.. _hierarchical-caching: Hierarchical Caching ******************** .. Licensed to the Apache Software Foundation (ASF) under one or more contributor license agreements. See the NOTICE file distributed with this work for additional information regarding copyright ownership. The ASF licenses this file to you under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at http://www.apache.org/licenses/LICENSE-2.0 Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License. .. toctree:: :maxdepth: 2 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 one or more parent caches so that if one parent is unavailable, then another parent is availale to service requests. This is called `Parent Failover`_. Traffic Server will support parent caching for HTTP and HTTPS requests. **Note:** 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. Simply set parent proxy rules in :file:`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). .. figure:: ../static/images/admin/cachehrc.jpg :align: center :alt: Parent caching Parent caching **Note:** 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 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 file:`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: Configuring Traffic Server to Use a Parent Cache ------------------------------------------------ To configure Traffic Server to use one or more parent caches, you must complete the following steps: - Enable the parent caching option. - 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. .. note:: You need to configure the child cache only. No additional configuration is needed for the Traffic Server parent cache. Configure Traffic Server to use a parent cache by editing the following variable :ts:cv:`proxy.config.http.parent_proxy_routing_enable` in :file:`records.config` file. Edit the :file:`parent.config` file located in the Traffic Server ``config`` directory to set parent proxy rules to 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 :option:`traffic_line -x` to apply the configuration changes. .. XXX As of yet, this is unsupported. .. # ICP Peering # {#ICPPeering} .. 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 :file:`records.config` file: .. * :ts:cv:`proxy.config.icp.enabled` * :ts:cv:`proxy.config.icp.icp_port` * :ts:cv:`proxy.config.icp.multicast_enabled` * :ts:cv:`proxy.config.icp.query_timeout` .. Edit :file:`icp.config` file located in the Traffic Server `config` directory: For each ICP peer you want to identify, enter a separate rule in the :file:`icp.config` file. .. Run the command :option:`traffic_line -x` to apply the configuration changes.