Carter Kozak created HTTPCLIENT-2142:
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Summary: Design expressive idle connection validation support
Key: HTTPCLIENT-2142
URL: https://issues.apache.org/jira/browse/HTTPCLIENT-2142
Project: HttpComponents HttpClient
Issue Type: New Feature
Reporter: Carter Kozak
This ticket is based on a discussion from the mailing list:
http://mail-archives.apache.org/mod_mbox/hc-dev/202103.mbox/%3C60fa1a2bad046869a2da9269b7e90b9484a050ca.camel%40apache
.org%3E
h3. Background
Currently several connection managers support a validation interval for idle
connections which is used to check if they're still open before attempting to
send a request. The java Socket API provides no way for a client to detect if
the remote side has closed a connection without attempting a blocking read
operation. In my experience the most common failure is that a remote server
_cleanly_ closes idle connections sooner than the client expects (either due to
a low idle connection timeout or a server shutdown) resulting in failed
requests and retries.
A stale check on a java socket takes at minimum 1 millisecond due to the
millisecond socket timeout resolution, and the only way (that I'm aware of) to
check if a connection has been closed is to do a blocking read. A
SocketTimeoutException tells us the connection is intact, while other
IOExceptions suggest the connection is unusable. In many environments a 1ms
check is far more expensive than the median request to a remote server and
causes additional context switching.
When the validation interval is set to a higher value to avoid performance
pitfalls, idle connections may build up in the connection pool. If the remote
server unexpectedly restarts, the client may need to churn through hundreds of
requests burning through retries before the higher validation interval is
reached or remaining connections are closed due to reaching the keepalive
timeout.
h3. Potential Ideas
h4. Route state awareness
Track connection health state per route such that stale checks are required
before using a connection that has been idle longer than any connection that
has been determined to be stale. This requires functionality to detect
{{ConnectionClosedException}} and {{NoHttpResponseException}} produced by
attempted requests, and send it back to the connection pool. This allows
retryable requests to be executed optimistically (in cases where it's safe to
do so) and only pre-validate after a failure has occurred to limit the number
of retries that may be consumed.
h4. Request-aware stale checking predicate
Provide a configurable mechanism to force stale checks based on the incoming
request, for example I may always want to validate connections before making
requests to non-idempotent endpoints or when by request body is not considered
repeatable, but have more risk tolerance when I'm confident a retry is
possible. Perhaps also force stale connection checks for all retries.
h4. Alternative forms of stale checks
We may consider making the stale-check mechanism itself configurable, a
blocking read to be used, or an http2 ping when available, or even a full
{{OPTIONS}} or {{HEAD}} request.
It's not entirely clear if any of this should be shared between the classic and
async clients. I think the nio interfaces provide support for the common case
(cleanly closed connections) which is a deficiency of the blocking socket API.
What do you think? Additional ideas are appreciated.
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