13.6.4.Notes on the Sample's Dynamics

When the network initially starts all the publications are at level 0 and in sync. When transactions are fed into the network at a sufficiently slow rate all the servers get to process all transactions in real time. Note that the structure is such that every server does everybody else's work in addition to its own. Thus the insertion rate of the network can't be expected to be higher than that of an individual server. However read load can be spread across servers, so that this type of configuration is effective for balancing query load but not for balancing update load.

As we increase the transaction rate at each server we reach a point at which the queue of locally committed but un-replicated transactions grows faster than the other servers will absorb the feed. The servers will each eventually disconnect all synced replication to stop the queue from growing. Once the queue that no longer grows goes empty the subscribers get disconnected. At this point all servers only process their own load without any other distraction.

Next each server will notice that it is disconnected from the network and will attempt a resync as a result of the periodic scheduled call to log_sync. Each server will then re-establish a connection to every other server and start resyncing. This will lead to the network being again in sync if the per server transaction rate slows down sufficiently to allow replicators to catch up. If this does not happen the syncing can stay in progress indefinitely, until it either reaches sync or is terminated.

Typically a server's capacity for processing local transactions is greater than its capacity for replaying replication feed. This is because one thread is responsible for all replay activity while many threads can process local transactions.

The net result of this scheduling policy is that even a heavily replicated network will scale to high peak loads and will automatically return to sync state as soon as the peak is over. If guaranteed transaction level synchronicity must be maintained between servers then the application should not be written using transactional replication but rather with distributed transactions, where each commit makes sure the transaction is fully processed on each participant before returning to the client. This is however up to several times slower and will stop the entire network if a single node fails.