I wonder what's the best practice to make a db update thread safe.
I am calling a stateless session bean's
public void addReservation(Reservation reservation)
Before adding this reservation - I would like to check if there are still valid places for that reservation, using an entity bean's method
public boolean isThereRoomForAnotherReseravtion()
I read here that synchronized method will not solve the problem.
I thought of using stored procedure with a validation rule for the solution, but it doesn't sound right.
Is there a standard practice for making write procedure thread safe?
Regards,
Danny
No Java synchronization can actually help you since there can always be some second database client (maybe even your own application in clustered environment) that can access the database simultaneously.
You need to go deeper on the database layer and use database locking facilities like optimistic/pessimistic locking. JPA 1.0 supports the former, 2.0 supports both. The choice depends on how much concurrency you suspect to experience.
Related
In Broadleaf, both SessionOrderLockManager and DatabaseOrderLockManager can be used by CartStateFilter to serialize user requests.
It appears that the former locks on the session associated with the request, and the latter locks on the order embedded in the request body.
My question is that, why both implementations exist? Are there differences in the provided semantics? Are there scenarios where session-based locks won't suffice? (I found through git history that the database lock implementation is introduced after.)
Many thanks!
p.s. I'm not familiar with HTTP sessions. Please correct me if any statement is wrong.
[Edit] Here's another related question: why use locks at all? It seems that marking the service methods #Transactional suffices.
I have done a small experiment with #FlowScoped beans, whose purpose, as I understand, is to make easier creating "wizard-type" web applications, gradually accumulating data over a sequence of pages, then, once all the data is ready, writing it to the persistent storage (this is just an example, nothing prevents of course to write to the persistent storage during intermediate steps). As I saw, the calls to a #FlowScoped bean are not synchronized, and thus there is in principle the possibility of corrupting the data stored in the bean (by doing a double submit, or launching by any other means two almost simultaneous HTTP requests, which invoke the methods of the bean). This unlike #ConversationScoped beans the calls to which are synchronized.
What puzzles me is that about #SessionScoped beans I have found several links which speak about the need to synchronize the access to a #SessionScoped bean (or recommending not to use them at all, apart from user data which changes rarely), but I have not found anything like that about #FlowScoped beans.
What is considered then to be a "best practice" for using #FlowScoped beans? Am I missing something?
EDIT
#FlowScoped seems, at least to me, to be motivated in part by Spring WebFlow, with which I have some experience, and which, as I know, offers integration with JSF 2 (not all JSF 2.2 features seem to be implemented, but it seems that PrimeFaces is usable, for example). I know that Spring WebFlow + JSF is actually used in "real world" applications, and the issue of thread safety of flow scoped objects is handled there elegantly together with double submit issues (flow execution id must be supplied with each HTTP request, and it expires and a new one is returned after a HTTP request which invokes a Spring WebFlow "action" method: therefore one cannot invoke concurrently more than one "action" method for the same user and flow id).
So I want to understand, what is the best practice in the case of JSF 2.2 if I wish to use the #FlowScoped beans to construct an application "flow" (without using Spring WebFlow). Do I really need to synchronize the access to #FlowScoped beans myself, or there is some standard way to deal with such issues?
I developing a stateless REST API that makes use of token based authentication, where I'm manually adding an Authentication object to the security context by calling SecurityContextHolder.getContext().setAuthentication(authentication) from within a custom security filter. I've been experiencing problems with the context not being set correctly which I believe is due to this :
Storing the SecurityContext between requests
In an application which receives concurrent requests in a single session, the same SecurityContext instance will be shared between threads. Even though a ThreadLocal is being used, it is the same instance that is retrieved from the HttpSession for each thread. This has implications if you wish to temporarily change the context under which a thread is running. If you just use SecurityContextHolder.getContext(), and call setAuthentication(anAuthentication) on the returned context object, then the Authentication object will change in all concurrent threads which share the same SecurityContext instance. ...
You can customize the behaviour of SecurityContextPersistenceFilter to create a completely new SecurityContext for each request, preventing changes in one thread from affecting another.
So the question is - how do you change the behaviour of the SecurityContextPersistenceFilter?
I'd like the security context to not be associated with the http session, but don't want to set the session creation policy to stateless, because I still want to implement CSRF protection etc.
I had this exact question this afternoon, and this open question matched my search exactly, so I thought I would add the little I learned.
We had threads that were accessing the same SecurityContext. I was unable to figure out how to customize the behavior of the SecurityContextPersistenceFilter directly (and in the pattern of the framework), however there were two ways that I could get it to be thread safe.
The first solution was to ensure that an empty context was created in our main authentication filter. This covered all of our authenticated requests, so it would work for our solution.
SecurityContextHolder.createEmptyContext();
The second thing that worked for me was to change our WebSecurityConfig to be stateless, which I know doesn't work for the OP, but added here for completeness.
http.authorizeRequests()
.anyRequest().authenticated()
.and()
.sessionManagement().sessionCreationPolicy(SessionCreationPolicy.STATELESS)
...
Both these solutions work independently for our particular configuration. I'm certain there is a 3rd solution that would read better, but I don't know what it is but would like to.
This is my first time posting. I welcome any feedback.
If I have a #ManagedBean that's #SessionScoped, why would I use a #Stateful EJB? I used it before for shopping carts and maintaining a conversational state, but since a managed bean will be kept during the user session I can store state there, then call SLSB for bussiness logic. Is that correct? If it is, then stateful ejbs will be left for more specific applications such as when you need transactions etc?
Very often stateless session beans can be used for a lot of business problems.
Stateful does not necessarily means only a remote server keeps state, although this is certainly one of the options. A remote Swing client could first send a bunch of data to a stateful session bean, hold on to the stub and then subsequently send some commands that operate on this data. This saves the client from having to send the same (large amount of) data each and every time.
In the remote use case, it indeed somewhat mirrors the usage of the HTTP session when web clients (browsers) are used. The major difference is that the session is per bean here, while with the HTTP session, the session is a scope shared by many beans. Since the HTTP session is based on cookies, and cookies are global for a domain for the entire browser, the HTTP session can not directly support multiple sessions from the same client (e.g. per tab or per window). This is trivial with stateful session beans.
However...
Remote Swing clients talking to remote EJBs are not that common.
In the context you described in your question, you will typically use local EJBs and you will store most state in the HTTP session (be careful with sharing!) and these days in the view scope or conversation scope.
So, finally, when to use stateful session beans in this scenario?
One important use case is the extended persistence context in JPA. Normally with a transaction scoped entity manager, when an entity crosses the transactional boundary of an EJB method call it will be detached. If you want to (optimistically) lock an entity between user interactions, this is undesirable. You'll lose the lock.
With an extended persistence context, the entity remains attached and the locks valid when you return from a call to the stateful session bean. This is very useful for preview functionality to assure that nobody else has made any changes to the entity when you okay after the preview. Or indeed for a shopping cart where you want to assure that for some time the item can't be sold to anyone else while in the cart.
I'm developing a java EE web app using JSF with a shopping cart style process, so I want to collect user input over a number of pages and then do something with it.
I was thinking to use an EJB 3 stateful session bean for this, but my research leads me to believe that a SFSB is not tied to a client's http session, so I would have to manually keep track of it via an httpSession, some side questions here . . .
1) Why is it called a session bean, as far as I can see it has nothing to do with a session, I could achieve the same by storing a pojo in a session.
2) What's the point of being able to inject it, if all I'm gonna be injecting' is a new instance of this SFSB then I might as well use a pojo?
So back to the main issue I see written all over that JSF is a presentation technology, so it should not be used for logic, but it seems the perfect option for collecting user input.
I can set a JSF session scoped bean as a managed property of all of my request beans which means it's injected into them, but unlike a SFSB the JSF managed session scoped bean is tied to the http session and so the same instance is always injected as long as the http session hasn't been invalidated.
So I have multiple tiers
1st tier) JSF managed request scoped beans that deal with presentation, 1 per page.
2nd tier) A JSF managed session scoped bean that has values set in it by the request beans.
3rd tier) A stateless session EJB who executes logic on the data in the JSF session scoped bean.
Why is this so bad?
Alternative option is to use a SFSB but then I have to inject it in my initial request bean and then store it in the http session and grab it back in each subsequent request bean - just seems messy.
Or I could just store everything in the session but this isn't ideal since it involves the use of literal keys and casting . etc .. etc which is error prone. . . and messy!
Any thoughts appreciated I feel like I'm fighting this technology rather than working with it.
Thanks
Why is it called a session bean, as far as I can see it has nothing to do with a session, I could achieve the same by storing a pojo in a session.
From the old J2EE 1.3 tutorial:
What Is a Session Bean?
A session bean represents a single
client inside the J2EE server. To
access an application that is deployed
on the server, the client invokes the
session bean's methods. The session
bean performs work for its client,
shielding the client from complexity
by executing business tasks inside the
server.
As its name suggests, a session bean
is similar to an interactive session.
A session bean is not shared--it may
have just one client, in the same way
that an interactive session may have
just one user. Like an interactive
session, a session bean is not
persistent. (That is, its data is not
saved to a database.) When the client
terminates, its session bean appears
to terminate and is no longer
associated with the client.
So it has to do with a "session". But session not necessarily means "HTTP session"
What's the point of being able to inject it, if all I'm gonna be injecting' is a new instance of this SFSB then I might as well use a pojo?
Well, first of all, you don't inject a SFSB in stateless component (injection in another SFSB would be ok), you have to do a lookup. Secondly, choosing between HTTP session and SFSB really depends on your application and your needs. From a pure theoretical point of view, the HTTP session should be used for presentation logic state (e.g. where you are in your multi page form) while the SFSB should be used for business logic state. This is nicely explained in the "old" HttpSession v.s. Stateful session beans thread on TSS which also has a nice example where SFSB would make sense:
You may want to use a stateful session
bean to track the state of a
particular transaction. i.e some one
buying a railway ticket.
The web Session tracks the state of
where the user is in the html page
flow. However, if the user then gained
access to the system through a
different channel e.g a wap phone, or
through a call centre you would still
want to know the state of the ticket
buying transaction.
But SFSB are not simple and if you don't have needs justifying their use, my practical advice would be to stick with the HTTP session (especially if all this is new to you). Just in case, see:
Stateless and Stateful Enterprise Java Beans
Stateful EJBs in web application?
So back to the main issue I see written all over that JSF is a presentation technology, so it should not be used for logic, but it seems the perfect option for collecting user input.
That's not business logic, that's presentation logic.
So I have multiple tiers (...)
No. You have probably a client tier, a presentation tier, a business tier, a data tier. What you're describing looks more like layers (not even sure). See:
Can anybody explain these words: Presentation Tier, Business Tier, Integration Tier in java EE with example?
Spring, Hibernate, Java EE in the 3 Tier architecture
Why is this so bad?
I don't know, I don't know what you're talking about :) But you should probably just gather the multi page form information into a SessionScoped bean and call a Stateless Session Bean (SLSB) at the end of the process.
1) Why is it called a session bean, as far as I can see it has nothing to do with a session, I could achieve the same by storing a pojo in a session.
Correction: an EJB session has nothing to do with a HTTP session. In EJB, roughly said, the client is the servlet container and the server is the EJB container (both running in a web/application server). In HTTP, the client is the webbrowser and the server is the web/application server.
Does it make more sense now?
2) What's the point of being able to inject it, if all I'm gonna be injecting' is a new instance of this SFSB then I might as well use a pojo?
Use EJB for transactional business tasks. Use a session scoped managed bean to store HTTP session specific data. Neither of both are POJO's by the way. Just Javabeans.
Why shouldn't I use a JSF SessionScoped bean for logic?
If you aren't taking benefit of transactional business tasks and the abstraction EJB provides around it, then just doing it in a simple JSF managed bean is indeed not a bad alternative. That's also the normal approach in basic JSF applications. The actions are however usually to be taken place in a request scoped managed bean wherein the session scoped one is been injected as a #ManagedProperty.
But since you're already using EJB, I'd question if there wasn't a specific reason for using EJB. If that's the business requirement from upper hand, then I'd just stick to it. At least, your session-confusion should now be cleared up.
Just in case you're not aware of this, and as a small contribution to the answers you have, you could indeed anotate a SFSB with #SessionScoped, and CDI will handle the life cycle of the EJB... This would tie an EJB to the Http Session that CDI manages. Just letting you know, because in your question you say:
but my research leads me to believe that a SFSB is not tied to a client's http session, so I would have to manually keep track of it via an httpSession, some side questions here . . .
Also, you could do what you suggest, but it depends on your requirements, until CDI beans get declarative transaction support or extended persistence contexts etc, you'll find yourself writing a lot of boilerplate code that would make your bean less clean. Of course you can also use frameworks like Seam (now moving to DeltaSpike) to enhance certain capabilities of your beans through their extensions.
So I'd say yes, at first glance you may feel it's not necessary to use a stateful EJB, but certain use cases may be better solve through them. If a user adds a product to his cart, and another user adds this same product later, but there is only one unit in stock, who gets it? the one who does the checkout faster? or the one who added it first? What if you want to access your entity manager to persist a kart in case the user decides to randomly close his browser or what if you have transactions that spawn multiple pages and you want every step to be synchronized to the db? (To keep a transaction open for so long is not advisable but maybe there could be a scenario where this is needed?) You could use SLSB but sometimes it's better and cleaner to use a SFSB..