Locking In CFML

Locking in CFML

- Why

- How

- What

- When}to lock?

Understand Locking

Locking in CFML

Agenda The problem

Critical ressources

Shared Scope Variables

<CFLOCK>

Nested locks

Locks and pointers

Name Locks

Lock Administration Client Variables *

Restrict number of simultaneous requests *

* page not in CF-Europe presentation* page not in CF-Europe presentation

Q & A *

Links *

Locking in CFML

Symptoms of a locking problem

- unexplained “losses” of session or application variables

- server crashes

- CF-Server consuming more and more RAM

- slow applications

Allthough these symptoms do not indicate that there MUST be locking issues, wrong locking (or no locking at all) is one of the most likely causes for the mentioned problems.

Allthough these symptoms do not indicate that there MUST be locking issues, wrong locking (or no locking at all) is one of the most likely causes for the mentioned problems.

Locking in CFML

MultithreadingThe ability of a program to perform multiple tasks at the same time.

Exanple: eMail client

Read messages and download new messages from the server at the same time

Locking in CFML

MultithreadingAdvantages- performance / saves time

- (system-) security

Drawbacks- programs are more

complicated to write

- not easy to implement

Locking in CFML

Multithreading in ColdFusion

ColdFusion can handle multiple requests at the same time

Every request is assigned to a thread

Additional requests will be queued

Within a thread the request is serialized

Number of Worker Threads can be set in CF-Administrator

Locking in CFML

Critical Ressources

Shared Scope Variables

Files

Component objects (COM, CORBA, Java)

All ressources that could cause problems or loose performance if they are used by more than one client at the same time

Concurre

nt Acc

ess

Locking in CFML

Shared Scope Variables

Server-Scope

Application-Scope

Session-Scope

Available for EVERY client of EVERY application on that server

Available for EVERY client of ONE SINGLE application

Available for ONE SINGLE client of ONE SINGLE application

Frames, multiple submits, reload/redirection, etc. can cause concurrent access with session variables!

Frames, multiple submits, reload/redirection, etc. can cause concurrent access with session variables!

Locking in CFML

<CFLOCK>

Identification

Type

Error Handling

NAME or SCOPE

TYPE=“Exclusive”vs.

TYPE=“ReadOnly”

TIMEOUT and THROWONTIMEOUT

Category Attributes

Locking in CFML

<CFLOCK> and Shared Scope Variables

Lock EVERY SINGLE access

Lock the entire scope

use the correct locking type

lock only what needs to be locked

Explained on pages “pointers and structures” and Q&A!

Explained on pages “pointers and structures” and Q&A!

Locking in CFML

Example: store query recordset in application variable

Wrong:<CFLOCK scope="Application" Timeout="10" type="Exclusive"> <CFQUERY datasource="#DSN#" name="application.customers"> SELECT * FROM tblCustomers </CFQUERY></CFLOCK>

Better:<CFQUERY datasource="#DSN#" name=“customers"> SELECT * FROM tblCustomers</CFQUERY><CFLOCK scope="Application" Timeout="10" type="Exclusive"> <CFSET application.customers = customers></CFLOCK>

Locking in CFML

Client Variables and Locking

client variables are NOT stored in server RAM, but in DB, Registry oder Cookies

operating system or DB engine will take care of concurrent access we do not need to lock client variables with ColdFusion

Locking in CFML

Nested Locks Deadlocks possible

<CFLOCK scope="Application" Timeout="10" type="ReadOnly"> <CFLOCK scope="Application" Timeout="10" type="Exclusive"> Code... </CFLOCK></CFLOCK>

Template 1:<CFLOCK scope="Application" Timeout="10" type="Exclusive">

<CFLOCK scope="Session" Timeout="10" type="Exclusive">Code...

</CFLOCK></CFLOCK>

Template 2:<CFLOCK scope="Session" Timeout="10" type="Exclusive"> <CFLOCK scope="Application" Timeout="10" type="Exclusive"> Code... </CFLOCK></CFLOCK>

Only nested locks make deadlocks possible!

Only nested locks make deadlocks possible!

Consider what might happen if those templates are executed at the same instant!

Consider what might happen if those templates are executed at the same instant!

Locking in CFML

Avoiding Nested Locks<CFLOCK scope="Application" Timeout="10" type="Exclusive">

<CFLOCK scope="Session" Timeout="10" type="Exclusive"> <CFSET session.DSN = application.DSN> <CFSET application.bgcolor = session.bgcolor>

</CFLOCK></CFLOCK>

<CFLOCK scope="Application" Timeout="10" type="ReadOnly"> <CFSET dsn = application.DSN></CFLOCK> <CFLOCK scope="Session" Timeout="10" type="ReadOnly"> <CFSET bgcolor = session.bgcolor></CFLOCK> <CFLOCK scope="Application" Timeout="10" type="Exclusive">     <CFSET application.bgcolor = bgcolor></CFLOCK> <CFLOCK scope="Session" Timeout="10" type="Exclusive"> <CFSET session.DSN = DSN></CFLOCK>

Same result without nested locks:

Locking in CFML

Nested LocksAvoid nested locks if at all possible

(performance issues, danger of deadlocks)

1. session scope

If you can’t avoid nesting, always lock in the following order:

2. application scope

3. server scope

“Local out” approach

Locking in CFML

Locking of Pointers

Pointer: points to a structure (is NOT a real copy!)

<CFSET application.userData = session>

Changing the pointer also changes initial structure.

<CFSET myPointer = myStruct>

Shared scope variables are structures!

<CFLOCK scope="Application" Timeout="10" type="Exclusive"> <CFSET application.userData = session> </CFLOCK>

<CFLOCK scope="Session" Timeout="10" type="ReadOnly"> <CFLOCK scope="Application" Timeout="10" type="Exclusive"> <CFSET application.userData = session> </CFLOCK> </CFLOCK>

<CFLOCK scope="Application" Timeout="10" type="ReadOnly"> <CFSET temp = application.userData.bgcolor> </CFLOCK> <CFSET temp = application.userData.bgcolor>

<CFLOCK scope="Session" Timeout="10" type="ReadOnly"> <CFLOCK scope="Application" Timeout="10" type="ReadOnly"> <CFSET temp = application.userData.bgcolor> </CFLOCK> </CFLOCK>

<CFSET application.userData.bgcolor = "##EEEEEE"> <CFLOCK scope="Application" Timeout="10" type="Exclusive"> <CFSET application.userData.bgcolor = "##EEEEEE"> </CFLOCK>

<CFLOCK scope="Session" Timeout="10" type="Exclusive"> <CFLOCK scope="Application" Timeout="10" type="Exclusive"> <CFSET application.userData.bgcolor = "##EEEEEE"> </CFLOCK> </CFLOCK>

<CFSET application.myVar = “123“> does not only access the key “myVar”, but accesses the structure “applciation”. So lock the entire scope! Always!!

<CFSET application.myVar = “123“> does not only access the key “myVar”, but accesses the structure “applciation”. So lock the entire scope! Always!!

Locking in CFML

Name LocksName idetifies the lock and denies access toprotected ressource for all locks with the

samenameUse it for all critical ressources except sharedscope variables, e.g.

Verity

<CFFILE>

COM-Objects

Recommended: use a naming convention!

Locking in CFML

Limit the number of simultaneous requests

Some ressources will dramatically loose performance if they receive too

many simultaneous requests. Some others (e.g. some FTP servers) will only

accept a certain number of simultaneous requests from the same client.

Name locks can be used to limit the number of simulatneous requests to

a certain ressource, even if concurrent access is not a problem.

<CFSET myLockName = "Lock_FTP_MyServer_" & RandRange(1,3)><CFLOCK name=“#myLockName#" timeout="10" type=“Exclusive"> <CFFTP . . .></CFLOCK>

You need to create a certain number of possible lock names, for example

using the functions for random numbers:

Because only three different lock names are possible, there will never be

more than three simultaneous requests from CF to FTP server.

Locking in CFML

Lock-Administration

Single Threaded SessionsAll thread with the same sessionID are serialized

concurrent access with session variables impossible

performance drawbacks (e.g. with frames) template timeouts more likely to

occur

Locking in CFML

Lock-Administration

Variable Scope Lock Settings

No automatic checking or locking

Full checking

Automatic read locking

Locking in CFML

Lock-Administration

Variable Scope Lock SettingsNo automatic checking or locking

developer is responsible for proper locking

good performance but dangerous

Use this setting for production servers

with TESTED applications

Locking in CFML

Lock-Administration

Variable Scope Lock SettingsFull checking

every unlocked access throws an exception

secure, but small performance drawbacks

use it for development servers

for shared Servern

Name Locks also throw exceptions

Bug warning!

Not locking IsDefined() when using shared scope variables will NOT cause an exception! But, IsDefined(“shared_scope_var”) MUST be locked, too!!

Bug warning!

Not locking IsDefined() when using shared scope variables will NOT cause an exception! But, IsDefined(“shared_scope_var”) MUST be locked, too!!

Locking in CFML

Lock-Administration

Variable Scope Lock SettingsAutomatic read locking

every read access is automatically locked

quite secure, but has serious performance drawbacks useful if you need to add locks to an older

application

write acesses must be locked manually

Locking in CFML

Summary

1. Lock EVERY access

2. If possible sum up accesses in a single lock, but,

3. Lock only what needs to be locked

4. For Shared Scope Variables always use the SCOPE attribute

5. Use the correct locking type

6. Avoid server scope on shared servers

Locking in CFML

Summary (continued)

8. THROWONERROR=“Yes” ist useful, but, you need to catch exceptions with CFTRY/CFCATCH

9. Avoid pointers between different scopes vermeiden. Better use StructCopy() oder Duplicate().

10. If pointer can not be avoided: lock both scopes.

11. For production servers use “No automatic checking or

locking” setting (with TESTED applications only!)12. For development server use “Full checking” setting

7. Avoid nested locks; if you need to nest locks, use local out approach

Locking in CFML

Q & A<CFLOCATION URL=“page.cfm“ ADDTOKEN=“Yes“> appends session.URLToken to the URL. Don‘t I have to lock that kind of access to shared scope variables, too?

ADDTOKEN=“YES” does append CLIENT.URLToken instead of SESSION.URLToken. In the documentation of CFLOCATION only one small remark reveals the difference: “clientManagement must be enabled”. So it is a client variable and we don’t have to lock those.

ADDTOKEN=“YES” does append CLIENT.URLToken instead of SESSION.URLToken. In the documentation of CFLOCATION only one small remark reveals the difference: “clientManagement must be enabled”. So it is a client variable and we don’t have to lock those.

Why do I have to lock the entire scope when using shared scope variables? Shouldn‘t it be sufficient to lock the single variable I try to access?

When accessing a shared scope variable, we always access the structure as a whole. There‘s no use in locking a single element, if the bigger context can still be compromised. You should ALWAYS lock the entire scope.

Due to security restrictions you can not use structure functions with the server scope. (<CFLOOP collection=“#server#“ item=“key“> will not work either). But still it is a structure and must be locked entirely when using a server variable.

When accessing a shared scope variable, we always access the structure as a whole. There‘s no use in locking a single element, if the bigger context can still be compromised. You should ALWAYS lock the entire scope.

Due to security restrictions you can not use structure functions with the server scope. (<CFLOOP collection=“#server#“ item=“key“> will not work either). But still it is a structure and must be locked entirely when using a server variable.

Locking in CFML

Q & AWhy do I have to lock EVERY single access to shared scope variables? One single unlocked access surely will not harm the locked variables...

WRONG! It is very important to understand how locks work. A lock only prevents simulatneous accesses from within other locks with the same NAME or SCOPE attribute. If you lock the application scope, it is not protected against unlocked access, but only against access from within other locks with SCOPE=“application”.

That’s why name locks are a bit difficult to use. If two locks have different names, they still can access the same ressource simultaneously and they can still cause concurrent access problems. Using a naming convention for locks can help you to manage lock names more easily.

WRONG! It is very important to understand how locks work. A lock only prevents simulatneous accesses from within other locks with the same NAME or SCOPE attribute. If you lock the application scope, it is not protected against unlocked access, but only against access from within other locks with SCOPE=“application”.

That’s why name locks are a bit difficult to use. If two locks have different names, they still can access the same ressource simultaneously and they can still cause concurrent access problems. Using a naming convention for locks can help you to manage lock names more easily.

What kind of naming convention should I use for name locks?

Actually that doesn‘t matter at all, as long as you follow your convention strictly. For example a lock name could be “Lock” + protected ressource, e.g.

“Lock_files” or “Lock_ftp”.

Actually that doesn‘t matter at all, as long as you follow your convention strictly. For example a lock name could be “Lock” + protected ressource, e.g.

“Lock_files” or “Lock_ftp”.

Locking in CFML

Links

Best practices:http://www.macromedia.com/v1/Handlers/index.cfm?ID=20370&Method=Full

A comprehensive guide:http://www.depressedpress.com/DepressedPress/Content/ColdFusion/Guides /Locking/Index.cfm

BF on CF: Lock it or loose it!http://www.sys-con.com/coldfusion/article.cfm?id=135

Locking in CFML

Still got questions?

Christoph SchmitzeMail: info@procept.net

Latest version of this presentation is available for download athttp://www.procept.net