feat(cache): add Redis Cluster support for HA deployments

[krhoward-amd]

Implements Redis Cluster client support in slurm-web agent to enable high-availability caching across distributed Redis clusters.

Problem

Slurm-web currently supports only standalone Redis instances for caching. In high-availability deployments with Redis Cluster (3+ node clustered Redis), slurm-web agents fail to connect because they use the standard redis.Redis() client instead of the cluster-aware redis.cluster.RedisCluster() client.

Solution

This commit adds optional Redis Cluster support while maintaining full backwards compatibility with standalone Redis deployments.

Core Changes

slurmweb/cache.py:

• Import RedisCluster and ClusterNode from redis.cluster
• Add cluster_mode and cluster_nodes optional parameters to CachingService
• Implement cluster mode initialization with RedisCluster client
• Parse cluster_nodes from "host:port" string format
• Add connection validation with fail-fast error handling

slurmweb/apps/agent.py:

• Pass cluster_mode and cluster_nodes parameters to CachingService
• Use getattr() with defaults for backwards compatibility

conf/vendor/agent.yml:

• Add cluster_mode boolean parameter (default: false)
• Add cluster_nodes list parameter with string content type
• Document configuration with examples

Features

• Opt-in design: Cluster mode disabled by default (cluster_mode=false)
• Automatic failover: Cluster continues if a Redis node fails
• Load distribution: Requests distributed across cluster nodes
• Backwards compatible: Existing standalone configurations work unchanged
• Fail-fast validation: Connection tested at initialization

Configuration Example

[cache]
enabled = yes
cluster_mode = yes
cluster_nodes =
    10.0.0.1:6379
    10.0.0.2:6379
    10.0.0.3:6379
jobs = 30
nodes = 30

Testing

Tested on production environment:

• Slurm-web 6.0.0
• Redis cluster: 3 nodes
• Slurm controllers: 2 nodes
• OS: Ubuntu 24.04
• Verified backward compatibility with standalone mode

Implementation Notes

• Uses "host:port" string format for RFL schema compatibility (list content type must be str, not dict)
• skip_full_coverage_check=True allows partial cluster visibility
• decode_responses=False maintains pickle serialization compatibility
• Connection validated with ping() at initialization

Closes: #[issue-number]

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[krhoward-amd]

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