June 4, 2018

Sreekanth B

CouchDB Advanced Interview Questions And Answers

How Do You Compare Mongodb, Couchdb And Couchbase?

MongoDB and CouchDB are document oriented database.
MongoDB and CouchDB are the most typical representative of the open source NoSQL database.
They have nothing in common other than are stored in the document outside.
MongoDB and CouchDB, the data model interface, object storage and replication methods have many different.

How Is Pouchdb Different From Couchdb?

PouchDB is also a CouchDB client, and you should be able to switch between a local database or an online CouchDB instance without changing any of your application’s code.

However, there are some minor differences to note:

View Collation – CouchDB uses ICU to order keys in a view query; in PouchDB they are ASCII ordered.

View Offset – CouchDB returns an offset property in the view results. In PouchDB, offset just mirrors the skip parameter rather than returning a true offset.

So Is Couchdb Now Going To Written In Java?

Erlang is a great fit for CouchDB and I have absolutely no plans to move the project off its Erlang base. IBM/Apache’s only concerns are we remove license incompatible 3rd party source code bundled with the project, a fundamental requirement for any Apache project. So some things may have to replaced in the source code (possibly Mozilla Spidermonkey), but the core Erlang code stays.

An important goal is to keep interfaces in CouchDB simple enough that creating compatible implementations on other platforms is feasible. CouchDB has already inspired the database projects RDDB and Basura. Like SQL databases, I think CouchDB needs competition and a ecosystem to be viable long term. So Java or C++ versions might be created and I would be delighted to see them, but it likely won’t be me who does it.

How Fast Are Couchdb Views?

It would be quite hard to give out any numbers that make much sense. From the architecture point of view, a view on a table is much like a (multi­column) index on a table in an RDBMS that just performs a quick look­up. So this theoretically should be pretty quick. The major advantage of the architecture is, however, that it is designed for high traffic. No locking occurs in the storage module (MVCC and all that) allowing any number of parallel readers as well as serialized writes. With replication, you can even set up multiple machines for a horizontal scale­out and data partitioning (in the future) will let you cope with huge volumes of data.


Why Does Couchdb Not Use Mnesia?

The first is a storage limitation of 2 Giga bytes per file.

The second is that it requires a validation and fix up cycle after a crash or power failure, so even if the size limitation is lifted, the fix up time on large files is prohibitive.

Mnesia replication is suitable for clustering, but not disconnected, distributed edits. Most of the “cool” features of Mnesia aren’t really useful for CouchDB.

Also Mnesia isn’t really a general-purpose, large scale database. It works best as a configuration type database, the type where the data isn’t central to the function of the application, but is necessary for the normal operation of it. Think things like network routers, HTTP proxies and LDAP directories, things that need to be updated, configured and reconfigured often, but that configuration data is rarely very large.

How Much Stuff Can Be Stored In Couchdb?

For node partitioning, basically unlimited. The practical scaling limits for a single database instance,  are not yet known.

Where Are The Couchdb Logfiles Located?

For a default linux/unix installation the logfiles are located here:

/usr/local/var/log/couchdb/couch.log

This is set in the default.ini file located here:

/etc/couchdb/default.ini

If you've installed from source and are running couchdb in dev mode the logfiles are located here:

YOUR­COUCHDB­SOURCE­DIRECTORY/tmp/log/couch.log

What Does Ibm’s Involvement Mean For Couchdb And The Community?

The main consequences of IBM’s involvement are:

The code is now being Apache licensed, instead of GPL.
Damien is going to be contributing much more time!

Mention The Main Features Of Couchdb?

JSON Documents – Everything stored in CouchDB boils down to a JSON document.
RESTful Interface – From creation to replication to data insertion, every management and data task in CouchDB can be done via HTTP.
N-Master Replication – You can make use of an unlimited amount of ‘masters’, making for some very interesting replication topologies.
Built for Offline – CouchDB can replicate to devices (like Android phones) that can go offline and handle data sync for you when the device is back online.
Replication Filters – You can filter precisely the data you wish to replicate to different nodes.

What Language Is Couchdb Written In ?

Erlang, a concurrent, functional programming language with an emphasis on fault tolerance.

Early work on CouchDB was started in C++ but was replaced by Erlang OTP platform. Erlang has so far proven an excellent match for this project.

CouchDB’s default view server uses Mozilla’s Spidermonkey JavaScript library which is written in C. It also supports easy integration of view servers written in any language

Can I Talk To Couchdb Without Going Through The Http Api?

CouchDB's data model and internal API map the REST/HTTP model so well that any other API would basically reinvent some flavor of HTTP. However, there is a plan to refractor CouchDB's internals so as to provide a documented Erlang API.

Erlang Has Been Slow To Adopt Unicode. Is Unicode Or Utf­8 A Problem With Couchdb?

CouchDB uses Erlang binaries internally. All data coming to CouchDB must be UTF­8 encoded.

What Is Couchdb?

CouchDB is a database that completely embraces the web. Store your data with JSON documents. Access your documents and query your indexes with your web browser, via HTTP. Index, combine, and transform your documents with JavaScript. CouchDB works well with modern web and mobile apps. You can even serve web apps directly out of CouchDB. And you can distribute your data, or your apps, efficiently using CouchDB’s incremental replication. CouchDB supports master-master setups with automatic conflict detection.

CouchDB comes with a suite of features, such as on-the-fly document transformation and real-time change notifications, that makes web app development a breeze. It even comes with an easy to use web administration console. You guessed it, served up directly out of CouchDB! We care a lot about distributed scaling. CouchDB is highly available and partition tolerant, but is also eventually consistent. And we care a lot about your data. CouchDB has a fault-tolerant storage engine that puts the safety of your data first.

What Does Couch Mean?

It's an acronym, Cluster Of Unreliable Commodity Hardware. This is a statement of Couch's long term goals of massive scalability and high reliability on fault prone hardware. The distributed nature and flat address space of the database will enable node partitioning for storage scalability (with a map/reduce style query facility) and clustering for reliability and fault tolerance.

Is Couchdb Ready For Production?

Yes. There are many companies using CouchDB.

What Platforms Are Supported?

Most POSIX systems, this includes GNU/Linux and OS X.
Windows is not officially supported but it should work

What Is The Use Of Couchdb?

CouchDB allows you to write a client side application that talks directly to the Couch without the need for a server side middle layer, significantly reducing development time. With CouchDB, you can easily handle demand by adding more replication nodes with ease. CouchDB allows you to replicate the database to your client and with filters you could even replicate that specific user’s data.

Having the database stored locally means your client side application can run with almost no latency. CouchDB will handle the replication to the cloud for you. Your users could access their invoices on their mobile phone and make changes with no noticeable latency, all whilst being offline. When a connection is present and usable, CouchDB will automatically replicate those changes to your cloud CouchDB.

CouchDB is a database designed to run on the internet of today for today’s desktop-like applications and the connected devices through which we access the internet.

How Can I Get A List Of The Design Documents In A Database?

Query the _all_docs view with

startkey="_design/"&endkey="_design0".

How Do I Do Sequences?

With replication sequences are hard to realize. Sequences are often used to ensure unique identifiers for each row in a database table. CouchDB generates unique ids from its own and you can specify your own as well, so you don't really need a sequence here. If you use a sequence for something else, you might find a way to express in CouchDB in another way.

How Do I Use Replication?

POST /_replicate with a post body of
{"source":"$source_database"
,
"target":"$target_database"}

Where $source_database and $target_database can be the names of local database or full URIs of remote databases. Both databases need to be created before they can be replicated from or to.

How Do I Review Conflicts Occurred During Replication?

Use a view like this:

map: function(doc) {if(doc._conflicts){emit(null,null);}}

How Can I Spread Load Across Multiple Nodes?

Using an http proxy like nginx, you can load balance GETs across nodes, and direct all POSTs, PUTs and DELETEs to a master node. CouchDB's triggered replication facility can keep multiple read-only  servers in sync with a single master server, so by replicating from master ­> slaves on a regular basis, you can keep your content up to date.

What Is Couchdb Kit?

The Couchdb Kit is used to provide a structure for your Python applications to manage and access Couchdb. This kit provides full featured  and easy client to manage and access Couchdb. It helps you to maintain databases,  to view access, Couchdb server and doc managements. Mostly python objects are reflected by the objects for convenience. The Database and server objects are used easily  as using a dict.

How Do I Use Transactions With Couchdb?

CouchDB uses an “Optimistic concurrency” model. In the simplest terms, this just means that you send a document version along with your update, and CouchDB rejects the change if the current document version doesn’t match what you’ve sent.

You can re-frame many normal transaction based scenarios for CouchDB. You do need to sort of throw out your RDBMS domain knowledge when learning CouchDB, though.

It’s helpful to approach problems from a higher level, rather than attempting to mold Couch to a SQL based world.

Can Views Update Documents Or Databases?

No. Views are always read­only to databases and their documents.

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