I am trying to put together my first Google App Engine applicaton and having ploughed through the docs and some examples I decided that I quite like the Objectify approach. However, I also like GWT but passing an Objectify object to a GWT class causes issues.
So I tried Objectify-gwt but there just seems to be tumbleweed blowing thorugh that project. Any examples seem to date from 2010/2011 but the project seems to have been forked off the main Objectify branch recently (April 2014) so I was just wondering if it has a future.
If not, what is a good pattern for passing data between an Objectify back end and a GWT front-end? Do I really have to have two sets of objects, one in the persistence layer and one in the UI?
objectify-gwt is a very small bit of code which doesn't require much in the way of updates. That said, someone just submitted a pull request that fixes a problem with Gradle builds. It's overdue for a new release. But unless GAE adds some new data types, objectify-gwt is pretty much done. I wouldn't worry about the future of objectify-gwt any more than I would worry about the future of GWT in general.
That said, I think the value of passing raw entity types back and forth across the GWT-RPC boundry is overrated. It tends to produce an Anemic Domain Model. When I was actively using GWT, I tended to do this with simple embedded components and not with whole entities. But every project is different; maybe it is a good approach for you.
Related
Such questions have been asked before - but all of the answers are outdated now.
I am looking forward to work on a Scala based webapp. I understand this question can be split into two, but I am posting them as one because they rely on same context, there being a dependency on the hosting platform and frameworks used.
I have read multiple (awesome) debates on Play! and Lift, but cannot find a good comparison between Play! 2.1 and Lift. How do I decide which one is better for my scenario (a social network website) ?
Similarly, this discussion has some very good arguments as to which platform to use for if I go with Lift, but it's from 2010 and seems outdated. The recommended provider (stax.net) is dead (or I guess it's merged with cloudbees.com). I am personally inclined towards GAE, as they are quick to start with, but unsure if the issues still prevail :
Support for actors (I am not sure if Akka helps us solve this problem)
Requests for a given session being served by different JVMs without notice to running app
Quoting David Pollak (lead author of Lift) :
GAE is slow and non-scalable, despite Google's claims (everyone I've
spoken with that have tried to scale GAE apps have failed and gone
elsewhere). GAE locks you into a tremendously suboptimal storage
mechanism. GAE is free, but so is Stax and there are many inexpensive
options including SliceHost. Next up, you've got Amazon EC2 and
RackSpace. So, I haven't found a good reason for anyone to use GAE.
And if there's no good reason to use GAE, devoting a pile of resources
to code around the GAE JVM incompatibilities (e.g., no new threads)
seems like a waste.
Another issue if I go with GAE is lack of Play! 2.1 support. I still don't see a module for that. Another issue is difficulty to migrate to other databases (although I hear migrating to MongoDB should be relatively easier) in the future. Worst case would be to move out of GAE and use AppScale.
Personally I use Lift, Cloudbees, and MongoLab as my first choice for most of my projects. I tried several cloud hosting services to no avail (Heroku and RedHat in particular. I don't think I tried GAE due to the post from David Pollak that you have already referenced). To use cloudbees, you just need an sbt plugin. Then it is as easy as running the cloudbees-deploy target. Within a minute, your code is up and running. I was floored by how easy it was. I went with Mongo primarily because of this excellent g8 template (note, there is now an SQL equivalent)
Another thing I really like about Cloudbees and MongoLab is they both have free services. It's great for me because I only work on these projects in my free time, so I don't want to spend any money while my ideas are half-baked.
As for Lift, I can't compare it much to Play. I downloaded/installed play and was immediately turned off by how MVC it is. I felt that the view-first approach, albeit foreign to me, seemed to be a much more intuitive and powerful way to build web applications. I love how Lift doesn't obscure from me the fact that I am indeed developing a web application. I often feel that MVC frameworks try to keep all of the HTML/CSS/JS etc at an arms-length.
The question is quite open so I will share my experience and opinion regarding Scala web app development as it might help you with your decision.
I built my first scala web application using Scalatra and Scalate using Jetty as the server. The app is hosted on an Amazon EC2 instance and I've had no problems with this... it's been running since the end of 2011 with only one small blip that took 10 mins to resolve. I found it a good experience for learning to use Scala in web applications.
http://www.scalatra.org/
Typesafe (http://typesafe.com) appear to have opted for the Play Framework and so for my next scala based web app I am likely to go for Play. A book I have been reading on the Play Framework is "Play for Scala". It has just been published this month (Oct 2013).
http://www.manning.com/hilton/
My impression is that Lift was the go-to framework in the past but that this has shifted to the Play Framework.
Is it possible to share a model through other apps? if possible, how to do this.
Yeah, maybe we can implement API for those apps, But if apps can share their models for each other without any external libs, it would be cool :-)
You could possibly broaden your definition of "other applications" in the original question to include other versions of the same application (where these versions are really your "other applications"). If so, then this could be possible by deploying each of your "other applications" as a different version of the same application. That way they should be able to share the same data store. I've not yet attempted this myself but from what I read this should be possible. Might get some more info if anyone posts here, or if I end up trying this myself I'll let you know.
Update: I tried this out and it works but with one minor and one possibly significant issue. The minor issue is that you have to work out a way to duplicate the same data model across your two apps (or at least as much as you need). The bigger issue is that datastore commits made in one application may not be visible to the other application for quite some time, and that amount of time varies depending on where/how you're deploying.
There is no way (yet?) for an App to open its datastore to other Apps, if that is what you mean.
You'd have to go through an HTTP interface (which could probably be derived from the model classes directly, and thus shared). The remote_api standardizes this somewhat.
Is there an open source, reference implementation of a simple, correctly designed, best practiced, GWT + GAE open source application? I would like to see the best practices on the following:
Recommended GWT Page navigation, session handling
GWT best practice UI (validation, controls etc)
Work with detached objects, conversations, wizards
Effective and "as intended" integration between GWT and GAE
MVC implementation with GWT and GAE, where is the controller? (Spring?)
Using domain POJOs in the UI without writing data transfer objects?
JDO (or JPA) and DAO layer - correct mapping, transaction and service layers
A permission layer, (e.g. like Spring security/ACEGI)
Integration with client side libraries (JQuery / Sancha)
JUnit layer for both server and client side
All the samples I found so far handled a very narrow aspect of the above, and I'm sure there has to be one good "starting point" such as the one's you can find for Spring / Hibernate or RoR projects
Have you looked at Roo?
This can be used to quickly build a GWT app based on one of a number of JPA providers and databases.
It doesn't offer all the items listed but has quite a variety of optional add-ons
Unfortunately I don't know of any such reference implementation, and I don't think you're likely to find such an all-encompassing example is because:
It's much easier to understand an example that focuses on one thing, like focusing on using JDO in a GWT app, or using Spring security in a GWT app, or how to do client-side validation in a GWT app. There's no reason one sample has to include such disparate features.
In the same vein, what if I wanted this exact list of features in a reference implementation, but using something else instead of JDO? Should I expect someone to write everything all over again, with that one part replaced? It's much better to write samples and documentation for each piece of the puzzle and leave it up to developers to glue that knowledge together.
Some of the things you listed aren't exactly common usages, e.g., integration with JQuery and Sencha. That's not to say it's not possible, and there are some (focused) examples out there if you look, but it's not the main use case for GWT developers, so I don't think it makes sense in a reference implementation.
The sort of app you're talking about is very complex, something that would take multiple engineers months to write and then would have to be maintained and updated. All for a sample? Some people aren't willing to invest that kind of energy even when there's money involved!
That being said, there are some great GWT sample apps (with source available) here. Even though they don't encompass everything you listed, they'll hopefully be helpful.
There is a "Google App Engine Java and GWT Application Development" A code of the application they build during the book comes "Licensed under the Apache License, Version 2.0" so (I guess this is qualify as open source but IANAL) it worth looking at. At least it does helps me as I am working through the same set of questions. It written Nov 2010 so it is still relevant as Google Platform evolves at astonishing speed.
Declaimer - I got no relation to this book except that I am reading it.
What is the current status of grails and google app engine deployment. I am new to app engine but wonder worth exploring it. Some specific qns are
the latest plugin, which has high user rating, has any restrictions? or it work seamlessly with all gorm features
is there any issue with high startup time for grails application. How is it in real world scenario? (with a typical small and large scale application)
what about other grails plugins (like, shiro, joda time, nimble etc). I guess they wont play well. So using those libraries directly is the better option
If decided to give up goole-app as a deployment option, how easy to switch to a normal environment. The JPA support ensures the compatibility with other traditional DBs?
Not sure what else are major issues.. probably, this is the foundation for a good discussion.
thanks.
I got few good response from grails mailing list, and the conclusion shares the comment by David. see the thread here
Couple of relevant responses:
From Tomas Lin:
I would suggest looking into Gaelyk if you really want to build a
project on the App Engine. It is built from the ground up with the App
Engine as the target engine, so it can bypass problems like long
loadtimes due to Spring and Hibernate. The newly introduced plugin
mechanism guarantees that your Gaelyk applications can be extended in
a way guaranteed to work on GAE.
Gaelyk has it's own native entity persistence DSL, which is a little
cleaner that the JPA/JDO abstractions on top of the App Engine.
I currently see many HardDeadlineExceeded exceptions with the App
Engine and Grails. It is just not designed to work well with Spring
right now. Hopefully this will improve with the later releases of
Groovy, Grails and the Spring / Google partnership for GAE for
business, but I wouldn't consider Grails on GAE production ready.
Even with Gaelyk, there are reports of slow performance. So imagine
the difficulties that arise with the much bigger Grails stack.
The app-engine comes with it's own implementation of a user / security
management system based on GMail accounts. If you just want to provide
an admin / non-admin implementation, this is supported in the
appengine configuration. Cannot comment on Shiro.
Be aware that one of the major restrictions of the App Engine is the
inability to write a file, so even basic file uploading in Spring
becomes problematic since the default mechanism writes to a temporary
file. I would imagine that most of the plugins would not work out of
the box without digging into their code and changing it.
I think the biggest issue here is lack of support for native JDBC. JPA
is not as well supported as plain JDBC GORM, things like named queries
would probably not work out of the box without retrofitting. If you
want to use the latest and greatest parts of Grails, it might be
worthwhile to consider other hosting solutions.
From Aaron Eischeid
1.The GAE plugin and the JPA-GORM plugins combined do not get you all GORM features seamlessly. Though you should get basics like .save(), .delete(), and maybe .list() the dynamic finders etc. are going to be out (at least for now). I could be way off here, but I think most/all Hibernate dependent features are out or replaced by something else (since it relies on SQL under the hood and GAE doesn't currently have SQL based DB...) so for example any criteria builders are a no go. It is unclear to me how much of the dot drilling you can do on objects. For example, not sure if you could do something like:
def b = new Book()
def stores = b.authors.publishers.bookstores
One place I could use some pointers is how to use JPA in the domain classes. I am sure there is good info out there, but I just haven't found it yet.
unsure
grails plugins that include domain classes or manipulate your current domain classes are bound to have issues since you have to construct your domain classes differently to play nice with JPA which is necessary because Googles Datastore isn't quite like a relational DB. On the flip side. you can use Google's built in security so you shouldn't necessarily need plugins like Acegi or Shiro.
This probably boils down to the different levels of GORM that you can use in controllers and services and the different ways you define domain classes. Some refactoring seems inevitable unless JPA plays just as nice with SQL DB's as it does with Googles Datastore. If JPA can move like that then transferring should be easy, but by using JPA-GORM you give up some stuff you would probably want if you weren't benefiting from due to being on GAE.
Eager to hear what others have to say,
Aaron
I would like to develop a web-app requiring data persistence using GWT and GAE. As I understand it, my only (or at least by far the most convenient) option for data persistence is GAE's Datastore, using JDO or JPA annotated objects. I would also like to be able to send my objects back and forth client-server using GWT Remote Procedure Calls (RPC), therefore my objects must be able to "detach". However, GWT RPC serialization cannot handle detached JDO/JPA objects and it doesn't appear as though it will in the near future.
My question: what is the simplest and most direct solution to this? Being able to share the same objects client/server with server-side persistence would be extremely convenient.
EDIT
I should clarify that I still wish to use GWT RPC with GAE's Datastore. I am just looking for the best solution that would allow all these technologies to work together.
Try use http://gilead.sourceforge.net/
I've recently found Objectify, which is designed to be a replacement for JDO. Not much experience with it yet but its simpler to use than JDO, seems more lightweight, and claims to get around the need for DTOs with GWT, though I haven't tried that particular feature yet.
Ray Cromwell has a temporary hack up. I've tried it, and it works.
It forces you to use Transient instead of Detachable entities, because GWT can't serialize a hidden Object[] used by DataNucleus; This means that the objects you send to the client can't be inserted back into the datastore, you must retrieve the actual datastore object, and copy all the persistent fields back into it. Ray's method uses reflection to iterate over the methods, retrieve the getBean() and setBean() methods, and apply the entity setBean() with your transient gwt object's getBean().
You should strive to use JDO, the JPA isn't much more than a wrapper class for now. To use this hack, you must have both getter and setter methods for every persistent field, using PROPER getBean and setBean syntax for every "bean" field. Well, ALMOST PROPER, as it assumes all getters will start with "get", when the default boolean field use is "is".
I've fixed this issue and posted a comment on Ray's blog, but it's awaiting approval and I'm not sure if he'll post it. Basically, I implemented a #GetterPrefix(prefix=MethodPrefix.IS) annotation in the org.datanucleus package to augment his work.
In case it doesn't get posted, and this is an issue, email x_AT_aiyx_DOT_info Re: #GetterPrefix for JDO and I'll send you the fix.
Awhile ago I wrote a post Using an ORM or plain SQL?
This came up last year in a GWT
application I was writing. Lots of
translation from EclipseLink to
presentation objects in the service
implementation. If we were using
ibatis it would've been far simpler to
create the appropriate objects with
ibatis and then pass them all the way
up and down the stack. Some purists
might argue this is Bad™. Maybe so (in
theory) but I tell you what: it
would've led to simpler code, a
simpler stack and more productivity.
which basically matches your observation.
But of course that isn't an option with Google App Engine so you're pretty much stuck having a translation layer between client-side objects and your JPA entities.
JPA entities are quite rigid so they're not really appropriate for sending back and forth between the client anyway. Typically you want little bits from several entities when doing this (thus ending up with some sort of presentation-layer value object). That is your path forward.
Try this. It is a module for serializing GAE core types and send them to the GWT client.
You can consider using JSON. GWT has necessary API to parse & generate JSON string in the client side. You get a lot of JSON API for server side. I tried with google-gson, which is fine. It converts your JSON string to POJO model and viceversa. Hope this helps you providing a decent solution for your requirement
Currently, I use the DTO (DataTransferObject) pattern. Not necessarily as clean and plenty more boilerplate but GAE still requires a fair amount of boilerplate at current. ;)
I have a Domain Object mapped (usually) one-to-one with a DTO. When a client needs Domain info, a DAO(DataAccessObject) coughs up a DTO representation of the Domain object and sends that across the wire. When a DTO comes back, I hand the DAO the DTO which then updates all the appropriate Domain Objects.
Not as clean as being able to pass Domain Objects directly across the wire obviously but the limitations of GAE's JDO implementation and GWT's Serialization process means this is the cleanest way for me to handle this currently.
I believe Google's official answer for this is GWT 2.1 RequestFactory.
Given that you are using GWT and GAE, I'd suggest you stick to the official Google framework... I have a similar GWT / GAE based app and that's what I am doing.
By the way, setting up RequestFactory is a bit of pain in the ass. The current Eclipse plug-in doesn't include all the jars but I was able to find the help I needed, in Stackoverflow
I've been using Objectify as well, and I really like it. You still have to do some dancing around with pre/postLoad methods to translate e.g. Text to String and back.
since GWT ultimately compiles to JavaScript, for detached persistence it would need one of a few services available. the best known are HTML5 stores and Gears (both use SQLite!). of course, neither is widely deployed, so you'd have to convince your users to either use a modern browser or install a little-known plugin. be sure to degrade to a usable subset if the user doesn't comply
What about directly using Datastore API to load/store POJO domain objects?
It should be comparable to DTO approach, meaning e.g. that you have to manually handle all fields (if you don't use tricks like reflection-based automation) while it should give you more flexibility and full access to all Datastore features.