Locking In CFML

1. Locking In CFML

2. Locking in CFML } • Understand Locking - Why - What to lock? - How - When

3. Locking in CFML • The problem • Locks and pointers • Agenda • Critical ressources • Name Locks • Shared Scope Variables • Restrict number of simultaneous requests * • Client Variables * • Lock Administration • <CFLOCK> • Q & A * • Nested locks • Links * * page not in CF-Europe presentation

4. Locking in CFML • Symptoms of a locking problem - slow applications • unexplained “losses” of session • or application variables • CF-Server consuming more • and more RAM 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. - server crashes

5. Locking in CFML The ability of a program to perform multiple tasks at the same time. • Multithreading Exanple: eMail client Read messages and download new messages from the server at the same time

6. Locking in CFML Advantages • Multithreading - performance / saves time - (system-) security Drawbacks • programs are more • complicated to write - not easy to implement

7. Locking in CFML ColdFusion can handle multiple requests at the same time • Multithreading in ColdFusion Every request is assigned to a thread Number of Worker Threads can be set in CF-Administrator Additional requests will be queued Within a thread the request is serialized

8. Locking in CFML • Critical Ressources • Shared Scope Variables Concurrent Access • 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

9. Locking in CFML • Shared Scope Variables • Server-Scope Available for EVERY client of EVERY application on that server • Application-Scope Available for EVERY client of ONE SINGLE application • Session-Scope Available for ONE SINGLE client of ONE SINGLE application Frames, multiple submits, reload/redirection, etc. can cause concurrent access with session variables!

10. Locking in CFML • <CFLOCK> Category Attributes Identification NAME or SCOPE Type TYPE=“Exclusive” vs. TYPE=“ReadOnly” Error Handling TIMEOUT and THROWONTIMEOUT

11. Locking in CFML • <CFLOCK> and Shared Scope Variables • Lock EVERY SINGLE access Explained on pages “pointers and structures” and Q&A! • Lock the entire scope • use the correct locking type • lock only what needs to be locked

12. 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>

13. 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

14. Locking in CFML Only nested locks make deadlocks possible!  Deadlocks possible • Nested Locks Template 1: <CFLOCK scope="Application"Timeout="10"type="Exclusive"> <CFLOCK scope="Session"Timeout="10"type="Exclusive"> Code... </CFLOCK> </CFLOCK> <CFLOCK scope="Application"Timeout="10"type="ReadOnly"> <CFLOCK scope="Application"Timeout="10"type="Exclusive"> Code... </CFLOCK> </CFLOCK> Consider what might happen if those templates are executed at the same instant! Template 2: <CFLOCK scope="Session"Timeout="10"type="Exclusive"> <CFLOCK scope="Application"Timeout="10"type="Exclusive"> Code... </CFLOCK> </CFLOCK>

15. Locking in CFML <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> • Avoiding Nested Locks Same result without nested locks: <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>

16. Locking in CFML • Avoid nested locks if at all possible (performance issues, danger of deadlocks) • Nested Locks • If you can’t avoid nesting, always lock in the following order: 1. session scope 2. application scope 3. server scope  “Local out” approach

17. Locking in CFML <CFSET myPointer = myStruct> Pointer: points to a structure (is NOT a real copy!) • Locking of Pointers Changing the pointer also changes initial structure. Shared scope variables are structures! <CFSET application.myVar = “123“> does not only access the key “myVar”, but accesses the structure “applciation”. So lock the entire scope! Always!! <CFSET application.userData = session> <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>

18. Locking in CFML Name idetifies the lock and denies access to protected ressource for all locks with the same name • Name Locks Use it for all critical ressources except shared scope variables, e.g. • Verity • <CFFILE> • COM-Objects Recommended: use a naming convention!

19. Locking in CFML 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. • Limit the number of simultaneous requests Name locks can be used to limit the number of simulatneous requests to a certain ressource, even if concurrent access is not a problem. You need to create a certain number of possible lock names, for example using the functions for random numbers: <CFSET myLockName = "Lock_FTP_MyServer_" & RandRange(1,3)> <CFLOCK name=“#myLockName#" timeout="10"type=“Exclusive"> <CFFTP . . .> </CFLOCK> Because only three different lock names are possible, there will never be more than three simultaneous requests from CF to FTP server.

20. Locking in CFML Single Threaded Sessions • Lock-Administration All 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

21. Locking in CFML Variable Scope Lock Settings • Lock-Administration • No automatic checking or locking • Full checking • Automatic read locking

22. Locking in CFML Variable Scope Lock Settings • Lock-Administration • No automatic checking or locking  developer is responsible for proper locking  good performance but dangerous • Use this setting for production servers with TESTED applications

23. Locking in CFML Bug warning! Not locking IsDefined() when using shared scope variables will NOT cause an exception! But, IsDefined(“shared_scope_var”) MUST be locked, too!! Variable Scope Lock Settings • Lock-Administration • Full checking  every unlocked access throws an exception  Name Locks also throw exceptions  secure, but small performance drawbacks  use it for development servers  for shared Servern

24. Locking in CFML Variable Scope Lock Settings • Lock-Administration • Automatic read locking  every read access is automatically locked  write acesses must be locked manually  quite secure, but has serious performance drawbacks • useful if you need to add locks to an older application

25. 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

26. Locking in CFML 7. Avoid nested locks; if you need to nest locks, use local out approach • 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

27. Locking in CFML <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? • Q & A 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.

28. Locking in CFML Why do I have to lock EVERY single access to shared scope variables? One single unlocked access surely will not harm the locked variables... • Q & A 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”.

29. 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

30. Locking in CFML • Still got questions? Christoph Schmitz eMail: info@procept.net Latest version of this presentation is available for download at http://www.procept.net