Ubuntu Manpage: ns_proxy - Proxy-process Tcl interface

Provided by: aolserver4-dev_4.5.1-18.1_amd64

NAME

       ns_proxy - Proxy-process Tcl interface

SYNOPSIS

       ns_proxy active pool
       ns_proxy cleanup
       ns_proxy config pool ?-opt val -opt val ...
       ns_proxy eval handle script ?timeout?
       ns_proxy get pool ?-handle n -timeout ms?
       ns_proxy ping handle
       ns_proxy release handle
       ns_proxy recv handle
       ns_proxy send handle script
       ns_proxy wait handle ?timeout?
_________________________________________________________________

DESCRIPTION

This command provides a simple, robust proxy mechanism to evaluate Tcl scripts in a
separate, pipe-connected process. This approach can be useful both to isolate potentially
thread-unsafe code outside the address space of a multithreaded process such as AOLserver
or to enable separation and timeout of potentially misbehaving, long running scripts.

The command is provided by the nsproxy dynamic library which can be loaded into an
interpreter via the Tcl load command, for example:

load /usr/local/lib/libnsproxy.so
ns_proxy ...

It can also possible to load the library into all interpreters of an AOLserver virutal
server by specifying an nsproxy.so entry in the server's module config entry, for example:

ns_section ns/server/server1/modules
ns_param nsproxy nsproxy.so

When loaded, the library adds the single ns_proxy command with takes multiple options as
described below. Proxies are normally created on demand when requested and connected to
the parent process via pipes used to send scripts and receive response. The processes
remain active until the parent process exits, effectively closing all pipes to the slave
processes.

ns_proxy active pool
Returns a list of all currently evaluating scripts in proxies for the given pool.
The output is a list which includes two elements, the string name of the proxy
handle and the string for the script being executed. It is also possible to view
the currently evaluating scripts with the Unix ps command as the proxy slave
process re-writes it's command argument space with the request script before
evaluation and clears it after sending the result.

ns_proxy cleanup
Releases any handles from any pools currently owned by a thread. This command is
intended to be used as part of a garbage collection step at the end of a
transaction. Calling this command within AOLserver is not necessary as the module
registers a trace to release all handles via the Ns_TclRegisterTrace facility when
interpreters are deallocated after a transaction, for example, at the end of a
connection.

ns_proxy config pool ?-opt val -opt val ...
Configures options for the pool specified by pool. The pool is created with
default options if it does not already exist. The result of ns_proxy config is a
list of the current options in the form -opt val -opt val .... Configurable
options include:

-init script
Specifies a script to evaluate when proxies are started. This can be used to load
additional libraries and/or source script files. The default is no script.

-reinit script
Specifies a script to evaluate after being allocated and before being returned to
the caller. This can be used to re-initalizes shared state. The default is no
script.

-min n
Sets the minimum number of proxy slave processes to pre-start before any
allocations. This defaults to 0 which results in on-demand start the first time
proxies are requested. Setting it to a higher number can be useful if
initialization takes a significant amount of time.

-max n
Sets the maximum number of proxy slave processes. Requests for proxies beyond the
maximum will result in requesting threads waiting for existing proxies to be
available instead of creating new proxy processes. Setting this value to 0
disables the pool, causing all subsequent allocation requests to fail immediately
(currently allocated proxies, if any, remain valid).

-exec program
Specifies the filename of a slave proxy program. This defaults to nsproxy in the
bin subdirectory of the AOLserver process. It is possible to create a custom
program and enter the proxy event loop with the Ns_ProxyMain application startup
routine; see the source code for details.

-getimeout ms
Specifies the maximum time to wait to allocate handles from the pool. The default
is 500 milliseconds, i.e., 1/2 of a second.

-evaltimeout ms
Specifies the maximum time to wait for a script to be evaluated in a proxy. This
parameter can be overridden on a per-call basis with the optional timeout parameter
to ns_proxy eval. The default is 100 milliseconds, i.e., 1/10 of a second which
assumes scripts are evaluated with minimal delay.

-sendtimeout ms

-recvtimeout ms
Specifies the maximium time to wait to send a script and receive a result from a
proxy. The default is 100 milliseconds, i.e., 1/10 of a second which assumes
minimal delay sending and receiving reasonably sized scripts and results over the
connecting pipe.

-waittimeout ms
Specifies the maximum time to wait for a proxy to exit. The wait is performed in a
dedicated reaper thread. The reaper will close the connection pipe and wait the
given timeout. If the timeout is exceeded, the reaper will send a SIGTERM signal
and finally a SIGKILL signal to ensure the process eventually exits. The default
is 100 milliseconds which should be ample time for a graceful exit unless the
process is hung executing a very long, misbehaving script, resulting in a more
disruptive SIGTERM or SIGKILL.

ns_proxy eval handle script ?timeout?
Evalutes script in the proxy specified by handle. The optional timeout argument
specifies a maximum number of milliseconds to wait for the command to complete
before raising an error (see ERROR HANDLING below for details on handling errors).

ns_proxy send handle script
Sends script in the proxy specified by handle. Unlike with ns_proxy eval, this
option will return immediately while the script continues to execute in the proxy
process. A later ns_proxy wait followed by an ns_proxy recv is expected.

ns_proxy wait handle ?timeout?
Waits for a script sent via ns_proxy send in the proxy specified by the handle
argument to complete. The optional timeout parameter specifies the number of
milliseconds to wait for the script to complete, the default is an indefinite wait.

ns_proxy recv handle
Receives a response from a script that was sent via ns_proxy send and waited on via
ns_proxy wait.

ns_proxy get pool ?-handle n -timeout ms? Returns one or
more handles to proxies from the specified pool. The pool will be created with
default options if it does not already exist. The optional -handle n arguments can
be used to specify the number of handles to allocate, the default being 1. The
optional -timeout ms arguments specifies the maximum amount of time in milliseconds
to wait for the handles to become availale before raising an error (see ERROR
HANDLING below for details on handling errors). Requesting more than one handle in
a single call if more than one handle is required is necessary as it is an error to
request handles from a pool from which handles are already owned in the thread.
This restriction is to avoid a possible deadlock condition and is similar to the
manner in which the ns_db gethandles command operates.

ns_proxy ping handle
This command sends a null request to the proxy specified by the handle argument.
The proxy will be verified alive and restarted if necessary. This command is not
normally required as the ns_proxy eval command will also verify and restart proxies
as needed.

ns_proxy release handle
This command can be used to release a single proxy specified by the handle
argument. All handles owned by a thread to the cooresponding pool must be returned
before any handles can be allocated again. Within AOLserver, a call to this
routine is recommended for clarity but not stricting necessary. As described
above, AOLserver installs a trace to release all handles at the end of every
connection during interprepter deallocation.

ERROR HANDLING

       Errors  generated  by a script evaluated in a proxy interpreter are completely returned to
       the calling interpreter, including mapping the errorInfo and  errorInfo  global  variables
       from  the  proxy to the parent if present and raising a Tcl exception.  This enables proxy
       code to look very similar to that which may use the internal eval command.

       Errors raised by a failure to communicate with the proxy  process  due  to  a  timeout  or
       unexpected  process  exit  are  also  communicated  back  to the parent interpreter as Tcl
       exceptions.  To distinguish between these cases,  communication  related  errors  set  the
       errorCode  global  variable  with the first element NSPROXY.  The second element is one of
       the following:

       EDeadlock
              The interpreter attempted to allocate handles from a pool  from  which  it  already
              owns one or more handles.

       EExec  The slave program specified by the -exec program option could not be started.

       ERange Insufficient handles available in pool.

       ERecv  There was an error receiving the result from the slave process.

       ESend  There was an error sending the script to the slave process.

       ETimeout
              The  timeout  specified  for the pool by the -evaltimeout option or as the optional
              argument to the current call to ns_proxy eval was exceeded.

EXAMPLES

       The following demonstrates sending a script to a remote proxy:

                set handle [ns_proxy get myproxy]
                ns_proxy eval $handle {info patchlevel}
                ns_proxy release $handle

       The following demonstrates an asyncronous request:

                set handle [ns_proxy get myproxy]
                ns_proxy send $handle {long running script}
                ... continue other work ...
                ns_proxy wait $handle
                set result [ns_proxy recv $handle]
                ns_proxy release $handle

       The following demonstrates using multiple proxies:

                ns_proxy config myproxy -max 10
                set handles [ns_proxy get myproxy -handle 10]
                foreach h $handles {
                   ns_proxy eval $h {puts "alive: [pid]"}
                }
                ns_proxy cleanup

KEYWORDS