Synchronous replication vs asynchronous replication

Evidian SafeKit

Data loss or not on application failover?

There is a significant difference between synchronous replication vs asynchronous replication. According the choice, you may have data loss on application failover.

Synchronous replication as implemented by the SafeKit software is essential for failover of transactional applications. With synchronous replication, all committed data on the disk of the primary server are on the disk of the secondary server. With asynchronous replication, committed data on the disk of the primary server can be lost in case of failure because not copied to the secondary server. There is also an alternative solution named semi-synchronous replication, with committed data on the secondary server but not necessary on its disk.

To help you to take the right decision if you have to choose between synchronous replication vs asynchronous replication, we explain now the technical mechanisms and the impact on application failover.

Synchronous replication

With synchronous replication as implemented by the SafeKit software, when a disk IO is performed by the application or by the file system cache on the primary server, SafeKit waits for the IO acknowledgement from the local disk and from the secondary server, before sending the IO acknowledgement to the application or to the file system cache. This mechanism is essential for failover of transactional applications when they commit their transactions.

Asynchronous replication

With asynchronous replication implemented by most solutions, the IOs are placed in a queue on the primary server but the primary server does not wait for the IO acknowledgments of the secondary server. So, all data that did not have time to be copied across the network on the secondary server is lost if the primary server fails. In particular, a transactional application loses committed transactions in case of failure.

Semi-synchronous replication

With semi-synchronous replication, SafeKit always waits for the acknowledgement of the two servers before sending the acknowledgement to the application or the file system cache. But in the semi-synchronous case, the secondary sends the acknowledgement to the primary upon receipt of the IO and writes to disk after. In the synchronous case, the secondary writes the IO to disk and then sends the acknowledgement to the primary.

Conclusion

With asynchronous replication, there is data loss on failure. Even with the semi-synchronous replication, there is data loss in the special case of a simultaneous double power outage of both servers, with inability to restart on the former primary server and the requirement to re-start on the secondary server. So be very careful when choosing synchronous replication vs asynchronous replication. Always prefer a synchronous or a semi-synchronous replication for a critical application.

Video: Synchronous replication vs asynchronous replication >

Other differentatiors to consider when choosing a high availability cluster with synchronous or asynchonous replication

Best practices of a mirror cluster with replication and failover

Evidian SafeKit mirror cluster with real-time file replication and failover

All clustering features  All clustering features

Like  A SafeKit cluster runs on Windows and Linux without the need for expensive shared or replicated disk bays

Like  SafeKit includes all clustering features: synchronous real-time file replication, monitoring of server/network/software failures, automatic application restart, virtual IP address switched in case of failure to reroute clients

Dislike  This is not the case with replication-only solutions like replication at the database level which implements only replication

Like   The cluster configuration is very simple and made by means of application modules. There is no domain controller or active directory to configure as with Microsoft cluster

Like  SafeKit implements quick application restart in case of failure: around 1 mn or less (see RTO/RPO here)

Dislike  Quick application restart is not ensured with full virtual machines replication. In case of hypervisor failure, a full VM must be rebooted on a new hypervisor with a recovery time depending on the OS reboot as with VMware HA or Hyper-V cluster

Synchronous replication  Synchronous replication

Like  The real-time replication is synchronous with no data loss on failure

Dislike  This is not the case with asynchronous replication

Fully automated failback procedure  Automatic failback

Like  After a failure when a server reboots, the replication failback procedure is fully automatic and the failed server reintegrates the cluster without stopping the application on the only remaining server

Dislike  This is not the case with most replication solutions particularly with replication at the database level. Manual operations are required for resynchronizing a failed server. The application may even be stopped on the only remaining server during the resynchonization of the failed server

Replication of any type of data 

Like  The replication is working for databases but also for any files which shall be replicated

Dislike  This not the case for replication at the database level

File replication vs disk replication  File replication vs disk replication

Like  The replication is based on file directories that can be located anywhere (even in the system disk)

Disike  This is not the case with disk replication where special application configuration must be made to put the application data in a special disk

File replication vs shared disk  File replication vs shared disk

Like  The servers can be put in two remote sites

Dislike  This is not the case with shared disk solutions

Remote sites and virtual IP address  Remote sites

Like  All SafeKit clustering features are working for 2 servers in remote sites. Performances of replication depends on the interconnect latency for real-time synchronous replication and on the bandwidth for resynchronizing data on a failed server

Like  If both servers are connected to the same IP network through an extended LAN between two remote sites, the virtual IP address of SafeKit is working with rerouting at level 2

Like  If both servers are connected to two different IP networks between two remote sites, the virtual IP address can be configured at the level of a load balancer. SafeKit offers a health check: the load balancer is configured with a URL managed by SafeKit which returns OK on the primary server and NOT FOUND else. This solution is implemented for SafeKit in the Cloud but it can be also implemented with a load balancer on premise

Quorum  Quorum

Like  With remote sites, the solution works with only 2 servers and for the quorum (network isolation), a simple split brain checker to a router is offered to support a single execution

Like  This is not the case for most clustering solutions where a 3rd server is required for the quorum

Active/active cluster  Active active mirror cluster

Like  The secondary server is not dedicated to the restart of the primary server. The cluster can be active-active by running 2 different mirror modules

Dislike  This is not the case with a fault-tolerant system where the secondary is dedicated to the execution of the same application synchronized at the instruction level

Uniform high availability solution  Uniform high availability solution

Like  SafeKit implements a mirror cluster with replication and failover. But it imlements also a farm cluster with load balancing and failover. Thus a N-tiers architecture can be made highly available and load balanced with the same solution on Windows and Linux (same installation, configuration, administration with the SafeKit console or with the command line interface). This is unique on the market

Dislike  This is not the case with an architecture mixing different technologies for load balancing, replication and failover

High availability architectures comparison

Feature

SafeKit cluster

Other clusters

Software clustering vs hardware clustering  A software cluster with SafeKit installed on two servers

Like  A simple software cluster with the SafeKit package just installed on two servers		 Hardware clustering with external shared storage Network load balancers or dedicated proxy servers



Dislike  Complex hardware clustering with external storage or network load balancers
Shared nothing vs a shared disk cluster  SafeKit shared-nothing cluster: easy to deploy even in remote sites

Like  SafeKit is a shared-nothing cluster: easy to deploy even in remote sites		 Shared disk cluster: complex to deploy

Dislike  A shared disk cluster is complex to deploy
Application High Availability vs Full Virtual Machine High Availability 

Like  Application HA supports hardware failure and software failure with a quick recovery time (RTO around 1 mn or less).
Smooth upgrade of application and OS possible server by server (version N and N+1 can coexist)		 Virtual machines high availability supports only hardware failure with an recovery time depending on the OS reboot

Dislike  Full virtual machines HA supports only hardware failure with a VM reboot and a recovery time depending on the OS reboot.
Smooth upgrade not possible
High availability vs fault tolerance SafeKit high availability vs fault-tolerance

Like  No dedicated server with SafeKit. Each server can be the failover server of the other one.
Software failure with restart in another OS environment.
Smooth upgrade of application and OS possible server by server (version N and N+1 can coexist)		 Fault tolerance system

Dislike  Secondary server dedicated to the execution of the same application synchronized at the instruction level.
Software exception on both servers at the same time.
Smooth upgrade not possible
Synchronous replication vs asynchronous replication 

Like  SafeKit implements real-time synchronous replication with no data loss in case of failure		 Asynchronous replication with data loss on failure

Dislike  With asynchronous replication, there is data loss on failure
Byte-level file replication vs block-level disk replication  SafeKit cluster with byte-level file replication: simply replicates directories even in the system disk

Like  SafeKit implements real-time byte-level file replication and is simply configured with application directories to replicate even in the system disk		 Cluster with block-level disk replication: complex and require to put application data in a special disk

Dislike  Block-level disk replication is complex to configure and requires to put application data in a special disk
Heartbeat, failover and quorum to avoid 2 master nodes  Simple quorum in a SafeKit cluster with a split brain checker configured on a router

Like  To avoid 2 masters, SafeKit proposes a simple split brain checker configured on a router		 Complex quorum in other clusters: third machine, special quorum disk, remote hardware reset

Dislike  To avoid 2 masters, other clusters require a complex configuration with a third machine, a special quorum disk, a special interconnect
Virtual IP address
primary/secondary, network load balancing, failover  		No special network configuration in a SafeKit cluster

Like  No dedicated proxy servers and no special network configuration are required in a SafeKit cluster for virtual IP addresses		 Special network configuration in other clusters

Dislike  Special network configuration is required in other clusters for virtual IP addresses. Note that SafeKit offers a health check adapted to load balancers

SafeKit Modules for Plug&Play High Availability Solutions

SafeKit Modules for Plug&Play High Availability Solutions

Network load balancing and failover: click on the blue buttons

Farm modules

Windows

Linux

New application
IIS-
Apache
Amazon AWS farm
Microsoft Azure farm
Google GCP farm
Cloud generic farm

Real-time file replication and failover: click on the blue buttons

Mirror modules

Windows

Linux

New application
Microsoft SQL Server-
Oracle
MariaDB
MySQL
PostgreSQL
Firebird
Hyper-V-
KVM-
Docker-
Elasticsearch-
Milestone XProtect-
Genetec SQL Server-
Hanwha Wisenet SSM-
Nedap AEOS-
Amazon AWS mirror
Microsoft Azure mirror
Google GCP mirror
Cloud generic mirror

Demonstrations of SafeKit High Availability Software

SafeKit Webinar

This webinar presents in 10 minutes Evidian SafeKit.

In this webinar, you will understand:

  • mirror and farm clusters
  • cost savings against hardware clustering solutions
  • best use cases
  • the integration process for a new application

Microsoft SQL Server Cluster

This video shows a mirror module configuration with synchronous real-time replication and failover.

The file replication and the failover are configured for Microsoft SQL Server but it works in the same manner for other databases.

Free trial here

Apache Cluster

This video shows a farm module configuration with load balancing and failover.

The load balancing and the failover are configured for Apache but it works in the same manner for other web services.

Free trial here

Hyper-V Cluster

This video shows a Hyper-V cluster with full replications of virtual machines.

Virtual machines can run on both Hyper-V servers and they are restarted in case of failure.

Free trial here