Quick Summary

This plugin adds concurrent update functionality to grails domain classes by annotating a class with @ConcurrentUpdate. By annotating the class, it gets a custom validator for the version field, which is being compared to the persistent value. If the persistent value is greater, this means that another user has modified the object in the meantime.

Additionally the plugin provides a taglib to recover the other users changes.

<conup:storedValue bean="${yourbean}" field="somefield"/>

Introduction

The concurrent-update plugin provides functionality for the case of concurrently updating a business object by two different users. The scenario is as follows.

Lets assume a domain class Announcement which contains one property message.

class Announcement {

    String message

    static constraints = {
        message(blank: false)
    }
}

When scaffolding the controller code for this domain class, we get the optimistic lock code in the update action. Personally, I think, the code to check for optimistic locking belongs to the validation of the domain class.

Annotating a domain class

When annotating a domain class with @ConcurrentUpdate, the plugin attaches a custom validator to the domain class which checks the version.

import com.rosowski.grails.plugin

@ConcurrentUpdate
class Announcement {
    String title
    String message
    ...
}

So what happens if a user A is loading an object and while happily editing his/her object, another user B has saved his/her changes to the object? The custom validator provided by the annotation checks if the persistent version of the object is greater than the version stored in the hidden field. If so, the validator returns the error code optimistic.locking.failure. For the version field to automatically be part of the autobinding process, we needed to provide a BindEventListener.

Retrieving the other users changes

There are two ways of retrieving user changes in a concurrent update situation. First, by using the ConcurrentUpdateService.getPersistentValues(). This service adds an extra field for each persistent property of the domain class with a trailing "_persisted" added to the field name. Taking the above example class Announcement, we would end up with two extra properties added dynamically to the instance, title_persisted and message_persisted.

Additionally, the plugin provides a taglib with which the changes from user B can be applied to the object by the editing user A. Lets see how we can add the tag to a typical scaffolded view.

<conup:storedValue bean="${announcement}" field="message"/>

So if user A hits 'save' and another version has already been saved by user B, user A gets a warning and the ability to apply the changes from user B to the object (for each field where the tag has been used).

GOTCHAS

Make sure you have erased the version check in the controller code, otherwise the custom validator won't be hit.
Don't flush the save in the controller, since this leads to a duplicate validation of the object.

TODO

handle date fields

drosowski/grails-concurrent-update

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