How do we use CachedMap in Trident.
My use case is such that, I will be doing some database and want to store that information in cache or trident state.
Next time when I get any tuple, I want to query the trident state or cache before querying database.
I don't get required data in trident state/cache, I will be querying the database and updating the trident state/cache.
Please let me know how do we achieve this or if there any link to help me out with this scenario.
CachedMap wraps your IBackingMap and also implements IBackingMap. It calls your store when it doesn't have a cached value.
new CachedMap(myIBackingMap, cacheSize)
See example usage in trident-memcached
Related
I would like to override the Query Async Method with some Custom code, so I can access an external api, get some data and then use this data to query the db tables to get the results, this should support all the default sync and paging features provided by the base method. I'm using the latest 5.0 DataSync library. Old versions returned a IQueryable so this was easy to do, but now it returns an action result. Is there any solution? Could not find any docs.
e.g. I get a set of guids from api. I query the db tables with these guids and get all the matching rows and return back to client.
I know how to override the method, call external api's, get data and query the db tables, but this provides me with a custom data format not the one that the query async gives by default with paging support.
The way to accomplish this is to create a new repository, swapping out the various CRUD elements with your own implementation. You can use the EntityTableRepository as an example. You will note that the "list" operation just returns an IQueryable, so you can do what you need to do and then return the updated IQueryable.
I have a problem that i need to solve using DATANUCLEUS (JDO), maybe a limitation of something that was not covered by JDO specs.
I need to catch when objects are loaded in a Query - there is NO InstanceLifecycleListener for this! (Queried objects should be treated as LOADED objects after all - they can be changed, detached, etc)
Query query=pm.newQuery(...);
Collection col=(Collection)query.execute();
Another way to do it would be to catch when objects are made TRANSIENT. I found no way of doing that either!
pm.makeTransientAll(col,true);
Any ideas?
For detached Query elements, i use the DetachLifecycleListener over the PMF, that listens to the DETACH method. That's the only way I've found to make InstanceLifecycleListener work on queries.
Here's what i found out with a lot of DEBUG:
1) Queries DO NOT trigger any InstanceLifecycleListener (except when elements are DETACHED) if the DEFAULT fetch group (aka METADATA default-fetch-group) is REMOVED from the PersistentManager prior to the query. In other words: one cannot use the pm.getFetchPlan().clearGroups() method, or one has to ADD the DEFAULT fetch group back before executing the query. SO... if there is a need to define a programmatic fetch group, it needs to take into account that the DEFAULT fetch group will (must!) be present (aka, all the fields defined in the metadata!) for the InstanceLifecycleListener to work.
The workaround would be to DEFINE the DEFAULT fetch group with ONLY the KEY (it's needed) and just change the logic to work with that. What i did exactly... The result?
2) the getObjectById method STOPS triggering the postLoad!!! After debug i found that for this to work, the DEFAULT fetch plan MUST define another field apart from the KEY one!
There is a total lack of information about the way the FETCH GROUPS affect the way the InstanceLifecycleListener are triggered, and Apache JDO (at least) should revise the way this works (or at least the DOCS), so that JDO users don't go nuts with implementation.
I am writing an application which communicates with an API and stores the response in a Meteor Collection so I can have the power of mongo to sort/filter.
I would like to clear the collection for every new result set. But a Meteor Collection is persistent.
What is the preferred way of clearing the collection? I know you can drop the meteor collection, but is that the preferred method?
Help appreciated. Thank you!
I would go about creating a local mongo collection which will be available on client side only. To create a client-side collection, just don't give it a name argument.
//This collection is client-only, and will not be sync with server
myCollection = new Mongo.Collection();
//To be more explicit, you can use `null` for the name:
myCollection = new Mongo.Collection(null);
Once you are done using the data empty the collection
myCollection.remove({});
myCollection.remove({}) is the syntax for removing all documents from a collection. This will only work on the server unless the collection is a client-side collection as per #Nakib's example. Otherwise documents can only be deleted by _id on the client side. Normally your allow/deny rules should block any attempt to delete anything on the client as it provides a great attack vector.
Not completely familiar with the Meteor best practice but if you were going to clear out an array in javascript the best practice would be to run the following.
myArrary.length = 0;
For more information I recommend this blog post by David Walsh where he details the reasoning behind zeroing out an array as follows:
Setting the length equal to zero empties the existing array, not
creating another array! This helps you to avoid pointer issues with
arrays as well.
What to do when I want to make changes on my data Collection but I do not want to persist it? In other words, I want to make changes on minimongo, locally, but I do not want to spread it to world.
Use _collection.
MyCollection = new Meteor.Collection('my-collection');
// Subscribe as you see fit
Meteor.subscribe('my-publication');
// Now, to make updates locally you can access the documents in the collection without
// making any calls to the sever.
MyCollection._collection.insert({key:value});
MyCollection._collection.update({key:value}, {key:value});
Works with the usual mini-mongo operations.
This is undocumented and might change in future releases of Meteor without notice.
According to the docs, we can create a Collection and set its name as null. It will create an unmanaged (unsynchronized) local collection.
Unfortunately, it seems to not be possible to make local changes in synchronized collections.
You can create what I call a "local mirror" of a shared collection. Here's a gist with baisc functionality: https://gist.github.com/belisarius222/4715531
The idea is that you wire up a new local collection (new Meteor.Collection(null)) so that any change in the shared collection gets applied to the local collection too.
I will try to be as concise as possible. I'm using Flex/Hibernate technologies for my app. I also use Cairngorm micro-architecture for Flex. Because i'm beginner, i have probably misunderstand something about Caringorm's ModelLocator purpose. I have following problem...
Suppose that we have next data model:
USER ----------------> TOPIC -------------> COMMENT
1 M 1 M
User can start many topics, topics can have many comments etc. It is pretty simple model, just for example. In hibernate, i use EAGER fetching strategy for unidirectional USER->TOPIC and TOPIC->COMMENT relations(here is no question about best practices etc, this is just example of problem).
My ModelLocator looks like this:
...
public class ModelLocator ....
{
//private instance, private constructor, getInstance() etc...
...
//app state
public var users:ArrayCollection;
public var selectedUser:UserVO;
public var selectedTopic:TopicVO;
}
Because i use eager fetching, i can 'walk' through all object graph on my Flex client without hitting the database. This is ok as long as i don't need to insert, update, or delete some of the domain instances. But when that comes, problems with synchronization arise.
For example, if i want to show details about some user from some UserListView, when user(actor) select that user in list, i will take selected index in UserList, get element from users ArrayCollection in ModelLocator at selected index and show details about selected user.
When i want to insert new User, ok, I will save that user in database and in IResponder result method i will add that user in ModelLocator.users ArrayCollection.
But, when i want to add new topic for some user, if i still want to use convenience of EAGER fetching, i need to reload user list again... And to add topic to selected user... And if user is in some other location(indirectly), i need to insert topic there also.
Update is even worst. In that case i need to write even some logic...
My question: is this good way of using ModelLocator in Cairngorm? It seems to me that, because of mentioned, EAGER fetching is somehow pointless. In case of using EAGER fetching, synchronization on Flex client can become big problem. Should I always hit database in order to manipulate with my domain model?
EDIT:
It seems that i didn't make myself clear enough. Excuse me for that.
Ok, i use Spring in technology stack also and DTO(DVO) pattern with flex/spring (de)serializer, but i just wanted to stay out of that because i'm trying to point out how do you stay synchronized with database state in your flex app. I don't even mention multi-user scenario and poling/pushing topic which is, maybe, my solution because i use standard request-response mechanism. I didn't provide some concrete code, because this seems conceptual problem for me, and i use standard Cairngorm terms in order to explain pseudo-names which i use for class names, var names etc.
I'll try to 'simplify' again: you have flex client for administration of above mentioned domain(CRUD for each of domain classes), you have ListOfUsersView(shows list of users with basic infos about them), UserDetailsView(shows user details and list of user topics with delete option for each of topic), InsertNewUserTopicView(form to insert new topic) etc.
Each of view which displays some infos is synchronized with ModelLocator state variables, for example:
ListOfUsersView ------binded to------> users:ArrayCollection in ModelLocator
UserDetailsView ------binded to------> selectedUser:UserVO in ModelLocator
etc.
View state transition look like this:
ListOfUsersView----detailsClick---->UserDetailsView---insertTopic--->InsertTopicView
So when i click on "Details" button in ListOfUsersView, in my logic, i get index of selected row in ListOfUsers, after that i take UserVO object from users:ArrayCollection in ModelLocator at mentioned index, after that i set that UserVO object as selectedUser:UserVO in ModelLocator and after that i change view state to UserDetailsView(it shows user details and selectedUser.topics) which is synchronized with selectedUser:UserVO in ModelLocator.
Now, i click "Insert new topic" button on UserDetailsView which results in InsertTopicView form. I enter some data, click "Save topic"(after successful save, UserDetailsView is shown again) and problem arise.
Because of my EAGER-ly fetched objects, i didn't hit the database in mentioned transitions and because of that there are two places for which i need to be concerned when insert new topic for selected user: one is instance of selectedUser object in users:ArrayCollection (because my logic select users from that collection and shows them in UserDetailsView), and second is selectedUser:UserVO(in order to sync UserDetailsView which comes after successfull save operation).
So, again my question arises... Should i hit database in every transition, should i reload users:ArrayCollection and selectedUser:UserVO after save in order to synchronize database state with flex client, should i take saved topic and on client side, without hitting the database, programmatically pass all places which i need to update or...?
It seems to me that EAGER-ly fetched object with their associations is not good idea. Am i wrong?
Or, to 'simplify' :) again, what should you do in the mentioned scenario? So, you need to handle click on "Save topic" button, and now what...?
Again, i really try to explain this as plastic as possible because i'm confused with this. So, please forgive me for my long post.
From my point of view the point isn't in fetching mode itself but in client/server interaction. From my previous experience with it I've finally found some disadvantages of using pure domain objects (especially with eager fetching) for client/server interaction:
You have to pass all the child collections maybe without necessity to use them on a client side. In your case it is very likely you'll display topics and comments not for all users you get from server. The most like situation you need to display user list then display topics for one of the selected users and then comments for one of the selected topics. But in current implementation you receive all the topics and comments even if they are not needed to display. It is very possible you'll receive all your DB in a single query.
Another problem is it can be very insecure to get all the user data (or some other data) with all fields (emails, addresses, passwords, credit card numbers etc).
I think there can be other reasons not to use pure domain objects especially with eager fetching.
I suggest you to introduce some Mapper (or Assembler) layer to convert your domain objects to Data Transfer Objects aka DTO. So every query to your service layer will receive data from your DAO or Active Record and then convert it to corresponding DTO using corresponding Mapper. So you can get user list without private data and query some additional user details with a separate query.
On a client side you can use these DTOs directly or convert them into client domain objects. You can do it in your Cairngorm responders.
This way you can avoid a lot of your client side problems which you described.
For a Mapper layer you can use Dozer library or create your own lightweight mappers.
Hope this helps!
EDIT
What about your details I'd prefer to get user list with necessary displayable fields like first name and last name (to display in list). Say a list of SimpleUserRepresentationDTO.
Then if user requests user details for editing you request UserDetailsDTO for that user and fill tour selectedUser fields in model with it. The same is for topics.
The only problem is displaying list of users after user details editing. You can:
Request the whole list again. The advantage is you can display changes performed by other users. But if the list is too long it can be very ineffective to query all the users each time even if they are SimpleUserRepresentationDTO with minimal data.
When you get success from server on user details saving you can find corresponding user in model's user list and replace changed details there.
Tell you the truth, there's no good way of using Cairngorm. It's a crap framework.
I'm not too sure exactly what you mean by eager fetching (or what exactly is your problem), but whatever it is, it's still a request/response kind of deal and this shouldn't be a problem per say unless you're not doing something right; in which case I can't see your code.
As for frameworks, I recommend you look at RobotLegs or Parsley.
Look at the "dpHibernate" project. It implements "lazy loading" on the Flex client.