rawlinp opened a new pull request #6671:
URL: https://github.com/apache/trafficcontrol/pull/6671


   Currently, logging in a user can take up to 4 different DB queries, and
   most route handlers require at least 1 DB query to look up the user and
   role of the requester.
   
   By periodically reading users and roles from the database (using the
   interval defined by `user_cache_refresh_interval_sec` in cdn.conf) and
   storing them in memory, Traffic Ops can quickly check the user's data
   and role without having to read from the database at request time.
   
   This makes logins extremely efficient because they require no database
   queries at request time. Additionally, overall database load is reduced
   because most route handlers have reduced the number of queries they make
   by 1.
   
   From a security perspective, making logins extremely efficient is
   advantageous because it reduces the effectiveness of denial of service
   attacks targeting the login endpoint (which would normally make at least
   1 database query to check that the user is allowed to log in). With this
   feature enabled, a DoS attack against the login endpoint would not
   exhaust limited database resources. Note: other endpoints are not as
   susceptible to DoS attacks due to requiring a valid cookie before any
   database queries are made.
   
   The downside to enabling this feature is that updates to Users and/or
   Roles are not enforced by Traffic Ops immediately when the updates are
   made. If `user_cache_refresh_interval_sec` is N, it may take up to N
   seconds before Traffic Ops will enforce updates to Users and/or Roles.
   However, as long as this interval is sufficiently short, this should not
   be a problem.
   
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   ## Which Traffic Control components are affected by this PR?
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   - Documentation
   - Traffic Ops
   - Ansible
   
   ## What is the best way to verify this PR?
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   Ensure the automated tests pass. Manual: configure 
`user_cache_refresh_interval_sec` in cdn.conf to 5, start TO, and change a 
user's role (or its role's permissions), while using that user to make requests 
for endpoints with the corresponding permissions that are being changed. Ensure 
that the changed permissions are enforced within 5 seconds of making the change 
via the API.
   
   ## PR submission checklist
   - [x] This PR has tests <!-- If not, please delete this text and explain why 
this PR does not need tests. -->
   - [x] This PR has documentation <!-- If not, please delete this text and 
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   - [x] This PR has a CHANGELOG.md entry <!-- A fix for a bug from an ATC 
release, an improvement, or a new feature should have a changelog entry. -->
   - [x] This PR **DOES NOT FIX A SERIOUS SECURITY VULNERABILITY** (see [the 
Apache Software Foundation's security guidelines](https://apache.org/security) 
for details)
   
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