Configuring the Cache¶

The Traffic Server Cache¶

The Traffic Server cache consists of a high-speed object database called the object store. The object store indexes objects according to URLs and associated headers. This enables Traffic Server to store, retrieve, and serve not only web pages, but also parts of web pages - which provides optimum bandwidth savings. Using sophisticated object management, the object store can cache alternate versions of the same object (versions may differ because of dissimilar language or encoding types). It can also efficiently store very small and very large documents, thereby minimizing wasted space. When the cache is full, Traffic Server removes stale data to ensure the most requested objects are kept readily available and fresh.

Traffic Server is designed to tolerate total disk failures on any of the cache disks. If the disk fails completely, then Traffic Server marks the entire disk as corrupt and continues using the remaining disks. An alarm is then created to indicate which disk failed. If all of the cache disks fail, then Traffic Server goes into proxy-only mode.

You can perform the following cache configuration tasks:

• Change the total amount of disk space allocated to the cache: refer to Changing Cache Capacity.
• Partition the cache by reserving cache disk space for specific protocols and origin servers/domains: refer to Partitioning the Cache.
• Delete all data in the cache: refer to Clearing the Cache.
• Override cache directives for a requested domain name, regex on a url, hostname or ip, with extra filters for time, port, method of the request (and more). ATS can be configured to never cache; always cache; ignore no-cache directives, etc. These are configured in cache.config.

The RAM Cache¶

Traffic Server maintains a small RAM cache of extremely popular objects. This RAM cache serves the most popular objects as quickly as possible and reduces load on disks, especially during temporary traffic peaks. You can configure the RAM cache size to suit your needs, as described in Changing the Size of the RAM Cache below.

The RAM cache supports two cache eviction algorithms, a regular LRU (Least Recently Used) and the more advanced CLFUS (Clocked Least Frequently Used by Size, which balances recentness, frequency and size to maximize hit rate – similar to a most frequently used algorithm). The default is to use CLFUS, and this is controlled via proxy.config.cache.ram_cache.algorithm.

Both the LRU and CLFUS RAM caches support a configuration to increase scan resistance. In a typical LRU, if you request all possible objects in sequence, you will effectively churn the cache on every request. The option proxy.config.cache.ram_cache.use_seen_filter can be set to add some resistance against this problem.

In addition, CLFUS also supports compressing in the RAM cache itself. This can be useful for content which is not compressed by itself (e.g. images). This should not be confused with Content-Encoding: gzip, this feature is only thereto save space internally in the RAM cache itself. As such, it is completely transparent to the User-Agent. The RAM cache compression is enabled with the option proxy.config.cache.ram_cache.compress. The default is 0, which means no compression. Other possible values are 1 for fastlz, 2 for libz and 3 for liblzma.

Changing the Size of the RAM Cache¶

Traffic Server provides a dedicated RAM cache for fast retrieval of popular small objects. The default RAM cache size is automatically calculated based on the number and size of the cache partitions you have configured. If you’ve partitioned your cache according to protocol and/or hosts, then the size of the RAM cache for each partition is proportional to the size of that partition.

You can increase the RAM cache size for better cache hit performance. However, if you increase the size of the RAM cache and observe a decrease in performance (such as increased latencies), then it’s possible that the operating system requires more memory for network resources. In such instances, you should return the RAM cache size to its previous value.

To change the RAM cache size:

1. Stop Traffic Server.
2. Set the variable proxy.config.cache.ram_cache.size to specify the size of the RAM cache. The default value of -1 means that the RAM cache is automatically sized at approximately 1MB per gigabyte of disk.
3. Restart Traffic Server. If you increase the RAM cache to a size of 1GB or more, then restart with the trafficserver command (refer to Start Traffic Server).

Changing Cache Capacity¶

You can increase or reduce the total amount of disk space allocated to the cache without clearing the content. To check the size of the cache (in bytes), enter the command traffic_line -r proxy.process.cache.bytes_total.

Partitioning the Cache¶

You can manage your cache space more efficiently and restrict disk usage by creating cache volumes with different sizes for specific protocols. You can further configure these volumes to store data from specific origin servers and/or domains. The volume configuration must be the same on all nodes in a cluster.

Configuring the Cache Object Size Limit¶

By default, Traffic Server allows objects of any size to be cached. You can change the default behavior and specify a size limit for objects in the cache via the steps below:

1. Set proxy.config.cache.max_doc_size to specify the maximum size allowed for objects in the cache in bytes. 0 (zero) if you do not want a size limit.
2. Run the command traffic_line -x to apply the configuration changes.

Clearing the Cache¶

When you clear the cache, you remove all data from the entire cache - including data in the host database. You should clear the cache before performing certain cache configuration tasks, such as partitioning. You cannot clear the cache when Traffic Server is running.

To clear the cache:

1. Stop Traffic Server (refer to Stopping Traffic Server)

2. Enter the following command to clear the cache:

   traffic_server -Cclear
   

   The clear command deletes all data in the object store and the host database. Traffic Server does not prompt you to confirm the deletion.

3. Restart Traffic Server (refer to Starting Traffic Server).

Removing an Object From the Cache¶

Traffic Server accepts the custom HTTP request method PURGE when removing a specific object from cache. If the object is found in the cache and is successfully removed, then Traffic Server responds with a 200 OK HTTP message; otherwise, a 404 File Not Found message is returned.

In the following example, Traffic Server is running on the domain example.com and you want to remove the image remove_me.jpg from cache. Because by default we do not permit PURGE requests from any other IP, we connect to the daemon via localhost:

$ curl -X PURGE -H 'Host: example.com' -v "http://localhost/remove_me.jpg"
* About to connect() to localhost port 80 (#0)
* Trying 127.0.0.1... connected
* Connected to localhost (127.0.0.1) port 80 (#0)

> PURGE /remove_me.jpg HTTP/1.1
> User-Agent: curl/7.19.7
> Host: example.com
> Accept: */*
>
< HTTP/1.1 200 Ok
< Date: Thu, 08 Jan 2010 20:32:07 GMT
< Connection: keep-alive

The next time Traffic Server receives a request for the removed object, it will contact the origin server to retrieve it (i.e., it has been purged from the Traffic Server cache).

Note: The procedure above only removes an object from a specific Traffic Server cache. Users may still see the old (removed) content if it was cached by intermediary caches or by the end-users’ web browser.

Inspecting the Cache¶

Traffic Server provides a Cache Inspector utility that enables you to view, delete, and invalidate URLs in the cache (HTTP only). The Cache Inspector utility is a powerful tool that’s capable of deleting all the objects in your cache; therefore, make sure that only authorized administrators are allowed to access this utility, see Controlling Client Access to the Proxy Cache and the @src_ip option in remap.config.