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Pros of Enhanced AGI
It has the simplicity of process-based AGI, with the addition of a simple read-
only stream of the channel’s audio. This is the only variant that offers this feature.
Cons of Enhanced AGI
Since a new process must be spawned to run your application for every call, it has
the same efficiency concerns as regular, process-based AGI.
For an alternative way of gaining access to the audio outside
Asterisk, consider using JACK. Asterisk has a module for
JACK integration, called app_jack. It provides the JACK()
dialplan application and the JACK_HOOK() dialplan function.
FastAGI—AGI over TCP
FastAGI is the term used for AGI call control over a TCP connection. With process-
based AGI, an instance of an AGI application is executed on the system for every call,
and communication with that application is done over stdin and stdout. With
FastAGI, a TCP connection is made to a FastAGI server. Call control is done using
the same AGI protocol, but the communication is over the TCP connection and does
not require a new process to be started for every call. The AGI protocol is discussed
in more detail in “AGI Communication Overview” on page 319. Using FastAGI is
much more scalable than process-based AGI, though it is also more complex to
implement.
To use FastAGI you invoke the AGI() application in the Asterisk dialplan, but instead
of providing the name of the application to execute, you provide an agi:// URL. For
example:
exten => 238,1,AGI(agi://127.0.0.1)
AGI Variants
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The default port number for a FastAGI connection is 4573. A different port number
can be appended to the URL after a colon. For example:
exten => 238,1,AGI(agi://127.0.0.1:4574)
Just as with process-based AGI, arguments can be passed to a FastAGI application. To
do so, add them as additional arguments to the AGI() application, delimited by
commas:
exten => 238,1,AGI(agi://192.168.1.199,arg1,arg2,arg3)
FastAGI also supports the usage of DNS SRV records, if you provide a URL in the
form of hagi://. By using SRV records, your DNS servers can return multiple hosts
that Asterisk can attempt to connect to. This can be used for high availability and
load balancing. In the following example, to find a FastAGI server to connect to,
Asterisk will perform a DNS lookup for _agi._tcp.shifteight.org:
exten => 238,1,AGI(hagi://shifteight.org)
In this example, the DNS servers for the shifteight.org domain would need at least
one SRV record configured for _agi._tcp.shifteight.org.
Pros of FastAGI
It’s more efficient than process-based AGI. Rather than spawning a process per
call, a FastAGI server can be built to handle many calls.
DNS can be used to achieve high availability and load balancing among FastAGI
servers to further enhance scalability.
Cons of FastAGI
It is more complex to implement a FastAGI server than to implement a process-
based AGI application.
Async AGI—AMI-Controlled AGI
Async AGI allows an application that uses the Asterisk Manager Interface (AMI) to
asynchronously queue up AGI commands to be executed on a channel. This can be
especially useful if you are already making extensive use of the AMI and would like to
enhance your application to handle call control, rather than writing a detailed Aster‐
isk dialplan or developing a separate FastAGI server.
More information on the Asterisk Manager Interface can be found
in Chapter 17.
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Chapter 18: Asterisk Gateway Interface
Async AGI is invoked by the AGI() application in the Asterisk dialplan. The argu‐
ment to AGI() should be agi:async, as shown in the following example:
exten => 239,AGI(agi:async)
Additional information on how to use async AGI over the AMI can be found in the
next section.
Pros of async AGI
An existing AMI application can be used to control calls using AGI commands.