To test network partitioning tests, we isolated one zone from the network. For a short period during this network partitioned state, the old master received some writes until the active ProxySQL node had its configuration updated by the Consul Template. This happens because the traffic is now switched to the active ProxySQL node by the internal load balancer and the active ProxySQL node has an outdated configuration since it did not receive any updates during the network partitioning.This was particularly relevant to errant transactions where the master failover happened to be in the same zone as the active ProxySQL node. In this scenario, when the network was restored the failed node came back, it still received some writes despite no longer being the master.
When it detects an unhealthy virtual appliance, Gateway Load Balancer reroutes traffic away from that instance to a healthy one, so you experience graceful failover during both planned and unplanned down time.
MHA Manager is the software that manages the failover (automatic or manual), takes decisions on when and where to failover, and manages slave recovery during promotion of the candidate master for applying differential relay logs. If the master database dies, MHA Manager will coordinate with MHA Node agent as it applies differential relay logs to the slaves that do not have the latest binlog events from the master. The MHA Node software is a local agent that will monitor your MySQL instance and allow the MHA Manager to copy relay logs from the slaves. Typical scenario is that when the candidate master for failover is currently lagging and MHA detects it do not have the latest relay logs. Hence, it will wait for its mandate from MHA Manager as it searches for the latest slave that contains the binlog events and copies missing events from the slave using scp and applies them to itself.
If you notice that the automatic failover is not working as expected either during testing or in production, see: Troubleshooting automatic failover problems in SQL Server 2012 Always On environments.
The redis-cli cluster support is very basic, so it always uses the fact thatRedis Cluster nodes are able to redirect a client to the right node.A serious client is able to do better than that, and cache the map betweenhash slots and nodes addresses, to directly use the right connection to theright node. The map is refreshed only when something changed in the clusterconfiguration, for example after a failover or after the system administratorchanged the cluster layout by adding or removing nodes.
As you can see during the failover the system was not able to accept 578 reads and 577 writes, however no inconsistency was created in the database. This maysound unexpected as in the first part of this tutorial we stated that RedisCluster can lose writes during the failover because it uses asynchronousreplication. What we did not say is that this is not very likely to happenbecause Redis sends the reply to the client, and the commands to replicateto the replicas, about at the same time, so there is a very small window tolose data. However the fact that it is hard to trigger does not mean that itis impossible, so this does not change the consistency guarantees providedby Redis cluster.
Your Windows Server failover cluster should now be working. You can testmanually moving cluster resources between your instances. You're not done yet,but this is a good checkpoint to validate that everything you've done so far isworking.
In the following video I show the advances of a continuous available file share. The upload of the file will continue even during a cluster failover. The client is a Windows 10 1809. I upload an iso to the file share I created earlier. My upload speed it about 10-20Mbit/s WAN connection. During failover to a different cluster node, the upload stops for some seconds. After successful failover it continues uploading the ISO file.
It is recommended to enable GR If the Edge node is connected to a single dual supervisor system that supports forwarding traffic when the control plane is restarting. This will ensure that forwarding table data is preserved and forwarding will continue through the restarting supervisor or control plane. Enabling BFD with such a system would depend on the device-specific BFD implementation. If the BFD session goes down during supervisor failover, then BFD should not be enabled with this system. If the BFD implementation is distributed such that the BFD session would not go down in case of supervisor or control plane failure, then enable BFD as well as GR.
It is recommended to enable GR If the Edge node is connected to a single dual supervisor system that supports forwarding traffic when the control plane is restarting. This will ensure that the forwarding table is table is preserved and forwarding will continue through the restarting supervisor or control plane. Enabling BFD with such system depends on BFD implementation of hardware vendor. If the BFD session goes down during supervisor failover, then BFD should not be enabled with this system; however, if the BFD implementation is distributed such that that the BFD session would not go down in case of supervisor or control plane failure, then enable BFD as well as GR.
GitHub's engineering team has employed several strategies for HA over the years, gradually moving towards uniformity across the organization. Since this is not restricted to MySQL, requirements for an HA solution also include cross-datacenter availability and split brain prevention. There are different possible approaches for MySQL master discovery. Previously, GitHub utilized DNS and VIP for discovery of the MySQL master node. The client applications would connect to a fixed hostname, which would be resolved by DNS to point to a VIP. A VIP allows traffic to be routed to different hosts to provide mobility without tying it down to a single host. The VIP would always be owned by the current master node. However, there were potential issues with the VIP acquire-and-release process during failover events, including split-brain situations. When this happens, two different hosts can have the same VIP and traffic can be routed to the wrong one. In addition, DNS changes have to occur to handle a master node that is in a different data center, and that can take time to propagate due to DNS caching at clients. 1e1e36bf2d