[4/n] remove env var - ray serve proxy always on head node

[harshit-anyscale]

Summary

This PR removes the RAY_SERVE_ALWAYS_RUN_PROXY_ON_HEAD_NODE environment variable as it has no practical effect in production.

Why this env var is redundant

The env var was intended to "always run a proxy on the head node even if it has no replicas." However, the controller already unconditionally ensures the head node is included in the proxy nodes set, making this flag ineffective.

Code flow analysis

In controller.py _update_proxy_nodes() (lines 419-430):

def _update_proxy_nodes(self):
    new_proxy_nodes = self.deployment_state_manager.get_active_node_ids()
    new_proxy_nodes = new_proxy_nodes - set(
        self.cluster_node_info_cache.get_draining_nodes()
    )
    new_proxy_nodes.add(self._controller_node_id)  # <-- UNCONDITIONAL
    self._proxy_nodes = new_proxy_nodes

The head node (_controller_node_id) is always added to _proxy_nodes unconditionally.

In proxy_state.py update() (where the env var was checked):

if RAY_SERVE_ALWAYS_RUN_PROXY_ON_HEAD_NODE:
    proxy_nodes.add(self._head_node_id)

This check happens after the controller already added the head node. Since proxy_nodes is a set, adding the same node again is a no-op.

Consequence

• Setting RAY_SERVE_ALWAYS_RUN_PROXY_ON_HEAD_NODE=0 had no effect because the controller adds the head node regardless.
• Setting RAY_SERVE_ALWAYS_RUN_PROXY_ON_HEAD_NODE=1 (the default) was also a no-op since the head node was already present.

Changes

• Removed RAY_SERVE_ALWAYS_RUN_PROXY_ON_HEAD_NODE definition from constants.py
• Removed the import and usage in proxy_state.py

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[abrarsheikh]

lg2m. @akyang-anyscale please review

[akyang-anyscale]

lgtm. looks like you may need to update the tests to pass in the head node id on .update(...)

[harshit-anyscale]

lgtm. looks like you may need to update the tests to pass in the head node id on .update(...)

done, fixed it.