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
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 is the process by which Cassandra applies hints to unavailable nodes.
For example, consider a mutation is to be made at
LOCAL_QUORUM against a keyspace with
Replication Factor of
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_windowin_ms of downtime (defaults to
3 h). 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_2is 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_2is unavailable and stores a hint to its local disk.
t3): Later, when
replica_2starts back up it sends a gossip message to all nodes, including the coordinator.
t4): The coordinator replays hints including the missed mutation against
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.
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.
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 are also stored for write requests that time out. The
write_request_timeout setting in
cassandra.yaml configures the
timeout for write requests.
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 is 10 ms.
Hints are enabled by default as they are critical for data consistency.
cassandra.yaml configuration file provides several settings for
Table 1. Settings for Hints
Enables/Disables hinted handoffs
A list of data centers that do not perform hinted handoffs even when handoff is otherwise enabled. Example:
Defines the maximum amount of time a node shall have hints generated after it has failed.
Maximum throttle in KiBs per second, per delivery thread. This will be reduced proportionally to the number of nodes in the cluster. (If there are two nodes in the cluster, each delivery thread will use the maximum rate; if there are 3, each will throttle to half of the maximum,since it is expected for two nodes to be delivering hints simultaneously.)
Number of threads with which to deliver hints; Consider increasing this number when you have multi-dc deployments, since cross-dc handoff tends to be slower
Directory where Cassandra stores hints.
How often hints should be flushed from the internal buffers to disk. Will not trigger fsync.
Maximum size for a single hints file, in megabytes.
Compression to apply to the hint files. If omitted, hints files will be written uncompressed. LZ4, Snappy, and Deflate compressors are supported.
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
Table 2. Nodetool Commands for Hints
Disables storing and delivering hints
Disables storing and delivering hints to a data center
Re-enables future hints storing and delivery on the current node
Enables hints for a data center that was previously disabled
Prints the max hint window in ms. New in Cassandra 4.0.
Prints current hinted handoff window
Pauses hints delivery process
Resumes hints delivery process
Sets hinted handoff throttle in kb per second, per delivery thread
Sets the specified max hint window in ms
Status of storing future hints on the current node
Truncates all hints on the local node, or truncates hints for the endpoint(s) specified.
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.
Sometimes a node may be down for more than the normal
max_hint_window, (default of three hours), but the hardware and
data itself will still be accessible. In such a case you may consider
max_hint_window dynamically via the
nodetool setmaxhintwindow command added in Cassandra 4.0
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
max_hint_window setting in
by a rolling restart.
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.