etcd

The M3 stack leverages etcd as a distributed key-value storage to:

1. Update cluster configuration in realtime
2. Manage placements for our distributed / sharded tiers like M3DB and M3Aggregator
3. Perform leader-election in M3Aggregator

and much more!

Overview

M3DB ships with support for running embedded etcd (called seed nodes), and while this is convenient for testing and development, we don’t recommend running with this setup in production.

Both M3 and etcd are complex distributed systems, and trying to operate both within the same binary is challenging and dangerous for production workloads.

Instead, we recommend running an external etcd cluster that is isolated from the M3 stack so that performing operations like node adds, removes, and replaces are easier.

While M3 relies on etcd to provide strong consistency, the operations we use it for are all low-throughput so you should be able to operate a very low maintenance etcd cluster. A 3-node setup for high availability should be more than sufficient for most workloads.

Configuring an External etcd Cluster

M3DB

Most of our documentation demonstrates how to run M3DB with embedded etcd nodes. Once you’re ready to switch to an external etcd cluster, all you need to do is modify the M3DB config to remove the seedNodes field entirely and then change the endpoints under etcdClusters to point to your external etcd nodes instead of the M3DB seed nodes.

For example this portion of the config

config:
    service:
        env: default_env
        zone: embedded
        service: m3db
        cacheDir: /var/lib/m3kv
        etcdClusters:
            - zone: embedded
              endpoints:
                  - http://m3db_seed1:2379
                  - http://m3db_seed2:2379
                  - http://m3db_seed3:2379
    seedNodes:
        initialCluster:
            - hostID: m3db_seed1
              endpoint: http://m3db_seed1:2380
            - hostID: m3db_seed2
              endpoint: http://m3db_seed2:2380
            - hostID: m3db_seed3
              endpoint: http://m3db_seed3:2380

would become

config:
    service:
        env: default_env
        zone: embedded
        service: m3db
        cacheDir: /var/lib/m3kv
        etcdClusters:
            - zone: embedded
              endpoints:
                  - http://external_etcd1:2379
                  - http://external_etcd2:2379
                  - http://external_etcd3:2379

Note: M3DB placements and namespaces are stored in etcd so if you want to switch to an external etcd cluster you’ll need to recreate all your placements and namespaces. You can do this manually or use etcdctl’s Mirror Maker functionality.

M3Coordinator

M3Coordinator does not run embedded etcd, so configuring it to use an external etcd cluster is simple. Just replace the endpoints under etcdClusters in the YAML config to point to your external etcd nodes instead of the M3DB seed nodes. See the M3DB example above for a detailed before/after comparison of the YAML config.

etcd Operations

Embedded etcd

If you’re running M3DB seed nodes with embedded etcd (which we do not recommend for production workloads) and need to perform a node add/replace/remove then follow our placement configuration guide and pay special attention to follow the special instructions for seed nodes.

External etcd

Just follow the instructions in the etcd docs.