Replication

In TomP2P, there are indirect and direct replication mechanisms available. The direct replication can be described as peers constantly publishing their content. This means that a single peer is responsible for its content and periodically republishes the content. If this peer stops doing so, the content will eventually time out and gets removed. The information about responsibilities need to be stored along with the data in the Storage class. The indirect replication can be described as peers publishing content for others. The peer closest to a location ID is considered as the responsible peer and replicates data if necessary.

Direct Replication

The direct replication can be turned on by setting DHTBuilder.setRefreshSeconds(int refreshSeconds) to a value greater than 0. This tells TomP2P to refresh the entry every refreshSeconds seconds. The information what content has to be refreshed is stored in the Storage class, which can be either memory or disk-based. The add and put methods are similar with respect to the direct replication except that add calculates a hash of the data on the sender, while put does not. The time-to-live (TTL) in combination with storage tells all other peers (except the sender) to invalidate the entry after this time, if no refresh has been performed. The sender never expires “its” content unless the sender calls remove.

The TTL and refreshSeconds for the remove operation has a different meaning. The refresh tells the sender to remove the entry every refreshSeconds until TTL has been reached. These values are used on the sender only.

The tracker does only direct replication and never active replication. It has a fixed timing, which can be set with TrackerStorageMemory.setTrackerTimoutMillis(TTL). for the TrackerStorage and its TTL * 0.75 for the refresh interval.

The following example executes 5 times put

PutBuilder putBuilder = peer.put(Number160.ONE).data(new Data("test"));
JobScheduler replication = new JobScheduler(peer.peer());
Shutdown shutdown = replication.start(putBuilder, 1000, -1, new AutomaticFuture() {
 @Override
 public void futureCreated(BaseFuture future) {
  System.out.println("put again...");
 }
});
Thread.sleep(NINE_SECONDS);
shutdown.shutdown();

Indirect Replication

Since version 3.2, TomP2P supports indirect replication. This feature can be enabled for put/get storage with DHTBuilder.setDefaultStorageReplication() and for tracker storage TrackerBuilder.setDefaultTrackerReplication(). Indirect replication is triggered by two events:

• One event is if a peer x gets to know a new peer y. Then peer x checks all its stored data for which peer x is responsible for. If peer y is the new responsible for this stored data, the content will be handed over to peer y and peer y will be the new responsible peer. If peer y leaves the system that was responsible for content and peer x becomes the new responsible peer, peer x checks if there are enough replicas for this content. If not enough replicas are stored on peers, peer x starts to replicate the content.
• The second event is triggered by a timer. Peer x checks periodically all stored data for enough replicas in the network. If not enough replicas are stored on peers, peer x starts to replicate the content.

new IndirectReplication(peer).start();