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.

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).

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. Enable the parent caching option by adjusting proxy.config.http.parent_proxy_routing_enable in records.config.

    CONFIG proxy.config.http.parent_proxy_routing_enable INT 1
  2. 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.

  3. Edit parent.config to set parent proxy rules which will specify the parent cache to which you want missed requests to be forwarded.

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.

ICP Peering

The Internet Cache Protocol (ICP) is used by proxy caches to exchange information about their content. ICP query messages ask other caches if they are storing a particular URL; ICP response messages reply with a hit or miss answer. A cache exchanges ICP messages only with specific ICP peers, which are neighboring caches that can receive ICP messages. An ICP peer can be a sibling cache (which is at the same level in the hierarchy) or a parent cache (which is one level up in the hierarchy).

If Traffic Server has ICP caching enabled, then it sends ICP queries to its ICP peers when the HTTP request is a cache miss. If there are no hits but parents exist, then a parent is selected using a round-robin policy. If no ICP parents exist, then Traffic Server forwards the request to its HTTP parents. If there are no HTTP parent caches established, then Traffic Server forwards the request to the origin server.

If Traffic Server receives a hit message from an ICP peer, then Traffic Server sends the HTTP request to that peer. However, it might turn out to be a cache miss because the original HTTP request contains header information that is not communicated by the ICP query. For example, the hit might not be the requested alternate. If an ICP hit turns out to be a miss, then Traffic Server forwards the request to either its HTTP parent caches or to the origin server.

To configure a Traffic Server node to be part of an ICP cache hierarchy, you must perform the following tasks:

  • Determine if the Traffic Server can receive ICP messages only, or if it can send and receive ICP messages.
  • Determine if Traffic Server can send messages directly to each ICP peer or send a single message on a specified multicast channel.
  • Specify the port used for ICP messages.
  • Set the ICP query timeout.
  • Identify the ICP peers (siblings and parents) with which Traffic Server can communicate.

To configure Traffic Server to use an ICP cache hierarchy edit the following variables in records.config file:

Edit icp.config file located in the Traffic Server config directory: For each ICP peer you want to identify, enter a separate rule in the icp.config file.

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