Data replication is the process of copying or transferring the data from a database from one server of a database to a database of another server. It helps to improve data availability and accessibility. This process also facilitates data recovery and data sharing. Data replication is performed in order to provide duplication of the whole data in case of any data failure or system failure, which will basically cause harm or loss to the data. So the recovery event tends to be fast, accurate and cost-effective. Data replication also enables accurate sharing of the information or data so that all the users will have access to the consistency of the data in real time.
There are various methods of traditional data replication, they are:
- Synchronous replication
- Asynchronous replication
- Semi-synchronous replication
Synchronous Replication
- In synchronous replication, when a disk performs I/O by the application or by the file system cache on the primary server, then the program waits for the I/O acknowledgement from the local disk and from the secondary server, before sending the I/O acknowledgement to the application or to the file of the system cache.
- Hence, this mechanism is useful and essential for the failover of transactional applications when they commit their transactions.
Asynchronous Replication
- In asynchronous replication, the I/O are been placed in a queue on the primary server, but the primary server does not wait for I/O acknowledgements of the secondary server.
- So basically, the data which does not have time to be copied across the network on the secondary server is been lost if the primary server fails. In context, a transactional application losses the committed transactions in case of failure.
Semi-Synchronous Replication
- In semi-synchronous replication, the program always waits for the acknowledgement of two servers before sending the acknowledgement to the application or the file system cache.
- But in the semi-synchronous replication case, the secondary sends an acknowledgement to the primary upon receipt of the I/O and writes to the disk after. whereas, In synchronous case, the secondary write the I/O to the disk and then tends to send the acknowledgement to the primary.
- In asynchronous replication, there is loss on the failure side. Also, there is a loss of data in semi-synchronous but only in special cases.
- So, always prefer a synchronous or a semi-synchronous replication for a critical application.
Challenges
- Consistency: Ensuring that all replicas of the data are in a consistent state can be difficult, especially in distributed systems where updates may not be immediately propagated to all replicas.
- Scalability: As the number of replicas increases, the amount of network traffic and processing power required to keep them in sync, can become a bottleneck.
- Performance: Ensuring that data is replicated quickly and efficiently can be difficult, especially in large, distributed systems.
- Data Integrity: Ensuring that data is not corrupt or lost during replication can be difficult, especially when replicating over a network.
- Failover: Automating failover to a replica in the event of a primary data loss can be a difficult task.
- Cost: Data replication can be a costly process, both in terms of hardware and software resources and in terms of the time and effort required to set up and maintain the replication infrastructure.
Advantages
- High Availability: Data replication can help ensure that data is always available, even in the event of a failure of a single node or site.
- Improved Performance: Replicating data across multiple nodes can help distribute the load and improve performance, especially in distributed systems.
- Increased Scalability: Data replication can help scale a system by allowing multiple nodes to handle the increased load.
- Improved Disaster Recovery: Data replication can help ensure that data is available in the event of a disaster, such as a natural disaster or power outage.
- Increased Security: Replicating data across multiple nodes can help protect against data loss or unauthorized access.
- Better Backup: Replicated data can act as a backup, allowing for the recovery of lost or corrupted data.
- Better Analytics: Having multiple copies of data can enable more robust analytics and data mining.
- Better compliance: Replicating data can help organizations to maintain compliance with different regulations and standards.
Disadvantages
- Increased complexity: Data replication can add complexity to a system, requiring additional hardware and software resources and specialized knowledge to set up and maintain.
- Increased overhead: Data replication can add overhead to a system, requiring additional network and storage resources, and impacting performance.
- Increased costs: Data replication can be a costly process, both in terms of hardware and software resources and in terms of the time and effort required to set up and maintain the replication infrastructure.
- Consistency issues: Ensuring that all replicas of the data are in a consistent state can be difficult, especially in distributed systems where updates may not be immediately propagated to all replicas.
- Security concerns: Securing replicated data and ensuring that only authorized users have access to it can be a challenge.
- Data integrity issues: Ensuring that data is not corrupt or lost during replication can be difficult, especially when replicating over a network.
- Potential data loss: If a replica is lost or corrupted, the data it contains may be lost as well.
- Dependence on the network: Data replication relies on the quality and reliability of the network infrastructure and can be affected by network issues such as latency or bandwidth limitations.