Recalc Strategy underestimates function requests per second

[ema-pe]

There is a bug in the Recalc strategy when the agent retrieves statistics from the HAProxy stick table: the values are reset every time HAProxy restarts, which turns to an underestimation of the request counters. This behavior is intentional in HAProxy but affects the agent's weight calculation logic.

A partial fix is introduced in commit 15f8a65: the agent now reads the rate directly from the HAProxy stick table for the last 1-second window. This is not a complete fix since it only accounts for the most recent second, but it may be sufficient as the agent bases its decisions on 1-second data during each recalculation period (default: 1 minute).

See:

dfaas/dfaasagent/agent/loadbalancer/recalcstrategy.go

Lines 181 to 192 in 15f8a65

	for _, stEntry := range stContent {
	// This stick table contains a single key, "80", which tracks both
	// the number of requests within the given time window and the rate
	// per second during the recalculation period. We only use the rate.
	//
	// Note: Do not use http_req_cnt, as HAProxy restarts every
	// recalculation period, causing all counters to reset.
	//
	// FIXME: The http_req_rate value is taken from the previous
	// 1-second period, not averaged over the entire recalculation
	// period. This is a known limitation of the current strategy.
	strategy.userRates[funcName] = float64(stEntry.HTTPReqRate)

dfaas/dfaasagent/agent/loadbalancer/haproxycfgrecalc.tmpl

Lines 25 to 32 in 15f8a65

	# Stick table for invocations of function {{$funcName}} from users, not from
	# other DFaaS nodes. We just use one row for all clients. Denied requests are
	# not counted here.
	#
	# Among all stick tables, this is the only one used by the DFaaS agent to
	# calculate forwarding weights.
	backend st_users_func_{{$funcName}}
	stick-table type integer size 10 expire {{$.SecsRecalc}}s store http_req_cnt,http_req_rate(1s)