Updating Configuration

You can make changes to the service after it has been launched. Configuration management is handled by the scheduler process, which in turn handles deploying Cassandra itself.

After making a change, the scheduler will be restarted, and it will automatically deploy any detected changes to the service, one node at a time. For example, a given change will first be applied to the node-0 pod, then node-1, and so on.

Nodes are configured with a “Readiness check” to ensure that the underlying service appears to be in a healthy state before continuing with applying a given change to the next node in the sequence. However, this basic check is not foolproof and reasonable care should be taken to ensure that a given configuration change will not negatively affect the behavior of the service.

Some changes, such as decreasing the number of nodes or changing volume requirements, are not supported after initial deployment. See Limitations.

The instructions below describe how to update the configuration for a running DC/OS service.

Enterprise DC/OS 1.10

Enterprise DC/OS 1.10 introduces a convenient command line option that allows for easier updates to a service’s configuration, as well as allowing users to inspect the status of an update, to pause and resume updates, and to restart or complete steps if necessary.

Prerequisites

• Enterprise DC/OS 1.10 or newer.
• Service with a version greater than 2.0.0-x.
• The DC/OS CLI installed and available.
• The service’s subcommand available and installed on your local machine.
  • You can install just the subcommand CLI by running dcos package install --cli beta-cassandra.
  • If you are running an older version of the subcommand CLI that doesn’t have the update command, uninstall and reinstall your CLI.

    dcos package uninstall --cli beta-cassandra
    dcos package install --cli beta-cassandra
    

Preparing configuration

If you installed this service with Enterprise DC/OS 1.10, you can fetch the full configuration of a service (including any default values that were applied during installation). For example:

$ dcos beta-cassandra describe > options.json

Make any configuration changes to this options.json file.

If you installed this service with a prior version of DC/OS, this configuration will not have been persisted by the the DC/OS package manager. You can instead use the options.json file that was used when installing the service.

Note: You need to specify all configuration values in the options.json file when performing a configuration update. Any unspecified values will be reverted to the default values specified by the DC/OS service. See the “Recreating options.json” section below for information on recovering these values.

Recreating options.json (optional)

If the options.json from when the service was last installed or updated is not available, you will need to manually recreate it using the following steps.

First, we’ll fetch the default application’s environment, current application’s environment, and the actual template that maps config values to the environment:

1. Ensure you have jq installed.
2. Set the service name that you’re using, for example:

   $ SERVICE_NAME=beta-cassandra
   

3. Get the version of the package that is currently installed:

   $ PACKAGE_VERSION=$(dcos package list | grep $SERVICE_NAME | awk '{print $2}')
   

4. Then fetch and save the environment variables that have been set for the service:

   $ dcos marathon app show $SERVICE_NAME | jq .env > current_env.json
   

5. To identify those values that are custom, we’ll get the default environment variables for this version of the service:

   $ dcos package describe --package-version=$PACKAGE_VERSION --render --app $SERVICE_NAME | jq .env > default_env.json
   

6. We’ll also get the entire application template:

   $ dcos package describe $SERVICE_NAME --app > marathon.json.mustache
   

Now that you have these files, we’ll attempt to recreate the options.json.

1. Use JQ and diff to compare the two:

   $ diff <(jq -S . default_env.json) <(jq -S . current_env.json)
   

2. Now compare these values to the values contained in the env section in application template:

   $ less marathon.json.mustache
   

3. Use the variable names (e.g. ``) to create a new options.json file as described in Initial service configuration.

Starting the update

Once you are ready to begin, initiate an update using the DC/OS CLI, passing in the updated options.json file:

$ dcos beta-cassandra update start --options=options.json

You will receive an acknowledgement message and the DC/OS package manager will restart the Scheduler in Marathon.

See Advanced update actions for commands you can use to inspect and manipulate an update after it has started.

Open Source DC/OS, Enterprise DC/OS 1.9 and Earlier

If you do not have Enterprise DC/OS 1.10 or later, the CLI commands above are not available. For Open Source DC/OS of any version, or Enterprise DC/OS 1.9 and earlier, you can perform changes from the DC/OS GUI.

To make configuration changes via scheduler environment updates, perform the following steps:

1. Visit to access the DC/OS web interface.
2. Navigate to Services and click on the service to be configured (default PKGNAME).
3. Click Edit in the upper right. On DC/OS 1.9.x, the Edit button is in a menu made up of three dots.
4. Navigate to Environment (or Environment variables) and search for the option to be updated.
5. Update the option value and click Review and run (or Deploy changes).
6. The Scheduler process will be restarted with the new configuration and will validate any detected changes.
7. If the detected changes pass validation, the relaunched Scheduler will deploy the changes by sequentially relaunching affected tasks as described above.

To see a full listing of available options, run dcos package describe --config beta-cassandra in the CLI, or browse the DC/OS Apache Cassandra Service install dialog in the DC/OS Dashboard.

Adding a Node

The service deploys 3 nodes by default. This may be customized at initial deployment or after the cluster is already running via the NODES environment variable. Shrinking the cluster is not supported. If you decrease this value, the scheduler will complain about the configuration change until it’s reverted back to its original value or a larger one.

Resizing a Node

The CPU and Memory requirements of each node may be increased or decreased as follows:

• CPU (1.0 = 1 core): CASSANDRA_CPUS
• Memory (in MB): CASSANDRA_MEMORY_MB. To prevent out of memory errors, you must ensure that the TASKCFG_ALL_CASSANDRA_HEAP_SIZE environment variable is less than $CASSANDRA_MEMORY_MB.

Note: volume requirements (type and/or size) can not be changed after initial deployment.

Performing Cassandra Cleanup and Repair Operations

You may manually trigger certain nodetool operations against your Cassandra instance using the CLI or the HTTP API.

Cleanup

You may trigger a nodetool cleanup operation across your Cassandra nodes using the cleanup plan. This plan requires the following parameters to run:

• CASSANDRA_KEYSPACE: the Cassandra keyspace to be cleaned up.

To initiate this plan from the command line:

dcos cassandra plan start cleanup -p CASSANDRA_KEYSPACE=space1

To view the status of this plan from the command line:

dcos cassandra plan status cleanup
cleanup (IN_PROGRESS)
└─ cleanup-deploy (IN_PROGRESS)
   ├─ node-0:[cleanup] (COMPLETE)
   ├─ node-1:[cleanup] (STARTING)
   └─ node-2:[cleanup] (PENDING)

When the plan is completed, its status will be COMPLETE.

The above plan start and plan status commands may also be made directly to the service over HTTP. To see the queries involved, run the above commands with an additional -v flag.

For more information about nodetool cleanup, see the Cassandra documentation.

Repair

You may trigger a nodetool repair operation across your Cassandra nodes using the repair plan. This plan requires the following parameters to run:

• CASSANDRA_KEYSPACE: the Cassandra keyspace to be cleaned up.

To initiate this command from the command line:

dcos cassandra plan start repair -p CASSANDRA_KEYSPACE=space1

To view the status of this plan from the command line:

dcos cassandra plan status repair
repair (STARTING)
└─ repair-deploy (STARTING)
   ├─ node-0:[repair] (STARTING)
   ├─ node-1:[repair] (PENDING)
   └─ node-2:[repair] (PENDING)

When the plan is completed, its status will be COMPLETE.

The above plan start and plan status commands may also be made directly to the service over HTTP. To see the queries involved, run the above commands with an additional -v flag.

For more information about nodetool repair, see the Cassandra documentation.

Updating Placement Constraints

Placement constraints may be updated after initial deployment using the following procedure. See Service Settings above for more information on placement constraints.

Let’s say we have the following deployment of our nodes

• Placement constraint of: [["hostname", "LIKE", "10.0.10.3|10.0.10.8|10.0.10.26|10.0.10.28|10.0.10.84"]]
• Tasks:

  10.0.10.3: node-0
  10.0.10.8: node-1
  10.0.10.26: node-2
  10.0.10.28: empty
  10.0.10.84: empty
  

10.0.10.8 is being decommissioned and we should move away from it. Steps:

1. Remove the decommissioned IP and add a new IP to the placement rule whitelist by editing PLACEMENT_CONSTRAINT:

    [["hostname", "LIKE", "10.0.10.3|10.0.10.26|10.0.10.28|10.0.10.84|10.0.10.123"]]
   

2. Redeploy node-1 from the decommissioned node to somewhere within the new whitelist: dcos beta-cassandra pod replace node-1
3. Wait for node-1 to be up and healthy before continuing with any other replacement operations.

Restarting a Node

This operation will restart a node, while keeping it at its current location and with its current persistent volume data. This may be thought of as similar to restarting a system process, but it also deletes any data that is not on a persistent volume.

1. Run dcos beta-cassandra pod restart node-<NUM>, e.g. node-2.

Replacing a Node

This operation will move a node to a new system and will discard the persistent volumes at the prior system to be rebuilt at the new system. Perform this operation if a given system is about to be offlined or has already been offlined.

Note: Nodes are not moved automatically. You must perform the following steps manually to move nodes to new systems. You can automate node replacement according to your own preferences.

1. Run dcos beta-cassandra pod replace node-<NUM> to halt the current instance with id <NUM> (if still running) and launch a new instance elsewhere.

For example, let’s say node-2’s host system has died and node-2 needs to be moved.

dcos beta-cassandra pod replace node-2

Pausing a Node

Pausing a node relaunches it in an idle command state. This allows the operator to debug the contents of the node, possibly making changes to fix problems. While these problems are often fixed by just replacing the node, there may be cases where an in-place repair or other operation is needed.

For example:

• A node which crashes immediately upon starting may need additional work to be performed.
• Some services may require that certain repair operations be performed manually when the task itself isn’t running. Being able to put the node in an offline but accessible state makes it easier to resolve these situations.

After the node has been paused, it may be started again, at which point it will be restarted and will resume running task(s) where it left off.

Here is an example session where an index-1 node is crash looping due to some corrupted data in a persistent volume. The operator pauses the index-1 node, then uses task exec to repair the index. Following this, the operator starts the node and it resumes normal operation:

$ dcos beta-cassandra debug pod pause index-1
{
  "pod": "index-1",
  "tasks": [
    "index-1-agent",
    "index-1-node"
  ]
}

$ dcos beta-cassandra pod status
myservice
├─ index
│  ├─ index-0
│  │  ├─ index-0-agent (COMPLETE)
│  │  └─ index-0-node (COMPLETE)
│  └─ index-1
│     ├─ index-1-agent (PAUSING)
│     └─ index-1-node (PAUSING)
└─ data
   ├─ data-0
   │  └─ data-0-node (COMPLETE)
   └─ data-1
      └─ data-1-node (COMPLETE)

... repeat "pod status" until index-1 tasks are PAUSED ...

$ dcos task exec --interactive --tty index-1-node /bin/bash
index-1-node$ ./repair-index && exit

$ dcos beta-cassandra debug pod resume index-1
{
  "pod": "index-1",
  "tasks": [
    "index-1-agent",
    "index-1-node"
  ]
}

$ dcos beta-cassandra pod status
myservice
├─ index
│  ├─ index-0
│  │  ├─ index-0-agent (RUNNING)
│  │  └─ index-0-node (RUNNING)
│  └─ index-1
│     ├─ index-1-agent (STARTING)
│     └─ index-1-node (STARTING)
└─ data
   ├─ data-0
   │  └─ data-0-node (RUNNING)
   └─ data-1
      └─ data-1-node (RUNNING)

... repeat "pod status" until index-1 tasks are RUNNING ...

In the above example, all tasks in the node were being paused and started, but it’s worth noting that the commands also support pausing and starting individual tasks within a node. For example, dcos beta-cassandra debug pod pause index-1 -t agent will pause only the agent task within the index-1 node.

Seed nodes

Cassandra seed nodes are those nodes with indices smaller than the seed node count. By default, Cassandra is deployed with a seed node count of two. So, node-0 and node-1 are seed nodes. When a replace operation is performed on one these nodes, all other nodes must be restarted to be brought up to date regarding the ip address of the new seed node. This operation is performed automatically.

For example if node-0 needed to be replaced we would execute:

dcos beta-cassandra pod replace node-0

which would result in a recovery plan like the following:

$ dcos beta-cassandra --name=cassandra plan show recovery
recovery (IN_PROGRESS)
└─ permanent-node-failure-recovery (IN_PROGRESS)
   ├─ node-0:[server] (COMPLETE)
   ├─ node-1:[server] (STARTING)
   └─ node-2:[server] (PENDING)

Note: Only the seed node is being placed on a new node, all other nodes are restarted in place with no loss of data.

Upgrading Service Version

The instructions below show how to safely update one version of a service to the next.

Viewing available versions

The update package-versions command allows you to view the versions of a service that you can upgrade or downgrade to. These are specified by the service maintainer and depend on the semantics of the service (i.e. whether or not upgrades are reversal).

For example, for DC/OS Apache Cassandra Service, run:

$ dcos beta-cassandra update package-versions

Upgrading or downgrading a service

1. Before updating the service itself, update its CLI subcommand to the new version:

   $ dcos package uninstall --cli beta-cassandra
   $ dcos package install --cli beta-cassandra --package-version="1.1.6-5.0.7"
   

2. Once the CLI subcommand has been updated, call the update start command, passing in the version. For example, to update DC/OS Apache Cassandra Service to version 1.1.6-5.0.7:

   $ dcos beta-cassandra update start --package-version="1.1.6-5.0.7"
   

If you are missing mandatory configuration parameters, the update command will return an error. To supply missing values, you can also provide an options.json file (see Updating configuration):

$ dcos beta-cassandra update start --options=options.json --package-version="1.1.6-5.0.7"

See Advanced update actions for commands you can use to inspect and manipulate an update after it has started.

Advanced update actions

The following sections describe advanced commands that be used to interact with an update in progress.

Monitoring the update

Once the Scheduler has been restarted, it will begin a new deployment plan as individual pods are restarted with the new configuration. Depending on the high availability characteristics of the service being updated, you may experience a service disruption.

You can query the status of the update as follows:

$ dcos beta-cassandra update status

If the Scheduler is still restarting, DC/OS will not be able to route to it and this command will return an error message. Wait a short while and try again. You can also go to the Services tab of the DC/OS GUI to check the status of the restart.

Pause

To pause an ongoing update, issue a pause command:

$ dcos beta-cassandra update pause

You will receive an error message if the plan has already completed or has been paused. Once completed, the plan will enter the WAITING state.

Resume

If a plan is in a WAITING state, as a result of being paused or reaching a breakpoint that requires manual operator verification, you can use the resume command to continue the plan:

$ dcos beta-cassandra update resume

You will receive an error message if you attempt to resume a plan that is already in progress or has already completed.

Force Complete

In order to manually “complete” a step (such that the Scheduler stops attempting to launch a task), you can issue a force-complete command. This will instruct to Scheduler to mark a specific step within a phase as complete. You need to specify both the phase and the step, for example:

$ dcos beta-cassandra update force-complete service-phase service-0:[node]

Force Restart

Similar to force complete, you can also force a restart. This can either be done for an entire plan, a phase, or just for a specific step.

To restart the entire plan:

$ dcos beta-cassandra update force-restart

Or for all steps in a single phase:

$ dcos beta-cassandra update force-restart service-phase

Or for a specific step within a specific phase:

$ dcos beta-cassandra update force-restart service-phase service-0:[node]