topicctl

README

topicctl

A tool for easy, declarative management of Kafka topics. Includes the ability to "apply" topic changes from YAML as well as a repl for interactive exploration of brokers, topics, consumer groups, messages, and more.

Motivation

Managing Kafka topics via the standard tooling can be tedious and error-prone; there is no standard, declarative way to define topics (e.g., YAML files that can be checked-in to git), and understanding the state of a cluster at any given point in time requires knowing and using multiple, different commands with different interfaces.

We created topicctl to make the management of our Kafka topics more transparent and user-friendly. The project was inspired by kubectl and other tools that we've used in non-Kafka-related contexts.

See this blog post for more details.

Getting started

Installation

Run GO111MODULE="on" go get github.com/segmentio/topicctl/cmd/topicctl.

You can also build and install the binary by running make install in the root of the repo.

Quick tour

1. Start up a 6 node Kafka cluster locally:

docker-compose up -d

1. Run the net alias script to make the broker addresses available on localhost:

./scripts/set_up_net_alias.sh

1. Apply the topic configs in examples/local-cluster/topics:

topicctl apply --skip-confirm examples/local-cluster/topics/*yaml

1. Send some test messages to the topic-default topic:

topicctl tester --zk-addr=localhost:2181 --topic=topic-default

1. Open up the repl (while keeping the tester running in a separate terminal):

topicctl repl --cluster-config=examples/local-cluster/cluster.yaml

1. Run some test commands:

get brokers
get topics
get partitions topic-default
get offsets topic-default
tail topic-default

1. Increase the number of partitions in the topic-default topic by changing the partitions: ... value in topic-default.yaml to 9 and re-applying:

topicctl apply examples/local-cluster/topics/topic-default.yaml

1. Bring down the local cluster:

docker-compose down

Usage

Subcommands

apply

topicctl apply [path(s) to topic config(s)]

The apply subcommand ensures that the actual state of a topic in the cluster matches the desired state in its config. If the topic doesn't exist, the tool will create it. If the topic already exists but its cluster state is out-of-sync, then the tool will initiate the necessary changes to bring it into compliance.

See the Config formats section below for more information on the expected file formats.

bootstrap

topicctl [flags] bootstrap

The bootstrap subcommand creates apply topic configs from the existing topics in a cluster. The output can be sent to either a directory (if the --output flag is set) or stdout.

check

topicctl check [path(s) to topic config(s)]

The check command validates that each topic config has the correct fields set and is consistent with the associated cluster config. Unless --validate-only is set, it then checks the topic config against the state of the topic in the corresponding cluster.

get

topicctl get [flags] [operation]

The get subcommand lists out the instances and/or details of a particular resource type in the cluster. Currently, the following operations are supported:

Subcommand	Description
get balance [optional topic]	Number of replicas per broker position for topic or cluster as a whole
get brokers	All brokers in the cluster
get config [broker or topic]	Config key/value pairs for a broker or topic
get groups	All consumer groups in the cluster
get lags [topic] [group]	Lag for each topic partition for a consumer group
get members [group]	Details of each member in a consumer group
get partitions [topic]	All partitions in a topic
get offsets [topic]	Number of messages per partition along with start and end times
get topics	All topics in the cluster

repl

topicctl repl [flags]

The repl subcommand starts up a shell that allows running the get and tail subcommands interactively.

reset-offsets

topicctl reset-offsets [topic] [group] [flags]

The reset-offsets subcommand allows resetting the offsets for a consumer group in a topic. The partition and offset values are set in the flags.

tail

topicctl tail [flags] [topic]

The tail subcommand tails and logs out topic messages using the APIs exposed in kafka-go. It doesn't have the full functionality of kafkacat (yet), but the output is prettier and it may be easier to use in some cases.

tester

topicctl tester [flags]

The tester command reads or writes test messages in a topic. For testing/demonstration purposes only.

Specifying the target cluster

There are two patterns for specifying a target cluster in the topicctl subcommands:

1. --cluster-config=[path], where the refererenced path is a cluster configuration in the format expected by the apply command described above or
2. --zk-addr=[zookeeper address] and --zk-prefix=[optional prefix for cluster in zookeeper]

All subcommands support the cluster-config pattern. The second is also supported by the get, repl, reset-offsets, and tail subcommands since these can be run independently of an apply workflow.

Version compatibility

We've tested topicctl on Kafka clusters with versions between 0.10.1 and 2.4.1, inclusive. If you run into any compatibility issues, please file a bug.

Config formats

topicctl uses structured, YAML-formatted configs for clusters and topics. These are typically source-controlled so that changes can be reviewed before being applied.

Clusters

Each cluster associated with a managed topic must have a config. These configs can also be used with the get, repl, and tail subcommands instead of specifying a ZooKeeper address.

The following shows an annotated example:

meta:
  name: my-cluster                      # Name of the cluster
  environment: stage                    # Cluster environment
  region: us-west-2                     # Cloud region of the cluster
  description: |                        # A free-text description of the cluster (optional)
    Test cluster for topicctl.

spec:
  versionMajor: v0.10                   # Version
  bootstrapAddrs:                       # One or more broker bootstrap addresses
    - my-cluster.example.com:9092
  zkAddrs:                              # One or more cluster zookeeper addresses
    - zk.example.com:2181
  zkPrefix: my-cluster                  # Prefix for zookeeper nodes
  zkLockPath: /topicctl/locks           # Path used for apply locks (optional)
  clusterID: abc-123-xyz                # Expected cluster ID for cluster (optional, used as
                                        #   safety check only)

Note that the name, environment, region, and description fields are used for description/identification only, and don't appear in any API calls. They can be set arbitrarily, provided that they match up with the values set in the associated topic configs.

Topics

Each topic is configured in a single YAML file. The following is an annotated example:

meta:
  name: topics-test                     # Name of the topic
  cluster: my-cluster                   # Name of the cluster
  environment: stage                    # Environment of the cluster
  region: us-west-2                     # Region of the cluster
  description: |                        # Free-text description of the topic (optional)
    Test topic in my-cluster.

spec:
  partitions: 9                         # Number of topic partitions
  replicationFactor: 3                  # Replication factor per partition
  retentionMinutes: 360                 # Number of minutes to retain messages (optional)
  placement:
    strategy: in-zone                   # Placement strategy, see info below
    picker: randomized                  # Picker method, see info below (optional)
  settings:                             # Miscellaneous other config settings (optional)
    cleanup.policy: delete
    max.message.bytes: 5242880

The cluster, environment, and region fields are used for matching against a cluster config and double-checking that the cluster we're applying in is correct; they don't appear in any API calls.

See the Kafka documentation for more details on the parameters that can be set in the settings field. Note that retention time can be set in either this section or via retentionMinutes but not in both places. The latter is easier, so it's recommended.

Placement strategies

The tool supports the following per-partition, replica placement strategies:

Strategy	Description
any	Allow any replica placement
balanced-leaders	Ensure that the leaders of each partition are evenly distributed across the broker racks
in-rack	Ensure that the followers for each partition are in the same rack as the leader; generally this is done when the leaders are already balanced, but this isn't required
static	Specify the placement manually, via an extra staticAssignments field
static-in-rack	Specify the rack placement per partition manually, via an extra staticRackAssignments field

Picker methods

There are often multiple options to pick from when updating a replica. For instance, with an in-rack strategy, we can pick any replica in the target rack that isn't already used in the partition.

Currently, topicctl supports the following methods for this replica "picking" process:

Method	Description
cluster-use	Pick based on broker frequency in the topic, then break ties by looking at the frequency of each broker across all topics in the cluster
lowest-index	Pick based on broker frequency in the topic, then break ties by choosing the lowest-index broker
randomized	Pick based on broker frequency in the topic, then break ties randomly. The underlying random generator uses a consistent seed (generated from the topic name, partition, and index), so the choice won't vary between apply runs.

If no picking method is set in the topic config, then randomized is used by default.

Note that these all try to achieve in-topic balance, and only vary in the case of ties. Thus, the placements won't be significantly different in most cases.

In the future, we may add pickers that allow for some in-topic imbalance, e.g. to correct a cluster-wide broker inbalance.

Rebalancing

If apply is run with the --rebalance flag set, then topicctl will do a full broker rebalance after the usual apply steps. This process will check the balance of the brokers for each index position (i.e., first, second, third, etc.) in each partition and make replacements if there are any brokers that are significantly over- or under-represented.

The rebalance process can optionally remove brokers from a topic too. To use this feature, set the --to-remove flag. Note that this flag has no effect unless --rebalance is also set.

Rebalancing is not done by default on all apply runs because it can be fairly disruptive and generally shouldn't be necessary unless the topic started off in an inbalanced state or there has been a change in the number of brokers.

Tool safety

The bootstrap, get, repl, and tail subcommands are read-only and should never make any changes in the cluster.

The apply subcommand can make changes, but under the following conditions:

1. A user confirmation is required for any mutation to the cluster
2. Topics are never deleted
3. Partitions can be added but are never removed
4. All apply runs are interruptable and idempotent (see sections below for more details)
5. Partition changes in apply runs are locked on a per-cluster basis
6. Leader changes in apply runs are locked on a per-topic basis
7. Partition replica migrations are protected via "throttles" to prevent the cluster network from getting overwhelmed
8. Before applying, the tool checks the cluster ID in ZooKeeper against the expected value in the cluster config. This can help prevent errors around applying in the wrong cluster when multiple clusters are accessed through the same address, e.g localhost:2181.

The reset-offsets command can also make changes in the cluster and should be used carefully.

Idempotency

Apply runs are designed to be idemponent- the effects should be the same no matter how many times they are run, assuming everything else in the cluster remains constant (e.g., the number of brokers, each broker's rack, etc.). Changes in other topics should generally not effect idempotency, unless, possibly, if the topic is configured to use the cluster-use picker.

Interruptibility

If an apply run is interrupted, then any in-progress broker migrations or leader elections will continue and any applied throttles will be kept in-place. The next time the topic is applied, the process should continue from where it left off.

Cluster access details

Most topicctl functionality interacts with the cluster through ZooKeeper. Currently, only the following depend on broker APIs:

1. Group-related get commands: get groups, get lags, get members
2. get offsets
3. reset-offsets
4. tail
5. apply with topic creation

In the future, we may shift more functionality away from ZooKeeper, at least for newer cluster versions; see the "Feature roadmap" section below for more details.

Feature roadmap

The following are in the medium-term roadmap:

1. Use broker APIs exclusively for newer cluster versions: This is needed for a future world where Kafka doesn't use ZooKeeper at all. Even before that happens, though, doing everything through broker APIs simplifies the configuration and is also needed to run topicctl in environments where users aren't given direct ZK access.
2. Support TLS for communication with cluster: This is fairly straightforward assuming that (1) is done. It allows topicctl to be run in environments that don't permit insecure cluster access.

Development

Run tests

First, set up docker-compose and the associated network alias:

docker-compose up -d
./scripts/set_up_net_alias.sh

This will create a 6 node, 3 rack cluster locally with the brokers accessible on 169.254.123.123.

Then, run:

make test

You can change the Kafka version of the local cluster by setting the KAFKA_IMAGE_TAG environment variable when running docker-compose up -d. See the wurstmeister/kafka dockerhub page for more details on the available versions.

Run against local cluster

To run the get, repl, and tail subcommands against the local cluster, set --zk-addr=localhost:2181 and leave the --zk-prefix flag unset.

To test out apply, you can use the configs in pkg/config/test-configs/local.