Active-Active Geo-Distributed Redis (CRDB)
In Redis Enterprise, active-active geo-distribution is based on CRDT technology. The Redis Enterprise implementation of CRDT is called Conflict-Free Replicated Database (CRDB). With CRDBs, applications can read and write to the same data set from different geographical locations seamlessly and with latency less than 1 ms, without changing the way the application connects to the database.
CRDBs also provide disaster recovery and accelerated data read-access for geographically distributed users.
Considerations for Conflict-free Replicated Databases (CRDBs)
CRDBs are based on multi-master replication that is configured to run on each database. A CRDB is made up of instances of the data that are each stored on an RS cluster.
Before configuring a CRDB, you must:
- If the CRDB spans a WAN, establish a VPN between each networks that hosts a cluster with a CDRB instance.
Setup RS clusters for each CRDB instance.
All clusters must have the same RS version.
Configure FQDNs in a DNS server for connections to the cluster.
CRDBs are not compatible with the Discovery Service for inter-cluster communications, but are compatible with local application connections.
Configure the network so that all nodes in each cluster can connect to the proxy port and the cluster admin port (9443) of each cluster.
Confirm that a network time service is configured and running on each node in all clusters.
CRDB Current Limitations
- RS is limited to five Participating Clusters or CRDB Instances per CRDB.
- An existing database cannot be changed into a CRDB. To move data from an existing database to a CRDB you must create a new CRDB and migrate the data.
- CRDBs do not support Redis modules.
- CRDBs require FQDNs or mDNS (development only). Discovery Service is not supported with CRDBs.
- CRDBs are not compatible with Replica Of.
Network Time Service (NTP or Chrony)
For CRDBs, you must use a time service like NTP or Chrony. This is critical to minimize time drift both intercluster and intracluster for CRDBs on an ongoing basis.
There may be times that the OS system time is used for conflict resolution between CRDB Instances, although that rarely happens. The built-in vector clocks tell RS the order of operations, or identifies that the data operations were concurrent. When there is no option to intelligently handle conflicting writes, OS timestamps are used in resolving the conflict. For example, in certain cases "string type" uses timestamps to resolve conflicts.
The RS installation checks if there is a network time service installed, running, and configured to start on boot.
- If no network time service is found, the installation asks if you want to "tune the system".
- If you answer yes, you are prompted to install and configure a network time service.
- If you answer yes, the NTP is installed.
2017-10-30 11:24:07 [?] Do you want to automatically tune the system for best performance [Y/N]? Y 2017-10-30 11:24:15 [?] Cluster nodes must have their system time synchronized. Do you want to set up NTP time synchronization now [Y/N]? Y 2017-10-30 11:24:19 [.] Making sure NTP is installed and time is set.
RS assumes that networking between the clusters is already configured when you create a CRDB. For security purposes, recommend that you configure a secure VPN between all clusters that host a CRDB instance. The setup of the CRDB fails if there is no connectivity between the clusters.
For initial configuration and ongoing maintenance of a CRDB, every node must have access to the REST API ports of every other node. You must also open ports for VPNs and Security groups.
For synchronization, CRDBs operate over the standard endpoint ports. The endpoint port that you configure when you create the CRDB is the endpoint port of the proxy for that CRDB on each cluster.
You can set the data persistence configuration, including AOF (Append-Only File) data persistence and snapshot, for each participating cluster.
Each node in a cluster containing a CRDB instance hosts a process called syncer. The syncer process:
- Connects to the other cluster proxy
- Reads data from that database
- Writes the data to the master shard of that database
Some replication capabilities are also included in open source redis.
The Master at the top of the master-slaves tree creates a replication ID. This replication ID is identical for all slaves in that tree. When a new master is appointed, the replication ID changes but a partial sync from the previous ID is still possible. In a partial sync, the backlog of operations since the offset are transferred as raw operations. In a full sync, the data from the master is transferred to the slave as an RDB file which is followed by a partial sync.
Partial synchronization requires a backlog large enough to store the data operations until connection is restored.
Syncer in Active-Active Replication (CRDB)
In the case of a CRDB:
- Multiple past replication IDs and offsets are stored to allow for multiple syncs
- The Active-Active backlog is also sent to the slave during a full sync
The scenarios in which CRDB updates to other instances use partial synchronization are:
- Failover of master shard to slave shard
- Restart or crash of slave shard that requires sync from master
- Migrate slave shard to another node
- Migrate master shard to another node as a slave using failover and slave migration
- Migrate master shard and preserve roles using failover, slave migration, and second failover to return shard to master
Syncer in Active-Passive Replication (Replica Of)
In Replica Of, the master node does not transfer the replication backlog to its slave. Whenever a synchronization is necessary, the slave has no backlog and can only do a full sync. But, in a controlled failover the demoted master still has the replication backlog, and when the syncer connects, it can do a partial sync.