Disaster Recovery 101: Architecting Auto-Failovers for DynamoDB and Ruby Deployments on OVH

Establishing Multi-Region DynamoDB Replication

For robust disaster recovery, a multi-region strategy for DynamoDB is paramount. This involves enabling global tables, which automatically replicate data across multiple AWS regions. While OVHcloud does not directly offer DynamoDB, we’ll architect this assuming a hybrid cloud scenario where critical data stores reside on AWS, and application frontends are deployed on OVHcloud. The core principle remains: ensure your data is available in a geographically distinct location.

The setup for DynamoDB Global Tables is primarily managed through the AWS Console or AWS CLI. The key is to create identical table structures in your primary and secondary regions and then associate them as replicas. This is a declarative process, not one requiring complex scripting for the replication itself.

Automating Ruby Application Failover on OVHcloud

Our Ruby application will be deployed across multiple OVHcloud regions. The failover mechanism will rely on a combination of DNS-level health checks and an intelligent load balancer or proxy. We’ll use HAProxy for its flexibility and performance in managing traffic routing based on service availability.

HAProxy Configuration for Multi-Region Failover

We’ll configure HAProxy to monitor health endpoints on our Ruby application instances in each region. If a primary region becomes unresponsive, HAProxy will automatically reroute traffic to the secondary region. This requires careful configuration of backend servers, health checks, and the frontend listener.

HAProxy Configuration Snippet

This configuration assumes you have two distinct OVHcloud regions, e.g., ‘GRA’ (Gravelines) and ‘RBX’ (Roubaix), each with a set of application servers. A health check endpoint, typically a simple HTTP GET request to /health, is crucial.

global
    log 127.0.0.1 local0
    maxconn 4096
    daemon

defaults
    mode http
    timeout connect 5000ms
    timeout client 50000ms
    timeout server 50000ms
    option httplog
    option dontlognull

listen stats
    bind *:8080
    mode http
    stats enable
    stats uri /haproxy?stats
    stats auth admin:StrongPassword123!

frontend main_frontend
    bind *:80
    acl is_gra_region dst -i 
    acl is_rbx_region dst -i 

    # Default to primary region if available
    use_backend app_servers_gra if is_gra_region
    # Fallback to secondary region if primary is down
    use_backend app_servers_rbx if is_rbx_region

backend app_servers_gra
    balance roundrobin
    option httpchk GET /health HTTP/1.1\r\nHost:\ primary.example.com
    server app1_gra 192.168.1.10:3000 check port 3000 inter 2000 rise 2 fall 3
    server app2_gra 192.168.1.11:3000 check port 3000 inter 2000 rise 2 fall 3

backend app_servers_rbx
    balance roundrobin
    option httpchk GET /health HTTP/1.1\r\nHost:\ secondary.example.com
    server app1_rbx 192.168.2.10:3000 check port 3000 inter 2000 rise 2 fall 3
    server app2_rbx 192.168.2.11:3000 check port 3000 inter 2000 rise 2 fall 3

Explanation:

global and defaults: Standard HAProxy settings for logging, connection limits, and timeouts.
listen stats: Exposes HAProxy’s statistics dashboard for monitoring. Secure this with strong authentication.
frontend main_frontend: Listens on port 80. The acl directives are placeholders for how you might direct traffic initially, but the core failover logic is in the backend definitions. In a true multi-region setup, you’d likely use external DNS health checks to point to the HAProxy instance in the *active* region.
backend app_servers_gra and app_servers_rbx: Define the groups of application servers in each region.
balance roundrobin: Distributes traffic evenly among healthy servers in a backend.
option httpchk GET /health ...: Configures HAProxy to send an HTTP GET request to the /health endpoint on each server. The rise and fall parameters define how many successful checks are needed to consider a server up (rise 2) and how many failures trigger an outage (fall 3).
server ... check port ... inter ...: Defines individual application servers, their IP addresses, ports, and the health check interval.

Integrating with DNS for Global Failover

While HAProxy handles failover *within* a region or between HAProxy instances if they are in different availability zones, true multi-region failover often starts at the DNS level. OVHcloud’s DNS services, or a third-party DNS provider like AWS Route 53 or Cloudflare, can be configured with health checks.

The strategy is to have a primary DNS record (e.g., app.example.com) pointing to the public IP of the HAProxy instance in your primary OVHcloud region. A secondary DNS record, also pointing to the HAProxy instance in the secondary region, is configured with a lower priority or as a failover target.

DNS Failover Logic (Conceptual)

Primary DNS Record: app.example.com (A record) -> IP of HAProxy in GRA. Configured with a health check targeting the HAProxy instance’s public IP and a specific port (e.g., 80).
Secondary DNS Record: app.example.com (A record) -> IP of HAProxy in RBX. Configured with a lower priority and the same health check.

When the health check for the primary DNS record fails, the DNS provider automatically switches traffic to the secondary record. This directs users to the HAProxy instance in the secondary region, which then takes over routing traffic to its local application servers.

Orchestrating Data Synchronization and Application State

The failover of the application layer is only half the battle. Data consistency is critical. If your DynamoDB tables are replicated globally, this is largely handled. However, for any data not in DynamoDB (e.g., local file storage, caches, or relational databases), a separate replication strategy is needed.

For relational databases like PostgreSQL or MySQL, consider setting up cross-region read replicas or logical replication. For file storage, tools like rsync or cloud-native object storage replication (if applicable) are essential. The key is to ensure that data written in the primary region is eventually consistent or immediately available in the secondary region.

Example: Database Replication (Conceptual)

If using PostgreSQL on OVHcloud instances:

# On the primary database server (e.g., in GRA)
sudo -u postgres psql -c "ALTER SYSTEM SET wal_level = replica;"
sudo -u postgres psql -c "ALTER SYSTEM SET max_wal_senders = 5;"
sudo -u postgres psql -c "ALTER SYSTEM SET archive_mode = on;"
sudo -u postgres psql -c "ALTER SYSTEM SET archive_command = 'cp %p /var/lib/postgresql/wal_archive/%f';"
sudo systemctl restart postgresql

# Configure replication user and permissions in pg_hba.conf
# Example entry for replication from RBX:
# host    replication     replicator      /32      md5

# On the secondary database server (e.g., in RBX)
# Perform an initial data sync (e.g., using pg_basebackup)
sudo -u postgres pg_basebackup -h  -D /var/lib/postgresql/data -U replicator -P -v -W

# Configure recovery settings in postgresql.conf
# Example entries:
# restore_command = 'ssh replicator@ "cp /var/lib/postgresql/wal_archive/%f"'
# recovery_target_timeline = 'latest'

sudo systemctl start postgresql
sudo -u postgres psql -c "SELECT pg_wal_replay_resume();" # If it stopped during setup

This is a simplified illustration. Production setups require robust WAL archiving, robust security, and potentially streaming replication with automatic failover tools like Patroni.

Monitoring and Alerting for Proactive Recovery

Automated failover is only effective if you are alerted when it occurs or, ideally, before it’s necessary. Implement comprehensive monitoring across all layers:

Application Performance Monitoring (APM): Track response times, error rates, and resource utilization of your Ruby applications. Tools like New Relic, Datadog, or Prometheus/Grafana are essential.
Infrastructure Monitoring: Monitor CPU, memory, disk I/O, and network traffic on your OVHcloud instances.
HAProxy Stats: Regularly scrape HAProxy’s stats endpoint to track backend health, connection counts, and error rates.
DNS Health Checks: Monitor the success/failure of your DNS health checks.
DynamoDB Metrics: Utilize AWS CloudWatch to monitor DynamoDB read/write capacity, latency, and throttled requests.

Configure alerts for critical thresholds. For instance, if more than 50% of application servers in a region become unhealthy, or if the DNS health check fails for an extended period, trigger an alert to your operations team. This allows for manual intervention if automation fails or for post-mortem analysis.