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Run Memcached as a private, disposable cache beside your Rails application: install Ubuntu’s package, bind the daemon to loopback (or a private interface for a separate cache host), then point Rails’ :mem_cache_store at the address through Dalli. Never expose TCP or UDP port 11211 to the public internet.
This guide covers installation, systemd management, network hardening, Rails configuration, verification, cache design, capacity decisions, and recovery when the cache is unavailable or has been cleared.
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For a Rails application and Memcached on the same Ubuntu VPS, the safest starting topology is:
- Memcached runs as the operating-system service named
memcached. - The daemon listens only on
127.0.0.1, or on a Unix socket where your client setup supports it. - Rails uses
ActiveSupport::Cache::MemCacheStorewith the Dalli client and the endpoint127.0.0.1:11211. - Cached values have an expiry and can be regenerated from the database or another authoritative source.
- Port 11211 is not reachable from the public network.
If you put Memcached on a second VPS, bind it to that host’s private address and allow port 11211 only from the Rails server’s private address or security group. The application should continue to work when a cache entry is missing or the cache service is restarted.
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Install Memcached and enable its service
Use Ubuntu’s package manager rather than compiling a daemon manually. The package supplies the service integration and configuration expected by systemd.
- Refresh package metadata:
sudo apt-get update - Install Memcached:
sudo apt-get install memcached - Enable it at boot and start it immediately:
sudo systemctl enable --now memcached
Installing the package does not make an application cache anything by itself. The Memcached project explicitly notes that applications must be programmed to use the service; a running daemon alone does not accelerate database queries or page rendering.
Inspect the service and binary
Confirm that systemd considers the service healthy and inspect the command line that the package actually starts:
systemctl status memcached
systemctl cat memcached
memcached --versionsystemctl cat is useful because distribution packages can pass options through a unit file, an environment file, or an init-script compatibility layer. Check the effective command before changing a setting so that an override does not silently replace your intended configuration.
Bind and harden the Memcached daemon
Memcached is designed for trusted application networks, not hostile internet traffic. Its own configuration guidance warns that it does little to defend against random internet connections. Treat an internet-facing listener as a serious security error, not as a normal deployment option.
Local-only baseline for one VPS
Edit the package configuration file, commonly /etc/memcached.conf, and establish a local-only baseline such as:
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-m 256
-p 11211
-l 127.0.0.1
-U 0Choose -m for the workload and available RAM; do not copy 256 blindly. The explicit -U 0 disables UDP. Newer Memcached releases disable UDP by default, but stating the intent in configuration prevents an accidental re-enable during a package or configuration change.
Understand the important daemon options
| Option or concern | Purpose | How to apply it |
|---|---|---|
-l / --listen |
Chooses the address on which Memcached accepts connections. | Use 127.0.0.1 for a same-host Rails app. For a remote cache, use only the private interface address. |
-p |
Selects the TCP port. | 11211 is the conventional port. Restrict it with binding and firewall rules. |
-s |
Creates a Unix-domain socket instead of relying on TCP. | Use this for a same-host design when your Rails/Dalli configuration supports the socket path and its permissions. |
-U |
Controls the UDP listener. | Set -U 0 when UDP is unnecessary. |
-m |
Sets the memory limit for cached data. | Size it against available VPS memory and the cache working set; the Ubuntu Noble manpage’s 64 MB figure is a default, not a recommendation. |
| Connection limit | Caps simultaneous client connections. | The Ubuntu Noble manpage lists 1,024 as the default. Compare that with Rails worker, thread, and process counts before changing it. |
| Item-size limit | Limits the size of an individual value. | Measure serialized Rails values and review the daemon’s effective limit; the supplied Ubuntu reference does not establish a universal value for your package. |
The 64 MB memory and 1,024-connection numbers documented for Ubuntu Noble in 2024 describe package defaults. They are not sizing guidance for every VPS, Rails version, or traffic pattern.
Restart and verify the listener
After editing the configuration, restart the service and check both its state and network listeners:
sudo systemctl restart memcached
sudo systemctl is-active memcached
sudo ss -ltnup | grep 11211On a same-host deployment, the socket listing should show a listener on loopback rather than an address such as 0.0.0.0:11211 or [::]:11211. If you use a Unix socket, verify the socket file and its ownership and permissions instead of expecting a TCP listener.
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When Memcached runs on another VPS, replace 127.0.0.1 with the cache server’s private address. Enforce both layers:
- Bind Memcached only to the private interface.
- Allow TCP 11211 in the host firewall or cloud security group only from the Rails server’s private address (or its private security group).
- Do not create an inbound rule for
0.0.0.0/0or the public IPv6 equivalent. - Keep the private route available from every Rails process that must share the cache.
SASL can authenticate clients when the build and client support it, but authentication is not encryption. The official SASL documentation says it does not provide encryption and is not a justification for internet exposure. Private networking and firewalling remain mandatory.
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Configure Rails to use Memcached
Rails exposes Memcached through ActiveSupport::Cache::MemCacheStore. The store uses Dalli, so add the gem when it is not already in the bundle:
gem "dalli"Run bundle install through your normal deployment process, commit the resulting dependency files, and deploy the same bundle to every Rails process that should use the cache.
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Use an explicit production configuration
In config/environments/production.rb, or in an initializer loaded by production, configure the store with an environment-controlled server list and namespace:
config.cache_store = :mem_cache_store,
ENV.fetch("MEMCACHE_SERVERS", "127.0.0.1:11211").split(","),
{ namespace: ENV.fetch("CACHE_NAMESPACE", "myapp-production") }The server argument accepts multiple addresses. A comma-separated value lets you supply more than one Memcached server without changing application code:
MEMCACHE_SERVERS=10.0.2.15:11211,10.0.2.16:11211
CACHE_NAMESPACE=myapp-productionSet these variables in the service manager, deployment environment, or secret configuration outside source control. Do not commit private addresses, credentials, or environment-specific namespaces to a public repository.
Understand Rails’ fallback behavior
Rails’ caching guide documents this resolution order when servers are not passed directly: it checks MEMCACHE_SERVERS and then falls back to 127.0.0.1:11211. An explicit configuration is preferable because it makes the production endpoint and namespace visible in application configuration rather than relying on an implicit default.
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Choose a namespace deliberately
Use one namespace for processes that must share entries and different namespaces for environments that must remain isolated. For example, myapp-production and myapp-staging prevent a staging deploy from reading production entries when both environments point at the same cache host.
A namespace is not a security boundary. It prevents key collisions at the application level; it does not encrypt values or stop a client that already has access to Memcached from reading other keys.
Verify the complete path from Rails
Test with the same environment variables, bundle, and Unix user that the production service uses. A shell test as a different user can hide permission, endpoint, or environment mistakes.
- Open a production console:
RAILS_ENV=production bundle exec rails console - Write a short-lived probe, read it, and delete it:
cache = Rails.cache cache.write("memcached_probe", "ok", expires_in: 60.seconds) cache.read("memcached_probe") cache.delete("memcached_probe") - Confirm that the read returns
"ok"and that a read after deletion returnsnil.
This verifies Rails serialization, the Dalli connection, the configured namespace, and basic write/read/delete behavior. It does not prove that every production payload fits the item-size limit or that the cache will remain available under peak load.
Check the daemon without making it public
From the Rails VPS, query the local listener directly:
printf "versionrnquitrn" | nc 127.0.0.1 11211The daemon should return a version response and close the connection. For a private remote cache, replace the address with the private endpoint; never run this test against a public address that you have opened for general internet access.
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Review recent service messages when a test fails:
sudo journalctl -u memcached --since "10 minutes ago"Write cache code that survives misses
Memcached is disposable state. It can evict entries, restart without preserving them, or become temporarily unreachable. The database, API, or rendered source object remains authoritative; Rails code must regenerate a value whenever Rails.cache.read returns nil.
Use fetch for read-through behavior
Rails’ fetch API combines a lookup with a block that repopulates a miss:
Rails.cache.fetch("article:#{article.id}", expires_in: 5.minutes) do
ArticleSerializer.new(article).as_json
endChoose an expiry that matches how long stale data is acceptable. Short expiries reduce staleness but increase regeneration work; long expiries reduce backend load but require explicit invalidation when source data changes.
Design keys and serialization
- Include the record or query identity in every key, such as
article:42. - Include a representation or schema version when the serialized shape can change, for example
article:v2:42. - Keep keys deterministic and short enough to be practical for your client and monitoring tools.
- Store values that can be regenerated; do not use Memcached as the only copy of payments, sessions, audit records, or other durable data.
- Test serialization for the largest realistic object, not only the small probe string.
MemCacheStore supports Memcached-specific options on write and fetch. Review those options against your Dalli and Rails versions instead of assuming that every backend-specific setting has identical semantics.
Choose a deployment pattern
| Pattern | Best fit | Required controls | Main trade-off |
|---|---|---|---|
| Loopback TCP on the Rails VPS | One application host with modest cache requirements. | -l 127.0.0.1, firewall default-deny policy, and -U 0. |
Simple and low latency, but cache capacity is tied to that VPS. |
| Unix socket on the same VPS | Same-host communication where socket permissions and client support are easy to operate. | Socket ownership, mode, path monitoring, and a Rails/Dalli endpoint configured for the socket. | Avoids TCP exposure, but permissions and deployment paths require care. |
| Private remote Memcached host | Several Rails servers sharing one cache. | Private binding, private routing, host firewall or security group restricted to Rails addresses, and monitoring of network failures. | Shared capacity, but introduces network latency and a cache-host failure domain. |
| Multiple private servers | Workloads that need more capacity or distribution. | Explicit server list, consistent namespaces, private access rules, and a plan for partial server loss. | More capacity and moving parts; key distribution and failure behavior must be tested. |
Rails documents MemCacheStore as a centralized shared cache suitable for multiple application processes and servers. Sharing a cache does not remove the need for namespace isolation or miss-safe application code.
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Size memory, connections, and expirations from measurements
Start with the effective package defaults, then compare them with observed workload rather than selecting numbers by habit.
- Memory: The Ubuntu Noble manpage lists 64 MB as the default limit. Increase it only when the VPS has headroom after Ruby, the database client, web server, and operating-system needs are accounted for.
- Connections: The documented default is 1,024 simultaneous connections. Compare it with the number of Rails processes and threads, plus health checks and administrative clients.
- Item size: Measure the serialized size of representative values. A single oversized object can fail even when total memory is mostly unused; the supplied Ubuntu reference does not establish one universal item-size figure for all packages.
- Expiry: Set
expires_infor data that becomes stale. An expiry is a correctness control, not merely a memory-saving setting. - Eviction: Expect old entries to disappear as memory fills. A miss must trigger regeneration, and regeneration should not overload the database when many keys expire together.
- Namespace: Keep staging, production, and incompatible serialization versions separate.
After changing a limit, restart Memcached and verify the effective command line again. A restart clears disposable cache contents, so schedule it with the expectation that the first requests will repopulate entries.
Troubleshoot common failures
| Symptom | Likely causes | Checks and fixes |
|---|---|---|
| Systemd service is inactive | Configuration syntax, a port conflict, or a failed package start. | Run systemctl status memcached, inspect systemctl cat memcached, and read journalctl -u memcached. Correct the option or conflict, then restart. |
| Rails cannot connect | Wrong MEMCACHE_SERVERS, missing Dalli gem, DNS/private-route failure, firewall denial, or a listener on another address. |
Print the environment seen by the Rails service, confirm the bundle, test the endpoint with nc, inspect ss on the cache host, and verify private firewall rules. |
| Connection works locally but not remotely | Memcached is bound to loopback, or the private route/security rule is missing. | Bind to the cache host’s private interface only and permit TCP 11211 from the Rails private address or security group. |
| Every read is a miss | Different namespaces, inconsistent keys, immediate expiry, or different server lists across processes. | Log the resolved namespace and key format, compare environment variables across processes, and inspect expiry values. |
| Large values fail while small probes work | The serialized object exceeds the effective item-size limit. | Measure the serialized payload, split or redesign the value, or adjust the daemon limit deliberately after checking memory impact. |
| Cache disappears after maintenance | Memcached restart or eviction removed disposable entries. | Confirm that application code rebuilds values on misses. Never treat the cache as durable storage. |
| Security scan reports port 11211 exposed | A wildcard bind or permissive cloud firewall/security group. | Set a loopback or private bind, disable UDP when unused, remove public ingress, restart, and recheck with ss and an external port scan. |
Operational checklist for production
- Install Memcached from Ubuntu’s package manager and manage it with systemd.
- Record the effective version and startup options after each package or configuration change.
- Bind locally or to a private interface; never expose 11211 to untrusted networks.
- Disable UDP explicitly with
-U 0unless a documented requirement says otherwise. - Restrict remote access with host firewall and cloud security-group rules.
- Keep SASL’s limitation in mind: authentication is not encryption.
- Configure Rails’
:mem_cache_storewith Dalli and an explicit server list. - Set
MEMCACHE_SERVERSandCACHE_NAMESPACEoutside source control. - Use separate namespaces for staging and production.
- Give cache entries intentional expirations and use
fetchto regenerate misses. - Test read, write, and delete from a production-like Rails console.
- Monitor service status, logs, memory pressure, connection usage, evictions, and miss-driven backend load.
- Document how the application behaves during a cache restart or complete cache loss.
Sources and scope of the documented defaults
The installation and security guidance above follows the Memcached project’s 2024 server and configuration guides. The 64 MB memory and 1,024-connection defaults are from Canonical’s Ubuntu Noble memcached manpage published in 2024. Rails’ server fallback and MemCacheStore behavior are documented in the Ruby on Rails caching guide from 2024. Package versions, defaults, and service-unit details can change with a later Ubuntu release, so inspect the installed unit and manpage on the VPS before relying on a value as a sizing target.
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