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:
- query schedulers
These components can optionally be connected into a hash ring:
- index gateway
In an architecture that has three distributors and three ingestors 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
Define ring configuration within the
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,
ingester.lifecycler.ring block for ingesters,
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.
About the ruler ring
The ruler ring is used to determine which rulers evaluate which rule groups.
About the index gateway ring
The index gateway ring is used to determine which gateway is responsible for which tenant’s indexes when queried by rulers or queriers.