Replication is a method that ensures that multiple database instances are storing the same data. To enable replication, there must be at least two instances of Memgraph in a cluster. Each instance has one of either two roles: main or replica. The main instance is the instance that accepts writes to the database and replicates its state to the replicas. In a cluster, there can only be one main. There can be one or more replicas. None of the replicas will accept write queries, but they will always accept read queries (there is an exception to this rule and is described below). Each instance will always be reachable using the standard supported communication protocols. The replication will replicate WAL data. All data is transported through a custom binary protocol that will try remain backward compatible, so that replication immediately allows for zero downtime upgrades.
Each replica can be configured to accept replicated data in one of the following modes:
When the data is replicated to a replica synchronously, all of the data of a currently pending transaction must be sent to the synchronous replica before the transaction is able to commit its changes.
This mode has a positive implication that all data that is committed to the main will always be replicated to the synchronous replica. It also has a negative performance implication because non-responsive replicas could grind all query execution to a halt.
This mode is good when you absolutely need to be sure that all data is always consistent between the main and the replica.
When the data is replicated to a replica asynchronously, all pending transactions are immediately committed and their data is replicated to the asynchronous replica in the background.
This mode has a positive performance implication in which it won't slow down query execution. It also has a negative implication that the data between the main and the replica is almost never in a consistent state (when the data is being changed).
This mode is good when you don't care about consistency and only need an eventually consistent cluster, but you care about performance.
When the data is replicated to a replica semi-synchronously, the data is replicated using both the synchronous and asynchronous methodology. The data is always replicated synchronously, but, if the replica for any reason doesn't respond within a preset timeout, the pending transaction is committed and the data is replicated to the replica asynchronously.
This mode has a positive implication that all data that is committed is mostly replicated to the semi-synchronous replica. It also has a negative performance implication as the synchronous replication mode.
This mode is useful when you want the replication to be synchronous to ensure that the data within the cluster is consistent, but you don't want the main to grind to a halt when you have a non-responsive replica.
Addition of a new replica
Each replica, when added to the cluster (in any mode), will first start out as an asynchronous replica. That will allow replicas that have fallen behind to first catch-up to the current state of the database. When the replica is in a state that it isn't lagging behind the main it will then be promoted to a semi-synchronous or synchronous replica. Replicas that are added as asynchronous replicas will remain asynchronous.