Skip to Navigation
Data Subscriptions: Revisions Support
Data subscription supports subscribing not only on documents, but also on all their revisions.
Revision support should be defined in the subscription. It also requires revisions to be configured on the collection in question.
While regular subscriptions process single documents, subscription on documents revisions processes pairs of subsequent document revisions.
Such functionality allows keeping track of each change that was performed on a document, and even to compare two subsequent versions of a document.
Both document revisions are accessible in the filtering and the projection process.
Revisions processing order
An audit trail entry is called a Document Revision and is comprised of a document snapshot.
In data subscription, Documents Revisions will be processed in pairs of subsequent entries.
Example:
Let us assume a user document that looks like:
{
Name:'James',
Age:'21'
}
-
We update the User document twice, in separate operations:
- We update the 'Age' field to the value of 22
- We update the 'Age' field to the value of 23
Data subscription's revision processing mechanism will receive pairs of revision in the following order:
| # | Previous | Current |
|---|---|---|
| 1 | null |
{ Name:'James', Age:'21' } |
| 2 | { Name:'James', Age:'21' } |
{ Name:'James', Age:'22' } |
| 3 | { Name:'James', Age:'22' } |
{ Name:'James', Age:'23' } |
Warning
As seen above, in order for subscriptions on revisions to work properly, it needs the revisions entries to be available, otherwise, there will be no data to process. Therfore, it's crucial to make sure that the revisions configuration allows storing documents revisions enough time, without discarding unprocessed revisions
Simple declaration and usage
Creation:
name = store.subscriptions().createForRevisions(Order.class);SubscriptionCreationOptions options = new SubscriptionCreationOptions();
options.setQuery("from orders (Revisions = true)");
name = store.subscriptions().createForRevisions(Order.class, options);Consumption:
SubscriptionWorker<Revision<Order>> revisionWorker = store
.subscriptions().getSubscriptionWorkerForRevisions(Order.class, name);
revisionWorker.run(x -> {
for (SubscriptionBatch.Item<Revision<Order>> documentsPair : x.getItems()) {
Order prev = documentsPair.getResult().getPrevious();
Order current = documentsPair.getResult().getCurrent();
processOrderChanges(prev, current);
}
});Revisions processing and projection
Creation:
SubscriptionCreationOptions options = new SubscriptionCreationOptions();
options.setQuery("declare function getOrderLinesSum(doc) {" +
" var sum = 0;" +
" for (var i in doc.Lines) { sum += doc.Lines[i]; } " +
" return sum;" +
"}" +
"" +
" from orders (Revisions = true) " +
" where getOrderLinesSum(this.Current) > getOrderLinesSum(this.Previous) " +
" select {" +
" previousRevenue: getOrderLinesSum(this.Previous)," +
" currentRevenue: getOrderLinesSum(this.Current)" +
"}");
name = store.subscriptions().create(options);Consumption:
SubscriptionWorker<Revision<Order>> revisionWorker = store
.subscriptions().getSubscriptionWorkerForRevisions(Order.class, name);
revisionWorker.run(x -> {
for (SubscriptionBatch.Item<Revision<Order>> documentsPair : x.getItems()) {
Order prev = documentsPair.getResult().getPrevious();
Order current = documentsPair.getResult().getCurrent();
processOrderChanges(prev, current);
}
});
Related Articles