I have a collection of documents that need to be processed by multiple client nodes.
Basically, each document should be processed by only 1 client node.
So what I'm thinking of is creating a unique clientId for each node and set the clientId to the document being processed to tell other clients that this document is being processed.
I already implemented this approach using Mongodb a couple of years ago by using the findAndModify operator which guarantees the atomicity of both setting the clientId to the document and returning it.
Now I'm looking for a maybe similar approach in Couchbase but couldn't find.
Any idea on how to do it?
I think what you are searching for is the method called getAndLock. It will guarantee that only one server is reading this document.
Updating the document with an attribute might be a bad idea if the server fails during this process, as no other server will take over those documents that have already have been assigned to the faulty one.
We have handeled similar kind of scenario in our project. What we do is to create one document and save other processing doc in that. If that document is not present in that, then it you can prevent it getting updated by other client.
If I add the new document in Couchbase Sync Gateway and try to fetch from the view, I do not get that document immediately, but after some time I get that document.
What I miss??
I did some testing and fined this post.
Thanks #shirshir for suggesting that. You're right - for Couchbase
Server 3.0 and higher, the indexable flag is no longer required on
write for a 'read-your-own-write' scenario - you should be able to do
a standard write and then a query with stale=false to see that data.
For Couchbase Server versions 2.x I believe you'd still need to force
persistence (with Indexable) on the write to ensure you're going to
see the results in your stale=false query.
I also use that flag and now it works fine.
I am coming from object relation database background, I understand Couchbase is schema-less, but data migration will still happen as the application develop.
In SQL we have management tool to alter table, or I can write migration script with SQL to do migration from version 1 table to version 2 table.
But in document, say we have json Document UserProfile:
UserProfile
{
"Owner": "Rich guy!",
"Car": "Cool car"
}
We might want to add a last visit field there, allow user have multiple car, so the new updated document will become follows:
UserProfile
{
"Owner": "Rich guy!",
"Car": ["Cool car", "Another car"],
"LastVisit": "2015-09-29"
}
But for easier maintenance, I want all other UserProfile documents to follow the same format, having "Car" field as an array.
From my experience in SQL, I could write migration script which support migrating different version of table. Migrate from version 1 table to version 2...N table.
So how can I should I write such migration code? I will have to really just writing an app (executable) using Couchbase SDK to migrate all the documents each time?
What will be the good way for doing migration like this?
Essentially, your problem breaks down into two parts:
Finding all the documents that need to be updated.
Retrieving and updating said documents.
You can do this in one of two ways: using a view that gives you the document ids, or using a DCP stream to get all the documents from the bucket. The view only gives you the ids of the documents, so you basically iterate over all the ids, and then retrieve, update and store each one using regular key-value methods. The DCP protocol, on the other hand, gives you the actual documents.
The advantage of using a view is that it's very simple to implement, works with any language SDK, and it lets you write your own logic around the process to make it more robust and safe. The disadvantage is having to build a view just for this, and also that if the data keeps changing, you must retrieve the ENTIRE view result at once, because if you try to page over the view with offsets, the ordering of results can change, thus giving you an inconsistent snapshot of the data.
The advantage of using DCP to stream all documents is that you're guaranteed to get a consistent snapshot of your data even if it's constantly changing, and also that you get the whole document directly as part of the stream, so you don't need to retrieve it separately - just update and store back to the database. The disadvantage is that it's currently only implemented in the Java SDK and is considered an experimental feature. See this blog for a simple implementation.
The third - and most convenient for an SQL user - way to do this is through the N1QL query language that's introduced in Couchbase 4. It has the same data manipulation commands as you would expect in SQL, so you could basically issue a command along the lines of UPDATE myBucket SET prop = {'field': 'value'} WHERE condition = 'something'. The advantage of this is pretty clear: it both finds and updates the documents all at once, without writing a single line of program code. The disadvantage is that the DML commands are considered "beta" in the 4.0 release of Couchbase, and that if the data set is too large, then it might not actually work due to timing out at some point. And of course, that fact that you need Couchbase 4.0 in the first place.
I don't know of any official tool currently to help with data model migrations, but there are some helpful code snippets depending on the SDK you use (see e.g. bulk updates in java).
For now you will have to write your own script. The basic process is as follow:
Make sure all your documents have a model_version attribute that you increment after each migration.
Before a migration update your application code so it can handle both the old and the new model_version, and so that new documents are written in the new model.
Write a script that iterate through all the old model documents in your bucket(you need a view that emits the document key), make the update you want, increment model_version and save the document back.
In a high concurrency environment it's important to have good error handling and monitoring, you could have for example a view that counts how many documents are in each model_version.
You can use Couchmove, which is a java migration tool working like Flyway DB.
You can execute N1QL queries with this tool to migrate your documents and keep tracking of your changes.
If I understood correctly, the crux here is getting and then 'update every CB docs'. This can be done with a view, provided that you understand that views are only 'eventually consistent' (unlike read/write actions which are strongly consistent).
If (at migration-time) no new documents are added to your bucket, then your view would be up-to-date and should return the entire set of documents to be migrated. easy.
On the other hand, if new documents continue to be written into your bucket, and these documents need to be migrated, then you will have to run your migration code continually to catch all these new docs (since the view wont return them until it is updated, a few seconds later).
In this 2nd scenario, while migration is happening, your bucket will contain a heterogeneous collection of docs: some that have been migrated already, some that are about to be migrated and some that your view has not 'seen' yet (because they were recently added) and would only be migrated once you re-run the migration code.
To make the migration process efficient, you'll need to find a way to differentiate between already-migrated items and yet-to-be-migrated items. You can add a field to each doc with its 'version number' and update it during the migration. Your view should be defined to only select documents with older 'version number' and ignore already-migrated items.
I suggest you read more about couchbase views - here and on their site.
Regarding your migration: There are two aspects here: (1) getting the list of document ids that need to be updated and (2) the actual update.
The actual update is simple: you retrieve the doc and save it again with the new format. There's no explicit schema. Where once you added column in SQL and populated it, you now just add a field in the json-doc (of all the docs). All migrated docs should have this field. Side note: Things get little more complicated if (while you're migrating) the document can be updated by another process. This requires special handling (read aboud CAS if that's the case).
Getting all the relevant doc-keys requires that you define a view and query it. Its beyond the scope of this answer (and is very well documented). Once you have all the keys, you simply iterate them one by one and update them.
With N1QL, Couchbase provides the same schema migration capabilities as you have in RDBMS or object-relational database. For the example in your question, you can place the following query in a migration script:
UPDATE UserProfile
SET Car = TO_ARRAY(Car),
LastVisit = NOW_STR();
This will migrate all the documents in your bucket to your new schema. Note that update statements in Couchbase provide document-level atomicity, not statement-level atomicity. But since this update is idempotent (repeatable), you can run it multiple times if you run into errors. Note: similar to the last paragraph of David's answer above.
One of the resources my app uses is data in two JSON files that are pulled from a third party and that are constantly updated with fresh content.
Each of these files have a specific structure that doesn't change.
However, sometimes the third party creates structural changes that may mess with my app.
My question is: how can I monitor their structure so I can detect changes as they occur?
Thanks!
For this reason you can use json schema and validate the files agains it. If they have the schema than you're, good just need to validate. If not than you have to do it based on a correct json. There are online generators for that.
Background
I have a (glorified) CRUD application that I'd like to enable HTML5 offline support with. The cache-manifest system looks simple yet powerful, but I'm curious about how I can allow users to access data while offline.
For example, suppose I have these pages for the entity "Case" (i.e. this is CRM case-management software):
http://myapplication.com/Case
http://myapplication.com/Case/{id}
http://myapplication.com/Case/Create
The first URI contains a paged listing of all cases, using the querystring parameters pageIndex and pageSize, e.g. /Case?pageIndex=2&pageSize=20.
The second URI is the template for editing individual cases, e.g. /Case/1 or /Case/56.
Finally, /Case/Create is the form used to create cases.
The Problem
I would like all three to be available offline.
/Case
The simple way would be to add /Case to the cache-manifest, however that would break paging (as the links wouldn't work).
I think I could instead add something like /Case/AllData which is an XML resource, which is cached and if offline then a script on /Case would use this XML data to populate the list and provide for pagination.
If I go for the latter, how can I have this XML data stored in the in-browser SQL database instead of as a cached resource? I think using the SQL database would be more resilient.
/Case/{id}
This is more complicated. There is the simple solution of manually adding /Case/1, /Case/2, /Case/3 etc... to /Case/1234, but there can be hundreds or even thousands of cases so this isn't very practical.
I think the system should provide access to the 30 most recent cases, for example. As above, how can I store this data in the database?
Also, how would this work? If I don't explicitly add /Case/34 to the manifest and the user clicks on to /Case/34 how can I get the browser to load a page that my JavaScript will populate based on the browser's SQL database data and not display the offline message?
/Case/Create
This one is more simple - as it's just an empty page and on the <form>'s submit action my script would detect if it's offline, and if it is offline then it would add it to the browser's SQL database. Does this sound okay?
Thanks!
I think you need to be looking at a LocalStorage database (though it does have some downsides), but there are other alternatives such as WebSQL and IndexedDB.
Also I don't think you should be using numeric Id's if you are allowing people to create as you will get Primary Key conflicts, it is probably best to use something like a GUID.
Another thing you need is the ability to push those new cases onto the server. there could be multiple...
Can they be edited? If they can I think you really need to be thinking about synchronization and conflict resolution hard very hard if that is the case.
Shameless self promotion, I have a project that is designed to handle these very issues, though it's not done, it's close. You can see it (with an ugly but very functional) demo at https://github.com/forbesmyester/SyncIt