• Status
• Scope
• Goals
• Approach
• Timeline
• Mailing list / Slack channels
• Related JIRA tickets
• Motivation
• Audience
• Proposed Changes
• New or Changed Public Interfaces
• Compatibility, Deprecation, and Migration Plan
• Test Plan
• Rejected Alternatives

Status

Current state: Under Discussion

Discussion thread: TBD

JIRA: TBD

Released: <Cassandra Version>

Please keep the discussion on the mailing list rather than commenting on the wiki (wiki discussions get unwieldy fast).

Scope

todo

Goals

• easy way to enforce system-wide soft and hard limits to prevent anti-patterns of bad usage and in the long run make it not possible to severely degrade the performance of a node/cluster through user actions (too many MVs/secondary indexes per table, ...), thus increasing stability/availability
• as a C* developer it should be easy to add new Guardrails
• Guardrails are disabled by default and there should be no overhead when Guardrails are disabled

Non-Goals

• enforcing limits on a per-user-basis
• setting limits dynamically while nodes are running

Timeline

todo

Mailing list / Slack channels

Mailing list: 

• TBD

Slack channel: 

• TBD

Discussion threads: 

• TBD

Related JIRA tickets

JIRA(s): 

• here

Motivation

Operators of C* want to provide uptime SLAs and prevent users from applying anti-patterns that could potentially bring down a node or severely degrade performance.

Guardrails are a tool to achieve this by setting soft and hard limits that stop users from employing bad practices. For example, when running C* in a cloud environment, as an operator you want to guarantee that certain SLAs can be met by guarding the system from users that would perform CL=ALL writes in a multi-dc cluster.

Audience

Operators of C*

Proposed Changes

The goal of this feature is to have an easy way for operators to enforce system-wide soft and hard limits that ensure good practices, foster availability, and guard the system from wrong usage patterns.

Operators are generally more interested in the overall health of a node/cluster, and so enforcing soft/hard limits on a per-user-basis is not a goal.

The specific guardrails proposed in this spec are intended to be a starting point with the expectation that more guardrails will be defined over time. All guardrails should be of a form that is enforceable when an operation takes place without introducing significant latency.

Guardrail Classes and Configuration

• Guardrail: general class definition of a Guardrail that guards against a particuluar usage/condition
• Guardrails: Entry point for Guardrails, storing all defined Guardrails and additional helper methods
• GuardrailsConfig: Configuration settings for Guardrails, which are populated from cassandra.yaml. This contains a main setting enabled, controlling if Guardrails are globally active or not, and individual settings to control each Guardrail
• cassandra.yaml: allows configuring individual Guardrails

Overview of proposed Guardrails

Guardrails can be in the form of:

• numeric threshold with soft/hard limits
• size threshold with soft/hard limits in bytes
• percentage threshold with soft/hard limits
• have a boolean enabled/disabled flag that triggers a failure
• list of disallowed values that trigger a failure

Reaching a soft limit should issue a warning, whereas reaching a hard limit issues a failure (InvalidRequestException).

Below is an overview of a set of proposed guardrails with some example limits (which are subject to change):

compression, compaction

Configuration of Guardrails

Guardrails will be configured via cassandra.yaml settings as shown below (-1 means disabled):

# guardrails
  # enabled: false
  # column_value_size_failure_threshold_in_kb: -1
  # columns_per_table_failure_threshold: -1
  # secondary_index_per_table_failure_threshold: -1
  # materialized_view_per_table_failure_threshold: -1
  # tables_warn_threshold: -1
  # tables_failure_threshold: -1
  # table_properties_disallowed: 
  # write_consistency_levels_disallowed:
  # partition_size_warn_threshold_in_mb: -1
  # partition_keys_in_select_failure_threshold: -1
  # disk_usage_percentage_warn_threshold: -1
  # disk_usage_percentage_failure_threshold: -1
  # in_select_cartesian_product_failure_threshold: -1
  # user_timestamps_enabled: true
  # read_before_write_list_operations_enabled: true
  # fields_per_udt_failure_threshold: -1
  # collection_size_warn_threshold_in_kb: -1
  # items_per_collection_warn_threshold: -1

Migrating existing cassandra.yaml warn/fail thresholds

Once Guardrails are implemented, it would make sense to move existing warn/fail thresholds to using guardrails. A few that come to mind are:

• tombstone_warn_threshold
• tombstone_failure_threshold
• batch_size_warn_threshold_in_kb
• batch_size_fail_threshold_in_kb
• unlogged_batch_across_partitions_warn_threshold
• compaction_large_partition_warning_threshold_mb

Distinction from Capability Restrictions

Guardrails allow C* operators to impose system-wide restrictions that are configured through yaml. Capability restrictions are focused on imposing restrictions on particular users and offer a new CQL API to do so. Both concepts are not mutually exclusive and are complementary.

Guardrails and Diagnostic Events

In their initial form, Guardrails would issue warnings/failures to the log file. It would make sense to also emit such events as Diagnostic Events to help troubleshooting these issues.

Test Plan

• unit and integration tests for every single guardail will be added (handling erroneous input, proper notification of warnings/failures, guardrail boundary settings, no warnings/failures when disabled, ...)

Performance Characteristics and Testing

• there should be no overhead when Guardrails are disabled and this should be tested in existing read/write benchmarks
• a certain amount of performance degradation is acceptable (to which degree has to be defined). This featureset prioritizes stability and predictability of performance over achieving the highest possible throughput or latency on a cluster, and as such we need to figure out acceptable performance tradeoffs for having these enabled.