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plugins/forward: Add max_concurrent option (#3640)

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* count and limit concurrent queries

Signed-off-by: Chris O'Haver <cohaver@infoblox.com>

* add option

Signed-off-by: Chris O'Haver <cohaver@infoblox.com>

* return servfail when limit exceeded

Signed-off-by: Chris O'Haver <cohaver@infoblox.com>

* docs

Signed-off-by: Chris O'Haver <cohaver@infoblox.com>

* docs

Signed-off-by: Chris O'Haver <cohaver@infoblox.com>

* docs

Signed-off-by: Chris O'Haver <cohaver@infoblox.com>

* review feedback

Signed-off-by: Chris O'Haver <cohaver@infoblox.com>

* move atomic counter to beginning of struct

Signed-off-by: Chris O'Haver <cohaver@infoblox.com>

* add comment for ErrLimitExceeded

Signed-off-by: Chris O'Haver <cohaver@infoblox.com>

* rename option to max_concurrent

Signed-off-by: Chris O'Haver <cohaver@infoblox.com>

* add metric

Signed-off-by: Chris O'Haver <cohaver@infoblox.com>

* response REFUSED; incl max in error; add more docs

Signed-off-by: Chris O'Haver <cohaver@infoblox.com>

* avoid err setup race

Signed-off-by: Chris O'Haver <cohaver@infoblox.com>

* respond SERVFAIL; doc memory usage

Signed-off-by: Chris O'Haver <cohaver@infoblox.com>
This commit is contained in:
Chris O'Haver
2020-02-04 07:59:08 -05:00
committed by GitHub
parent 8724a134c4
commit 22cd28a798
5 changed files with 85 additions and 2 deletions
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9
plugin/forward/README.md
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@@ -50,6 +50,7 @@ forward FROM TO... {
tls_servername NAME
policy random|round_robin|sequential
health_check DURATION
max_queries MAX
}
~~~
@@ -83,6 +84,11 @@ forward FROM TO... {
* `round_robin` is a policy that selects hosts based on round robin ordering.
* `sequential` is a policy that selects hosts based on sequential ordering.
* `health_check`, use a different **DURATION** for health checking, the default duration is 0.5s.
* `max_concurrent` **MAX** will limit the number of concurrent queries to **MAX**. Any new query that would
raise the number of concurrent queries above the **MAX** will result in a SERVFAIL response. This
response does not count as a health failure. When choosing a value for **MAX**, pick a number
at least greater than the expected *upstream query rate* * *latency* of the upstream servers.
As an upper bound for **MAX**, consider that each concurrent query will use about 2kb of memory.
Also note the TLS config is "global" for the whole forwarding proxy if you need a different
`tls-name` for different upstreams you're out of luck.
@@ -102,7 +108,8 @@ If monitoring is enabled (via the *prometheus* plugin) then the following metric
* `coredns_forward_healthcheck_failure_count_total{to}` - number of failed health checks per upstream.
* `coredns_forward_healthcheck_broken_count_total{}` - counter of when all upstreams are unhealthy,
and we are randomly (this always uses the `random` policy) spraying to an upstream.
* `max_concurrent_reject_count_total{}` - counter of the number of queries rejected because the
number of concurrent queries were at maximum.
Where `to` is one of the upstream servers (**TO** from the config), `rcode` is the returned RCODE
from the upstream.