Evgeny Kuzin <evgeny.kuzin@outlook.com> writes:
> We've been running into an issue with "target_session_attrs" when using dns-based service discovery. Currently, when
libpqconnects to a host with multiple A-records and the connection succeeds but is rejected due to target_session_attrs
mismatch(e.g., connecting to a read-only server with target_session_attrs=read-write), it skips all remaining addresses
forthat hostname and moves directly to the next host in the connection string.
> Looking at git history, I found this was a deliberate choice by Robert Haas in commit 721f7bd3cbc (2016), where he
noted"I changed Mithun's patch to skip all remaining IPs for a host if we reject a connection based on this new
parameter."The original mailing list discussion is at [1], though I wasn't able to find a clear explanation of why this
approachwas preferred over trying all addresses.
> This makes it impractical to use a single multi-A-record DNS name pointing to all cluster members with
target_session_attrs=read-writeto find the primary - only the first responding IP is tried before giving up on that
hostname.
> The attached patch changes the behavior to try all addresses for a hostname before moving to the next host, matching
theexisting behavior for connection failures. This would enable simpler DNS-based service discovery without requiring
externaltools like Consul or explicit multi-host connection strings.
TBH, I'd say that your DNS setup is broken and you should fix it.
It makes no sense to have the same DNS entry pointing to both
read-write and read-only hosts. The proposed patch will mainly
result in useless connection attempts in more-sanely-constructed
setups.
regards, tom lane
