1. John Rushford John_Rushford@cable.comcast.com
Apache Traffic Server Multi-Site Origin and Secondary Consistent Hash Feature
John Rushford

2. Redundancy for Comcast VOD and Linear video.

3. ATS existing Parent Selection
Used to create cache hierarchies.
Define multiple parent caches in parent.config.
Parent cache failover.
Consistent hash and round robin parent selection algorithms.
Parent selection occurs on remapped host names.

4. Feature Additions to ATS.
Update Parent Selection to support selection and failover of multiple origin servers.
Add simple retry of origin servers on 4xx http responses.
Add dead server retry of origin servers on 5xx http responses.
Add a secondary consistent hash table to ParentSelection.

5. Multi-site origin configuration: Parent.config
New configuration parameter parent_is_proxy (true/false).
May be used with forward or reverse proxies. At Comcast use is on the mid-tier forward proxy caches.
parent_is_proxy true (default), indicates that the listed parents are proxies.
parent_is_proxy false, indicates that the listed parents are origin servers.
dest_domain=“cdnlab.comcast.net” parent=“orig1.cdnlab.comcast.net:80|1.0;orig2.cdnlab.comcast.net:80|1.0” parent_is_proxy=false round_robin=consistent_hash go_direct=false

6. Simple retry Checks the response from an origin server.
By default, a 404 response will retry parents until a successful response is obtained. The client will finally see a 404 response if all parents have been tried without success.
Each parent is tried multiple in succession.
May configure a list of response codes that should trigger simple retry.

7. Dead server retry. Checks the response from an origin server.
By default, a 503 response will trigger a retry until a successful response is obtained from one of the parents.
Each parent is tried multiple multiple times in succession.
The parent that returns a 503 will be marked unavailable for parent selection for a period of 5 minutes by default.

8. Simple and dead server retry config.
Configure globally in records.config. All are overridable with the header_rewrite_plugin per remap.
proxy.config.http.parent_origin.simple_retry_enabled INT 1 (default is 0 - disabled).
proxy.config.http.parent_origin.simple_retry_response_codes STRING 404 (default). May be a comma separated list of response codes).
proxy.config.http.parent_origin.dead_server_retry_enabled INT 1 (default 0 – disabled)
proxy.config.http.parent_origin.dead_server_retry_response_codes STRING 503 (default). May be a comma separated list of response codes.
map
cond %{REMAP_PSEUDO_HOOK} set-config proxy.config.http.parent_origin.simple_retry_enabled 1

9. Secondary consistent hash
The secondary list is optional.
Add a secondary list of parents when round_robin = consistent_hash to use for failover in a cache hierarchy.
A connection failure, results in a lookup for a new parent on the secondary hash list taking advantage of content affinity built up on the secondary list of servers.
dest_domain=“cdnlab.comcast.net” parent=“orig1.cdnlab.comcast.net:80|1.0;orig2.cdnlab.comcast.net: 80|1.0” secondary_parent=“orig1.site2.cdnlab.comcast.net:80|1.0;orig2.site 2.cdnlab.comcast.net:80|1.0” round_robin=consistent_hash go_direct=false

10. Parent Selection refactor