Jovan Attisha created HTTPCLIENT-2327:
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Summary: MemcachedHttpAsyncCacheStorage propagates
CancellationException from cancelled cache requests to caller
Key: HTTPCLIENT-2327
URL: https://issues.apache.org/jira/browse/HTTPCLIENT-2327
Project: HttpComponents HttpClient
Issue Type: Bug
Components: HttpCache
Reporter: Jovan Attisha
Spymemcached provides options to handle failure modes when an in-flight request
to a Memcached instance fails (for example, when deploying to a Memcached
server that is currently serving requests). One of the modes, and the most
logical for our scenario in particular, is
[Cancel|https://github.com/couchbase/spymemcached/blob/master/src/main/java/net/spy/memcached/FailureMode.java#L51-L54].
This, as you would expect, cancels any futures that are in-flight when a
Memcached node becomes unreachable.
The issue that we're having is that the
{{[MemcachedHttpAsyncCacheStorage|https://github.com/apache/httpcomponents-client/blob/master/httpclient5-cache/src/main/java/org/apache/hc/client5/http/impl/cache/memcached/MemcachedHttpAsyncCacheStorage.java#L184]}}
(for restore linked here, as this is our issue, but also for other methods),
doesn't handle the {{CancellationException}} gracefully, and instead propagates
this Exception up to the caller. Since this is a cache for an HTTP client, I
would expect the client to handle the cancellation as a "cache miss" (I would
almost expect this behavior for {_}all exceptions{_}, though I'm likely not
thinking of some edge cases here) and proceed to make the request to the
resource originally requested, since the cache is a mere optimization, and a
failure in the cache should not impact the actual request to the resource we're
targeting.
We did POC this by extending the
{{[MemcachedHttpAsyncCacheStorage|https://github.com/apache/httpcomponents-client/blob/master/httpclient5-cache/src/main/java/org/apache/hc/client5/http/impl/cache/memcached/MemcachedHttpAsyncCacheStorage.java#L184]}}
and overriding the {{restore}} method to explicitly handle
{{{}CancellationException{}}}s and complete the future to treat it as a cache
miss, but before we proceed with this, we want to understand if:
1. The Apache HTTPClient agrees that this is the desired behavior
2. If so, would the Apache HTTP Client accept this behavior in the client
itself, rather than us extending to override this behavior
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