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By Michael Yoon, Jon Salz and Lars Pind.
-</authorblurb> - -+
When using AOLserver, remember that there are effectively two types of global namespace, not one: -
- -Server-global: As you'd expect, there is +
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
@@ -25,60 +15,34 @@
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.
+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 +
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 behaviour, however, "script-global" is a more -appropriate term.
- -+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
-asychronously, in its own thread. It almost always seems like a good idea to
+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
+
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 +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 +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
@@ -95,14 +59,7 @@
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
@@ -111,30 +68,17 @@
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 +
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
- -+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 \ @@ -147,16 +91,11 @@ 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 $last_name" @@ -166,33 +105,21 @@ return [array get user_info] } -- - -Using Arrays and Pass-By-Reference
- -+
Using Arrays and Pass-By-Reference
Sometimes pass-by-value incurs too much overhead, and you'd rather pass-by-reference. Specifically, if you're writing a proc that uses arrays internally to build up some value, there are many entries in the array, and you're planning on iterating over the proc many times. In this case, pass-by-value is expensive, and you'd use pass-by-reference. -
- -The transformation of the array into a list and back to an +
- -The transformation of the array into a list and back to an array takes, in our test environment, approximately 10 microseconds per entry of 100 character's length. Thus you can process about 100 entries per millisecond. The time depends almost completely on the number of entries, and -almost not at all on the size of the entries.
+almost not at all on the size of the entries.
You implement pass-by-reference in Tcl by taking the name of an array
as an argument and upvar it.
-
-- ad_proc ad_get_user_info { -array:required user_id @@ -209,38 +136,23 @@ doc_body_append "$user_info(namelink) ($user_info(emaillink))" -- - -We prefer pass-by-value over pass-by-reference. Pass-by-reference makes +
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
upvars through several layers of the call stack, you'll have
-a hard time debugging.
Multisets: Using ns_sets and Pass-By-Reference
+a hard time debugging.
Multisets: Using ns_sets 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, +
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_sets, since they
+exact properties.
You always use pass-by-reference with ns_sets, 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 @@ -257,22 +169,14 @@ 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:
-Consider for example a loop over the entries in a
- - - -ns_setas -compared to an array:- # ns_set variant set size [ns_set size $myset] for { set i 0 } { $i < $size } { incr i } { @@ -284,15 +188,10 @@ puts "$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 @@ -305,18 +204,10 @@ baz $baz ] -- -+
ns_sets are designed to be lightweight, so memory consumption
should not be a problem. However, when using ns_set get to
perform lookup by name, they perform a linear lookup, whereas arrays use a
hash table, so ns_sets are slower than arrays when the number of
entries is large.
-
($Id$)
- -