Automatic Spark Master election

Spark Master elections are automatically managed, and do not require any manual configuration.

DSE Analytics datacenters communicate with each other to elect one of the nodes as the Spark Master and another as the reserve Master. The Master keeps track of each Spark Worker and application, storing the information in a system table. If the Spark Master node fails, the reserve Master takes over and a new reserve Master is elected from the remaining Analytics nodes.

Each Analytics datacenter elects its own master.

For dsetool commands and options, see dsetool.

Determining the Spark Master address

You do not need to specify the Master address when configuring or using Spark with DSE Analytics. Configuring applications with a valid URL is sufficient for DSE to connect to the Master node and run the application. The following commands give information about the Spark configuration of DSE:

To view the URL used to configure Spark applications:

dse client-tool spark master-address

dse://10.200.181.62:9042?connection.local_dc=Analytics;connection.host=10.200.181.63;

To view the current address of the Spark Master in this datacenter:
```
dse client-tool spark leader-address
```
```
10.200.181.62
```

Workloads for Spark Master are flagged as Workload: Analytics(SM).

dsetool ring

Address          DC                   Rack         Workload             Graph  Status  State    Load             Owns                 Token                                        Health [0,1]                                                                                                                 0
10.200.181.62    Analytics            rack1        Analytics(SM)        no     Up      Normal   111.91 KiB       ?                    -9223372036854775808                         0.10

Query the dse_leases.leases table to list all the masters from each data center with Analytics nodes:

select * from dse_leases.leases ;

name        | dc                   | duration_ms | epoch   | holder
-------------------+----------------------+-------------+---------+---------------
 Leader/master/6.0 | Analytics            |       30000 |  805254 | 10.200.176.42
 Leader/master/6.0 | SearchGraphAnalytics |       30000 | 1300800 | 10.200.176.45
 Leader/master/6.0 | SearchAnalytics      |       30000 |       7 | 10.200.176.44

Ensure that the replication factor is configured correctly for the dse_leases keyspace

If the dse_leases keyspace is not properly replicated, the Spark Master might not be elected.

Every time you add a new datacenter, you must manually increase the replication factor of the dse_leases keyspace for the new DSE Analytics datacenter. If DataStax Enterprise or Spark security options are enabled on the cluster, you must also increase the replication factor for the dse_security keyspace across all logical datacenters.

The initial node in a multi datacenter has a replication factor of 1 for the dse_leases keyspace. For new datacenters, the first node is created with the dse_leases keyspace with an replication factor of 1 for that datacenter. However, any datacenters that you add have a replication factor of 0 and require configuration before you start DSE Analytics nodes. You must change the replication factor of the dse_leases keyspace for multiple analytics datacenters. See Setting the replication factor for analytics keyspaces.

Monitoring the lease subsystem

All changes to lease holders are recorded in the dse_leases.logs table. Most of the time, you do not want to enable logging.

To turn on logging, ensure that the lease_metrics_options is enabled in the dse.yaml file:
```
lease_metrics_options:
    enabled:true
    ttl_seconds: 604800
```

Look at the dse_leases.logs table:

select * from dse_leases.logs ;

 name              | dc  | monitor       | at                              | new_holder    | old_holder
-------------------+-----+---------------+---------------------------------+---------------+------------
 Leader/master/6.0 | dc1 | 10.200.180.44 | 2018-05-17 00:45:02.971000+0000 | 10.200.180.44 |
 Leader/master/6.0 | dc1 | 10.200.180.49 | 2018-05-17 02:37:07.381000+0000 | 10.200.180.49 |

When the lease_metrics_option is enabled, you can examine the acquire, renew, resolve, and disable operations. Most of the time, these operations should complete in 100 ms or less:

select * from dse_perf.leases ;

 name              | dc  | monitor       | acquire_average_latency_ms | acquire_latency99ms | acquire_max_latency_ms | acquire_rate15 | disable_average_latency_ms | disable_latency99ms | disable_max_latency_ms | disable_rate15 | renew_average_latency_ms | renew_latency99ms | renew_max_latency_ms | renew_rate15 | resolve_average_latency_ms | resolve_latency99ms | resolve_max_latency_ms | resolve_rate15 | up   | up_or_down_since
-------------------+-----+---------------+----------------------------+---------------------+------------------------+----------------+----------------------------+---------------------+------------------------+----------------+--------------------------+-------------------+----------------------+--------------+----------------------------+---------------------+------------------------+----------------+------+---------------------------------
 Leader/master/6.0 | dc1 | 10.200.180.44 |                          0 |                   0 |                      0 |              0 |                          0 |                   0 |                      0 |              0 |                       24 |               100 |                  100 |            0 |                          8 |                  26 |                     26 |              0 | True | 2018-05-03 19:30:38.395000+0000
 Leader/master/6.0 | dc1 | 10.200.180.49 |                          0 |                   0 |                      0 |              0 |                          0 |                   0 |                      0 |              0 |                        0 |                 0 |                    0 |            0 |                         10 |                  32 |                     32 |              0 | True | 2018-05-03 19:30:55.656000+0000

If the log warnings and errors do not contain relevant information, edit the logback.xml file and add:
```
<logger name="com.datastax.bdp.leasemanager" level="DEBUG">
```
Restart the node for the debugging settings to take effect.

Troubleshooting

Perform these various lease holder troubleshooting activities before you contact DataStax Support.

Verify the workload status

Run the dsetool ring command:

dsetool ring

If the replication factor is inadequate or if the replicas are down, the output of the dsetool ring command contains a warning:

Address          DC                   Rack         Workload             Graph  Status  State    Load             Owns                 Token                                        Health [0,1]
                                                                                                                                      0
10.200.178.232   SearchGraphAnalytics rack1        SearchAnalytics      yes    Up      Normal   153.04 KiB       ?                    -9223372036854775808                         0.00
10.200.178.230   SearchGraphAnalytics rack1        SearchAnalytics(SM)  yes    Up      Normal   92.98 KiB        ?                    0                                            0.000

If the automatic Job Tracker or Spark Master election fails, verify that an appropriate replication factor is set for the dse_leases keyspace.

Use cqlsh commands to verify the replication factor of the analytics keyspaces

Describe the dse_leases keyspace:

DESCRIBE KEYSPACE dse_leases;

CREATE KEYSPACE dse_leases WITH replication =
{'class': 'NetworkTopologyStrategy', 'Analytics1': '1'}
AND durable_writes = true;

Increase the replication factor of the dse_leases keyspace:

ALTER KEYSPACE dse_leases WITH replication =
{'class': 'NetworkTopologyStrategy', 'Analytics1': '3', 'Analytics2':'3'}
;

Run nodetool repair.

Automatic Spark Master election

Determining the Spark Master address

Ensure that the replication factor is configured correctly for the dse_leases keyspace

Monitoring the lease subsystem

Troubleshooting

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