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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
SEE ALSO¶
eval(n)
KEYWORDS¶
threads, interpreters, proxy, initialization