Hierarchical Caching

Understanding Cache Hierarchies

A cache hierarchy consists of cache levels that communicate with each other. Traffic Server supports several types of cache hierarchies. All cache hierarchies recognize the concept of parent and child. A parent cache is a cache higher up in the hierarchy, to which Traffic Server can forward requests. A child cache is a cache for which Traffic Server is a parent.

Traffic Server supports the following hierarchical caching options:

Parent Caching

If a Traffic Server node cannot find a requested object in its cache, then it searches a parent cache (which itself can search other caches) before finally retrieving the object from the origin server. You can configure a Traffic Server node to use multiple parent caches so that if one parent is unavailable, the other parent caches will be checked in turn until either the request is serviced properly or no further parent caches are available and the origin server is contacted. This is called Parent Failover. Traffic Server supports parent caching for both HTTP and HTTPS requests.

If you do not want all requests to go to the parent cache, then simply configure Traffic Server to route certain requests (such as requests containing specific URLs) directly to the origin server. This may be achieved by setting parent proxy rules in parent.config.

The figure below illustrates a simple cache hierarchy with a Traffic Server node configured to use a parent cache. In the following scenario, a client sends a request to a Traffic Server node that is a child in the cache hierarchy (because it’s configured to forward missed requests to a parent cache). The request is a cache miss, so Traffic Server then forwards the request to the parent cache where it is a cache hit. The parent sends a copy of the content to the Traffic Server, where it is cached and then served to the client. Future requests for this content can now be served directly from the Traffic Server cache (until the data is stale or expired).

Parent caching

Parent caching

If the request is a cache miss on the parent, then the parent retrieves the content from the origin server (or from another cache, depending on the parent’s configuration). The parent caches the content and then sends a copy to Traffic Server (its child), where it is cached and served to the client.

Parent Selection Policies

There are several parent selection policies available to the Traffic Server child node.

  • Consistent Hash. The child node chooses a specific parent for each URI. This effectively makes the parent’s cache size the sum total of each of the individual parent cache sizes.

  • Round Robin. The child node cycles through the parents. The child can select the parent based on the client IP (“true”), or in a strict cycling (“strict”).

  • (Unnamed). The child node chooses the first live parent listed. There are two variants to this strategy. When the strategy is set to “false,” the child will use the first listed parent (for clarity called p1) until it is marked down, when the child will switch to the second listed parent (p2). Then, when the p1 is retested and found live, the child will restart using p1. When the selection strategy is set to “latched,” the child will use p1 until it is marked down, when it will switch to p2. When p1 is retested and found live, the child will continue using p2 until it is marked down.

Interaction with Remap.config

If remap rules are required (proxy.config.reverse_proxy.enabled), when a request comes in to a child node, its remap.config is evaluated before parent selection. This means that the client request is translated according to the remap rule, and therefore, any parent selection should be made against the remapped host name. This is true regardless of pristine host headers (proxy.config.url_remap.pristine_host_hdr) being enabled or not. The parent node will receive the translated request (and thus needs to be configured to accept it).

remap.config now also allows an alternative configuration that supports all the Parent Selection policies and failover mentioned here using a new remap.config tag, @strategy. This eliminates the need for a second lookup against the remapped host name required when using parent.config. See using NextHop strategies with remap.config, strategies.yaml


The client makes a request to Traffic Server for http://example.com. The origin server for the request is http://origin.example.com; the parent node is parent1.example.com, and the child node is configured as a reverse proxy.

If the child’s remap.config contains

map http://example.com http://origin.example.com

with the child’s parent.config containing

dest_domain=origin.example.com method=get parent="parent1.example.com:80 )

and parent cache (parent1.example.com) would need to have a remap.config line similar to

map http://origin.example.com http://origin.example.com

With this example, if parent1.example.com is down, the child node would automatically directly contact the origin.example.com on a cache miss.

Parent Failover

Traffic Server supports use of several parent caches. This ensures that if one parent cache is not available, another parent cache can service client requests.

When you configure your Traffic Server to use more than one parent cache, Traffic Server detects when a parent is not available and sends missed requests to another parent cache. If you specify more than two parent caches, then the order in which the parent caches are queried depends upon the parent proxy rules configured in the parent.config configuration file. By default, the parent caches are queried in the order they are listed in the configuration file.

Configuring Traffic Server to Use a Parent Cache

To configure Traffic Server to use one or more parent caches, you must perform the configuration adjustments detailed below.


You need to configure the child cache only. Assuming the parent nodes are configured to serve the child’s origin server, no additional configuration is needed for the nodes acting as Traffic Server parent caches.

  1. Identify the parent cache you want to use to service missed requests. To use parent failover, you must identify more than one parent cache so that when a parent cache is unavailable, requests are sent to another parent cache.

  2. Edit parent.config to set parent proxy rules which will specify the parent cache to which you want missed requests to be forwarded. Or as an alternative to parent.config, edit strategies.yaml to specify next hop parent proxy rules and hosts. Then in remap.config use the @strategy tag to select the next hop parent proxy rules. See strategies.yaml

The following example configures Traffic Server to route all requests containing the regular expression politics and the path /viewpoint directly to the origin server (bypassing any parent hierarchies):

url_regex=politics prefix=/viewpoint go_direct=true

The following example configures Traffic Server to direct all missed requests with URLs beginning with http://host1 to the parent cache parent1. If parent1 cannot serve the requests, then requests are forwarded to parent2. Because round-robin=true, Traffic Server goes through the parent cache list in a round-robin based on client IP address.:

dest_host=host1 scheme=http parent="parent1;parent2" round-robin=strict

Run the command traffic_ctl config reload to apply the configuration changes.