Introduction

Global data networking has become part of everyday life: Internet users request billions of documents and petabytes of data, on a daily basis, to and from all parts of the world. Information is free, abundant, and accessible. Unfortunately, global data networking can also be a nightmare for IT professionals as they struggle with overloaded servers and congested networks. It can be challenging to consistently and reliably accommodate society’s growing data demands.

Traffic Server is a high-performance web proxy cache that improves network efficiency and performance by caching frequently-accessed information at the edge of the network. This brings content physically closer to end users, while enabling faster delivery and reduced bandwidth use. Traffic Server is designed to improve content delivery for enterprises, Internet service providers (ISPs), backbone providers, and large intranets by maximizing existing and available bandwidth.

Traffic Server Deployment Options

To best suit your needs, Traffic Server can be deployed in several ways:

  • As a web proxy cache

  • As a reverse proxy

  • In a cache hierarchy

The following sections provide a summary of these Traffic Server deployment options.

Traffic Server as a Web Proxy Cache

As a web proxy cache, Traffic Server receives user requests for web content as those requests travel to the destined web server (origin server). If Traffic Server contains the requested content, then it serves the content directly. If the requested content is not available from cache, then Traffic Server acts as a proxy: it obtains the content from the origin server on the user’s behalf and also keeps a copy to satisfy future requests.

Traffic Server provides explicit proxy caching, in which the user’s client software must be configured to send requests directly to Traffic Server. Explicit proxy caching is described in the Explicit Proxy Caching chapter.

Traffic Server can also be employed as a transparent caching proxy server, in which the client software needs no special configuration or even knowledge of the proxy’s existence. This setup is described in the Transparent Proxying section.

Traffic Server as a Reverse Proxy

As a reverse proxy, Traffic Server is configured to be the origin server to which the user is trying to connect (typically, the origin server’s advertised hostname resolves to Traffic Server, which acts as the real origin server). The reverse proxy feature is also called server acceleration. Reverse proxy is described in more detail in Reverse Proxy and HTTP Redirects.

Traffic Server in a Cache Hierarchy

Traffic Server can participate in flexible cache hierarchies, in which Internet requests not fulfilled from one cache are routed to other regional caches, thereby leveraging the contents and proximity of nearby caches. In a hierarchy of proxy servers, Traffic Server can act either as a parent or a child cache to other Traffic Server systems or to similar caching products.

Traffic Server as a Load Balancer

Traffic Server can act as a layer 7 HTTP load balancer distributing requests across several servers. It can choose the next hop server using request attributes like the Host: header, URL attributes, scheme, method, and client IP address. It has a few selection strategies in place like weighted round robin, and URL consistent hashing.

Traffic Server Components

Traffic Server consists of several components that work together to form a web proxy cache you can easily monitor and configure.

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. Using sophisticated object management, the object store can cache alternate versions of the same object (perhaps in a different language or encoding type). It can also efficiently store very small and very large objects, thereby minimizing wasted space. When the cache is full, Traffic Server removes stale data to ensure that the most requested objects are 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 to use remaining disks. If all of the cache disks fail, then Traffic Server switches to proxy-only mode. You can partition the cache to reserve a certain amount of disk space for storing data for specific protocols and origin servers. For more information about the cache, see HTTP Proxy Caching.

The RAM Cache

Traffic Server maintains a small RAM cache that contains extremely popular objects. This RAM cache serves the most popular objects as fast as possible and reduces load on disks, especially during temporary traffic peaks. You can configure the RAM cache size to suit your needs. For detailed information, refer to Changing the Size of the RAM Cache.

The Host Database

The Traffic Server host database stores the domain name server (DNS) entries of origin servers to which Traffic Server connects to fulfill user requests. This information is used to adapt future protocol interactions and optimize performance. Along with other information, the host database tracks:

  • DNS information (for fast conversion of hostnames to IP addresses).

  • The HTTP version of each host (so advanced protocol features can be used with hosts running modern servers).

  • Host reliability and availability information (so users will not wait for servers that are not running).

The DNS Resolver

Traffic Server includes a fast, asynchronous DNS resolver to streamline conversion of hostnames to IP addresses. Traffic Server implements the DNS resolver natively by directly issuing DNS command packets rather than relying on slower, conventional resolver libraries. Since many DNS queries can be issued in parallel and a fast DNS cache maintains popular bindings in memory, DNS traffic is reduced.

Traffic Server Processes

Traffic Server contains two processes that work together to serve requests and manage, control, and monitor the health of the system.

  1. The traffic_server process is the transaction processing engine of Traffic Server. It is responsible for accepting connections, processing protocol requests, and serving documents from the cache or origin server.

  2. The traffic_manager process is the command and control facility of the Traffic Server, responsible for launching, monitoring, and reconfiguring the traffic_server process. The traffic_manager process is also responsible for the proxy autoconfiguration port, the statistics interface, and virtual IP failover.

    If the traffic_manager process detects a traffic_server process failure, it instantly restarts the process but also maintains a connection queue of all incoming requests. All incoming connections that arrive in the several seconds before full server restart are saved in the connection queue and processed in first-come, first-served order. This connection queueing shields users from any server restart downtime.

Administration Tools

Traffic Server offers the following administration options:

  • The traffic_ctl command-line interface is a text-based interface from which you can monitor Traffic Server performance and network traffic, as well as configure the Traffic Server system.

  • Various configuration files enable you to configure Traffic Server through a simple file-editing and signal-handling interface. Any changes you make through traffic_ctl are automatically made to the configuration files as well.

  • Finally, there is a clean C API which can be put to good use from a multitude of languages. The Traffic Server Admin Client demonstrates this for Perl.

Traffic Analysis Options

Traffic Server provides several options for network traffic analysis and monitoring:

  • traffic_ctl enables you to collect and process statistics obtained from network traffic information.

  • Transaction logging enables you to record information (in a log file) about every request Traffic Server receives and every error it detects. By analyzing the log files, you can determine how many clients used the Traffic Server cache, how much information each of them requested, and what pages were most popular. You can also see why a particular transaction was in error and what state the Traffic Server was in at a particular time. For example, you can see that Traffic Server was restarted.

    Traffic Server supports several standard log file formats, such as Squid and Netscape, and its own custom format. You can analyze the standard format log files with off-the-shelf analysis packages. To help with log file analysis, you can separate log files so that they contain information specific to protocol or hosts.

Traffic Server event and error logging, monitoring, and analysis is covered in greater detail in Monitoring.

Traffic Server Security Options

Traffic Server provides numerous options that enable you to establish secure communication between the Traffic Server system and other computers on the network. Using the security options, you can do the following:

  • Control client access to the Traffic Server proxy cache.

  • Configure Traffic Server to use multiple DNS servers to match your site’s security configuration. For example, Traffic Server can use different DNS servers, depending on whether it needs to resolve hostnames located inside or outside a firewall. This enables you to keep your internal network configuration secure while continuing to provide transparent access to external sites on the Internet.

  • Configure Traffic Server to verify that clients are authenticated before they can access content from the Traffic Server cache.

  • Secure connections in reverse proxy mode between a client and Traffic Server, and Traffic Server and the origin server, using the SSL termination option.

  • Control access via SSL (Secure Sockets Layer).

Traffic Server security options are described in more detail in Security.

Tuning Traffic Server

Finally, this last chapter on Performance Tuning discusses the vast number of options that allow administrators to optimally tune Apache Traffic Server for maximum performance.