ColdFusion: looking at an issue Mingo had with ehcache and cachePut and cacheGet

 G'day:

Bloody Coldfusion. OK so why am I writing about a ColdFusion issue? Well about 80% of it is "not having a great deal else to do today", about 10% of being interested in this issue Mingo found. And 10% it being an excuse to mess around with Docker a bit. I am currently - and slowly - teaching myself about Docker, so there's some practise for me getting a ColdFusion instance up and running on this PC (which I no-longer have any type of CFML engine installed on).

OK so what's the issue. Mingo posted this on Twitter:

Just in case you wanna run that code, here it is for copy and paste:

foo = { bar = 1 };
cachePut( 'foobar', foo );

foo.bar = 2;

writeDump( cacheGet( 'foobar' ) );

Obviously (?) what one would expect here is {bar:1}. What gets put into cache would be a copy of the struct right?

Well... um... here goes...

/opt/coldfusion/cfusion/bin $ ./cf.sh
ColdFusion started in interactive mode. Type 'q' to quit.
cf-cli>foo = { bar = 1 };
struct
BAR: 1

cf-cli>cachePut( 'foobar', foo );
cf-cli>foo.bar = 2;
2
cf-cli>writeDump( cacheGet( 'foobar' ) );
struct
BAR: 2

cf-cli>

... errr... what?

It looks like ColdFusion is like only putting a reference to the struct into cache. So any code changing the data in the struct is changing it in CFML as well as changing it in cache. This does not seem right.

I did a quick google and found a ticket in Adobe's system about this: CF-3989480 - cacheGet returns values by reference. The important bit to note is that it's closed with

Not a great explanation from Adobe there, fortunately Rob Bilson had commented further up with a link to a cached version of an old blog article of his, explaining what's going on: "Issue with Ehcache and ColdFusion Query Objects". It's a slightly different situation, but it's the same underlying "issue". Just to copy and paste the relevant bit from his article:

Update: It looks like this is actually expected behavior in Ehcache. Unfortunately, it's not documented in the ColdFusion documentation anywhere, but Ehcache actually has two configurable parameters (as of v. 2.10) called copyOnRead and copyOnWrite that determine whether values returned from the cache are by reference or copies of the original values. By default, items are returned by reference. Unfortunately we can't take advantage of these parameters right now as CF 9.0.1 implements Ehcache 2.0.

I decided to have a look what we could do about this on ColdFusion 2018, hoping that its embedded implementation of Ehcache has been updated since Rob wrote that in 2010.

Firstly I checked the Ehcache docs for these two settings: copyOnWrite and copyOnRead. This is straight forward (from "copyOnRead and copyOnWrite cache configuration"):

<cache name="copyCache"
    maxEntriesLocalHeap="10"
    eternal="false"
    timeToIdleSeconds="5"
    timeToLiveSeconds="10"
    copyOnRead="true"
    copyOnWrite="true">
  <persistence strategy="none"/>
  <copyStrategy class="com.company.ehcache.MyCopyStrategy"/>
</cache>

The docs also confirm these are off by default

Next where's the file?

/opt/coldfusion $ find . -name ehcache.xml
./cfusion/lib/ehcache.xml
/opt/coldfusion $

Cool. BTW I just guessed at that file name.

So in there we have this (way down at line 471):

<!--
Mandatory Default Cache configuration. These settings will be applied to caches
created programmtically using CacheManager.add(String cacheName).

The defaultCache has an implicit name "default" which is a reserved cache name.
-->
<defaultCache
    maxElementsInMemory="10000"
    eternal="false"
    timeToIdleSeconds="86400"
    timeToLiveSeconds="86400"
    overflowToDisk="false"
    diskSpoolBufferSizeMB="30"
    maxElementsOnDisk="10000000"
    diskPersistent="false"
    diskExpiryThreadIntervalSeconds="3600"
    memoryStoreEvictionPolicy="LRU"
    clearOnFlush="true"
    statistics="true"
/>

That looked promising, so I updated it to use copyOnWrite:

<defaultCache
    maxElementsInMemory="10000"
    eternal="false"
    timeToIdleSeconds="86400"
    timeToLiveSeconds="86400"
    overflowToDisk="false"
    diskSpoolBufferSizeMB="30"
    maxElementsOnDisk="10000000"
    diskPersistent="false"
    diskExpiryThreadIntervalSeconds="3600"
    memoryStoreEvictionPolicy="LRU"
    clearOnFlush="true"
    statistics="true"
    copyOnWrite="false"
/>

Whatever I put into cache, I want it to be decoupled from the code immediately, hence doing the copy-on-write.

I restarted CF and ran the code again:

/opt/coldfusion/cfusion/bin $ ./cf.sh
ColdFusion started in interactive mode. Type 'q' to quit.
cf-cli>foo = { bar = 1 };
struct

BAR: 1

cf-cli>cachePut( 'foobar', foo );
cf-cli>foo.bar = 2;
2

cf-cli>writeDump( cacheGet( 'foobar' ) );

struct
BAR: 1

cf-cli>

Yay! We are getting the "expected" result now: 1

Don't really have much else to say about this. I'm mulling over writing down what I did to get ColdFusion 2018 up and running via Docker instead of installing it. Let's see if I can be arsed...

Righto.

-- 

Adam