Synchronous replication vs asynchronous replication
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 first server are on the disk of the second server. With asynchronous replication, committed data on the disk of the first server can be lost in case of failure. There is also an alternative solution named semi-synchronous replication, with commited data on the second server but not necessary on 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 requires the bandwidth of a LAN between the servers, possibly with an extended LAN in two geographically remote computer rooms. Asynchronous replication can be implemented on a low speed WAN.
Synchronous replication
With synchronous file-based replication as implemented by the SafeKit high availability software, when a disk IO is performed by the application or by the file cache system on the primary server inside a replicated file, 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. Note that SafeKit makes byte-level file replication by replicating directories and not entire disks, which greatly simplifies the configuration of a cluster.
Note that many high availability solutions with synchronous replication of Microsoft SQL Server, PosgreSQL, flat files etc... are available with a Safekit free trial in the following article. Note also that SafeKit is able to automatically re-synchronize a failed server without any manual operation and without stopping the critical application. It is an important differentiator when comparing SafeKit to replication solutions whose failover only works once: reintegrating the failed server in the cluster requires complex and non-automated manual operations.
Asynchronous replication
With asynchronous file-based 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 second server is lost if the first server fails. In particular, a transactional application loses committed data in case of failure.
Semi-synchronous replication
With the semi-synchronous file-based replication as implemented by the SafeKit high availability software, the asynchrony is not made on the primary server but on the secondary one. In this solution, SafeKit always waits for the acknowledgement of the two servers before sending the acknowledgement to the application or the system cache. But on the secondary, there are 2 options asynchronous or synchronous.
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.
But be careful, the synchronous mode on the secondary server is required if we consider a simultaneous double power outage of two servers, with inability to restart the former primary server and requirement to re-start on the secondary.
Conclusion
You see that just delaying write on the secondary server has a direct impact on critical application failover. So be very careful when choosing synchronous replication vs asynchronous replication. Always prefer a synchronous or a semi-synchronous replication for a critical application.
Key differentiators of file replication and failover with the Evidian SafeKit mirror cluster
Evidian SafeKit mirror cluster with real-time file replication and failover |
| Synchronous replication |  |  The real-time replication is synchronous with no data loss on failure
 This is not the case with asynchronous replication
|
| After a server failure, fully automated failback procedure |  | After a server failure and a failover, the replication failback procedure
is fully automatic on the failed server and without stopping
the application
on the only remaining server 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 |
| All clustering features |  | The solution includes all clustering features:
server failure monitoring, network failure monitoring, software failure monitoring,
automatic
application
restart with a quick recovery time, a virtual IP address switched in case of failure to automatically reroute clients.
A clustering configuration is simply made by means of a high availability application module.
There is no domain controller or active directory to configure on Windows. The solution works on Windows and Linux This is not the case with replication-only solutions like replication at the database level 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 an unknown recovery time |
| Replication of any type of data |  | The replication is working for
databases
but also for any files which shall be replicated This not the case for replication at the database level |
| File replication vs disk replication |  | The replication is based on file directories that can be located anywhere (even in the system disk) 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 |  | The servers can be put in two remote sites This is not the case with shared disk solutions |
| Remote sites |  | With remote sites, the solution works with only 2 servers and for the quorum (network isolation), a simple split brain checker is offered This is not the case for most clustering solutions where a 3rd server is required for the quorum 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 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 Virtual IP address is not offered by replication-only solution |
| Uniform high availability solution |  | 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 me 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 This is not the case with an architecture mixing different technologies for load balancing, replication and failover |
FAQ on Evidian SafeKit (synchronous replication)
3 demonstrations [+]
Application high availability modules [+]
Cloud solutions [+]
Customers [+]
Video surveillance (CCTV) [+]
Transport [+]
Paris transport company (RATP) chose the SafeKit high availability and load balancing solution for the centralized control room of line 1 of the Paris subway.
20 SafeKit clusters are deployed on Windows and Linux.
Stéphane Guilmin, RATP, Project manager says:
“Automation of line 1 of the Paris subway is a major project for RATP, requiring a centralized command room (CCR) designed to resist IT failures. With SafeKit, we have three distinct advantages to meet this need. Firstly, SafeKit is a purely software solution that does not demand the use of shared disks on a SAN and network boxes for load balancing. It is very simple to separate our servers into separate machine rooms. Moreover, this clustering solution is homogeneous for our Windows and Unix platforms. SafeKit provides the three functions that we needed: load balancing between servers, automatic failover after an incident and real time data replication.”
And also, Philippe Marsol, Atos BU Transport, Integration Manager says:
“SafeKit is a simple and powerful product for application high availability. We have integrated SafeKit in our critical projects like the supervision of Paris metro Line 4 (the control room) or Marseille Line 1 and Line 2 (the operations center). Thanks to the simplicity of the product, we gained time for the integration and validation of the solution and we had also quick answers to our questions with a responsive Evidian team.”
Best use cases [+]
Distinctive advantages [+]
More on the mirror cluster [+]
Demonstration of real-time replication and failover [+]
What are the advantages [+]
- Low Complexity
- Plug&Play deployment with no specific skills
- Suitable for large deployments in many sites (very simple to deploy)
- 2 nodes (3 nodes replication possibility)
- No shared storage requirement
- No Domain Controller requirement
- Same solution on Windows and Linux
- Support Windows Server and Client OS editions
- Well documented API and support
- Synchronous data replication (no data loss in case of failure)
- Replicated directories can be in the system disk
- Supports multiple heartbeats and vitual IP addresses
- Offers configurable software, hardware and network checkers
- For the quorum, does not require a special disk or a third machine or a dedicated link between both servers
- Automatic failover of services with a recovery time in the order of one minute
- Automatic failback when a server comes back after a failure (no manual operation)
- A very simple console to deploy the solution and to maintain it afterwards for end-customer
- Supports human errors (40% of causes of unavailability) thanks to its simplicity
- Supports software failures (40% of causes of unavailability): regression on software update (version N and N+1 can coexist), Operating System frozen, software bug
- Supports hardware and environment failures (20% of causes of unavailability), including the complete failure of a computer room with 2 nodes in two remote sites
What is the recovery time (RTO) [+]
RTO is the time during which the application is unavailable in case of failure. RTO of the SafeKit mirror solution is in the order of 1 mn.
For a hardware failure, RTO = heartbeat timeout (default 30 s, can be changed in userconfig.xml) + time to restart services.
For a software failure or an administrator restart, RTO = time to (cleanly) stop services + time to restart them.
Be careful, with solutions that reboot a full virtual machine in case of failure, the RTO is unpredictable as manual operations may be required after a hardware crash to reboot the virtual machine.
What is the data loss (RPO) [+]
RPO reflects the data loss in case of failure. RPO of the SafeKit mirror solution is 0 as the replication is synchronous and real-time.
Be careful, with asynchronous replication, RPO is not 0 and there is data loss in case of failure when the application restarts on the secondary server.
More information on the architecture [+]
More on the farm cluster [+]
Demonstration of load balancing and failover [+]
What are the advantages [+]
- Low Complexity
- Plug&Play deployment with no specific skills
- Suitable for large deployments in many sites (very simple to deploy)
- 2 nodes or more
- No network load balancers requirement
- No proxy server requirement (above the farm cluster)
- No Domain Controller requirement
- No restriction in VMware due to multicast or unicast address
- Same solution on Windows and Linux
- Support Windows Server and Client OS editions
- Well documented API and support
- Supports multiple monitoring channels on multiple networks for server failure detection
- Supports multiple vitual IP addresses
- Offers configurable software, hardware and network checkers
- Offers the mirror cluster with synchronous real-time replication and failover
- Automatic failover with a recovery time in the order of a few seconds
- Automatic failback when a server comes back after a failure (no manual operation)
- A very simple console to deploy the solution and to maintain it afterwards for end-customer
- Supports human errors (40% of causes of unavailability) thanks to its simplicity
- Supports software failures (40% of causes of unavailability): regression on software update (version N and N+1 can coexist), Operating System frozen, software bug
- Supports hardware and environment failures (20% of causes of unavailability), including the complete failure of a computer room with 2 nodes in two remote sites
What is the recovery time (RTO) [+]
RTO is the time during which the application is unavailable in case of failure. RTO of the SafeKit farm solution is in the order of a few seconds on hardware failure.
For a hardware failure, RTO = failure detection timeout through monitoring channels (default a few seconds): after the timeout the load balancing filters are reconfigured.
For a software failure or an administrator restart, RTO = time to (cleanly) stop services + time to restart them.
More information on the architecture [+]
SafeKit Webinar [+]
Pricing - Free trial [+]
