets for a request into one or more response parcels and returns them to:
- The TDP, in a mainframe environment. The TDP and mainframe CLIv2 returns it to the client utility or program.
- The gateway, in a network environment. The gateway sends it to the client interface product (such as CLIv2 or the Teradata ODBC Driver). The client interface product returns the response to the client utility or program.