Model-based replace operation using PUT

doc/aaa/Persistent HTTP Connections from CLI.md

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+Persistent HTTP connections from CLI
+====================================
+
+The SONiC CLI provided by the management framework container runs an instance of
+klish, which provides a fixed set of commands. The CLI is simply a front-end to
+the REST server, and each command is mapped to a corresponding REST endpoint.
+
+# Command Flow
+
+When the user enters a specific command on the CLI, the corresponding `ACTION`
+tag in the CLI XML specification shells out to a Python script with any
+arguments and an optional template to format the returned values. This script
+connects to the REST server on the local machine over HTTPS, retrieves and
+formats the JSON response. The Python script then exits, terminating any HTTP
+connection that had been set up.
+
+This is the current behavior, even without RBAC support, which means that every
+command will need to set up a new HTTPS connection. However, when RBAC is
+enabled, it is not likely to cause a noticeable performance impact, since the
+system is already incurring the TLS overhead.
+
+As can be seen from the flow above, it is not possible to set up a persistent
+HTTP connection, since every command spawns a new connection.
+
+# Alternative Designs
+
+This section describes some alternative designs that will enable the CLI to
+create a persistent connection.
+
+## Proxy service
+
+As part of the management framework, we can add a "proxy" service that is
+spawned with the CLI. This service will set up a secure HTTP connection for the
+authenticated user, and create a local unix socket that is accessible only by
+that user. The CLI XML will remain unchanged, but the Python ApiClient class
+will be changed to connect to the local socket.
+
+This will still create independent HTTP connections, but they can be insecure
+connections, while the proxy service will transfer the connections from the
+insecure unix socket to the secure tunnel, which will reduce the TLS connection
+time.
+
+**Note:** The security considerations have not been completely mapped out, and
+this may open the system up to security holes.
+
+## Klish modification
+
+This approach considers modifying the Klish executable. When Klish is spawned,
+it will set up the HTTPS connection and keep it alive as long as the CLI is
+active. Each `ACTION` tag will call into klish functions that will connect to
+the existing HTTPS connection.
+
+This approach is the most secure option, however, it needs heavy modification to
+klish, and there are several unknowns at this time.
+
+## No modification - Buzznik
+
+This approach leaves the design as is for the Buzznik release. Every command
+will continue to create a new HTTPS connection, as it does today, and will tear
+down the connection on completion of the request.