# How Kuma chooses the right policy to apply

At any single moment, there might be multiple policies (of the same type) that match a connection between sources and destinations Dataplanes.

E.g., there might be a catch-all policy that sets the baseline for your organization

type: TrafficLog
mesh: default
name: catch-all-policy
sources:
  - match:
      service: '*'
destinations:
  - match:
      service: '*'
conf:
  backend: logstash

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Additionally, there might be a more focused use case-specific policy, e.g.

type: TrafficLog
mesh: default
name: web-to-backend-policy
sources:
  - match:
      service: web
      cloud:   aws
      region:  us
destinations:
  - match:
      service: backend
conf:
  backend: splunk

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What does Kuma do when it encounters multiple matching policies ?

The answer depends on policy type:

since TrafficPermission represents a grant of access given to a particular client service, Kuma conceptually "aggregates" all such grants by applying ALL TrafficPermission policies that match a connection between sources and destinations Dataplanes
for other policy types - TrafficRoute, TrafficLog, HealthCheck - conceptual "aggregation" would be too complex for users to always keep in mind; that is why Kuma chooses and applies only "the most specific" matching policy in that case

Going back to 2 TrafficLog policies described above:

for connections between web and backend Dataplanes Kuma will choose and apply web-to-backend-policy policy as "the most specific" in that case
for connections between all other dataplanes Kuma will choose and apply catch-all-policy policy as "the most specific" in that case

"The most specific" policy is defined according to the following rules:

a policy that matches a connection between 2 Dataplanes by a greater number of tags is "more specific"

E.g., web-to-backend-policy policy matches a connection between 2 Dataplanes by 4 tags (3 tags on sources and 1 tag on destinations), while catch-all-policy matches only by 2 tags (1 tag on sources and 1 tag on destinations)
a policy that matches by the exact tag value is more specific than policy that matches by a '*' (wildcard) tag value

E.g., web-to-backend-policy policy matches sources by service: web, while catch-all-policy matches by service: *
if 2 policies match a connection between 2 Dataplanes by the same number of tags, then the one with a greater total number of matches by the exact tag value is "more specific" than the other
if 2 policies match a connection between 2 Dataplanes by the same number of tags, and the total number of matches by the exact tag value is the same for both policies, and the total number of matches by a '*' (wildcard) tag value is the same for both policies, then a "more specific" policy is the one whose name comes first when ordered alphabetically

E.g.,

match by a greater number of tags

sources:
  - match:
      service: '*'
      cloud:   aws
      region:  us
destinations:
  - match:
      service: '*'

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is "more specific" than

sources:
  - match:
      service: '*'
destinations:
  - match:
      service: '*'

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match by the exact tag value

sources:
  - match:
      service: web
destinations:
  - match:
      service: backend

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is "more specific" than a match by a '*' (wildcard)

sources:
  - match:
      service: '*'
destinations:
  - match:
      service: '*'

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match with a greater total number of matches by the exact tag value

sources:
  - match:
      service: web
      version: v1
destinations:
  - match:
      service: backend

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is "more specific" than

sources:
  - match:
      service: web
      version: '*'
destinations:
  - match:
      service: backend

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when 2 matches are otherwise "equally specific"

name: policy-1
sources:
  - match:
      service: web
      version: v1
destinations:
  - match:
      service: backend

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policy-1 is considered "more specific" only due to the alphabetical order of names "policy-1" and "policy-2"

name: policy-2
sources:
  - match:
      service: web
destinations:
  - match:
      service: backend
      cloud:   aws

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