Every connection is associated with a connection session on the server and a group of related session settings like the autocommit flag or the read_only flag. The following session settings are tracked by the AWS Advanced Python Wrapper, and together they form a session state:
- autocommit
- read_only
Since the AWS Advanced Python Wrapper can transparently switch physical connection to a server (for instance, during a cluster failover), it's important to re-apply the current session state to a new connection during such switch.
The diagram above shows the process of switching one database connection A to a new connection B. After connection A is established, it's returned to the user application, and the user application may use this connection to query data from the database as well as to change some session settings.
For example, if the user application sets the read_only value on a connection, the AWS Advanced Python Wrapper intercepts this operation and stores a new session setting for the read_only setting. At the same time, the wrapper verifies if the original session setting is known or not. If the original setting is not known, the wrapper will fetch the value of read_only and store it as a pristine value in order to save the original session setting. Later, the wrapper may need the pristine value to restore the connection session state to its original state.
Before closing an existing connection, the AWS Advanced Python Wrapper may try to reset all changes to the session state made by the user application. Some application frameworks and connection pools may intercept calls that close connections and may perform additional connection configuration. Since the AWS Advanced Python Wrapper might change the internal physical connection to a server, a new physical connection's settings may become unexpected to the user application and may cause errors. It is also important to mention that calling close() on a connection while using connection pooling doesn't close the connection or stop communicating to a server. Instead, the connection is returned to a pool of available connections. Cleaning up a session state before returning a connection to a pool is necessary to avoid side effects and errors when a connection is retrieved from a pool to be reused.
Before closing a connection, the AWS Advanced Python Wrapper sets its session state settings with the pristine values that have been previously stored in the wrapper. If a pristine value isn't available, it means that there have been no changes to that particular setting made by the user application, and that it's safe to assume that this setting is in its original/unchanged state.
Session state reset is enabled by default, and can be disabled by using the reset_session_state_on_close configuration parameter.
When the wrapper needs to switch to a new connection, it opens a new connection and transfers a session state to it. All current session state values are applied to the new connection. Pristine values for a new connection are also fetched and stored if needed. When a new connection is configured, it replaces the current internal connection.
Session transfer is enabled by default, and can be disabled by using the transfer_session_state_on_switch configuration parameter.
It is possible to extend or replace existing logic of resetting session state and transferring session state with custom handlers. Use the following methods on aws_advanced_python_wrapper.states.SessionStateTransferHandlers class to set and reset custom handlers:
set_reset_session_state_on_close_funcclear_reset_session_state_on_close_funcset_transfer_session_state_on_switch_funcreset_transfer_session_state_on_switch_func