MySQL InnoDB Cluster is an Oracle High Availability solution that can be easily installed over MySQL to provide high availability with multi-master capabilities and automatic failover. In the previous post we presented the first component of InnoDB Cluster, group replication. Now we will go through the second component, MySQL Router. We will address MySQL Shell in a final instalment of this three-part series. By then, you should have a good overview of the features offeed by MySQL InnoDB Cluster.
This component is responsible for distributing the traffic between members of the cluster. It is a proxy-like solution to hide cluster topology from applications, so applications don’t need to know which member of a cluster is the primary node and which are secondaries.
The tool is capable of performing read/write splitting by exposing different interfaces. A common setup is to have one read-write interface and one read-only interface. This is default behavior that also exposes 2 similar interfaces to use x-protocol (i.e. used for CRUD operations and async calls).
The read and write split is done using a concept of roles: Primary for writes and Secondary for read-only. This is analogous to how members of cluster are named. Additionally, each interface is exposed via a TCP port so applications only need to know the IP:port combination used for writes and the one used for reads. Then, MySQL Router will take care of connections to cluster members depending on the type of traffic to server.
MySQL Router is a very simple tool, maybe too simple as it is a layer four load balance and lacks some of the advanced features that some of it’s competitors have (e.g.. ProxySQL).
Here is a short list of the most important features of MySQL Router:
- As mentioned, read and write split based on roles.
- Load balancing both for reads and writes use different algorithms.
- Configuration is stored in a configuration test file.
- Automatically detects cluster topology by connecting and retrieving information, based on this information the router configures itself with default rules.
- Automatically detects failing nodes and redirects traffic accordingly.
Algorithms used for routing
An important thing to mention is the routing_strategy algorithms that are available, as they are assigned by default depending on the routing mode:
- For PRIMARY mode (i.e. writer node – or nodes): uses the first-available algorithm that picks the first writer node from a list of writes and in case of failure moves to the next in the list. If the failing node comes back to life, it’s automatically added to the list of servers and become PRIMARY again when cluster assign this status. When no writers are available then write routing is stopped
- For read-only mode (i.e. read nodes): uses the round-robin algorithm between servers listed in the destinations variable. This mode splits read traffic between all servers in an even manner.
Additional routing_strategy algorithms :
- next-available: similar to first-available but in this case a failing node is marked as crashed and can’t get back into the rotation.
- round-robin-with-fallback: same as round-robin but it includes the ability in this case of using servers from the primary list (writers) to distribute the read traffic.
A sample configuration
For performance purposes it’s recommended to setup MySQL Router in the same place as the application, considering an instance per application server.
Here you can see a sample configuration file auto-generated by
$ cat /etc/mysqlrouter/mysqlrouter.conf
# File automatically generated during MySQL Router bootstrap
level = INFO
We are almost done now, only one post left. The final post is about our third component MySQL Shell, so please keep reading.
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