Making sure data is safe and reliable is super important in today's world of apps and data-driven systems. MongoDB, a really popular type of NO-SQL type database, has a really important feature called WriteConcern that helps programmers control how write operations are acknowledged. It lets developers decide how many database instances need to acknowledge a write operation before considering it successful.
In this article, you will learn everything you need to know about WriteConcern MongoDB.
Understanding Write Operations in MongoDB
First, let's learn about how we write data in MongoDB before we get into WriteConcern. Writing data in MongoDB means adding, changing, or deleting information in a single document or a group of documents in the database.
When we write data, MongoDB stores it in memory and saves it to the disk so that it stays safe even if something unexpected happens. However, the speed at which this saving process happens can be influenced by things like how powerful the device is and how fast the network is.
Understanding WriteConcern in MongoDB
WriteConcern in MongoDB is all about making sure that when you write something to the database, you know it's been done correctly. It's like getting a thumbs-up from the database to say, "Yep, we got it!" You can choose different levels of WriteConcern to control how much acknowledgement you want from the database. Here are the options you can use:
{ w: <value>, j: <boolean>, wtimeout: <number> }
1. w: The w option defines the number of database instances that must acknowledge the write operation.
2. j: The j option specifies whether the write operation should be acknowledged after writing to the on-disk journal.
3. wtimeout: This option sets a time limit for acknowledging the write operation.
The w option makes the following w: <value> write concerns available:
1. "majority": This is the default option for most setups. It ensures that the write operations are securely committed to most of the database servers that hold the important data.
2. <number>: You can also choose a specific number. It means you want confirmation from many database servers that the write operation was successful.
3. <custom write concern name>: You can create your special write concern. It allows you to define specific requirements for confirming write operations based on custom settings.
The "j" option also ensures the write operation is safely written to the on-disk journal. It's like having an extra layer of protection.
There's the "w timeout" option to prevent write operations from taking forever. In milliseconds, it sets a time limit to finish the write operation. If it takes too long, there will be an error, even if the write operation eventually succeeds. But don't worry, changes made before the time limit will be done.
So, WriteConcern gives you control over how your data is stored, with options for different levels of confirmation and protection.
Implicit Default WriteConcerns
In the latest version of MongoDB (5.0), they changed the default way of handling write operations. They set the default write concern to "w: majority." But something is interesting to note if you're using a replica set with arbiters. An arbiter is a special type of member in a replica set. Unlike data-bearing members (like the primary and secondary nodes), an arbiter does not store data. Its main purpose is to participate in the election process for selecting a new primary node in case the current primary fails. It help maintain the fault tolerance and availability of the replica set without adding additional storage overhead.
In a replica set, they have this thing called a voting majority. It's like a rule they follow. The voting majority is calculated by dividing the number of voting members by two, rounding down to the nearest whole number, and then adding one to it. That's the magic formula!
If the count of data-bearing voting members is not greater than the voting majority count, MongoDB sets the default write concern to "w: 1." But for all other cases, the default write concern is "w: majority." It's like saying, "Hey, we need to make sure that most of the important guys agree before considering a write operation successful."
Levels of WriteConcern
MongoDB provides several levels of WriteConcern, each offering different guarantees and trade-offs. Let's explore each level in detail:
Level
Description
Unacknowledged WriteConcern
The fastest but least reliable level. MongoDB doesn't wait for any acknowledgement from the server regarding the success or failure of the write operation. Suitable for non-critical scenarios.
Acknowledged WriteConcern
The default level in MongoDB. MongoDB waits for acknowledgement from the server that the write operation has been successfully executed. Basic reliability is suitable for most applications.
Journaled WriteConcern
Enhances acknowledged level by adding an extra layer of durability. MongoDB waits for write operation acknowledgement and ensures data is written to a journal for additional durability.
Replicated WriteConcern
Provides increased durability by waiting for acknowledgement from the primary replica set members and replication to specified secondary members. Offers data durability and fault tolerance.
Majority WriteConcern
The highest level of durability in MongoDB. Guarantees acknowledgement by the majority of replica set members, ensuring data durability even in the event of failures affecting some members.
Additional Information on MongoDB WriteConcerns
Causally Consistent Sessions and Write Concerns:
Causal consistency is guaranteed by the client session with causally consistent sessions only if the associated read operations use the "majority" read concern and the associated write operations use the "majority" write concern.
Local Database Does Not Support Write Concern:
Write concerns are not supported in the local database. When performing an operation on a collection in the local database, any writing concern specified will be silently ignored by MongoDB.
Write Concern Provenance:
Starting from MongoDB 4.4, MongoDB tracks the provenance of the write concern, representing the specific source of the write concern.
Calculating Majority for Write Concern:
To calculate the majority for the "majority" write concern, the smaller value is taken from the following:
The majority of all available voting members, including arbiters.
The count of all data-bearing voting members.
Frequently Asked Questions
Can WriteConcern be customised in MongoDB?
Yes, you can modify a WriteConcern in MongoDB. It's similar to establishing the guidelines for how the database should perform write operations. To suit the requirements of your application, you can select from several levels of acknowledgement and durability or even design your criteria.
How can I handle WriteConcern errors in MongoDB?
To properly handle problems, you can employ strategies like retrying the action, keeping a log, and displaying error messages that make sense. These techniques will enable you to manage WriteConcern problems and guarantee that your data is securely and appropriately written.
Does WriteConcern affect read operations in MongoDB?
No, ReadConcern helps preserve consistency between read and write operations, whereas WriteConcern concentrates on the success and durability of writing operations in MongoDB. It all comes down to maintaining organisation and ensuring the accuracy of your data.
Conclusion
WriteConcern MongoDB is an important feature that makes sure your data stays safe and reliable. It helps control the acknowledgement level for write operations, meaning we can decide when a write operation is successful. In this article, we gave you an overview of WriteConcern in MongoDB and explain how it works in both standalone and replica set deployments.
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