keep estimate stream alive while DAS is silent - #332
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Problem
The cloud-optimised estimate stream relied on data-access-service
heartbeats to keep the client's connection alive. A DAS that fails
before opening its stream never heartbeats, so the connection goes
quiet for the 30-60s the failure takes and CloudFront drops it before
estimate-failedis written.Changes
skipped when an event was already sent in the last half interval, so
work that reports its own progress is not doubled up on.
probeClientwrites an SSE comment instead of a keep-alive event.DAS heartbeats still detect a client that has gone and unwind the
read, but no longer reach the browser as a second keep-alive.
detail, drop gatewayHTML error pages, and truncate anything long.