strategies.yaml

The strategies.yaml file identifies the next hop proxies used in an cache hierarchy and the algorithms used to select the next hop proxy. Use this file to perform the following configuration:

Set up next hop cache hierarchies, with multiple parents and parent failover

Traffic Server uses the strategies.yaml file only when one or more remap lines in remap.config specifies the use of a strategy with the @strategy tag. remap.config Example:

map http://www.foo.com http://www.bar.com @strategy='mid-tier-north'

After you modify the strategies.yaml file, run the traffic_ctl config reload command to apply your changes.

Format

The strategies.yaml is a YAML document with three top level namespaces: hosts, groups and strategies. These name spaces may be in separate files. When in separate files, use #include filename in the strategies.yaml so that they are concatenated by the strategy factory into a single YAML document in the order, hosts, groups, and the strategies.

Alternatively if the config parameter proxy.config.url_remap.strategies.filename refers to a directory, the NextHopStrategyFactory will alphanumerically concatenate all files in that directory that end in .yaml by name into a single document stream for parsing. The final document must be a valid YAML document with single strategies node and optionally a single hosts and groups node. Any #include filename strings are ignored when reading .yaml files in a directory.

Hosts definitions

The hosts definitions is a YAML list of hosts. This list is optional but if not used, the groups list must include complete definitions for hosts. See the group examples below.

In the example below, hosts is a YAML list of hosts. Each host entry uses a YAML anchor, &p1 and &p2 that may be used elsewhere in the YAML document to refer to hosts p1 and p2.

host: the host value is a Fully Qualified Domain Name string
protocol: a list of schemes, ports, and health check urls for the host. The scheme is optional; strategies with no scheme will match hosts with no scheme. Note the scheme is only used to match the strategy, the actual scheme used in the upstream request will be the scheme of the remap target, regardless of the strategy or host scheme.
healthcheck: health check information with the url used to check the hosts health by some external health check agent.
hash_string: a string to use for this host’s entry in the strategy. By default, the host is used (notably without a port).
- This is currently only used by consistent_hash policy strategies, but may be used by other policies in the future.
- There’s generally no benefit to giving any host any particular hash string, but this may be useful, for example:
  
  If multiple host objects share the same host FQDN, possibly on different ports
  
  If a parent server’s FQDN changes, to prevent changing a host’s position on the hash ring, and thus breaking the cache and sending different requests to different parents
  
  To force a change in the order of the hash ring for debugging purposes

Example:

hosts:
  - &p1
    host: p1.foo.com
    protocol:
      - scheme: http
        port: 80
        health_check_url: http://192.168.1.1:80
      - scheme: https
        port: 443
        health_check_url: https://192.168.1.1:443
  - &p2
    host: p2.new.foo.com
    hash_string: p2.original.foo.com
    protocol:
      - scheme: http
        port: 80
        health_check_url: http://192.168.1.2:80

Groups definitions

The groups definitions is a YAML list of host groups. host groups are used as the primary and secondary groups used by nexthop to choose hosts from. The first group is the primary group next hop chooses hosts from. The remaining groups are used failover. The strategies policy specifies how the groups are used.

Below are examples of group definitions. The first example is using YAML anchors and references. When using references, the complete YAML document must include the anchors portion of the document first.

The second example shows a complete groups definition without the use of a hosts name space and it’s YAML anchors.

The field definitions in the examples below are defined in the hosts section.

Example using YAML anchors and references:

groups:
  - &g1
    - <<: *p1
      weight: 1.5
    - <<: *p2
      weight: 0.5
  - &g2
    - <<: *p3
      weight: 0.5
    - <<: *p4
      weight: 1.5

Explicitly defined Example, no YAML references:

groups:
  - &g1
    - host: p1.foo.com
      protocol:
        - scheme: http
          port: 80
          health_check_url: http://192.168.1.1:80
        - scheme: https
          port: 443
          health_check_url: https://192.168.1.1:443
      weight: 0.5
    - host: p2.foo.com
      protocol:
        - scheme: http
          port: 80
          health_check_url: http://192.168.1.2:80
        - scheme: https
          port: 443
          health_check_url: https://192.168.1.2:443
      weight: 0.5
  - &g2
    - host: p3.foo.com
      protocol:
        - scheme: http
          port: 80
          health_check_url: http://192.168.1.3:80
        - scheme: https
          port: 443
          health_check_url: https://192.168.1.3:443
      weight: 0.5
    - host: p4.foo.com
      protocol:
        - scheme: http
          port: 80
          health_check_url: http://192.168.1.4:80
        - scheme: https
          port: 443
          health_check_url: https://192.168.1.4:443
      weight: 0.5

Strategies definitions

The strategies namespace defines a YAML list of strategies that may be applied to a remap entry using the @strategy tag in remap.config.

Each strategy in the list may using the following parameters:

strategy: The value is the name of the strategy.
policy: The algorithm the strategy uses to select hosts. Currently one of the following:
1. rr_ip: round robin selection using the modulus of the client IP
2. rr_strict: strict round robin over the list of hosts in the primary group.
3. first_live: always selects the first host in the primary group. Other hosts are selected when the first host fails.
4. latched: Same as first_live but primary selection sticks to whatever host was used by a previous transaction.
5. consistent_hash: hosts are selected using a hash_key.
hash_key: The hashing key used by the consistent_hash policy. If not specified, defaults to path which is the same policy used in the parent.config implementation. Use one of:
1. hostname: Creates a hash using the hostname in the request URL.
2. path: (default) Creates a hash over the path portion of the request URL.
3. path+query: Same as path but adds the query string in the request URL.
4. path+fragment: Same as path but adds the fragment portion of the URL.
5. cache_key: Uses the hash key from the cachekey plugin. defaults to path if the cachekey plugin is not configured on the remap.
6. url: Creates a hash from the entire request url.
go_direct: A boolean value indicating whether a transaction may bypass proxies and go direct to the origin. Defaults to true
parent_is_proxy: A boolean value which indicates if the groups of hosts are proxy caches or origins. true (default) means all the hosts used in the remap are Traffic Server caches. false means the hosts are origins that the next hop strategies may use for load balancing and/or failover.
cache_peer_result: A boolean value that is only used when the policy is ‘consistent_hash’ and a peering_ring mode is used for the strategy. When set to true, the default, all responses from upstream and peer endpoints are allowed to be cached. Setting this to false will disable caching responses received from a peer host. Only responses from upstream origins or parents will be cached for this strategy.
scheme: Indicates which scheme the strategy supports, http or https. Note this is only used to match hosts to strategies; the actual scheme used in the upstream request will be the scheme of the remap.config target, regardless of this value. The scheme is optional, and strategies without a scheme will match hosts without a scheme. This allows for omitting the scheme if hosts are not shared between strategies, or if all strategies using a given host use the same scheme.
failover: A map of failover information.
- max_simple_retries: Part of the failover map and is an integer value of the maximum number of retries for a simple retry on the list of indicated response codes. simple retry is used to retry an upstream request using another upstream server if the response received on from the original upstream request matches any of the response codes configured for this strategy in the failover map. If no failover response codes are configured, no simple retry is attempted.
- max_unavailable_retries: Part of the failover map and is an integer value of the maximum number of retries for a unavailable retry on the list of indicated markdown response codes. unavailable retry is used to retry an upstream request using another upstream server if the response received on from the original upstream request matches any of the markdown response codes configured for this strategy in the failover map. If no failover markdown response codes are configured, no unavailable retry is attempted. unavailable retry differs from simple retry in that if a failover for retry is done, the previously retried server is marked down for rety.
- ring_mode: Part of the failover map. The host ring selection mode. Use either exhaust_ring, alternate_ring or peering_ring
1. exhaust_ring: when a host normally selected by the policy fails, another host is selected from the same group. A new group is not selected until all hosts on the previous group have been exhausted
2. alternate_ring: retry hosts are selected from groups in an alternating group fashion.
3. peering_ring: This mode is only implemented for a policy of consistent_hash and requires that one or two host groups are defined. The first host group is a list of peer caches and “this” host itself, the (optional) second group is a list of upstream caches. Parents are always selected from the peer list however, if the selected parent is “this” host itself a new parent from the upstream list is chosen. If the second group is omitted, and go_direct is true, the upstream “list” has one element, the host in the remapped URL. In addition, if any peer host is unreachable or times out, a host from the upstream list is chosen for retries. Because the peer hosts may at times not have consistent up/down markings for the other peers, requests may be looped sometimes. So it’s best to use proxy.config.http.insert_request_via_str and proxy.config.http.max_proxy_cycles to stop looping.
- response_codes: Part of the failover map. This is a list of http response codes that may be used for simple retry.
- markdown_codes: Part of the failover map. This is a list of http response codes that may be used for unavailable retry which will cause a parent markdown.
- health_check: Part of the failover map. A list of health checks. passive is the default and means that the state machine marks down hosts when a transaction timeout or connection error is detected. passive is always used by the next hop strategies. active means that some external process may actively health check the hosts using the defined health check url and mark them down using traffic_ctl.
- self: Part of the failover map. This can only be used when ring_mode is peering_ring. This is the hostname of the host in the (first) group of peers that is the local host Traffic Server runs on. (self should only be necessary when the local hostname can only be translated to an IP address with a DNS lookup.)

Example:

#include unit-tests/hosts.yaml
#
strategies:
  - strategy: 'strategy-1'
    policy: consistent_hash
    hash_key: cache_key
    go_direct: false
    groups:
      - *g1
      - *g2
    scheme: http
    failover:
      ring_mode: exhaust_ring
      response_codes:
        - 404
      markdown_codes:
        - 503
      health_check:
        - passive
  - strategy: 'strategy-2'
    policy: rr_strict
    hash_key: cache_key
    go_direct: true
    groups:
      - *g1
      - *g2
    scheme: http
    failover:
      ring_mode: exhaust_ring
      response_codes:
        - 404
      markdown_codes:
        - 503
      health_check:
        - passive