Apache Kudu Administration

Kudu is easier to manage with Cloudera Manager than in a standalone installation. See Cloudera’s Kudu documentation for more details about using Kudu with Cloudera Manager.

Starting and Stopping Kudu Processes

These instructions are relevant only when Kudu is installed using operating system packages (e.g. rpm or deb).

  1. Start Kudu services using the following commands:

    $ sudo service kudu-master start
$ sudo service kudu-tserver start

  2. To stop Kudu services, use the following commands:

    $ sudo service kudu-master stop
$ sudo service kudu-tserver stop

Kudu Web Interfaces

Kudu tablet servers and masters expose useful operational information on a built-in web interface,

Kudu Master Web Interface

Kudu master processes serve their web interface on port 8051. The interface exposes several pages with information about the cluster state:

  • A list of tablet servers, their host names, and the time of their last heartbeat.

  • A list of tables, including schema and tablet location information for each.

  • SQL code which you can paste into Impala Shell to add an existing table to Impala’s list of known data sources.

Kudu Tablet Server Web Interface

Each tablet server serves a web interface on port 8050. The interface exposes information about each tablet hosted on the server, its current state, and debugging information about maintenance background operations.

Common Web Interface Pages

Both Kudu masters and tablet servers expose a common set of information via their web interfaces:

  • HTTP access to server logs.

  • an /rpcz endpoint which lists currently running RPCs via JSON.

  • pages giving an overview and detailed information on the memory usage of different components of the process.

  • information on the current set of configuration flags.

  • information on the currently running threads and their resource consumption.

  • a JSON endpoint exposing metrics about the server.

  • information on the deployed version number of the daemon.

These interfaces are linked from the landing page of each daemon’s web UI.

Kudu Metrics

Kudu daemons expose a large number of metrics. Some metrics are associated with an entire server process, whereas others are associated with a particular tablet replica.

Listing available metrics

The full set of available metrics for a Kudu server can be dumped via a special command line flag:

$ kudu-tserver --dump_metrics_json
$ kudu-master --dump_metrics_json

This will output a large JSON document. Each metric indicates its name, label, description, units, and type. Because the output is JSON-formatted, this information can easily be parsed and fed into other tooling which collects metrics from Kudu servers.

Collecting metrics via HTTP

Metrics can be collected from a server process via its HTTP interface by visiting /metrics. The output of this page is JSON for easy parsing by monitoring services. This endpoint accepts several GET parameters in its query string:

  • /metrics?metrics=<substring1>,<substring2>,…​ - limits the returned metrics to those which contain at least one of the provided substrings. The substrings also match entity names, so this may be used to collect metrics for a specific tablet.

  • /metrics?include_schema=1 - includes metrics schema information such as unit, description, and label in the JSON output. This information is typically elided to save space.

  • /metrics?compact=1 - eliminates unnecessary whitespace from the resulting JSON, which can decrease bandwidth when fetching this page from a remote host.

  • /metrics?include_raw_histograms=1 - include the raw buckets and values for histogram metrics, enabling accurate aggregation of percentile metrics over time and across hosts.

For example:

$ curl -s 'http://example-ts:8050/metrics?include_schema=1&metrics=connections_accepted'
[
    {
        "type": "server",
        "id": "kudu.tabletserver",
        "attributes": {},
        "metrics": [
            {
                "name": "rpc_connections_accepted",
                "label": "RPC Connections Accepted",
                "type": "counter",
                "unit": "connections",
                "description": "Number of incoming TCP connections made to the RPC server",
                "value": 92
            }
        ]
    }
]
$ curl -s 'http://example-ts:8050/metrics?metrics=log_append_latency'
[
    {
        "type": "tablet",
        "id": "c0ebf9fef1b847e2a83c7bd35c2056b1",
        "attributes": {
            "table_name": "lineitem",
            "partition": "hash buckets: (55), range: [(<start>), (<end>))",
            "table_id": ""
        },
        "metrics": [
            {
                "name": "log_append_latency",
                "total_count": 7498,
                "min": 4,
                "mean": 69.3649,
                "percentile_75": 29,
                "percentile_95": 38,
                "percentile_99": 45,
                "percentile_99_9": 95,
                "percentile_99_99": 167,
                "max": 367244,
                "total_sum": 520098
            }
        ]
    }
]
All histograms and counters are measured since the server start time, and are not reset upon collection.

Collecting metrics to a log

Kudu may be configured to periodically dump all of its metrics to a local log file using the --metrics_log_interval_ms flag. Set this flag to the interval at which metrics should be written to a log file.

The metrics log will be written to the same directory as the other Kudu log files, with the same naming format. After any metrics log file reaches 64MB uncompressed, the log will be rolled and the previous file will be gzip-compressed.

The log file generated has three space-separated fields. The first field is the word metrics. The second field is the current timestamp in microseconds since the Unix epoch. The third is the current value of all metrics on the server, using a compact JSON encoding. The encoding is the same as the metrics fetched via HTTP described above.

Although metrics logging automatically rolls and compresses previous log files, it does not remove old ones. Since metrics logging can use significant amounts of disk space, consider setting up a system utility to monitor space in the log directory and archive or delete old segments.

Common Kudu workflows

Migrating to Multiple Kudu Masters

For high availability and to avoid a single point of failure, Kudu clusters should be created with multiple masters. Many Kudu clusters were created with just a single master, either for simplicity or because Kudu multi-master support was still experimental at the time. This workflow demonstrates how to migrate to a multi-master configuration.

The workflow is unsafe for adding new masters to an existing multi-master configuration. Do not use it for that purpose.
The workflow presupposes at least basic familiarity with Kudu configuration management. If using Cloudera Manager (CM), the workflow also presupposes familiarity with it.
All of the command line steps below should be executed as the Kudu UNIX user, typically kudu.

Prepare for the migration

  1. Establish a maintenance window (one hour should be sufficient). During this time the Kudu cluster will be unavailable.

  2. Decide how many masters to use. The number of masters should be odd. Three or five node master configurations are recommendeded; they can tolerate one or two failures respectively.

  3. Perform the following preparatory steps for the existing master:

    • Identify and record the directory where the master’s data lives. If using Kudu system packages, the default value is /var/lib/kudu/master, but it may be customized via the fs_wal_dir configuration parameter.

    • Identify and record the port the master is using for RPCs. The default port value is 7051, but it may have been customized using the rpc_bind_addresses configuration parameter.

    • Identify the master’s UUID. It can be fetched using the following command:

      $ kudu fs dump uuid --fs_wal_dir=<master_data_dir> 2>/dev/null
      master_data_dir

      existing master’s previously recorded data directory

      Example
      $ kudu fs dump uuid --fs_wal_dir=/var/lib/kudu/master 2>/dev/null
4aab798a69e94fab8d77069edff28ce0

    • Optional: configure a DNS alias for the master. The alias could be a DNS cname (if the machine already has an A record in DNS), an A record (if the machine is only known by its IP address), or an alias in /etc/hosts. The alias should be an abstract representation of the master (e.g. master-1).

      Without DNS aliases it is not possible to recover from permanent master failures, and as such it is highly recommended.

  4. Perform the following preparatory steps for each new master:

    • Choose an unused machine in the cluster. The master generates very little load so it can be colocated with other data services or load-generating processes, though not with another Kudu master from the same configuration.

    • Ensure Kudu is installed on the machine, either via system packages (in which case the kudu and kudu-master packages should be installed), or via some other means.

    • Choose and record the directory where the master’s data will live.

    • Choose and record the port the master should use for RPCs.

    • Optional: configure a DNS alias for the master (e.g. master-2, master-3, etc).

Perform the migration

  1. Stop all the Kudu processes in the entire cluster.

  2. Format the data directory on each new master machine, and record the generated UUID. Use the following command sequence:

    $ kudu fs format --fs_wal_dir=<master_data_dir>
$ kudu fs dump uuid --fs_wal_dir=<master_data_dir> 2>/dev/null
    master_data_dir

    new master’s previously recorded data directory

    Example
    $ kudu fs format --fs_wal_dir=/var/lib/kudu/master
$ kudu fs dump uuid --fs_wal_dir=/var/lib/kudu/master 2>/dev/null
f5624e05f40649b79a757629a69d061e

  3. If using CM, add the new Kudu master roles now, but do not start them.

    • If using DNS aliases, override the empty value of the Master Address parameter for each role (including the existing master role) with that master’s alias.

    • Add the port number (separated by a colon) if using a non-default RPC port value.

  4. Rewrite the master’s Raft configuration with the following command, executed on the existing master machine:

    $ kudu local_replica cmeta rewrite_raft_config --fs_wal_dir=<master_data_dir> <tablet_id> <all_masters>
    master_data_dir

    existing master’s previously recorded data directory

    tablet_id

    must be the string 00000000000000000000000000000000

    all_masters

    space-separated list of masters, both new and existing. Each entry in the list must be a string of the form <uuid>:<hostname>:<port>

    uuid

    master’s previously recorded UUID

    hostname

    master’s previously recorded hostname or alias

    port

    master’s previously recorded RPC port number

    Example
    $ kudu local_replica cmeta rewrite_raft_config --fs_wal_dir=/var/lib/kudu/master 00000000000000000000000000000000 4aab798a69e94fab8d77069edff28ce0:master-1:7051 f5624e05f40649b79a757629a69d061e:master-2:7051 988d8ac6530f426cbe180be5ba52033d:master-3:7051

  5. Start the existing master.

  6. Copy the master data to each new master with the following command, executed on each new master machine:

    $ kudu local_replica copy_from_remote --fs_wal_dir=<master_data_dir> <tablet_id> <existing_master>
    master_data_dir

    new master’s previously recorded data directory

    tablet_id

    must be the string 00000000000000000000000000000000

    existing_master

    RPC address of the existing master and must be a string of the form <hostname>:<port>

    hostname

    existing master’s previously recorded hostname or alias

    port

    existing master’s previously recorded RPC port number

    Example
    $ kudu local_replica copy_from_remote --fs_wal_dir=/var/lib/kudu/master 00000000000000000000000000000000 master-1:7051

  7. Start all of the new masters.

    Skip the next step if using CM.

  8. Modify the value of the tserver_master_addrs configuration parameter for each tablet server. The new value must be a comma-separated list of masters where each entry is a string of the form <hostname>:<port>

    hostname

    master’s previously recorded hostname or alias

    port

    master’s previously recorded RPC port number

  9. Start all of the tablet servers.

Congratulations, the cluster has now been migrated to multiple masters! To verify that all masters are working properly, consider performing the following sanity checks:

  • Using a browser, visit each master’s web UI. Look at the /masters page. All of the masters should be listed there with one master in the LEADER role and the others in the FOLLOWER role. The contents of /masters on each master should be the same.

  • Run a Kudu system check (ksck) on the cluster using the kudu command line tool. Help for ksck can be viewed via kudu cluster ksck --help.

Recovering from a dead Kudu Master in a Multi-Master Deployment

Kudu multi-master deployments function normally in the event of a master loss. However, it is important to replace the dead master; otherwise a second failure may lead to a loss of availability, depending on the number of available masters. This workflow describes how to replace the dead master.

Due to KUDU-1620, it is not possible to perform this workflow without also restarting the live masters. As such, the workflow requires a maintenance window, albeit a brief one as masters generally restart quickly.

Kudu does not yet support Raft configuration changes for masters. As such, it is only possible to replace a master if the deployment was created with DNS aliases. See the multi-master migration workflow for more details.
The workflow presupposes at least basic familiarity with Kudu configuration management. If using Cloudera Manager (CM), the workflow also presupposes familiarity with it.
All of the command line steps below should be executed as the Kudu UNIX user, typically kudu.

Prepare for the recovery

  1. Ensure that the dead master is well and truly dead. Take whatever steps needed to prevent it from accidentally restarting; this can be quite dangerous for the cluster post-recovery.

  2. Choose one of the remaining live masters to serve as a basis for recovery. The rest of this workflow will refer to this master as the "reference" master.

  3. Choose an unused machine in the cluster where the new master will live. The master generates very little load so it can be colocated with other data services or load-generating processes, though not with another Kudu master from the same configuration. The rest of this workflow will refer to this master as the "replacement" master.

  4. Perform the following preparatory steps for the replacement master:

    • Ensure Kudu is installed on the machine, either via system packages (in which case the kudu and kudu-master packages should be installed), or via some other means.

    • Choose and record the directory where the master’s data will live.

  5. Perform the following preparatory steps for each live master:

    • Identify and record the directory where the master’s data lives. If using Kudu system packages, the default value is /var/lib/kudu/master, but it may be customized via the fs_wal_dir configuration parameter.

    • Identify and record the master’s UUID. It can be fetched using the following command:

      $ kudu fs dump uuid --fs_wal_dir=<master_data_dir> 2>/dev/null
      master_data_dir

      live master’s previously recorded data directory

      Example
      $ kudu fs dump uuid --fs_wal_dir=/var/lib/kudu/master 2>/dev/null
80a82c4b8a9f4c819bab744927ad765c

  6. Perform the following preparatory steps for the reference master:

    • Identify and record the directory where the master’s data lives. If using Kudu system packages, the default value is /var/lib/kudu/master, but it may be customized via the fs_wal_dir configuration parameter.

    • Identify and record the UUIDs of every master in the cluster, using the following command:

      $ kudu local_replica cmeta print_replica_uuids --fs_wal_dir=<master_data_dir> <tablet_id> 2>/dev/null
      master_data_dir

      reference master’s previously recorded data directory

      tablet_id

      must be the string 00000000000000000000000000000000

      Example
      $ kudu local_replica cmeta print_replica_uuids --fs_wal_dir=/var/lib/kudu/master 00000000000000000000000000000000 2>/dev/null
80a82c4b8a9f4c819bab744927ad765c 2a73eeee5d47413981d9a1c637cce170 1c3f3094256347528d02ec107466aef3

  7. Using the two previously-recorded lists of UUIDs (one for all live masters and one for all masters), determine and record (by process of elimination) the UUID of the dead master.

Perform the recovery

  1. Format the data directory on the replacement master machine using the previously recorded UUID of the dead master. Use the following command sequence:

    $ kudu fs format --fs_wal_dir=<master_data_dir> --uuid=<uuid>
    master_data_dir

    replacement master’s previously recorded data directory

    uuid

    dead master’s previously recorded UUID

    Example
    $ kudu fs format --fs_wal_dir=/var/lib/kudu/master --uuid=80a82c4b8a9f4c819bab744927ad765c

  2. Copy the master data to the replacement master with the following command:

    $ kudu local_replica copy_from_remote --fs_wal_dir=<master_data_dir> <tablet_id> <reference_master>
    master_data_dir

    replacement master’s previously recorded data directory

    tablet_id

    must be the string 00000000000000000000000000000000

    reference_master

    RPC address of the reference master and must be a string of the form <hostname>:<port>

    hostname

    reference master’s previously recorded hostname or alias

    port

    reference master’s previously recorded RPC port number

    Example
    $ kudu local_replica copy_from_remote --fs_wal_dir=/var/lib/kudu/master 00000000000000000000000000000000 master-2:7051

  3. If using CM, add the replacement Kudu master role now, but do not start it.

    • Override the empty value of the Master Address parameter for the new role with the replacement master’s alias.

    • Add the port number (separated by a colon) if using a non-default RPC port value.

  4. Reconfigure the DNS alias for the dead master to point at the replacement master.

  5. Start the replacement master.

  6. Restart the existing live masters. This results in a brief availability outage, but it should last only as long as it takes for the masters to come back up.

Congratulations, the dead master has been replaced! To verify that all masters are working properly, consider performing the following sanity checks:

  • Using a browser, visit each master’s web UI. Look at the /masters page. All of the masters should be listed there with one master in the LEADER role and the others in the FOLLOWER role. The contents of /masters on each master should be the same.

  • Run a Kudu system check (ksck) on the cluster using the kudu command line tool. Help for ksck can be viewed via kudu cluster ksck --help.