Schema Registry System Requirements

This section describes the key considerations before going to production with your cluster. However, it is not an exhaustive guide to running your Schema Registry in production.

Hardware

If you’ve been following the normal development path, you’ve probably been playing with Schema Registry on your laptop or on a small cluster of machines laying around. But when it comes time to deploying Schema Registry to production, there are a few recommendations that you should consider. Nothing is a hard-and-fast rule.

Memory

Schema Registry uses Kafka as a commit log to store all registered schemas durably, and maintains a few in-memory indices to make schema lookups faster. A conservative upper bound on the number of unique schemas registered in a large data-oriented company like LinkedIn is around 10,000. Assuming roughly 1000 bytes heap overhead per schema on average, heap size of 1GB would be more than sufficient.

CPUs

CPU usage in Schema Registry is light. The most computationally intensive task is checking compatibility of two schemas, an infrequent operation which occurs primarily when new schemas versions are registered under a subject.

If you need to choose between faster CPUs or more cores, choose more cores. The extra concurrency that multiple cores offers will far outweigh a slightly faster clock speed.

Disks

Schema Registry does not have any disk resident data. It currently uses Kafka as a commit log to store all schemas durably and holds in-memory indices of all schemas. Therefore, the only disk usage comes from storing the log4j logs.

Network

A fast and reliable network is obviously important to performance in a distributed system. Low latency helps ensure that nodes can communicate easily, while high bandwidth helps shard movement and recovery. Modern data-center networking (1 GbE, 10 GbE) is sufficient for the vast majority of clusters.

Avoid clusters that span multiple data centers, even if the data centers are colocated in close proximity. Definitely avoid clusters that span large geographic distances.

Larger latencies tend to exacerbate problems in distributed systems and make debugging and resolution more difficult.

Often, people might assume the pipe between multiple data centers is robust or low latency. But this is usually not true and network failures might happen at some point. Please refer to our recommended Schema Registry Multi-DC Setup.

JVM

We recommend running the latest version of JDK 1.8 with the G1 collector (older freely available versions have disclosed security vulnerabilities).

If you are still on JDK 1.7 (which is also supported) and you are planning to use G1 (the current default), make sure you’re on u51. We tried out u21 in testing, but we had a number of problems with the GC implementation in that version.

Our recommended GC tuning looks like this:

-Xms1g -Xmx1g -XX:MetaspaceSize=96m -XX:+UseG1GC -XX:MaxGCPauseMillis=20 \
       -XX:InitiatingHeapOccupancyPercent=35 -XX:G1HeapRegionSize=16M \
       -XX:MinMetaspaceFreeRatio=50 -XX:MaxMetaspaceFreeRatio=80

Important Configuration Options

The following configurations should be changed for production environments. These options depend on your cluster layout.

Depending on how Schema Registry instances coordinate to choose the master, you can deploy Schema Registry with ZooKeeper (which can be shared with Kafka) or with Kafka itself. You should configure Schema Registry to use either Kafka-based or ZooKeeper-based master election:

  • Kafka-based master election is available since version 4.0. You can use it in cases where ZooKeeper is not available, for example on hosted or cloud environments, or if access to ZooKeeper has been locked down. To configure Schema Registry to use Kafka for master election, configure the kafkastore.bootstrap.servers setting.

    A list of Kafka brokers to connect to. For example, PLAINTEXT://hostname:9092,SSL://hostname2:9092

    The effect of this setting depends on whether you specify kafkastore.connection.url.

    If kafkastore.connection.url is not specified, then the Kafka cluster containing these bootstrap servers will be used both to coordinate Schema Registry instances (master election) and store schema data.

    If kafkastore.connection.url is specified, then this setting is used to control how Schema Registry connects to Kafka to store schema data and is particularly important when Kafka security is enabled. When this configuration is not specified, Schema Registry‘s internal Kafka clients will get their Kafka bootstrap server list from ZooKeeper (configured with kafkastore.connection.url). In that case, all available listeners matching the kafkastore.security.protocol setting will be used.

    By specifiying this configuration, you can control which endpoints are used to connect to Kafka. Kafka may expose multiple endpoints that all will be stored in ZooKeeper, but Schema Registry may need to be configured with just one of those endpoints, for example to control which security protocol it uses.

    • Type: list
    • Default: []
    • Importance: medium
  • ZooKeeper-based master election is available in all versions of Schema Registry, and if you have an existing Schema Registry deployment you may continue to use it for compatibility. To configure Schema Registry to use ZooKeeper for master election, configure the kafkastore.connection.url setting.

    ZooKeeper URL for the Kafka cluster

    • Type: string
    • Default: “”
    • Importance: high

If you configure both kafkastore.bootstrap.servers and kafkastore.connection.url, ZooKeeper will be used for master election. To migrate from ZooKeeper-based to Kafka-based master election, see the migration details.

Additionally, there are some configurations that may commonly need to be set in either type of deployment.

listeners

Comma-separated list of listeners that listen for API requests over either HTTP or HTTPS. If a listener uses HTTPS, the appropriate SSL configuration parameters need to be set as well.

Schema Registry identities are stored in ZooKeeper and are made up of a hostname and port. If multiple listeners are configured, the first listener’s port is used for its identity.

host.name

The host name advertised in ZooKeeper. Make sure to set this if running Schema Registry with multiple nodes.

  • Type: string
  • Default: “192.168.50.1”
  • Importance: high

Note

Configure min.insync.replicas on the Kafka server for the schemas topic that stores all registered schemas to be higher than 1. For example, if the kafkastore.topic.replication.factor is 3, then set min.insync.replicas on the Kafka server for the kafkastore.topic to 2. This ensures that the register schema write is considered durable if it gets committed on at least 2 replicas out of 3. Furthermore, it is best to set unclean.leader.election.enable to false so that a replica outside of the isr is never elected leader (potentially resulting in data loss).

The full set of configuration options are documented in Schema Registry Configuration Options.

Don’t Modify These Storage Settings

Schema Registry stores all schemas in a Kafka topic defined by kafkastore.topic. Since this Kafka topic acts as the commit log for Schema Registry database and is the source of truth, writes to this store need to be durable. Schema Registry ships with very good defaults for all settings that affect the durability of writes to the Kafka based commit log. Finally, kafkastore.topic must be a compacted topic to avoid data loss. Whenever in doubt, leave these settings alone. If you must create the topic manually, this is an example of proper configuration:

# kafkastore.topic=_schemas
  bin/kafka-topics --create --zookeeper localhost:2181 --topic connect-configs --replication-factor 3 --partitions 1 --config cleanup.policy=compact

kafkastore.topic

The durable single partition topic that acts as the durable log for the data. This topic must be compacted to avoid losing data due to retention policy.

  • Type: string
  • Default: “_schemas”
  • Importance: high

kafkastore.topic.replication.factor

The desired replication factor of the schema topic. The actual replication factor will be the smaller of this value and the number of live Kafka brokers.

  • Type: int
  • Default: 3
  • Importance: high

kafkastore.timeout.ms

The timeout for an operation on the Kafka store

  • Type: int
  • Default: 500
  • Importance: medium

Kafka & ZooKeeper

For recommendations on operationalizing Kafka and ZooKeeper, see Schema Registry Operations.

Migration from ZooKeeper master election to Kafka master election

It is not required to migrate from ZooKeeper-based election to Kafka-based master election.

If you choose to migrate from ZooKeeper-based to Kafka-based master election, make the following configuration changes in all Schema Registry nodes:

  • Remove kafkastore.connection.url
  • Remove schema.registry.zk.namespace if its configured
  • Configure kafkastore.bootstrap.servers
  • Configure schema.registry.group.id if you originally had schema.registry.zk.namespace for multiple Schema Registry clusters

If you configure both kafkastore.connection.url and kafkastore.bootstrap.servers, ZooKeeper will be used for master election.

Downtime for Writes

You can migrate from ZooKeeper based master election to Kafka based master election by following below outlined steps. These steps would lead to Schema Registry not being available for writes for a brief amount of time.

  • Make above outlined config changes on that node and also ensure master.eligibility is set to false in all the nodes
  • Do a rolling bounce of all the nodes.
  • Configure master.eligibility to true on the nodes that can be master eligible and bounce them

Complete downtime

If you want to keep things simple, you can take a temporary downtime for Schema Registry and do the migration. To do so, simply shutdown all the nodes and start them again with the new configs.

Backup and Restore

As discussed in schemaregistry_design, all schemas, subject/version and ID metadata, and compatibility settings are appended as messages to a special Kafka topic <kafkastore.topic> (default _schemas). This topic is a common source of truth for schema IDs, and you should back it up. In case of some unexpected event that makes the topic inaccessible, you can restore this schemas topic from the backup, enabling consumers to continue to read Kafka messages that were sent in the Avro format.

As a best practice, we recommend backing up the <kafkastore.topic>. If you already have a multi-datacenter Kafka deployment, you can backup this topic to another Kafka cluster using Confluent Replicator. Otherwise, you can use a Kafka sink connector to copy the topic data from Kafka to a separate storage (e.g. AWS S3). These will continuously update as the schema topic updates.

In lieu of either of those options, you can also use Kafka command line tools to periodically save the contents of the topic to a file. For the following examples, we assume that <kafkastore.topic> has its default value “_schemas”.

To backup the topic, use the kafka-console-consumer to capture messages from the schemas topic to a file called “schemas.log”. Save this file off the Kafka cluster.

bin/kafka-console-consumer --bootstrap-server localhost:9092 --topic _schemas --from-beginning --property print.key=true --timeout-ms 1000 1> schemas.log

To restore the topic, use the kafka-console-producer to write the contents of file “schemas.log” to a new schemas topic. This examples uses a new schemas topic name “_schemas_restore”. If you use a new topic name or use the old one (i.e. “_schemas”), make sure to set <kafkastore.topic> accordingly.

bin/kafka-console-producer --broker-list localhost:9092 --topic _schemas_restore --property parse.key=true < schemas.log