The document discusses caching and synchronization of server data across multiple views and users in Flex applications. It proposes storing all fetched entities from the server in a single cache, binding views to the cache, and updating views when the server sends a notice that an entity has changed. When an entity is requested but not in the cache, a wrapper object is returned and the entity is queued to be fetched. The approach requires unique entity keys, abilities to get entities by key or keys, and support for versioned entities on the server-side.

1. Caching and
Synchronization in Flex
Zachary Pinter
Senior Developer
EffectiveUI
360Flex
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2. The Problem
How do we keep live (changing) server data
synchronized across multiple views and
multiple users?
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3. The Approach
Fetch entities only once and only
when needed.
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4. The Approach
One definitive instance of a server-
side entity.
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5. The Approach
Update all relevant views when the
server sends notice that an entity
has changed.
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6. The Approach
Server-side agnostic.
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7. What’s the strategy?
Store all fetched entities in a single
cache (Dictionary).
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8. What’s the strategy?
Bind the views to the cache.
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9. What about memory use?
public class WeakReference Allows cache values to be
{ garbage-collected if they’re not
being referenced anywhere else.
private var dic:Dictionary;
public function WeakReference(obj:*)
{
dic = new Dictionary(true); Text
dic[obj] = 1;
}
public function getValue():* {
for (var item:* in dic) {
return item;
}
return null;
}
}
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10. The Cache
How do we add an entity to the cache?
public class EntityCache
{
public function updateEntity(entity:BaseVO, ...):BaseVO
{
//...
}
}
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11. Recurse through properties
Adding a user to the cache also adds its address.
UserVO
id: 1
firstname: “Zachary”
lastname: “Pinter” EntityCache
address: 1: UserVO
AddressVO 2: AddressVO
id: 2
line1: “4444 W 44th Ave”
city: “Denver”
state: “CO”
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12. Recurse through properties
Arrays behave the same way
UserVO
id: 1
firstname: “Zachary”
lastname: “Pinter”
addresses: [ EntityCache
AddressVO
1: UserVO
id: 2
label: “home” 2: AddressVO
line1: “4444 W 44th Ave” 3: AddressVO
city: “Denver”
state: “CO”
AddressVO
id: 3
label: “work”
line1: “5555 W 55th Ave”
city: “Denver”
state: “CO”]
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13. Finding an object’s properties
Spring Actionscript (formally Prana)
http://www.pranaframework.org/
var type:Type = Type.forName(classname);
for each (var accessor:Accessor in type.accessors) {
if (accessor.isStatic == false && accessor.access.name == quot;readwritequot;) {
result.push(accessor.name);
}
}
return result;
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14. Finding an object’s properties
Source generator
public class UserVO extends BaseVO {
public var username : String;
public var firstname : String;
public var lastname : String;
public var address : AddressVO;
override public function getProperties():Array {
return super.getProperties().concat(quot;usernamequot;,quot;firstnamequot;,quot;lastnamequot;,quot;addressquot;);
}
}
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15. Updating the cache
What if the entity is already in the cache?
EntityCache EntityCache.updateEntity(
1: UserVO(instance A) UserVO(instance B)
id: 1 id: 1
firstname: “Robert” firstname: “Bob”
lastname: “Smith” lastname: “Smith”
)
Copy the properties from
instance B into instance A
EntityCache
Leave instance A in the
1: UserVO(instance A)
cache
id: 1
firstname: “Bob” Discard instance B
lastname: “Smith”
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16. Updating the cache
What about arrays?
EntityCache
UserVO
id: 1
firstname: “Zachary”
lastname: “Pinter” EntityCache.updateEntity(
addresses: [ AddressVO(instance B)
AddressVO(instance A)
id: 2
id: 2
label: “home” label: “home”
line1: “4444 W 44th Ave” line1: “3333 W 33rd Ave”
city: “Denver” city: “Denver”
state: “CO” state: “CO”
AddressVO )
id: 3
label: “work”
line1: “5555 W 55th Ave”
city: “Denver”
state: “CO”]
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17. Updating the cache
What about arrays?
UserVO
id: 1 Since we update the existing instance,
firstname: “Zachary” all references to it will see the update.
lastname: “Pinter”
addresses: [
AddressVO(instance A)
id: 2
label: “home”
line1: “3333 W 33rd Ave”
city: “Denver”
state: “CO”
AddressVO
id: 3
label: “work”
line1: “5555 W 55th Ave”
city: “Denver”
state: “CO”]
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18. Updating the Cache
The flip side
UserVO
EntityCache id: 5
AddressVO(instance A) firstname: “Zachary”
lastname: “Pinter”
id: 2
addresses: [
label: “home” AddressVO(instance B)
line1: “5555 W 55th Ave” id: 2
city: “Denver” label: “home”
state: “CO” line1: “6666 W 66th Ave”
city: “Denver”
state: “CO”
Your cache already has an
AddressVO
AddressVO with id 2 in it, and you
id: 3
add a new UserVO to the cache that
label: “work”
references an updated instance of the
line1: “5555 W 55th Ave”
AddressVO
city: “Denver”
state: “CO”]
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19. Updating the Cache
UserVO
EntityCache id: 5
AddressVO(instance A) firstname: “Zachary”
lastname: “Pinter”
id: 2
addresses: [
label: “home” AddressVO(instance A)
line1: “6666 W 66th Ave” id: 2
city: “Denver” label: “home”
state: “CO” line1: “6666 W 66th Ave”
city: “Denver”
state: “CO”
The AddressVO in the cache is AddressVO
updated and the AddressVO in id: 3
UserVO.addresses is replaced with label: “work”
the instance from the cache. line1: “5555 W 55th Ave”
city: “Denver”
state: “CO”]
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20. Updating the Cache
Code looks something like this...
if (obj is Array) {
var arr:Array = obj as Array;
for (var i:int=0;i<arr.length;i++) {
if (arr[i] is BaseVO) {
var res:BaseVO = updateEntity(arr[i] as BaseVO,...);
if (res != null) arr[i] = res;
}
}
}
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21. Updating the Cache
ArrayCollection’s are slightly trickier
if (obj is ArrayCollection) {
var ac:ArrayCollection = obj as ArrayCollection;
ac.disableAutoUpdate();
for (i=0;i<ac.length;i++) {
if (ac.getItemAt(i) is BaseVO) {
var res:BaseVO = updateEntity(ac.getItemAt(i) as BaseVO,...);
if (res != null) {
ac.setItemAt(res,i);
}
}
}
ac.enableAutoUpdate();
}
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22. Now that we’ve added
an entity to the cache...
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23. The Cache
How do we get an entity?
What happens if we ask for an entity that isn’t in the cache?
public class EntityCache
{
public function getEntity(id:String):EntityWrapper
{
//...
}
}
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24. EntityWrapper
[Bindable]
When requesting an entity, a
public class EntityWrapper wrapper object is returned.
{
public var entityId:String; If that object is in the cache,
public var entity:BaseVO; EntityWrapper.entity will have a
value.
public function EntityWrapper(id:String)
{ If the object is not in the cache,
entityId = id;
} EntityWrapper.entity will be null
and a call to fetch the entity will
} be queued.
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25. Using the Cache
userWrapper = cache.getEntity(userid.text);
<mx:Label text=quot;Username: {userWrapper.entity.username}quot;/>
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26. Using the Cache - List Renderers
Benefits
‣ Faster initial query since you’re only grabbing the id’s
‣ Rows are lazy-fetched as you scroll
Drawbacks
‣ Sorting has to be handled by the server
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27. Using the Cache - List Renderers
What’s the code for the item renderer look like?
override public function set data(val:Object):void
{
if(val != data){
if (!val) {
wrapper = null;
} else {
wrapper = cache.getEntity(val);
}
super.data = val;
}
}
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28. Serverside Considerations
What does this require of your
backend infrastructure?
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29. Serverside Considerations
Globally Unique Keys
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30. Dictionary Keys
Only one cache, need to prevent collisions
Some Options:
‣ Globally unique sequence across all tables (UserVO with id 1, AddressVO with id 2)
‣ UUID’s (great for generating new id’s flex-side)
‣ Combine type with id (“UserVO-1”, “AddressVO-1”)
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31. Serverside Considerations
Ability to get an entity by its key
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32. Serverside Considerations
Ability to query for entities and only
return their keys (used by lazy lists)
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33. Bonus
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34. Serverside Considerations
Ability to get multiple entities in a
single call by passing multiple keys
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35. Serverside Considerations
Ability to get multiple entities in a
single call by passing multiple keys
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36. Serverside Considerations
Versioned Entities
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37. Demo
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38. Questions?
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39. Thanks!
Zachary Pinter
http://github.com/zpinter/cache-sync
http://slideshare.net/zpinter
http://zacharypinter.com
http://effectiveui.com
Twitter: zpinter
Want to work for a cool company? We’re hiring!
http://jobs.effectiveui.com/
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