Changed in version 4. New in version 4. MongoDB applies database operations on the primary and then records the operations on the primary's oplog. The secondary members then copy and apply these operations in an asynchronous process.
All replica set members contain a copy of the oplog, in the local. To facilitate replication, all replica set members send heartbeats pings to all other members. Any secondary member can import oplog entries from any other member. Each operation in the oplog is idempotent. That is, oplog operations produce the same results whether applied once or multiple times to the target dataset.
When you start a replica set member for the first time, MongoDB creates an oplog of a default size if you do not specify the oplog size. The default oplog size is MB of either physical memory or free disk space depending on the storage engine:. In most cases, the default oplog size is sufficient. However, most replica sets have much lower operation volumes, and their oplogs can hold much higher numbers of operations.
Before mongod creates an oplog, you can specify its size with the oplogSizeMB option. Once you have started a replica set member for the first time, use the replSetResizeOplog administrative command to change the oplog size.
The mongod only truncates an oplog entry if : The oplog has reached the maximum configured size, and The oplog entry is older than the configured number of hours based on the host system clock.
This article helps you wrap your head around MongoDB Oplog, after which it explores its features, its data operations, and the processing methodology. MongoDB uses a Transaction log internally just like many other databases. This is known as an Oplog in MongoDB. This serves as an internal log of every internal operation used for replication in a MongoDB Cluster. This can be accessed in a fashion similar to any other Collection in MongoDB. MongoDB allows for two operations: commands and data manipulation ops.
Before delving into the MongoDB commands and data manipulation ops, here is a look at the Oplog entry structure. The code source for the same is here. The above image gives you an idea of the most common and useful fields, with many more fields for other types of operation present. After MongoDB has rolled in new updates, MongoDB Oplog is no longer simply a special capped Collection that keeps a rolling record of all operations, that modify the data stored in your databases.
After version 4. This is done to avoid deleting the majority commit point. As MongoDB ushered in version 4. MongoDB would remove an entry only under the following conditions:. Every operation taking place in the MongoDB Oplog is idempotent. This means that these operations give the same results irrespective of whether it has been applied once or many times on the target dataset. Coming to its features, here are a few significant ones:. When a Replica Set member is started for the very first time, MongoDB creates an Oplog of a default size unless mentioned otherwise.
Here are the sizes for different OS Operating Systems :. Generally, the default size for the Oplog in MongoDB is more than enough. This allows you to resize it dynamically without having to restart the mongod process. It automatically truncates it, starting from the oldest entries. This is done to maintain the configured maximum MongoDB Oplog size. You can configure the Minimum Oplog Retention Period when starting the mongod process through either of the following two steps:.
Setting the Minimum Retention Period while the mongod is running, overrides any values that you may have set during its startup. You would have to update the value of the corresponding configuration file setting or command-line option to maintain those changes through a server restart. This can also help you determine any unwanted replication delays. Uday Uday 1, 4 4 gold badges 24 24 silver badges 43 43 bronze badges.
Add a comment. Active Oldest Votes. Improve this answer. Evgeni Nabokov 2, 1 1 gold badge 28 28 silver badges 33 33 bronze badges. What is the log that slaves use to save the oplog portion read on their server Starting in MongoDB 4.
EvgeniNabokov thanks! If any of these operations fails then other related operations must be rolled back to prevent inconsistency. For this MongoDB uses WiredTiger transactions: begin transaction apply change to collection update index add the change to the oplog commit the transaction Low-level - WiredTiger runs the transaction and adds the changes to journal file.
Maksym Iasynetskyi Maksym Iasynetskyi 81 1 1 silver badge 4 4 bronze badges. Journaling should be coherent to updating the log docs. Sign up or log in Sign up using Google. Sign up using Facebook. Sign up using Email and Password. Post as a guest Name.
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