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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 versus asynchronous replication means data loss or not on failover.

Synchronous replication

With synchronous replication as implemented by SafeKit, 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 as implemented by Carbonite Double Take, 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 >

How the SafeKit mirror cluster works?

Step 1. Real-time replication

Server 1 (PRIM) runs the application. Clients are connected to a virtual IP address. SafeKit replicates in real time modifications made inside files through the network.

File replication at byte level in a mirror cluster

The replication is synchronous with no data loss on failure contrary to asynchronous replication.
You just have to configure the names of directories to replicate in SafeKit. There are no pre-requisites on disk organization. Directories may be located in the system disk.

Step 2. Automatic failover

When Server 1 fails, Server 2 takes over. SafeKit switches the virtual IP address and restarts the application automatically on Server 2.
The application finds the files replicated by SafeKit uptodate on Server 2. The application continues to run on Server 2 by locally modifying its files that are no longer replicated to Server 1.

Failover in a mirror cluster

The failover time is equal to the fault-detection time (30 seconds by default) plus the application start-up time.

Step 3. Automatic failback

Failback involves restarting Server 1 after fixing the problem that caused it to fail.
SafeKit automatically resynchronizes the files, updating only the files modified on Server 2 while Server 1 was halted.

Failback in a mirror cluster

Failback takes place without disturbing the application, which can continue running on Server 2.

Step 4. Back to normal

After reintegration, the files are once again in mirror mode, as in step 1. The system is back in high-availability mode, with the application running on Server 2 and SafeKit replicating file updates to Server 1.

Return to normal operation in a mirror cluster

If the administrator wishes the application to run on Server 1, he/she can execute a "swap" command either manually at an appropriate time, or automatically through configuration.

Typical usage with SafeKit

Why a replication of a few Tera-bytes?

Resynchronization time after a failure (step 3)

  • 1 Gb/s network ≈ 3 Hours for 1 Tera-bytes.
  • 10 Gb/s network ≈ 1 Hour for 1 Tera-bytes or less depending on disk write performances.

Alternative

Why a replication < 1,000,000 files?

  • Resynchronization time performance after a failure (step 3).
  • Time to check each file between both nodes.

Alternative

  • Put the many files to replicate in a virtual hard disk / virtual machine.
  • Only the files representing the virtual hard disk / virtual machine will be replicated and resynchronized in this case.

Why a failover ≤ 32 replicated VMs?

  • Each VM runs in an independent mirror module.
  • Maximum of 32 mirror modules running on the same cluster.

Alternative

  • Use an external shared storage and another VM clustering solution.
  • More expensive, more complex.

Why a LAN/VLAN network between remote sites?

Alternative

  • Use a load balancer for the virtual IP address if the 2 nodes are in 2 subnets (supported by SafeKit, especially in the cloud).
  • Use backup solutions with asynchronous replication for high latency network.

SafeKit HA Solutions and Quick Installation Guides (with preconfigured .safe modules)

How to Configure High Availability (HA) for Database Clusters (Replication and Failover)

Quick installation guides for configuring High Availability (HA), replication, and failover for the following database clusters using SafeKit:

How to Configure High Availability (HA) for Full Virtual Machines (VMs) or Containers

Quick installation guides for configuring High Availability (HA) and failover for the following Virtual Machine and container technologies using SafeKit:

How to Configure High Availability (HA) for Physical Security Systems

Quick installation guides for configuring High Availability (HA) and failover for the following Physical Security and Video Management Systems (VMS) using SafeKit:

How to Configure High Availability (HA) for Siemens Products

Quick installation guides for configuring High Availability (HA) and failover for the following Siemens industrial and security products using SafeKit: