pytorch/torch/distributed/elastic/rendezvous/etcd_rendezvous.py

#!/usr/bin/env python3
# mypy: allow-untyped-defs

# Copyright (c) Facebook, Inc. and its affiliates.
# All rights reserved.
#
# This source code is licensed under the BSD-style license found in the
# LICENSE file in the root directory of this source tree.

import json
import logging
import sys
import threading
import time
from typing import Optional


try:
    import etcd  # type: ignore[import]
except ModuleNotFoundError:
    from . import _etcd_stub as etcd

from torch.distributed.elastic.rendezvous import (
    RendezvousClosedError,
    RendezvousError,
    RendezvousHandler,
    RendezvousInfo,
    RendezvousParameters,
    RendezvousStoreInfo,
    RendezvousTimeoutError,
)

from .etcd_store import cas_delay, EtcdStore
from .utils import parse_rendezvous_endpoint


__all__ = [
    "EtcdRendezvousRetryableFailure",
    "EtcdRendezvousRetryImmediately",
    "EtcdRendezvousHandler",
    "EtcdRendezvous",
    "create_rdzv_handler",
]

_log_fmt = logging.Formatter("%(levelname)s %(asctime)s %(message)s")
_log_handler = logging.StreamHandler(sys.stderr)
_log_handler.setFormatter(_log_fmt)

logger = logging.getLogger(__name__)
logger.propagate = False
logger.setLevel(logging.INFO)
logger.addHandler(_log_handler)


# Retryable failure exception means the we were too late to make
# a desired state transition (e.g. because of a race condition),
# and should now restart from the beginning.
# A small delay is recommended to avoid spamming Etcd.
class EtcdRendezvousRetryableFailure(Exception):
    pass


# Similar to retryable failure, but the new state we observed suggests we
# can re-try immediately, i.e. without a need for "safety delay".
class EtcdRendezvousRetryImmediately(Exception):
    pass


# Default timeout for the rendezvous.
_DEFAULT_TIMEOUT: int = 600  # 10 minutes

# Additional waiting time after reaching the minimum number of nodes
# in case the rendezvous is elastic (min != max).
_DEFAULT_LAST_CALL_TIMEOUT: int = 30  # 30 seconds

# Various constants used internally in EtcdRendezvous
CONST_ETCD_SETUP_TTL = 5
CONST_ETCD_FROZEN_TTL = 10
CONST_ETCD_JOINABLE_EPHEMERAL_TTL = 10

# Ephemeral node TTL for worker's keep-alive key:
CONST_WORKER_KEEPALIVE_TTL = 10

# TTL for the ephemeral run_id-specific directory. All rendezvous state data
# for a specific run_id (job instance) is contained within directory.
# Its only role is to clean-up rendezvous data from old runs (for the case when
# etcd server is persistent), and has no affect on correctness, but should be
# larger than any timeouts that a worker process is expected to survive:
CONST_RUNID_SUBROOT_TTL = 7200  # 2 hours


class EtcdRendezvousHandler(RendezvousHandler):
    """
    Implements a
    :py:class:`torch.distributed.elastic.rendezvous.RendezvousHandler` interface
    backed by
    :py:class:`torch.distributed.elastic.rendezvous.etcd_rendezvous.EtcdRendezvous`.
    ``EtcdRendezvousHandler`` uses a URL to configure the type of rendezvous to
    use and to pass implementation specific configurations to the rendezvous
    module. The basic etcd rendezvous configuration URL looks like the following
    ::

     etcd://<etcd_address>:<port>/<job_id>?min_workers=<min_workers>&max_workers=<max_workers>  # noqa: W605

     -- example --

     etcd://localhost:2379/1234?min_workers=1&max_workers=3

    The URL above is interpreted as follows:

    1. Use the rendezvous handler that is registered with the ``etcd``
       scheme
    2. The ``etcd`` endpoint to use is ``localhost:2379``
    3. ``job_id == 1234`` is used as the prefix in etcd (this allows one to
       share a common etcd server for multiple jobs so long as the
       ``job_ids`` are guaranteed to be unique). Note that the job id can be
       any string (e.g. does not need to be a number) as long as it is
       unique.
    4. ``min_workers=1`` and ``max_workers=3`` specifies a range for
       membership size - Torch Distributed Elastic starts running the job as
       long as the cluster size is greater than or equal to ``min_workers``
       and admits up to ``max_workers`` into the cluster.

    Below are a full list of the parameters that can be passed to etcd
    rendezvous:

    +--------------------------------------------+--------------------------+
    | Parameter                                  | Description              |
    +============================================+==========================+
    | min_workers                                | minimum number of        |
    |                                            | workers for the          |
    |                                            | rendezvous to be valid   |
    +--------------------------------------------+--------------------------+
    | max_workers                                | maximum number of        |
    |                                            | workers to admit         |
    +--------------------------------------------+--------------------------+
    | timeout                                    | total timeout within     |
    |                                            | which next_rendezvous is |
    |                                            | expected to succeed      |
    |                                            | (default 600s)           |
    +--------------------------------------------+--------------------------+
    | last_call_timeout                          | additional wait amount   |
    |                                            | ("last call") after min  |
    |                                            | number of workers has    |
    |                                            | been reached (defaults   |
    |                                            | to 30s)                  |
    +--------------------------------------------+--------------------------+
    | etcd_prefix                                | path prefix (from etcd   |
    |                                            | root), inside which all  |
    |                                            | etcd nodes will be       |
    |                                            | created (defaults to     |
    |                                            | ``/torchelastic/p2p``)   |
    +--------------------------------------------+--------------------------+
    """

    def __init__(self, rdzv_impl: "EtcdRendezvous", local_addr: Optional[str]):
        """
        Args:
            rdzv_impl: the implementation of the rendezvous
            local_addr: the local address of the current node
        """

        self._rdzv_impl = rdzv_impl
        self._local_addr = local_addr

    def __del__(self):
        # TODO: look into using weakref here instead.
        del self._rdzv_impl

    def get_backend(self) -> str:
        return "etcd"

    def next_rendezvous(self):
        rdzv_version, rank, world_size = self._rdzv_impl.rendezvous_barrier()

        logger.info("Creating EtcdStore as the c10d::Store implementation")
        store = self._rdzv_impl.setup_kv_store(rdzv_version)

        bootstrap_store_info = RendezvousStoreInfo.build(
            rank, store, local_addr=self._local_addr
        )
        return RendezvousInfo(store, rank, world_size, bootstrap_store_info)

    def is_closed(self):
        try:
            _, state = self._rdzv_impl.get_rdzv_state()
            return state["status"] == "closed"
        except etcd.EtcdKeyNotFound:
            # No rendezvous state, so it cannot be closed.
            return False

    def set_closed(self):
        self._rdzv_impl.set_closed()

    def num_nodes_waiting(self):
        try:
            _, state = self._rdzv_impl.get_rdzv_state()
            if state["status"] == "final":
                return state["num_workers_waiting"]
        except etcd.EtcdKeyNotFound:
            pass
        return 0

    def get_run_id(self) -> str:
        return self._rdzv_impl._run_id

    def shutdown(self) -> bool:
        try:
            self.set_closed()
            return True
        except BaseException:  # noqa: B036
            logger.warning("Shutdown failed", exc_info=True)
            return False


# TODO: we should probably handle a few additional errors,
# like EtcdLeaderElectionInProgress and EtcdWatcherCleared. These are
# only relevant for multi-node Etcd ensemble. A simple retry would work,
# but is verbose to add everywhere. Consider wrapping the client calls
# into auto-retry for these errors?
#
class EtcdRendezvous:
    """A rendezvous implementation that uses `etcd <https://etcd.io/>`__ as the backend store."""

    def __init__(
        self,
        client,
        prefix,
        run_id,
        num_min_workers,
        num_max_workers,
        timeout,
        last_call_timeout,
    ):
        self.client = client
        logger.info("Etcd machines: %s", self.client.machines)

        self._prefix = prefix
        self._run_id = run_id
        self._num_min_workers = num_min_workers
        self._num_max_workers = num_max_workers
        self._timeout = timeout
        self._last_call_timeout = last_call_timeout

        # For cleaning up TTL refresher threads (for ephemeral keys)
        self._lease_run_id_stop = None
        self._lease_this_rank_stop = None

        if not self._prefix.endswith("/"):
            self._prefix += "/"

        # Setup a permanent prefix dir, if didn't exist
        if self._prefix != "/":
            self.create_path_if_not_exists(self._prefix)

        # Lease a "sub-root" node specific to this job instance (run_id)
        self.create_path_if_not_exists(self.get_path(""), ttl=CONST_RUNID_SUBROOT_TTL)
        self._lease_run_id_stop = self.setup_lease_renewal(
            self.get_path(""), ttl=CONST_RUNID_SUBROOT_TTL
        )

        # Subdir for all rendezvous work
        self.create_path_if_not_exists(self.get_path("/rdzv"))

        # Create a rendezvous version counter, if doesn't exist
        try:
            self.client.write(
                key=self.get_path("/rdzv/version_counter"), value="0", prevExist=False
            )
        except etcd.EtcdAlreadyExist:
            pass

    def __del__(self):
        # TODO: look into using weakref here instead.
        if self._lease_run_id_stop is not None:
            self._lease_run_id_stop.set()

        if self._lease_this_rank_stop is not None:
            self._lease_this_rank_stop.set()

    def rendezvous_barrier(self):
        """
        Main entry point for next rendezvous.

        This method is blocking until rendezvous succeeds or a timeout occurs.

        Returns:
             ``(rdzv_version, rank, world_size)``

        Raises:
            RendezvousTimeoutError - timeout waiting for rendezvous
            RendezvousClosedError - rendezvous is or was closed while waiting
            RendezvousError - other persistent errors that
             render the rendezvous non-retryable
        """
        self._rendezvous_deadline = time.time() + self._timeout
        while True:
            if time.time() > self._rendezvous_deadline:
                raise RendezvousTimeoutError

            logger.info("Attempting to join next rendezvous")
            try:
                # Dis-own our lease in the previous rendezvous, if exists
                if self._lease_this_rank_stop is not None:
                    self._lease_this_rank_stop.set()

                return self.init_phase()

            except EtcdRendezvousRetryImmediately:
                # The type of failure suggests we can retry without delay
                pass

            except EtcdRendezvousRetryableFailure:
                # In case of retryable failure, wait a small delay
                # to avoid spamming etcd
                time.sleep(1)

            except RendezvousTimeoutError:
                logger.info("Rendezvous timeout occurred in EtcdRendezvousHandler")
                raise

            except RendezvousClosedError:
                logger.info(
                    "Rendezvous for run_id=%s was observed to be closed", self._run_id
                )
                raise

            except RendezvousError:
                raise

            except Exception as e:
                # In case of a general exception, wait a small delay
                # to avoid spamming etcd
                # FIXME: there are a few things that fall under this like
                # etcd.EtcdKeyNotFound, etc, which could be handled more explicitly.
                logger.info("Rendezvous attempt failed, will retry. Reason: %s", e)  # noqa: G200
                time.sleep(1)

    def init_phase(self):
        """
        Initially, the rendezvous state is expected to be one of:

        1. empty (non-existent) - in this case we try to create a new one.
        2. joinable - we try to join it.
        3. final - we announce ourselves as waiting, and go into monitoring mode

        Any other state is considered transitional, and will be retried after
        a short delay.

        Returns:
            ``(rdzv_version, rank, world_size)``

        Raises:
            RendezvousClosedError - current rendezvous was/is closed
            EtcdRendezvousRetryableFailure - observed some intermediate
             state, which is best handled by retrying later
        """
        try:
            active_version = self.try_create_rendezvous()
            state = json.loads(active_version.value)
            logger.info("New rendezvous state created: %s", state)
        except etcd.EtcdAlreadyExist:
            active_version, state = self.get_rdzv_state()
            # Note: it is possible for above query to fail (etcd.EtcdKeyNotFound),
            # but this is ok for us - just means we'll restart from beginning.
            logger.info("Observed existing rendezvous state: %s", state)

        if state["status"] == "closed":
            raise RendezvousClosedError

        if state["status"] == "joinable":
            return self.join_phase(state["version"])

        if state["status"] == "final":
            self.handle_existing_rendezvous(state["version"])
            raise EtcdRendezvousRetryImmediately

        self.try_wait_for_state_change(etcd_index=active_version.etcd_index + 1)
        raise EtcdRendezvousRetryableFailure

    def join_phase(self, expected_version):
        """
        We observed a rendezvous state in 'joinable' state, and attempt to join this
        particular version, and then wait for all other peers to join.
        """
        # Failure to join will propagate an exception, causing a re-entry.
        active_version, this_rank = self.join_rendezvous(expected_version)
        state = json.loads(active_version.value)
        logger.info(
            "Joined rendezvous version %s as rank %s. Full state: %s",
            state["version"],
            this_rank,
            state,
        )

        # If this worker was first to reach num_min_workers requirement,
        # and rendezvous is still joinable (therefore it is elastic),
        # then this worker will be responsible for waiting out the "last call"
        # timeout and closing (i.e. transitioning to 'frozen') the rendezvous
        # afterwards.
        # As a safety against a potential failure of this worker (during the
        # last call timeout), the rendezvous state is made ephemeral
        # when min_num_workers is reached.

        if this_rank == self._num_min_workers - 1 and state["status"] == "joinable":
            logger.info("Rank %s is responsible for join last call.", this_rank)
            last_call_deadline = time.time() + self._last_call_timeout
            self.handle_join_last_call(expected_version, last_call_deadline)
            logger.info("Rank %s finished join last call.", this_rank)

        # Wait for rendezvous state to be frozen, which means a fixed set of peers
        logger.info("Waiting for remaining peers.")
        active_version = self.wait_for_peers(expected_version)
        state = json.loads(active_version.value)

        assert state["version"] == expected_version, (
            "Logic error: failed to observe version mismatch"
        )

        return self.confirm_phase(expected_version, this_rank)

    def confirm_phase(self, expected_version, this_rank):
        """
        Once the rendezvous state transitions from 'joinable' to 'frozen',
        we have every participant confirm their membership and setup per-member
        keep-alive TTL keys, and then wait for all other participants to confirm,
        which would then successfully conclude this rendezvous.
        """
        logger.info("All peers arrived. Confirming membership.")
        self.confirm_membership(expected_version, this_rank)

        logger.info("Waiting for confirmations from all peers.")
        active_version = self.wait_for_final(expected_version)
        state = json.loads(active_version.value)

        logger.info(
            "Rendezvous version %s is complete. Final state: %s",
            state["version"],
            state,
        )

        # Rendezvous version number; our rank in it; world size
        return state["version"], this_rank, len(state["participants"])

    def handle_existing_rendezvous(self, expected_version):
        """
        Handle the case when there's an existing (state 'final) rendezvous already
        in place, and we have to announce ourselves waiting, and wait until
        the next rendezvous opportunity.
        """
        # If state is 'final' -> increment num_workers_waiting
        # Then, observe state changes:
        #   1. if it's no longer final -> bail out and re-try
        #   2. if keep alives are missing, destroy it and bail out.
        active_state = self.announce_self_waiting(expected_version)
        logger.info(
            "Added self to waiting list. Rendezvous full state: %s", active_state.value
        )

        self.wait_for_rendezvous_to_free(expected_version)
        logger.info(
            "Previously existing rendezvous state changed. Will re-try joining."
        )

    def try_create_rendezvous(self):
        """
        Create new rendezvous state or raise an exception that indicates an unexpected state (e.g. already exists).

        Raises:
             RendezvousError - on unexpected state
        """
        # Initially active_version is ephemeral - this is to handle the
        # possibility that might fail to complete the setup transaction,
        # i.e. the transition "setup" -> "joinable".
        active_version = self.client.write(
            key=self.get_path("/rdzv/active_version"),
            value=json.dumps({"status": "setup"}),
            prevExist=False,
            ttl=CONST_ETCD_SETUP_TTL,
        )

        try:
            version_counter = self.client.get(self.get_path("/rdzv/version_counter"))
            version_counter.value = str(int(version_counter.value) + 1)
            self.client.update(version_counter)
        except (etcd.EtcdKeyNotFound, etcd.EtcdCompareFailed) as e:
            raise RendezvousError(
                "Unexpected state of EtcdRendezvousHandler, worker needs to die."
            ) from e

        # Any failure below results in declaring a retryable rendezvous failure.
        # The ephemeral /rdzv/active_version will expire and someone can then
        # re-try the setup process.

        # Create directory node for participant data
        self.client.write(
            key=self.get_path(f"/rdzv/v_{version_counter.value}"),
            value=None,
            dir=True,
            prevExist=False,
        )

        # Publish rendezvous version and signal it is ready-to-be-joined.
        # If rendezvous was set closed just before this, a retry will happen,
        # where the closed condition will be handled.
        return self.client.test_and_set(
            key=self.get_path("/rdzv/active_version"),
            value=json.dumps(
                {
                    "status": "joinable",
                    "version": version_counter.value,
                    "participants": [],
                }
            ),
            prev_value=active_version.value,
        )

    def join_rendezvous(self, expected_version):
        """Helper method for the join phase."""
        # Use compare-and-swap to add self to rendezvous state:
        while True:
            cas_delay()
            active_version, state = self.get_rdzv_state()

            if state["status"] != "joinable":
                raise EtcdRendezvousRetryableFailure(
                    "Rendezvous state became non-joinable before we could join. "
                    "Must join next one."
                )

            if state["version"] != expected_version:
                raise EtcdRendezvousRetryImmediately(
                    "Rendezvous version changed. Must try join the new one."
                )

            assert len(state["participants"]) < self._num_max_workers, (
                "Logic error: joinable rendezvous should always have space left"
            )

            this_rank = len(state["participants"])
            state["participants"].append(this_rank)

            # When reaching min workers, or changing state to frozen, we'll set
            # the active_version node to be ephemeral.
            set_ttl: Optional[int] = None
            if len(state["participants"]) == self._num_max_workers:
                state["status"] = "frozen"
                state["keep_alives"] = []
                set_ttl = CONST_ETCD_FROZEN_TTL
            elif len(state["participants"]) >= self._num_min_workers:
                set_ttl = CONST_ETCD_JOINABLE_EPHEMERAL_TTL

            try:
                # Compare-and-swap.
                active_version = self.client.test_and_set(
                    key=self.get_path("/rdzv/active_version"),
                    value=json.dumps(state),
                    prev_value=active_version.value,
                    ttl=set_ttl,
                )
                # We succeeded joining.
                return active_version, this_rank

            except etcd.EtcdCompareFailed:
                logger.info("Join rendezvous CAS unsuccessful, retrying")

    def wait_for_peers(self, expected_version):
        """Helper method for the join phase."""
        active_version, state = self.get_rdzv_state()
        while True:
            if state["status"] == "frozen" and state["version"] == expected_version:
                # Success, all peers arrived.
                return active_version

            elif state["status"] == "joinable" and state["version"] == expected_version:
                # Continue waiting for any interesting events.
                active_version, state = self.try_wait_for_state_change(
                    etcd_index=active_version.etcd_index + 1
                )

            else:
                # No valid transition possible at this point
                raise EtcdRendezvousRetryableFailure(
                    "Rendezvous state transition no longer possible. Must re-enter."
                )

    def confirm_membership(self, expected_version, this_rank):
        """Helper method for the confirm phase."""
        # Compare-and-swap loop
        while True:
            cas_delay()
            active_version, state = self.get_rdzv_state()

            if state["status"] != "frozen":
                raise EtcdRendezvousRetryImmediately(
                    "Rendezvous no longer frozen, before we confirmed. "
                    "Must join next one"
                )
            if state["version"] != expected_version:
                raise EtcdRendezvousRetryImmediately(
                    "Rendezvous version changed. Must try join the new one."
                )

            this_lease_key = self.get_path(
                f"/rdzv/v_{expected_version}/rank_{this_rank}"
            )
            self.client.set(this_lease_key, value=None, ttl=CONST_WORKER_KEEPALIVE_TTL)

            state["keep_alives"].append(this_lease_key)
            if len(state["keep_alives"]) == len(state["participants"]):
                # Everyone confirmed (this rank is last to do so)
                state["status"] = "final"
                state["num_workers_waiting"] = 0
                finalize = True
            else:
                finalize = False

            try:
                # Compare-and-swap. If new state is still frozen, keep it ephemeral.
                active_version = self.client.test_and_set(
                    key=self.get_path("/rdzv/active_version"),
                    value=json.dumps(state),
                    prev_value=active_version.value,
                    ttl=None if finalize else CONST_ETCD_FROZEN_TTL,
                )

                self._lease_this_rank_stop = self.setup_lease_renewal(
                    this_lease_key, ttl=CONST_WORKER_KEEPALIVE_TTL
                )
                return active_version

            except etcd.EtcdCompareFailed:
                logger.info("Confirm membership CAS unsuccessful, retrying")

    def wait_for_final(self, expected_version):
        """Helper method for the confirm phase."""
        active_version, state = self.get_rdzv_state()
        while True:
            if state["status"] == "final" and state["version"] == expected_version:
                # Success. This rendezvous is final, and we accept it.
                return active_version

            elif state["status"] == "frozen" and state["version"] == expected_version:
                # Continue waiting for any interesting events.
                active_version, state = self.try_wait_for_state_change(
                    etcd_index=active_version.etcd_index + 1
                )

            else:
                # No valid transition possible at this point
                raise EtcdRendezvousRetryableFailure(
                    "Rendezvous state transition no longer possible. Must re-enter."
                )

    def announce_self_waiting(self, expected_version):
        """
        Announce this worker is waiting (via num_workers_waiting counter) to join next
        rendezvous, but only if state and version match.
        """
        while True:
            cas_delay()
            active_version, state = self.get_rdzv_state()

            if state["status"] != "final" or state["version"] != expected_version:
                raise EtcdRendezvousRetryImmediately

            # Increment counter to signal an additional waiting worker.
            state["num_workers_waiting"] += 1

            try:
                active_version = self.client.test_and_set(
                    key=self.get_path("/rdzv/active_version"),
                    value=json.dumps(state),
                    prev_value=active_version.value,
                )
                return active_version

            except etcd.EtcdCompareFailed:
                logger.info("Announce self as waiting CAS unsuccessful, retrying")

    def wait_for_rendezvous_to_free(self, expected_version):
        """
        When there's an existing valid rendezvous in state 'final', we have to wait until the next opportunity to join.

        Such opportunity may come from:

        1. rendezvous state changed by someone else, in which case we unblock and retry.
        2. rendezvous becomes invalid because at least one member failed to renew their
           leased keep_alive node. We detect this, and destroy the rendezvous.
        """
        active_version, state = self.get_rdzv_state()
        while True:
            if state["status"] != "final" or state["version"] != expected_version:
                return

            # Check if current rendezvous state is valid, in the sense that all
            # its members are alive (renewing their lease).
            # If not, try destroy this rendezvous, so a new one can be created.
            alive_members = self.client.get(
                self.get_path(f"/rdzv/v_{expected_version}")
            )
            keep_alive_keys = [ch.key for ch in alive_members.children]

            for key in state["keep_alives"]:
                if key not in keep_alive_keys:
                    # This participant didn't renew their lease. We'll declare this
                    # rendezvous version as dead (but only if it hadn't changed)
                    logger.info("Keep-alive key %s is not renewed.", key)
                    logger.info(
                        "Rendezvous version %s is incomplete. ", expected_version
                    )
                    logger.info("Attempting to destroy it.")

                    # Compare-and-delete operation. Throws if compare failed,
                    # which means rendezvous was already destroyed/re-created/closed,
                    # and we can try to re-enter the barrier.
                    self.client.delete(
                        key=self.get_path("/rdzv/active_version"),
                        prevValue=active_version.value,
                    )

                    logger.info(
                        "Destroyed rendezvous version %s successfully.",
                        expected_version,
                    )

                    # We can return (and retry) immediately
                    return

            # Existing rendezvous seems valid, no reason to destroy it.
            # We just have to wait until something changes and re-check.
            try:
                overall_timeout = (
                    max(self._rendezvous_deadline - time.time(), 0.0) + 1.0
                )
                self.client.watch(
                    key=self.get_path("/rdzv"),
                    index=active_version.etcd_index + 1,
                    recursive=True,
                    timeout=overall_timeout,
                )
            except (etcd.EtcdEventIndexCleared, etcd.EtcdWatchTimedOut):
                pass

            if time.time() > self._rendezvous_deadline:
                raise RendezvousTimeoutError
            active_version, state = self.get_rdzv_state()

    def handle_join_last_call(self, expected_version, deadline):
        """
        After we reach min number of workers, one particular worker takes on the
        responsibility of waiting an additional timeout before closing the join window.
        If the worker responsible for this fails, the rendezvous will be destroyed due
        to expiring TTL, and the other participants will re-rendezvous.

        Here we expect to see state <joinable, expected_version>
        Exit gracefully if either:

        1. state becomes <frozen, expected_version>
        2. timeout happens (reaching deadline), in which case
           we try the transition to <frozen, expected_version>

        Exit with exception otherwise.
        """
        active_version, state = self.get_rdzv_state()
        while True:
            if state["status"] == "frozen" and state["version"] == expected_version:
                # Worker set became frozen before last-call timeout. This is possible
                # when num_max_workers is reached before the timeout.
                return

            if state["status"] != "joinable" or state["version"] != expected_version:
                raise EtcdRendezvousRetryableFailure(
                    "Rendezvous state transition no longer possible. Must re-enter."
                )

            # If timeout occurred, attempt a state transition (joinable -> frozen)
            if time.time() >= deadline:
                state["status"] = "frozen"
                state["keep_alives"] = []
                try:
                    active_version = self.client.test_and_set(
                        key=self.get_path("/rdzv/active_version"),
                        value=json.dumps(state),
                        prev_value=active_version.value,
                        ttl=CONST_ETCD_FROZEN_TTL,
                    )
                    # We successfully made this rendezvous frozen.
                    return
                except etcd.EtcdCompareFailed:
                    logger.info(
                        "Join last-call transition CAS unsuccessful. Will retry"
                    )
                    cas_delay()
                    active_version, state = self.get_rdzv_state()
                    continue

            # Timeout did not occur, so we must refresh TTL, and wait for
            # further changes. Note: we only want TTL to be refreshed if
            # state is still joinable, hence we use CAS for that here,
            # even though we don't change any of the data.
            try:
                active_version = self.client.test_and_set(
                    key=self.get_path("/rdzv/active_version"),
                    value=active_version.value,
                    prev_value=active_version.value,
                    ttl=CONST_ETCD_JOINABLE_EPHEMERAL_TTL,
                )

                # Minimize "oversleeping":
                timeout = min(
                    CONST_ETCD_JOINABLE_EPHEMERAL_TTL / 2,
                    deadline - time.time() + 1.0,  # Oversleeping by 1s is ok.
                )
                active_version, state = self.try_wait_for_state_change(
                    etcd_index=active_version.etcd_index + 1, timeout=timeout
                )
            except etcd.EtcdCompareFailed:
                logger.info("Join last-call TTL refresh CAS unsuccessful, will retry")
                cas_delay()
                active_version, state = self.get_rdzv_state()

    def set_closed(self):
        """
        Mark rendezvous 'closed' for current run_id, which is used to signal other
        participants to not attempt to perform (re-)rendezvous. This is useful
        when one of the workers decides the job is complete.
        """
        while True:
            active_version, state = self.get_rdzv_state()

            if state["status"] == "closed":
                # Already closed by someone else.
                return

            state["status"] = "closed"
            try:
                self.client.test_and_set(
                    key=self.get_path("/rdzv/active_version"),
                    value=json.dumps(state),
                    prev_value=active_version.value,
                )
                return

            except etcd.EtcdCompareFailed:
                logger.info("Set closed CAS unsuccessful, retrying")
                cas_delay()

    def get_rdzv_state(self):
        active_version = self.client.get(key=self.get_path("/rdzv/active_version"))
        return active_version, json.loads(active_version.value)

    def try_wait_for_state_change(self, etcd_index, timeout=None):
        # Don't sleep past the overall deadline (at least more than by 1s)
        overall_timeout = max(self._rendezvous_deadline - time.time(), 0.0) + 1.0
        timeout = overall_timeout if timeout is None else min(timeout, overall_timeout)

        try:
            self.client.watch(
                self.get_path("/rdzv/active_version"), index=etcd_index, timeout=timeout
            )
        except (etcd.EtcdEventIndexCleared, etcd.EtcdWatchTimedOut):
            pass

        if time.time() > self._rendezvous_deadline:
            raise RendezvousTimeoutError

        # Unfortunately, we have to do another fetch in order to get last etcd_index.
        return self.get_rdzv_state()

    def get_path(self, path):
        if not path.startswith("/"):
            path = "/" + path

        return f"{self._prefix}run_{self._run_id}{path}"

    def create_path_if_not_exists(self, full_path, ttl=None):
        try:
            self.client.write(
                key=full_path, value=None, dir=True, prevExist=False, ttl=ttl
            )
        except etcd.EtcdAlreadyExist:
            pass

    def setup_lease_renewal(self, full_path, ttl):
        # NOTE: For ephemeral key TTL renewal (~lease) to work correctly,
        # make sure you don't call any long-blocking methods that do not
        # release the Python's GIL! An example of this is calling a pybind11
        # extension function that is blocking / long-running, but is not
        # doing a scoped release of the GIL.
        def lease_worker(client, path, ttl, stop_event):
            while True:
                try:
                    client.refresh(path, ttl=ttl)
                except etcd.EtcdKeyNotFound:
                    break
                except ConnectionRefusedError:
                    # This error usually occurs during test when the server already got terminated but the
                    # python garbage collector have not yet invoked the __del__ method.
                    break

                if stop_event.wait(timeout=ttl / 2):
                    break

        lease_stop_event = threading.Event()
        lease_thread = threading.Thread(
            target=lease_worker, args=(self.client, full_path, ttl, lease_stop_event)
        )

        lease_thread.daemon = True
        lease_thread.start()

        return lease_stop_event

    def store_extra_data(self, rdzv_version, key, value):
        node = self.get_path(f"/rdzv/v_{rdzv_version}/extra_data")
        try:
            # If first time we are storing anything:
            extra_data = self.client.write(
                key=node, value=json.dumps({key: value}), prevExist=False
            )
            return
        except etcd.EtcdAlreadyExist:
            pass

        # CAS loop, to make sure we don't lose concurrent stores.
        while True:
            # We never delete extra_data. Failure here should be fatal, no special handling.
            extra_data = self.client.get(node)

            new_extra_data_value = json.loads(extra_data.value)
            new_extra_data_value[key] = value

            try:
                extra_data = self.client.test_and_set(
                    key=node,
                    value=json.dumps(new_extra_data_value),
                    prev_value=extra_data.value,
                )
                return
            except etcd.EtcdCompareFailed:
                logger.info("Store extra_data CAS unsuccessful, retrying")
                time.sleep(0.1)

    def load_extra_data(self, rdzv_version, key, timeout=None):
        # 'extra_data' node itself, and the directory it is located in:
        node = self.get_path(f"/rdzv/v_{rdzv_version}/extra_data")
        node_dir = self.get_path(f"/rdzv/v_{rdzv_version}")

        # TODO: implement timeout
        # https://github.com/pytorch/elastic/issues/12
        while True:
            # Combined wait for the node itself, and the key inside it.
            root = self.client.get(node_dir)

            # Find the extra_data node, if it exists
            extra_data = [n for n in root.children if n.key == node]
            assert len(extra_data) <= 1

            # Node for extra_data exists, check the desired key inside it.
            if len(extra_data) == 1:
                extra_data_dict = json.loads(extra_data[0].value)
                if key in extra_data_dict:
                    return extra_data_dict[key]

            # The 'extra_data' node doesn't exist, or they key isn't published yet.
            # Wait for interesting events on the extra_data node and retry.
            try:
                self.client.watch(node, index=root.etcd_index + 1)
            except (etcd.EtcdEventIndexCleared, etcd.EtcdWatchTimedOut):
                pass

    def setup_kv_store(self, rdzv_version):
        store_path = self.get_path(f"/rdzv/v_{rdzv_version}/kv")
        self.create_path_if_not_exists(store_path)
        return EtcdStore(etcd_client=self.client, etcd_store_prefix=store_path)


def _create_etcd_client(params: RendezvousParameters) -> etcd.Client:
    """Create a new ``etcd.Client`` from the specified ``RendezvousParameters``."""
    hostname, port = parse_rendezvous_endpoint(params.endpoint, 2379)

    # The communication protocol
    protocol = params.config.get("protocol")
    if protocol is None:
        protocol = "http"
    else:
        if protocol != "http" and protocol != "https":
            raise ValueError("The etcd protocol must be HTTP or HTTPS.")

    # The SSL client certificate
    ssl_cert = params.config.get("cert")
    if ssl_cert is not None:
        cert_key = params.config.get("key")
        if cert_key is not None:
            # The etcd client expects the certificate key as the second element
            # of the `cert` tuple.
            ssl_cert = (ssl_cert, cert_key)

    # The root certificate
    ca_cert = params.config.get("cacert")

    return etcd.Client(
        hostname,
        port,
        protocol=protocol,
        cert=ssl_cert,
        ca_cert=ca_cert,
        allow_reconnect=True,
    )


# Handler for torch.distributed "static" registration
def create_rdzv_handler(params: RendezvousParameters) -> RendezvousHandler:
    """
    Usage:

    ::

    rdzv_params = RendezvousParameters(
                        backend="etcd",
                        endpoint="192.168.0.42:2379",
                        run_id="123",
                        min_nodes=4,
                        max_nodes=8,
                        timeout=300,
                        last_call_timeout=30,
                        etcd_prefix="custom_prefix",
                        protocol="https",
                        cacert="/etc/kubernetes/certs/ca.crt",
                        cert="/etc/kubernetes/certs/client.crt",
                        key="/etc/kubernetes/certs/client.key")
    # -- or --
    rdzv_params = RendezvousParameters(
                        backend="etcd",
                        endpoint="192.168.0.42:2379",
                        run_id="123",
                        min_nodes=4,
                        max_nodes=8)

    etcd_rdzv_handler = create_etcd_rendezvous_handler(rdzv_params)


    Where:
        run_id - unique id for this training job instance,
        min_nodes - min number of workers expected to join the rendezvous,
        max_nodes - max number of workers allowed to join the rendezvous,
                        defaults to min_workers is not specified.
        timeout - total timeout within which next_rendezvous is expected to
                      succeed; a RendezvousTimeoutError is raised otherwise;
                      Defaults is 600 (10 minutes).
        last_call_timeout - additional wait amount ("last call") after
                            min number of workers has been reached.
                            Defaults to 30 seconds.
        etcd_prefix - path prefix (from etcd root), inside which all
                      etcd nodes will be created.
                      Default is "/torchelastic/p2p".
        protocol - http (default) or https to access etcd.
        cacert - CA cert to access etcd, only makes sense with https.
        cert - client cert to access etcd, only makes sense with https.
        key - client key to access etcd, only makes sense with https.
    """
    client = _create_etcd_client(params)

    etcd_prefix = params.get("etcd_prefix", "/torchelastic/p2p")

    rdzv = EtcdRendezvous(
        client=client,
        prefix=etcd_prefix,
        run_id=params.run_id,
        num_min_workers=params.min_nodes,
        num_max_workers=params.max_nodes,
        timeout=params.get_as_int("timeout", _DEFAULT_TIMEOUT),
        last_call_timeout=params.get_as_int(
            "last_call_timeout", _DEFAULT_LAST_CALL_TIMEOUT
        ),
    )
    return EtcdRendezvousHandler(
        rdzv_impl=rdzv,
        local_addr=params.local_addr,
    )