autothrottle

README

Overview

Autothrottle is a service that discovers partition reassignment events (as part of a broker replacement, scale up, storage rebalance, etc.) and dynamically sets broker replication throttles. The goal is to run replications as fast as possible without overloading brokers or starving bandwidth from Kafka clients.

Autothrottle does this by running a loop that discovers topics undergoing replication, fetches metrics for each involved broker from the Datadog API, and calculates a throttle based on a configured table of known bandwidth limits specific to each instance according to its type. An updated throttle rate is determined at each loop interval and continuously applied in order to adapt bandwidth utilization to changing workloads. Autothrottle always leaves (a configurable) bandwidth headroom to allow increased consumer consumption; if consumers demand more traffic, autothrottle will back off the throttle rates. Conversely, if consumer traffic decreases while a reassignment is taking place, autothrottle will increase the replication bandwidth to minimize replication times.

In contrast to where Kafka's out of the box tooling allows users to manually set a static, global inbound and outbound rate, autothrottle determines path-optimal rates on an individual broker basis. It does this by examining a graph representation of replication flows and individually calculating optimal transfer rates for inbound and outbound transfers. This means that if a single broker is near saturation, it doesn't need to slow down the recovery pace of the entire replication group. This allows every broker to run at target network utilization thresholds in any direction, minimizing recovery time and hotspots.

All throttle control logic is applied directly to the cluster through ZooKeeper; autothrottle natively mirrors Kafka's throttle control implementation rather than wrapping CLI tools.

Finally, autothrottle was designed to work as a piggyback system that doesn't take ownership of your cluster. It can easily be overridden (through the admin API), stopped safely at any time, or even outright disabled. This allows users to quickly revert to using other tools if desired.

Additional features:

• Configurable portion of free headroom available for use by replication (--max-rate)
• Throttle rate change threshold to reduce propagating broker config updates (--change-threshold)
• User-supplied map of instance type and capacity values (--cap-map)
• Automatic throttle removal with periodic, cluster-wide cleanup
• Ability to dynamically set override replication rates with broker level granularity (via the HTTP API)
• Automatic fail-safe rates should loss of metrics visibility occur
• Emits Datadog events at each check interval that detail what topics are undergoing replication, a list of all brokers involved, and throttle rates applied

Installation

• go get github.com/DataDog/kafka-kit/cmd/autothrottle

Binary will be found at $GOPATH/bin/autothrottle

Compatibility

Tested with Kafka 0.10, 2.2-2.7, ZooKeeper 3.4, 3.5

Usage

Autothrottle prerequisites include:

• Datadog API and app key
• A metric string that returns the system.net.bytes_sent and system.net.bytes_recvd metric per host, scoped to the cluster that's being managed
• That each Kafka host is tagged with instance-type (the Datadog AWS integration default) and a broker ID tag (configurable via -broker-id-tag, defaults to broker_id)
• A map of instance types and available bandwidth (in MB/s), supplied as a json string via the --cap-map parameter (e.g. --cap-map '{"d2.2xlarge":120,"d2.4xlarge":240}')

Once running, autothrottle should clearly log what it's doing:

2020/02/27 22:28:12 Autothrottle Running
2020/02/27 22:28:13 Admin API: localhost:8080
2020/02/27 22:28:13 Topics with ongoing reassignments: [test0]
2020/02/27 22:28:13 Source brokers participating in replication: [1037 1039]
2020/02/27 22:28:13 Destination brokers participating in replication: [1033 1041]
2020/02/27 22:28:14 Replication throttle rate for broker 1037 [leader] (based on a 90% max free capacity utilization): 139.83MB/s
2020/02/27 22:28:14 Updated throttle on broker 1037 [leader]
2020/02/27 22:28:15 Replication throttle rate for broker 1039 [leader] (based on a 90% max free capacity utilization): 147.24MB/s
2020/02/27 22:28:15 Updated throttle on broker 1039 [leader]
2020/02/27 22:28:15 Replication throttle rate for broker 1041 [follower] (based on a 90% max free capacity utilization): 179.75MB/s
2020/02/27 22:28:15 Updated throttle on broker 1041 [follower]
2020/02/27 22:28:15 Replication throttle rate for broker 1033 [follower] (based on a 90% max free capacity utilization): 181.88MB/s
2020/02/27 22:28:15 Updated throttle on broker 1033 [follower]
2020/02/27 22:28:28 Topics with ongoing reassignments: [test0]
2020/02/27 22:28:28 Source brokers participating in replication: [1037 1039]
2020/02/27 22:28:28 Destination brokers participating in replication: [1033 1041]
2020/02/27 22:28:28 Replication throttle rate for broker 1039 [leader] (based on a 90% max free capacity utilization): 225.00MB/s
2020/02/27 22:28:28 Updated throttle on broker 1039 [leader]
2020/02/27 22:28:28 Replication throttle rate for broker 1041 [follower] (based on a 90% max free capacity utilization): 225.00MB/s
2020/02/27 22:28:28 Updated throttle on broker 1041 [follower]
2020/02/27 22:28:29 Replication throttle rate for broker 1033 [follower] (based on a 90% max free capacity utilization): 225.00MB/s
2020/02/27 22:28:29 Updated throttle on broker 1033 [follower]
2020/02/27 22:28:29 Replication throttle rate for broker 1037 [leader] (based on a 90% max free capacity utilization): 225.00MB/s
2020/02/27 22:28:29 Updated throttle on broker 1037 [leader]
...
2020/02/27 22:35:58 Topics done reassigning: [test0]
2020/02/27 22:35:58 No topics undergoing reassignment
2020/02/27 22:36:09 Throttle removed on broker 1039
2020/02/27 22:36:10 Throttle removed on broker 1037
2020/02/27 22:36:11 Throttle removed on broker 1033
2020/02/27 22:36:12 Throttle removed on broker 1041

Overlaying autothrottle Datadog events on a recovery dashboard:

Flags

The variables in brackets are optional env var overrides.

Usage of autothrottle:
  -api-key string
    	Datadog API key [AUTOTHROTTLE_API_KEY]
  -api-listen string
    	Admin API listen address:port [AUTOTHROTTLE_API_LISTEN] (default "localhost:8080")
  -app-key string
    	Datadog app key [AUTOTHROTTLE_APP_KEY]
  -broker-id-tag string
    	Datadog host tag for broker ID [AUTOTHROTTLE_BROKER_ID_TAG] (default "broker_id")
  -cap-map string
    	JSON map of instance types to network capacity in MB/s [AUTOTHROTTLE_CAP_MAP]
  -change-threshold float
    	Required change in replication throttle to trigger an update (percent) [AUTOTHROTTLE_CHANGE_THRESHOLD] (default 10)
  -cleanup-after int
    	Number of intervals after which to issue a global throttle unset if no replication is running [AUTOTHROTTLE_CLEANUP_AFTER] (default 60)
  -dd-event-tags string
    	Comma-delimited list of Datadog event tags [AUTOTHROTTLE_DD_EVENT_TAGS]
  -failure-threshold int
    	Number of iterations that throttle determinations can fail before reverting to the min-rate [AUTOTHROTTLE_FAILURE_THRESHOLD] (default 1)
  -interval int
    	Autothrottle check interval (seconds) [AUTOTHROTTLE_INTERVAL] (default 180)
  -max-rx-rate float
    	Maximum inbound replication throttle rate (as a percentage of available capacity) [AUTOTHROTTLE_MAX_RX_RATE] (default 90)
  -max-tx-rate float
    	Maximum outbound replication throttle rate (as a percentage of available capacity) [AUTOTHROTTLE_MAX_TX_RATE] (default 90)
  -metrics-window int
    	Time span of metrics required (seconds) [AUTOTHROTTLE_METRICS_WINDOW] (default 120)
  -min-rate float
    	Minimum replication throttle rate (MB/s) [AUTOTHROTTLE_MIN_RATE] (default 10)
  -net-rx-query string
    	Datadog query for broker inbound bandwidth by host [AUTOTHROTTLE_NET_RX_QUERY] (default "avg:system.net.bytes_rcvd{service:kafka} by {host}")
  -net-tx-query string
    	Datadog query for broker outbound bandwidth by host [AUTOTHROTTLE_NET_TX_QUERY] (default "avg:system.net.bytes_sent{service:kafka} by {host}")
  -version
    	version [AUTOTHROTTLE_VERSION]
  -zk-addr string
    	ZooKeeper connect string (for broker metadata or rebuild-topic lookups) [AUTOTHROTTLE_ZK_ADDR] (default "localhost:2181")
  -zk-config-prefix string
    	ZooKeeper prefix to store autothrottle configuration [AUTOTHROTTLE_ZK_CONFIG_PREFIX] (default "autothrottle")
  -zk-prefix string
    	ZooKeeper namespace prefix [AUTOTHROTTLE_ZK_PREFIX]

Detailed: Rate Calculations, Applying Throttles

The throttle rate is calculated by building a graph of destination (brokers where partitions are being replicated to) and source brokers (brokers where partitions are being replicated from) and determining a per-path rate based on the appropriate network utilization for the broker's role; source brokers (those sending out data) receive an outbound throttle based on their outbound network utilization and destination brokers (those receiving data) receive an inbound throttle based on their inbound network utilization. Autothrottle references the provided -cap-map to lookup the network capacity. Autothrottle compares the amount of ongoing network throughput against the capacity (subtracting any amount already allocated for replication in previous intervals) to determine headroom. If more headroom is available, the throttle will be raised to consume the -max-{tx,rx}-rate (defaults to 90%) percent of what's available. If it's negative (throughput exceeds the configured capacity), the throttle will be lowered.

Autothrottle fetches metrics and performs this check every -interval seconds. In order to reduce propagating updated throttles to brokers too aggressively, a new throttle won't be applied unless it deviates more than -change-threshold (defaults to 10%) percent from the previous throttle. Any time a throttle change is applied, topics are done replicating, or throttle rates cleared, autothrottle will write Datadog events tagged with name:autothrottle along with any additionally defined tags (via the -dd-event-tags param).

Autothrottle is also designed to fail-safe and avoid flying blind. If fetching metrics fails or returns partial data, autothrottle will log what's missing and revert brokers to a safety throttle rate of -min-rate (defaults to 10MB/s). In order to prevent flapping, a configurable number of sequential failures before reverting to the minimum rate can be set with the -failure-threshold param (defaults to 1).

Operations Notes

• Autothrottle currently assumes that exactly one instance is running per cluster. Multi-node / HA support is planned.
• Autothrottle is effectively stateless and safe to restart at any time. If restarted, the first iteration may temporarily lower an existing throttle since it doesn't have a known rate to use as a compensation value in calculating headroom.
• Autothrottle is safe to stop using at any time. All operations mimic existing internals/functionality of Kafka. Autothrottle intends to be a layer of metrics driven decision autonomy.
• It's easy to accidentally leave throttles applied when performing manual reassignments. Autothrottle automatically clears previously applied throttles when no replications are running, and does a global throttle clearing every -cleanup-after iterations.

Admin API

The administrative API allows overrides to be set at two levels: global and granularly on a per-broker basis. This feature may be useful if there's a failure in the backing metrics system or a manually set rate is simply preferred.

A global override applies a static inbound and outbound rate to all brokers that are handling a partition reassignment. When setting a throttle, an optional autoremove bool parameter can be specified. If set, the throttle override will be removed once the next reassignment completes.

$ curl -XPOST "localhost:8080/throttle?rate=200&autoremove=true"
throttle successfully set to 200MB/s, autoremove==true

$ curl "localhost:8080/throttle"
a throttle override is configured at 200MB/s, autoremove==true

$ curl -XPOST "localhost:8080/throttle/remove"
throttle successfully removed

A broker level override rate applies to both reassignment replication as well as recovery traffic. For instance, if a broker level override is set to 50MB/s and the broker is stopped for a period of time before being resumed, it will catch up at only 50MB/s.

$ curl -XPOST "localhost:8080/throttle/1001?rate=50"                 
broker 1001: throttle successfully set to 50MB/s, autoremove==false

$ curl "localhost:8080/throttle/1001"
broker 1001: a throttle override is configured at 50MB/s, autoremove==false

$ curl -XPOST "localhost:8080/throttle/remove/1001"
broker 1001: throttle removed

Two considerations to take note of:

• Broker level throttle rates are "out-of-band" from reassignments. When a global rate is in place, it's dynamically applied against any broker that participates in a reassignment, even if the reassignment does not occur until after the throttle is set. With a broker level override, it is directly associated with a specific broker and goes into effect immediately rather than eventually becoming active should a reassignment occur. This is done to ensure that activity such as a recovery or bootstrap can be throttled, which doesn't have any (easily accessible) registered state in ZooKeeper to watch. Due to this, autoremove has no effect because there is no event that would trigger the removal. This is an explicit design decision due to some complexity in how Kafka throttle internals function.
• Any broker level override will prevent a global throttle autoremove from taking place. This is also an explicit design decision because of number of states that we have to account for; encoding logic that does the right thing would possibly become more complex because "the right thing" is highly conditional. Instead, we impose this simple rule: any broker level override freezes all automatic throttle clearing while in effect.