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Switch Broker's default backpressure policy to drop_oldest, bump buffer sizes

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At every site where we've dug into backpressure disconnect findings, it has been
the case that the default values were too small. 8192, so 4x the old default,
suffices at every site to drown out premature disconnects.

With metrics now available for the send buffers regardless of backpressure
overflow policy, this also switches the default from "disconnect" to
"drop_oldest" (for both peers and websockets), meaning that peerings remain
untouched but the oldest queued message simply gets dropped when a new message
is enqueued. With this policy, the number of backpressure overflows is then
simply the count of discarded messages, something that users can tune to see
drop to zero in everyday use.  Another benefit is that marginal overflows cause
less message loss than when an entire buffer's worth (plus potentially more
in-flight messages) gets thrown out with a disconnect.
This commit is contained in:
Christian Kreibich 2025-04-24 16:15:56 -07:00
parent 5008f586ea
commit 841a40ff88
1 changed files with 4 additions and 4 deletions
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8
scripts/base/frameworks/broker/main.zeek
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@ -89,20 +89,20 @@ export {
## Max number of items we buffer at most per peer. What action to take when
## the buffer reaches its maximum size is determined by
## :zeek:see:`Broker::peer_overflow_policy`.
const peer_buffer_size = 2048 &redef;
const peer_buffer_size = 8192 &redef;
## Configures how Broker responds to peers that cannot keep up with the
## incoming message rate. Available strategies:
## - disconnect: drop the connection to the unresponsive peer
## - drop_newest: replace the newest message in the buffer
## - drop_oldest: removed the olsted message from the buffer, then append
const peer_overflow_policy = "disconnect" &redef;
const peer_overflow_policy = "drop_oldest" &redef;
## Same as :zeek:see:`Broker::peer_buffer_size` but for WebSocket clients.
const web_socket_buffer_size = 512 &redef;
const web_socket_buffer_size = 8192 &redef;
## Same as :zeek:see:`Broker::peer_overflow_policy` but for WebSocket clients.
const web_socket_overflow_policy = "disconnect" &redef;
const web_socket_overflow_policy = "drop_oldest" &redef;
## How frequently Zeek resets some peering/client buffer statistics,
## such as ``max_queued_recently`` in :zeek:see:`BrokerPeeringStats`.