About MongoDB Synchronization Latency issues

When your users complain that the changed information does not change, the deleted data is still displayed, you pinch the count, it is estimated that the database master never synchronized.

As with other databases that provide data synchronization, MongoDB also encounters synchronization latency issues in MongoDB's replica

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In sets mode, synchronization delay is also often a big problem that bothers us. In this paper, from the origin of synchronization delay, MongoDB synchronization delay problem in-depth analysis.

What is a synchronization delay?

First of all, to appear synchronization delay, must be in the case of data synchronization, in MongoDB, there are two ways of data redundancy, one is Master-slave mode, one is replica sets mode. Both of these patterns are essentially write on one node, and the other node synchronizes the write operation on the master node to its own side for execution. In MongoDB, all write operations will produce a oplog,oplog is every modification of the data will generate one, if you use a batch update command updated n multiple data, then sorry, Oplog will have a lot of bars, not one.

So the synchronization delay is the write operation on the master node after the execution, from the node has not brought Oplog to execute again. The greater the amount of this write operation, the greater the difference between the master node and the slave node, and the greater the synchronization delay.

Problems with synchronization Latency

First, the synchronization operation usually has two effects, one is read-write separation, the read operation is placed on the node to execute, thereby reducing the pressure of the primary node. For most scenarios, it is very useful to read and write less than the basic features. Another role is data backup, the same write operation in addition to the main node execution, on the slave node also executed, so we have more than the same data, once the primary node data because of various natural disasters can not be restored, we have at least from the node to be able to rely on. However, the master-slave delay problem may have a bad effect on the above two effects.

If the master-slave delay is too large, there will be a lot of data changes on the primary node that are not synchronized to the slave node. If the primary node fails, there are two things:

1. The primary node fails and cannot be recovered, and if the application is unable to tolerate the loss of this part of the data, we have to think of ways to retrieve this data change and then write it to the slave node. As you can imagine, even if it is possible, it is definitely a very disgusting job.

2. The master node can recover, but it takes a long time, this situation if the application can endure, we can directly let the service from the node, but for the user, for a period of time the data is lost, and if the application can not accept the inconsistency of data, then can only offline the entire business, and so on after the master node recovery service.

If you have only one slave node, when the master-slave delay is too large, because the primary node only saves the most recent part of Oplog, it may result in lean from the node, having to perform Resync operations, synchronizing the data from the master node. This will bring two questions.

1. When the full amount of synchronization from the node, the actual only the master node to save the complete data, if the primary node failure, it is likely that all the data are discarded.

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About MongoDB Synchronization Latency issues