Replication is one of the central features of CouchDB. In CouchDB 2.0, replication takes advantage of clustering to achieve scalability and high availability. Some configuration defaults have changed, some aspects work a bit differently, there were many bug fixes, performance improvements, and of course, a set of exciting new features were added.
Replicating in a Cluster
Just like in CouchDB 1.x, there are still two ways to start replications: one is to write a document in a “_replicator” database, which will create a persistent replication, the other is via an HTTP request to the “_replicate” endpoint. The former is the preferred way, as the replication tasks will persist if the cluster restarts and the other doesn’t.
In either case CouchDB 2.0 makes sure the task is running on only one node in the cluster. In case of persistent replications it runs on the node where the first shard of the replication document is located. This is a nice performance optimization – if the document is updated, only a node local change feed is needed to notify replicator code of the update. In case of a replication posted to “_replicate” endpoint, the task is assigned to a cluster node based on a hash of source and target parameters. In both cases replication tasks should be uniformly distributed across the cluster, and with each newly added node users will see a performance improvement.
When cluster configuration changes, for example, because nodes are added or removed, placement of persistent replication tasks is re-evaluated and some replications might end up running on a different node. This is done automatically and is transparent to the user. However, replications created via the “_replicate” endpoint stay running where they have initially started, and are not moved to new nodes; this goes along with their transient nature.
Remote vs. Local
An interesting aspect related to replications in a cluster is how sources and targets are handled. In CouchDB 1.x both “local” and “remote” versions of sources and targets would be useful. Local ones are specified by using just the database name, and it refers to a database local to the server. Remote ones use a full URL to refer to the database. This could be a database on the same server or a database in another part of the world. Because of clustering in 2.0, a “local” database has different semantics – it means a database which is not clustered and lives only on the current node where the replication task is running. These databases are usually accessed via the node local API endpoint (default port 5986) and mostly likely are not what users would want to access directly. In other words, in CouchDB 2.0 in most cases it is better to use full URLs when specifying targets and sources, even if referring to databases on same cluster.
Multiple Replicator Databases
One of the configuration changes in 2.0 means that it’s no longer possible to change the name of the replicator database, it is always “_replicator”. On the other hand, it is now possible to have multiple replicator databases. Any database which ends with “/_replicator” suffix will be considered a replicator database, and will be monitored and processed accordingly just like the main “_replicator”. This allows greater flexibility, for example, by having a temporary db called “dev/_replicator” used for testing or others experiments. When finished with it, one can just delete “dev/_replicator” and all those replications will be canceled and cleaned up from the system.
Another configuration change is the default checkpoint interval for replications went up from 5 seconds to 30 seconds. As replications make progress they periodically write checkpoints to both target and source databases. In 1.0 this happens every 5 seconds by default. In 2.0, because a cluster will usually run a larger number of replications, this default has been increased to 30 seconds. This is just a default, and the setting is configurable via the “checkpoint_interval” parameter.
A New Way to Filter
Saving the best for last, perhaps the nicest “feature” is the base replication protocol has not changed. It is possible to replicate between a CouchDB 2.0 cluster and CouchDB 1.x instances. One can still use a variety of custom replication topologies for which CouchDB is known for: push, pull, and bidirectional replications, and of course continue to replicate with our in-browser sister-project, PouchDB . Also what used to be single machine node can now be replaced by a fault tolerant and scalable cluster.
Nick Vatamaniuc is a software engineer at Cloudant and an Apache CouchDB committer.
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