Monday, September 19, 2011

Chaining Replication Clusters

MySQL built-in replication includes a concept called relay slave, which allows you to create hierarchical database clusters. You can do the same thing with Tungsten, and this can be done in more than one way. Let's start with two distinct clusters. We can follow the recipes in the Tungsten Cookbook to install a master / slave cluster in three separate hosts and a Tungsten sandbox containing another master/slave cluster. Now, we want to make the master in the sandbox a slave of the master in the first cluster, as illustrated in the figure below. Chaning clusters master to master Notice that the recipe works in exactly the same way for two distinct clusters on separate hosts. We are using one sandbox to minimize the number of hosts involved. To install the bridge between the two cluster, you go to the directory where Tungsten was installed for the master in the sandbox, and run ./tools/configure-service. The purpose of this command is to create a second service in the master, a service that will act as a slave, and fetch data from the other cluster. To do so, we need to provide some information to the service installer, the most important of which are:
  • the local-service-name, a.k.a. who's the boss, will tell the replicator that this service will live with the 'tsandbox' service, which is the local master.
  • the role tells the replicator that this service will fetch data;
  • the master-thl-host is the address where to find a master capable of feeding data to this new slave;
  • the master-thl-port and thl-port options make sure that the service uses one port for its own dispatching and another one to get data from the master.
cd $HOME/tsb2/db1
./tungsten/tools/configure-service -C \
  --local-service-name=tsandbox \
  --thl-port=12111 \
  --role=slave \
  --service-type=remote \
  --master-thl-host=r1 \
  --master-thl-port=2112 \
  --datasource=127_0_0_1 \
  --svc-start \  
  dragon
After this connection, every change in the first cluster master will be replicated to all its slaves, one of which happens to be a master, which will then distribute the same data to all its slaves. So we have a cascade hierarchical replication cluster, similar to what we can have with MySQL native replication. But Tungsten can do something more than that. In MySQL replication, you need to enable a slave to become a relay-slave. In Tungsten, you don't need to do it. Chaning clusters slave to master Using a very similar command, I can connect to a slave of the first cluster, instead of the master, and the final result will be exactly the same.
cd $HOME/tsb2/db1
./tungsten/tools/configure-service -C \
  --local-service-name=tsandbox 
  --thl-port=12111 \
  --role=slave \
  --service-type=remote \
  --master-thl-host=r3 \
  --master-thl-port=2112 \
  --datasource=127_0_0_1 \
  --svc-start \  
  dragon
In my presentations, I call this feature "slave with an attitude". Thanks to Tungsten global transaction ID, a slave can request data to any host. Since the data is not labeled in terms of log files and position (as it is in MySQL), but in terms of sequence numbers, a slave ch ask any server for a given sequence number, and that number identifies a transaction unequivocally.

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