Fault Tolerance

When a worker is executing a task, if the worker dies unexpectedly, either because the process crashed or because the machine failed, Ray will rerun the task until either the task succeeds or the maximum number of retries is exceeded. The default number of retries is 3 and can be overridden by specifying max_retries in the @ray.remote decorator. Specifying -1 allows infinite retries, and 0 disables retries.

You can experiment with this behavior by running the following code.

import numpy as np
import os
import ray
import time

ray.init(ignore_reinit_error=True)

@ray.remote(max_retries=1)
def potentially_fail(failure_probability):
    time.sleep(0.2)
    if np.random.random() < failure_probability:
        os._exit(0)
    return 0

for _ in range(3):
    try:
        # If this task crashes, Ray will retry it up to one additional
        # time. If either of the attempts succeeds, the call to ray.get
        # below will return normally. Otherwise, it will raise an
        # exception.
        ray.get(potentially_fail.remote(0.5))
        print('SUCCESS')
    except ray.exceptions.WorkerCrashedError:
        print('FAILURE')

Ray also implements lineage reconstruction to recover task outputs that are lost from the distributed object store. This can occur during node failures. Ray will first automatically attempt to recover the value by looking for copies of the same object on other nodes. If none are found, then Ray will automatically recover the value by re-executing the task that created the value. Arguments to the task are recursively reconstructed with the same method.

Note that lineage reconstruction can cause higher than usual driver memory usage because the driver keeps the descriptions of any tasks that may be re-executed in case of a failure. To limit the amount of memory used by lineage, set the environment variable RAY_max_lineage_bytes (default 1GB) to evict lineage if the threshold is exceeded.

To disable this behavior, set the environment variable RAY_lineage_pinning_enabled=0 during ray start or ray.init. With this setting, if there are no copies of an object left, an ObjectLostError will be raised.