Question

RocksDB's LOG file comes in handy when troubleshooting Flink with RocksDB. How can I configure RocksDB logging?

Answer

Note: This section applies to Flink 1.10 - 1.14

By default, Flink uses the log level HEADER_LEVEL for RocksDB. This essentially disabled RocksDB logging and leave only RocksDB configuration printed in the RocksDB log file. The main reason is that the RocksDB log file is not controllable in size prior to Flink 1.14. The way to configure RocksDB logging depends on the version of Flink you are using. Flink 1.13 or later supports changing RocksDB log level via configuration. Flink 1.14 additionally supports specifying the logging directory which can be located on a (separate) volume that is retained after container shutdown for debugging purposes. Thanks to the new RocksDB version, you can also configure log rotation in Flink 1.14 or later. For other scenarios, you will need to define your own custom 

Via Configuration in Flink 1.14 or later

For example:

state.backend.rocksdb.log.level: INFO_LEVEL
state.backend.rocksdb.log.max-file-size: 10MB
state.backend.rocksdb.log.file-num: 10
state.backend.rocksdb.log.dir: /rocksdb/logs

Refer to Flink's documentation for more details of these configuration options.

Via Configuration in Flink 1.13

For example:

state.backend.rocksdb.log.level: INFO_LEVEL

Custom Options Factory

If you are using older versions of Flink (<1.13), you want to configure logging directory in Flink 1.13, or you want to dump RocksDB statistics in RocksDB's LOG file, you can create a custom 

Important: DefaultConfigurableOptionsFactory was not really meant for being extended and may change among releases. If you plan to take this into production, you should write your own complete ConfigurableRocksDBOptionsFactory and set all the options you need in there.

Extending DefaultConfigurableOptionsFactory

First, if you have not done so yet, add a dependency to Flink's RocksDB state backend. For example, add this to your Maven project's pom.xml:

<dependency>
   <groupId>org.apache.flink</groupId>
   <artifactId>flink-statebackend-rocksdb_${scala.binary.version}</artifactId>
   <version>${flink.version}</version>
   <scope>provided</scope>
</dependency>

Then create your own Options Factory:

package com.ververica.troubleshooting;

import org.apache.flink.contrib.streaming.state.DefaultConfigurableOptionsFactory;
import org.rocksdb.DBOptions;
import org.rocksdb.InfoLogLevel;

public class MyCustomRocksDBOptionsFactory extends DefaultConfigurableOptionsFactory {
    private static final long serialVersionUID = 1L;

    private String dbLogDir = "";

    @Override
    public DBOptions createDBOptions(DBOptions currentOptions,
                                     Collection<AutoCloseable> handlesToClose) {
        currentOptions = super.createDBOptions(currentOptions, handlesToClose);

        currentOptions.setInfoLogLevel(InfoLogLevel.INFO_LEVEL);
        currentOptions.setStatsDumpPeriodSec(60);
        currentOptions.setDbLogDir(dbLogDir);

        return currentOptions;
    }

    @Override
    public String toString() {
        return this.getClass().toString() + "{" + super.toString() + '}';
    }

    /**
     * Set directory where RocksDB writes its info LOG file (empty = data dir, otherwise the
     * data directory's absolute path will be used as the log file prefix).
     */
    public void setDbLogDir(String dbLogDir) {
        this.dbLogDir = dbLogDir;
    }
}

Three points in createDBOptions are important here:

• setInfoLogLevel(InfoLogLevel.INFO_LEVEL) sets the logging level to INFO from which you would get a decent amount of logging data (increase if needed)
• setStatsDumpPeriodSec(60) dumps various RocksDB statistics every this many seconds: this includes compaction statistics.
• setDbLogDir(dbLogDir) specifies the path where to put the LOG file: depending on what you are trying to troubleshoot, you can just use a local directory or you may need to put this onto a distributed file system (or persistent volume) to survive node/pod/job restarts

Configuring Flink

With the custom Options Factory, you can configure Flink either programmatically or through its flink-conf.yaml.

(1) Programmatically

Note: The state backend interface is changed since Flink 1.13; we provide both versions below.

// Flink 1.13 or later
StreamExecutionEnvironment env = StreamExecutionEnvironment.getExecutionEnvironment();
EmbeddedRocksDBStateBackend stateBackend = new EmbeddedRocksDBStateBackend()
env.setStateBackend(stateBackend);
env.getCheckpointConfig().setCheckpointStorage("file:///path/to/checkpoints");

MyCustomRocksDBOptionsFactory options = new MyCustomRocksDBOptionsFactory();
options.setDbLogDir("/path/to/rocksdb/logging/");
stateBackend.setRocksDBOptions(options);

// Flink <1.13
RocksDBStateBackend stateBackend = new RocksDBStateBackend("file:///path/to/checkpoints");
MyCustomRocksDBOptionsFactory options = new MyCustomRocksDBOptionsFactory();
options.setDbLogDir("/path/to/rocksdb/logging/");
stateBackend.setRocksDBOptions(options);

(2) Via Flink configuration (flink-conf.yaml)

If you want to configure your options factory completely via flink-conf.yaml, we may extend the code above to update its settings from the configuration. The code below shows how to do this for the log directory but you can extend it to make further settings configurable.

Note: The interface slightly changed since Flink 1.11; we provide both versions below.

// Flink 1.11 or later
public static final ConfigOption<String> LOG_DIR =
        key("state.backend.rocksdb.log.dir")
            .stringType()
            .noDefaultValue()
            .withDescription("Location of RocksDB's info LOG file (empty = data dir, otherwise the " +
                "data directory's absolute path will be used as the log file prefix)");

@Override
public DefaultConfigurableOptionsFactory configure(ReadableConfig configuration) {
    DefaultConfigurableOptionsFactory optionsFactory =
            super.configure(configuration);

    this.dbLogDir = configuration.getOptional(LOG_DIR).orElse(this.dbLogDir);

    return optionsFactory;
}

// Flink 1.10
public static final ConfigOption<String> LOG_DIR =
        key("state.backend.rocksdb.log.dir")
            .stringType()
            .noDefaultValue()
            .withDescription("Location of RocksDB's info LOG file (empty = data dir, otherwise the " +
                "data directory's absolute path will be used as the log file prefix)");

@Override
public DefaultConfigurableOptionsFactory configure(Configuration configuration) {
    DefaultConfigurableOptionsFactory optionsFactory =
            super.configure(configuration);

    this.dbLogDir = configuration.getOptional(LOG_DIR).orElse(this.dbLogDir);

    return optionsFactory;
}

With the code additions from above, you can simply adapt your flink-conf.yaml and configure it like this:

state.backend.rocksdb.log.dir: /path/to/rocksdb/logging/
state.backend.rocksdb.options-factory: com.ververica.troubleshooting.MyCustomRocksDBOptionsFactory

Note: Configuring the options factory via flink-conf.yaml will apply the options factory to all jobs started in the Flink cluster. Make sure that this class is available cluster-wide or in all jobs started on this cluster!
(For Ververica Platform deployments, this will not be a problem since each deployment spawns its own Flink cluster.)

Related Information

• Flink docs: State Backends
• FLINK-15068 - Disable RocksDB's local LOG by default
• FLINK-15747 - Enable setting RocksDB log level from configuration
• FLINK-20911 - Support configuration of RocksDB log level
• FLINK-23812 - Support RocksDB log rotation via configuration
• RocksDB Tuning Guide: RocksDB Statistics