RDI-115-retry-logic

Why these changes are being introduced:
The harvester needs retry logic to account for the observed behavior of various repositories during testing.

How this addresses that need:
* Add retry-related values to config.py
* Calls those values when instantiating a sickle client in oai.py

Side effects of this change:
None

Relevant ticket(s):
* https://mitlibraries.atlassian.net/browse/RDI-115

harvester/config.py

@@ -4,6 +4,10 @@
 
 import sentry_sdk
 
+DEFAULT_RETRY_AFTER = 10
+MAX_RETRIES = 3
+RETRY_STATUS_CODES = (429, 500, 503)
+
 
 def configure_logger(logger: logging.Logger, verbose: bool) -> str:
     if verbose:

harvester/oai.py

@@ -7,6 +7,8 @@
 from sickle import Sickle
 from sickle.models import Record
 
+from harvester.config import DEFAULT_RETRY_AFTER, MAX_RETRIES, RETRY_STATUS_CODES
+
 logger = logging.getLogger(__name__)
 
 
@@ -20,7 +22,12 @@ def __init__(
         set_spec: Optional[str] = None,
     ) -> None:
         self.source_url = source_url
-        self.client = Sickle(self.source_url)
+        self.client = Sickle(
+            self.source_url,
+            default_retry_after=DEFAULT_RETRY_AFTER,
+            max_retries=MAX_RETRIES,
+            retry_status_codes=RETRY_STATUS_CODES,
+        )
         self.metadata_format = metadata_format
         self._set_params(metadata_format, from_date, until_date, set_spec)