When using AOLserver, remember that there are effectively two types of global namespace, not one:
Server-global: As you'd expect, there is only one server-global namespace per server, and variables set within it can be accessed by any Tcl code running subsequently, in any of the server's threads. To set/get server-global variables, use AOLserver 3's
nsv
API (which supersedesns_share
from the pre-3.0 API).Script-global: Each Tcl script (ADP, Tcl page, registered proc, filter, etc.) executing within an AOLserver thread has its own global namespace. Any variable set in the top level of a script is, by definition, script-global, meaning that it is accessible only by subsequent code in the same script and only for the duration of the current script execution.
The Tcl built-in command global
accesses
script-global, not
server-global, variables from within a procedure. This distinction
is important to understand in order to use global
correctly when programming
AOLserver.
Also, AOLserver purges all script-global variables in a thread (i.e., Tcl interpreter) between HTTP requests. If it didn't, that would affect (and complicate) our use of script-global variables dramatically, which would then be better described as thread-global variables. Given AOLserver's behavior, however, "script-global" is a more appropriate term.
ns_schedule_proc
and
ad_schedule_proc
each take a
-thread
flag to cause a
scheduled procedure to run asynchronously, in its own thread. It
almost always seems like a good idea to specify this switch, but
there's a problem.
It turns out that whenever a task scheduled with ns_schedule_proc -thread
or ad_schedule_proc -thread t
is run,
AOLserver creates a brand new thread and a brand new interpreter,
and reinitializes the procedure table (essentially, loads all
procedures that were created during server initialization into the
new interpreter). This happens every
time the task is executed - and it is a very expensive
process that should not be taken lightly!
The moral: if you have a lightweight scheduled procedure which
runs frequently, don't use the -thread
switch.
Note also that thread is initialized with a copy of what was installed during server startup, so if the procedure table have changed since startup (e.g. using the APM watch facility), that will not be reflected in the scheduled thread.
The return
command in Tcl
returns control to the caller procedure. This definition allows
nested procedures to work properly. However, this definition also
means that nested procedures cannot use return
to end an entire thread. This
situation is most common in exception conditions that can be
triggered from inside a procedure e.g., a permission denied
exception. At this point, the procedure that detects invalid
permission wants to write an error message to the user, and
completely abort execution of the caller thread. return
doesn't work, because the
procedure may be nested several levels deep. We therefore use
ad_script_abort
to abort the remainder
of the thread. Note that using return
instead of ad_script_abort
may raise some security
issues: an attacker could call a page that performed some DML
statement, pass in some arguments, and get a permission denied
error -- but the DML statement would still be executed because the
thread was not stopped. Note that return -code return
can be used in
circumstances where the procedure will only be called from two
levels deep.
Many functions have a single return value. For instance,
util_email_valid_p
returns a number: 1
or 0. Other functions need to return a composite value. For
instance, consider a function that looks up a user's name and
email address, given an ID. One way to implement this is to return
a three-element list and document that the first element contains
the name, and the second contains the email address. The problem
with this technique is that, because Tcl does not support
constants, calling procedures that returns lists in this way
necessitates the use of magic numbers, e.g.:
set user_info [ad_get_user_info $user_id] set first_name [lindex $user_info 0] set email [lindex $user_info 1]
AOLserver/Tcl generally has three mechanisms that we like, for returning more than one value from a function. When to use which depends on the circumstances.
Using Arrays and Pass-By-Value
The one we generally prefer is returning an array
get
-formatted list. It has all the nice properties of
pass-by-value, and it uses Tcl arrays, which have good native
support.
ad_proc ad_get_user_info { user_id } { db_1row user_info { select first_names, last_name, email from users where user_id = :user_id } return [list \ name "$first_names $last_name" \ email $email \ namelink "<a href=\"/shared/community-member?user_id=[ns_urlencode $user_id]\">$first_names $last_name</a>" \ emaillink "<a href=\"mailto:$email\">$email</a>"] } array set user_info [ad_get_user_info $user_id] doc_body_append "$user_info(namelink) ($user_info(emaillink))"
You could also have done this by using an array internally and
using array get
:
ad_proc ad_get_user_info { user_id } { db_1row user_info { select first_names, last_name, email from users where user_id = :user_id } set user_info(name) "$first_names $last_name" set user_info(email) $email set user_info(namelink) "<a href=\"/shared/community-member?user_id=[ns_urlencode $user_id]\">$first_names $last_name</a>" set user_info(emaillink) "<a href=\"mailto:$email\">$email</a>" return [array get user_info] }
Using Arrays and Pass-By-Reference
Sometimes pass-by-value incurs too much overhead, and you'd rather pass-by-reference. Specifically, if you're writing a proc that uses arrays internally to build up some value, there are many entries in the array, and you're planning on iterating over the proc many times. In this case, pass-by-value is expensive, and you'd use pass-by-reference.
The transformation of the array into a list and back to an array takes, in our test environment, approximately 10 microseconds per entry of 100 character's length. Thus you can process about 100 entries per millisecond. The time depends almost completely on the number of entries, and almost not at all on the size of the entries.
You implement pass-by-reference in Tcl by taking the name of an
array as an argument and upvar
it.
ad_proc ad_get_user_info { -array:required user_id } { upvar $array user_info db_1row user_info { select first_names, last_name, email from users where user_id = :user_id } set user_info(name) "$first_names $last_name" set user_info(email) $email set user_info(namelink) "<a href=\"/shared/community-member?user_id=[ns_urlencode $user_id]\">$first_names $last_name</a>" set user_info(emaillink) "<a href=\"mailto:$email\">$email</a>" } ad_get_user_info -array user_info $user_id doc_body_append "$user_info(namelink) ($user_info(emaillink))"
We prefer pass-by-value over pass-by-reference.
Pass-by-reference makes the code harder to read and debug, because
changing a value in one place has side effects in other places.
Especially if have a chain of upvar
s through several layers of the call
stack, you'll have a hard time debugging.
Multisets: Using ns_set
s and
Pass-By-Reference
An array is a type of set, which means you can't have multiple entries with the same key. Data structures that can have multiple entries for the same key are known as a multiset or bag.
If your data can have multiple entries with the same key, you
should use the AOLserver built-in ns_set
. You can also
do a case-insensitive lookup on an ns_set
, something you can't easily do
on an array. This is especially useful for things like HTTP
headers, which happen to have these exact properties.
You always use pass-by-reference with ns_set
s, since they don't have any
built-in way of generating and reconstructing themselves from a
string representation. Instead, you pass the handle to the set.
ad_proc ad_get_user_info { -set:required user_id } { db_1row user_info { select first_names, last_name, email from users where user_id = :user_id } ns_set put $set name "$first_names $last_name" ns_set put $set email $email ns_set put $set namelink "<a href=\"/shared/community-member?user_id=[ns_urlencode $user_id]\">$first_names $last_name</a>" ns_set put $set emaillink "<a href=\"mailto:$email\">$email</a>" } set user_info [ns_set create] ad_get_user_info -set $user_info $user_id doc_body_append "[ns_set get $user_info namelink] ([ns_set get $user_info emaillink])"
We don't recommend ns_set
as a general mechanism for passing
sets (as opposed to multisets) of data. Not only do they inherently
use pass-by-reference, which we dis-like, they're also somewhat
clumsy to use, since Tcl doesn't have built-in syntactic
support for them.
Consider for example a loop over the entries in a ns_set
as compared to an array:
# ns_set variant set size [ns_set size $myset] for { set i 0 } { $i < $size } { incr i } { puts "[ns_set key $myset $i] = [ns_set value $myset $i]" } # array variant foreach name [array names myarray] { puts "$myarray($name) = $myarray($name)" }
And this example of constructing a value:
# ns_set variant set myset [ns_set create] ns_set put $myset foo $foo ns_set put $myset baz $baz return $myset # array variant return [list foo $foo baz $baz ]
ns_set
s are designed to be
lightweight, so memory consumption should not be a problem.
However, when using ns_set get
to perform lookup by name, they perform a linear lookup, whereas
arrays use a hash table, so ns_set
s are slower than arrays when the
number of entries is large.