Consistent hash rings

Consistent hash rings are incorporated into Loki cluster architectures to

aid in the sharding of log lines
implement high availability
ease the horizontal scale up and scale down of clusters. There is less of a performance hit for operations that must rebalance data.

Hash rings connect instances of a single type of component when

there are a set of Loki instances in monolithic deployment mode
there are multiple read components or multiple write components in simple scalable deployment mode
there are multiple instances of one type of component in microservices mode

Not all Loki components are connected by hash rings. These components need to be connected into a hash ring:

distributors
ingesters
query schedulers
compactors
rulers

These components can optionally be connected into a hash ring:

index gateway

In an architecture that has three distributors and three ingesters defined, the hash rings for these components connect the instances of same-type components.

Each node in the ring represents an instance of a component. Each node has a key-value store that holds communication information for each of the nodes in that ring. Nodes update the key-value store periodically to keep the contents consistent across all nodes. For each node, the key-value store holds:

an ID of the component node
component address, used by other nodes as a communication channel
an indication of the component node’s health

Configuring rings

Define ring configuration within the common.ring_config block.

Use the default memberlist key-value store type unless there is a compelling reason to use a different key-value store type. memberlist uses a gossip protocol to propagate information to all the nodes to guarantee the eventual consistency of the key-value store contents.

There are additional configuration options for distributor rings, ingester rings, and ruler rings. These options are for advanced, specialized use only. These options are defined within the distributor.ring block for distributors, the ingester.lifecycler.ring block for ingesters, and the ruler.ring block for rulers.

About the distributor ring

Distributors use the information in their key-value store to keep a count of the quantity of distributors in the distributor ring. The count further informs cluster limits.

About the ingester ring

Ingester ring information in the key-value stores is used by distributors. The information lets the distributors shard log lines, determining which ingester or set of ingesters a distributor sends log lines to.

About the query scheduler ring

Query schedulers use the information in their key-value store for service discovery of the schedulers. This allows queriers to connect to all available schedulers, and it allows schedulers to connect to all available query frontends, effectively creating a single queue that aids in balancing the query load.

About the compactor ring

Compactors use the information in the key-value store to identify a single compactor instance that will be responsible for compaction. The compactor is only enabled on the responsible instance, despite the compactor target being on multiple instances.