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Top 10 Things an Ops/Sys Admin Must Know About Couchbase

04.26.2013
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The bottom line for any sysadmin is to keep the database and in case of Couchbase, keeping the cluster up and running 24x7. With demanding app requirements, your cluster needs to be properly configured, sized and monitored. This can be quite challenging as things can fail anytime without any prior warning. As an operator, everyday is different - full of surprises and challenges.

But, there are a few things every Couchbase Server operator must know. We hope that the following 10 things will come in handy and make your job easier:

1.  Keep your client libraries up-to-date

Always use the latest version of the client libraries compatible with the server. By using the latest client libraries, you can get the most out of your Couchbase cluster – recently tested code and the latest features surfaced through the client.

2.  Monitor, monitor and monitor

By using the admin dashboard or the REST API’s, you can monitor how your Couchbase cluster is doing. It is a good practice to monitor the following system metrics - cache hit ratio, disk reads, resident item ratio and disk write queue.

Cache miss  ratio should be low. This means that document keys are cached in memory making reads and writes faster. 

Resident item ratio shows the total number of active documents that reside in memory. Typically you want your working set (actively accessed documents) to be in memory for low latencies and an awesome user experience.

The number of disk reads will give you an idea of your disk I/O. It is good to keep this number low so that most reads are serviced out of RAM, which is faster.

The disk write queue will give you an idea of the number of items that need to be written to disk and the rate at which they are getting drained. If your disk writes queues are very high (millions of items) your cluster may not be sized accurately.

Here’s a link to our REST API. 

3. Split data and index files across separate disk devices

For optimum performance, separate out data and index files on different storage devices. By doing so, you would have sufficient disk I/O bandwidth for performance intensive operations such as compaction without any significant performance degradation.

Compaction is an important process that runs all the time to reclaim space given the append-only architecture in Couchbase. It can also be scheduled. Learn more about compaction here.

4.  Oh no! Rebalance failed?

Rebalancing a Couchbase cluster is a complicated operation. There are several reasons why rebalance can be slow or fail. We have worked hard to make this process more robust but if rebalance still fails  – wait for around 5 minutes and restart the process again. There should be no impact to the application since the cluster-map on the client is automatically updated during rebalance.

Remember, not to click the rebalance button many times in a row - it just restarts the process over again, wastes useful work that was done, and sometimes can cause havoc.

5.  Remember that swap rebalance is better than regular rebalance

Swap Rebalance optimizes the movement of data when you are adding and removing the same number of nodes in the same operation. Data is moved directly  from the nodes being removed to the nodes being added. This is more efficient than standard rebalancing which would normally move data across the entire cluster.

6.  Monitor the health of your system and integrate Couchbase with your ecosystem

If you’re already using an external monitoring system such as nagios to monitor your infrastructure, you can plug-in your existing monitoring system with Couchbase via the REST API.  Couchbase Server can notify and alert you so that you can check to ensure the health of your Couchbase Server cluster . Some of them include:

·  IP Address Changes If the IP address of a Couchbase Server in your cluster changes, you will be warned that the address is no longer available. You should check the IP address on the server, and update your clients or server configuration.

·  Metadata Overhead Indicates that a bucket is now using more than 50% of the allocated RAM for storing metadata and keys, reducing the amount of RAM available for data values. This is a helpful indicator that you may need to add nodes to your cluster.

·  Disk Usage Indicates that the available disk space used for persistent storage has reached at least 90% of capacity. This is a signal that you may need to add more disks to your cluster.

Couchbase also has a rich ecosystem of adapters such as a Hadoop connector and a plug-in for ElasticSearch.

7.  Size your cluster appropriately for RAM, Disk and CPU

Prior to production, it is very important to size your cluster and test it adequately. The sizing of your Couchbase cluster is going to be critical to its stability and performance.  For high-performance, your application will want as many reads as possible coming out of cache, and the system to have enough IO capacity to handle its writes.  There needs to be enough capacity in all the various areas to support everything else the system is doing while maintaining the required level of performance. 

8.  Data is replicated throughout the cluster. Optionally, indexes can be replicated as well.

Documents in Couchbase can be replicated to upto 3 times within a cluster. The number of replicas (up to 3) can be configured through the admin UI.  Mutations of documents in-memory are replicated from the active to the replica nodes. To query for a document, you can use document ID and Couchbase will lookup the document using the primary index. To query a subset of the data, you can use secondary indexes. Replication of indexes in Couchbase is optional and can be configured through the admin UI.

9.  Cross data center replication can be used for disaster recovery

  Couchbase XDCR allows you to replicate data across clusters. Data access across clusters is eventually consistent. Make sure you remember to size  your clusters for XDCR as you will need double the disk and I/O capacity as well as some more CPU.

10.  Tune indexing

When dealing with a large number of documents, there are couple of ways you can index them. The first is to index all the documents from scratch, which is a time consuming process. The other is to only update the index for documents that have changed, which is an incremental process. In Couchbase, indexes are built using incremental mapreduce making the index update or build process efficient. As an admin, you can tune the number of indexes that can be built in parallel, or the tune the time / interval between index builds. Try to group more views together in fewer number of design documents for better performance. Some more view best practices can be found here.

Just 10 things? No, of course not! Couchbase is a NoSQL database system and after you try it you will find that there’s a lot more you will learn. If you feel that I missed something important that should be added in the top 10 list, feel free to add them using the comments below. Finally, if you haven’t read the best practice guidelines for Couchbase Server 2.0, don’t forget to check them out here.  

Published at DZone with permission of Don Pinto, author and DZone MVB. (source)

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