{"id":44322,"date":"2015-09-23T22:42:30","date_gmt":"2015-09-23T19:42:30","guid":{"rendered":"http:\/\/www.javacodegeeks.com\/?p=44322"},"modified":"2023-12-06T15:08:16","modified_gmt":"2023-12-06T13:08:16","slug":"mongodb-replication-guide","status":"publish","type":"post","link":"https:\/\/www.javacodegeeks.com\/2015\/09\/mongodb-replication-guide.html","title":{"rendered":"MongoDB Replication Guide"},"content":{"rendered":"

This article is part of our Academy Course titled MongoDB \u2013 A Scalable NoSQL DB<\/a>.<\/em><\/p>\n

In this course, you will get introduced to MongoDB. You will learn how to install it and how to operate it via its shell. Moreover, you will learn how to programmatically access it via Java and how to leverage Map Reduce with it. Finally, more advanced concepts like sharding and replication will be explained. Check it out here<\/a>!<\/em><\/p>\n

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Table Of Contents<\/h4>\n
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1. Introduction<\/a><\/dt>\n
2. Configuring Replication<\/a><\/dt>\n
3. Replication and Sharding (Partitioning)<\/a><\/dt>\n
4. Replication commands and command helpers<\/a><\/dt>\n
5. What\u00b4s next<\/a><\/dt>\n<\/dl>\n<\/div>\n

<\/a>1. Introduction<\/h2>\n

Replication is a foundational technique to keep your data safe (by providing redundancy) and highly available all the time (by providing multiple instances serving the exact copy of the data). Replication helps a lot to recover from hardware failure and prevent service interruptions. Very often it is being used to off-load some work (for example, reporting or backup) from the primary servers to dedicated replicas.<\/p>\n

There are server classes of replication: Master \u2013 Master<\/strong> (or Active \u2013 Active<\/strong>) and Master \u2013 Slave<\/strong> (or Active \u2013 Passive<\/strong>). At the moment, MongoDB<\/a> implements Master \u2013 Slave<\/strong> (or Active \u2013 Passive<\/strong>) replication (only one node can accept write operations at a time) with automatic master<\/strong> (primary<\/strong>) election in case of failure.<\/p>\n

MongoDB<\/a> supports the replication in a form of replica sets<\/strong>: a group of MongoDB<\/a> instances that maintain the same (synchronized) data across multiple instances (servers).<\/p>\n

A replica set <\/strong>consist of a single primary<\/strong> MongoDB <\/strong>instance (which accepts all write operations) and one or more secondary<\/strong> instances which synchronize with primary so to have the same data set. To support replication, the primary<\/strong> logs all changes to its data sets in its oplog: <\/strong>a special capped collection<\/strong> that keeps a rolling record of all operations that modify the data stored in the databases. Consequently, the secondaries<\/strong> replicate the primary\u2019s oplog<\/strong> and apply the operations to their data sets so the databases are kept in sync (please refer to official documentation<\/a> for more details). Please notice that those operations are applied asynchronously<\/strong> so the secondary<\/strong> instances may not always return the most up-to-date data (please refer to official documentation<\/a> for more details), the fact known as replication lag<\/strong>.<\/p>\n

Optionally, each replica set<\/strong> could include one or more arbiters<\/strong>: MongoDB <\/strong>instances which do not maintain a data set but only exist to vote in elections by contributing to majority of votes. Interestingly, a primary<\/strong> instance may step down and become secondary<\/strong>, a secondary<\/strong> may be promoted to primary<\/strong> but arbiters <\/strong>never change their roles (please refer to official documentation<\/a> for more details).<\/p>\n

When a primary<\/strong> is not available to other members of the replica set<\/strong> for more than 10 seconds, the replica set<\/strong> will attempt to promote one of the secondary<\/strong> instances to become a new primary<\/strong> by starting the election process: the first secondary<\/strong> that receives a majority of the votes becomes primary <\/strong>(please refer to official documentation<\/a> for more details).<\/p>\n

Before introducing replica set<\/strong> (which is the recommended way to configure replication), MongoDB <\/strong>supported a bit different master \/ slave <\/strong>replication model, which at the moment is considered legacy (for more details please refer to official documentation<\/a>). We are not going to cover this model in this part of the tutorial.<\/p>\n

<\/a>2. Configuring Replication<\/h2>\n

It is worth mentioning that each secondary member in the replica set<\/strong> might be configured to serve a particular purpose:<\/p>\n