Hints

Hinting is a data repair technique applied during write operations. When replica nodes are unavailable to accept a mutation, either due to failure or more commonly routine maintenance, coordinators attempting to write to those replicas store temporary hints on their local filesystem for later application to the unavailable replica. Hints are an important way to help reduce the duration of data inconsistency. Coordinators replay hints quickly after unavailable replica nodes return to the ring. Hints are best effort, however, and do not guarantee eventual consistency like anti-entropy repair does.

Hints are useful because of how Apache Cassandra replicates data to provide fault tolerance, high availability and durability. Cassandra partitions data across the cluster using consistent hashing, and then replicates keys to multiple nodes along the hash ring. To guarantee availability, all replicas of a key can accept mutations without consensus, but this means it is possible for some replicas to accept a mutation while others do not. When this happens an inconsistency is introduced.

Hints are one of the three ways, in addition to read-repair and full/incremental anti-entropy repair, that Cassandra implements the eventual consistency guarantee that all updates are eventually received by all replicas. Hints, like read-repair, are best effort and not an alternative to performing full repair, but they do help reduce the duration of inconsistency between replicas in practice.

Hinted Handoff

Hinted handoff is the process by which Cassandra applies hints to unavailable nodes.

For example, consider a mutation is to be made at Consistency Level LOCAL_QUORUM against a keyspace with Replication Factor of 3. Normally the client sends the mutation to a single coordinator, who then sends the mutation to all three replicas, and when two of the three replicas acknowledge the mutation the coordinator responds successfully to the client. If a replica node is unavailable, however, the coordinator stores a hint locally to the filesystem for later application. New hints will be retained for up to max_hint_window_in_ms of downtime (defaults to 3 hours). If the unavailable replica does return to the cluster before the window expires, the coordinator applies any pending hinted mutations against the replica to ensure that eventual consistency is maintained.

• (t0): The write is sent by the client, and the coordinator sends it to the three replicas. Unfortunately replica_2 is restarting and cannot receive the mutation.

• (t1): The client receives a quorum acknowledgement from the coordinator. At this point the client believe the write to be durable and visible to reads (which it is).

• (t2): After the write timeout (default 2s), the coordinator decides that replica_2 is unavailable and stores a hint to its local disk.

• (t3): Later, when replica_2 starts back up it sends a gossip message to all nodes, including the coordinator.

• (t4): The coordinator replays hints including the missed mutation against replica_2.

If the node does not return in time, the destination replica will be permanently out of sync until either read-repair or full/incremental anti-entropy repair propagates the mutation.

Application of Hints

Hints are streamed in bulk, a segment at a time, to the target replica node and the target node replays them locally. After the target node has replayed a segment it deletes the segment and receives the next segment. This continues until all hints are drained.

Storage of Hints on Disk

Hints are stored in flat files in the coordinator node’s $CASSANDRA_HOME/data/hints directory. A hint includes a hint id, the target replica node on which the mutation is meant to be stored, the serialized mutation (stored as a blob) that couldn’t be delivered to the replica node, the mutation timestamp, and the Cassandra version used to serialize the mutation. By default hints are compressed using LZ4Compressor. Multiple hints are appended to the same hints file.

Since hints contain the original unmodified mutation timestamp, hint application is idempotent and cannot overwrite a future mutation.

Hints for Timed Out Write Requests

Hints are also stored for write requests that time out. The write_request_timeout_in_ms setting in cassandra.yaml configures the timeout for write requests.

write_request_timeout_in_ms: 2000

The coordinator waits for the configured amount of time for write requests to complete, at which point it will time out and generate a hint for the timed out request. The lowest acceptable value for write_request_timeout_in_ms is 10 ms.

Configuring Hints

Hints are enabled by default as they are critical for data consistency. The cassandra.yaml configuration file provides several settings for configuring hints:

Table 1. Settings for Hints

hinted_handoff_disabled_datacenters:
  - DC1
  - DC2

Configuring Hints at Runtime with nodetool

nodetool provides several commands for configuring hints or getting hints related information. The nodetool commands override the corresponding settings if any in cassandra.yaml for the node running the command.

Table 2. Nodetool Commands for Hints

Make Hints Play Faster at Runtime

The default of 1024 kbps handoff throttle is conservative for most modern networks, and it is entirely possible that in a simple node restart you may accumulate many gigabytes hints that may take hours to play back. For example if you are ingesting 100 Mbps of data per node, a single 10 minute long restart will create 10 minutes * (100 megabit / second) ~= 7 GiB of data which at (1024 KiB / second) would take 7.5 GiB / (1024 KiB / second) = 2.03 hours to play back. The exact math depends on the load balancing strategy (round robin is better than token aware), number of tokens per node (more tokens is better than fewer), and naturally the cluster’s write rate, but regardless you may find yourself wanting to increase this throttle at runtime.

If you find yourself in such a situation, you may consider raising the hinted_handoff_throttle dynamically via the nodetool sethintedhandoffthrottlekb command.

Allow a Node to be Down Longer at Runtime

Sometimes a node may be down for more than the normal max_hint_window_in_ms, (default of three hours), but the hardware and data itself will still be accessible. In such a case you may consider raising the max_hint_window_in_ms dynamically via the nodetool setmaxhintwindow command added in Cassandra 4.0 (CASSANDRA-11720). This will instruct Cassandra to continue holding hints for the down endpoint for a longer amount of time.

This command should be applied on all nodes in the cluster that may be holding hints. If needed, the setting can be applied permanently by setting the max_hint_window_in_ms setting in cassandra.yaml followed by a rolling restart.

Monitoring Hint Delivery

Cassandra 4.0 adds histograms available to understand how long it takes to deliver hints which is useful for operators to better identify problems (CASSANDRA-13234).

There are also metrics available for tracking Hinted Handoff <handoff-metrics> and Hints Service <hintsservice-metrics> metrics.