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1Scan for outdated or missing drivers - takes under a minute2Repair Windows errors before they cause bigger problems3Fix the driver behind crashes, sound loss and screen glitchesPut nginx behind Varnish, define every nginx node as a Varnish backend, and use the vmod_directors VMOD to distribute requests and remove failed nodes automatically. For cache-tier high availability, place a load balancer in front of multiple Varnish servers or deploy Varnish Enterprise Cluster; several nginx backends behind one Varnish process are not, by themselves, a highly available Varnish tier.
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This guide shows the topology, VCL, nginx settings, health probes, cache policy, invalidation, failover tests, and the trade-offs between independent Varnish nodes and a coordinated cluster.
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What the clustered design should look like
Separate the web tier into two decisions: how requests are distributed among origin servers, and how the cache tier remains available when a cache node fails.
Clients
|
Load balancer (recommended for multiple cache nodes)
|
Varnish cache nodes
|
nginx/origin nodes
|
Application workers, databases, object storageEach Varnish process can define several nginx or origin servers as backends. A director from vmod_directors chooses a backend for each request. This is the appropriate way to make one Varnish instance balance across multiple origins. Varnish documentation describes multiple backends being joined into clusters for load balancing.
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That arrangement protects the origin tier, but it does not protect the Varnish process itself. If one Varnish node is a single point of failure, put a network load balancer, reverse proxy, or equivalent traffic distributor in front of two or more Varnish nodes. The nodes will normally have independent caches, so the same object may be stored more than once. Varnish Enterprise Cluster is the alternative when coordinated cache ownership, inter-node health checks, encrypted node traffic, and cluster-aware deployment are worth the additional commercial configuration.
Two different meanings of “cluster”
- Origin cluster: one Varnish instance has several nginx backends and balances misses or passes among them.
- Cache cluster: several Varnish instances receive traffic through a front-end load balancer, or participate in an Enterprise Cluster that coordinates ownership and node health.
Use the first design when nginx capacity or origin failover is the problem. Use the second when losing a cache host must not interrupt service. You can use both at once.
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Choose the request-routing topology
| Design | Cache ownership | Failure behavior | Operational cost | Best fit |
|---|---|---|---|---|
| One Varnish, several nginx backends | One cache; no duplication between cache nodes | Origin failures can be bypassed, but the Varnish host remains a single point of failure | Lowest | Small deployments or a separate highly available Varnish host |
| Load balancer plus independent Varnish nodes | Each node owns its local cache; objects can be duplicated | Front-end routing removes a failed Varnish node; a miss may be slower on a cold node | Moderate; invalidations must reach every node | Simple, understandable cache-tier redundancy |
| Varnish Enterprise Cluster | Cluster membership and cache ownership are coordinated | Nodes use cluster health information and can route around failures | Highest; requires Enterprise configuration, node security, and cluster operations | Large installations where duplication and coordinated ownership matter |
Round-robin directors are easiest to reason about and spread successive requests across healthy origins. Hash-based routing is useful when requests should remain associated with a node, improving cache locality or application affinity. Select the algorithm deliberately: distribution and affinity are different goals.
Prepare nginx origin nodes
Give every nginx node a stable address and an identical virtual-host configuration. The Varnish host must be able to reach the port nginx listens on; do not point Varnish at a public load-balancer address that could send traffic back through Varnish and create a loop.
Create a dependency-light health endpoint
Use a path such as /health that returns HTTP 200 quickly without a login cookie, database query, or third-party call. A probe should answer whether nginx can accept traffic, not whether every downstream service is perfect. If the endpoint depends on a database that is temporarily slow, Varnish may eject a perfectly capable web node and cause an avoidable capacity loss.
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server {
listen 8080;
server_name origin.example.internal;
location = /health {
add_header Content-Type text/plain;
return 200 "okn";
}
location / {
proxy_set_header Host $host;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header X-Forwarded-Proto $scheme;
proxy_pass http://game_app;
}
}
upstream game_app {
server 10.0.20.11:3000 max_fails=3 fail_timeout=10s;
server 10.0.20.12:3000 max_fails=3 fail_timeout=10s;
}The upstream block is optional from Varnish’s perspective: nginx can serve static files, reverse-proxy application workers, or balance those workers itself. nginx’s HTTP load-balancing feature distributes requests among application servers to improve performance, scalability, and reliability. If you use NGINX Plus, its additional application health checks, activity monitoring, and on-the-fly server-group changes can complement (but do not replace) Varnish’s origin probes.
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- Deploy the same application release and nginx configuration to every origin.
- Use shared or consistently replicated assets when a request can land on any node.
- Keep session state in a shared store or use an explicit affinity strategy; do not assume round-robin requests return to the same worker.
- Allow Varnish addresses through the origin firewall, while blocking direct public access to the origin listener where possible.
- Return the same status and body from
/healthon every node.
Define Varnish backends and a director
Create one backend block per nginx node. The following example uses round-robin distribution and an HTTP probe. The IP addresses, port, limits, and timeout values are illustrative starting points; tune them to measured application behavior and origin capacity.
vcl 4.1;
import directors;
backend web01 {
.host = "10.0.10.11";
.port = "8080";
.connect_timeout = 2s;
.first_byte_timeout = 10s;
.between_bytes_timeout = 5s;
.max_connections = 300;
.probe = {
.url = "/health";
.timeout = 2s;
.interval = 5s;
.window = 8;
.threshold = 3;
.initial = 2;
}
}
backend web02 {
.host = "10.0.10.12";
.port = "8080";
.connect_timeout = 2s;
.first_byte_timeout = 10s;
.between_bytes_timeout = 5s;
.max_connections = 300;
.probe = {
.url = "/health";
.timeout = 2s;
.interval = 5s;
.window = 8;
.threshold = 3;
.initial = 2;
}
}
backend web03 {
.host = "10.0.10.13";
.port = "8080";
.connect_timeout = 2s;
.first_byte_timeout = 10s;
.between_bytes_timeout = 5s;
.max_connections = 300;
.probe = {
.url = "/health";
.timeout = 2s;
.interval = 5s;
.window = 8;
.threshold = 3;
.initial = 2;
}
}
director vdir;
sub vcl_init {
new vdir = directors.round_robin();
vdir.add_backend(web01);
vdir.add_backend(web02);
vdir.add_backend(web03);
}
sub vcl_recv {
if (req.method != "GET" && req.method != "HEAD") {
return (pass);
}
set req.backend_hint = vdir.backend();
}
sub vcl_backend_response {
set beresp.grace = 5m;
}Attach the director in vcl_init, then select its result with set req.backend_hint = vdir.backend(); in vcl_recv. Varnish removes a backend whose probe state is sick from the director’s rotation. When enough successful samples arrive, it becomes eligible again.
Set timeouts and connection limits from evidence
connect_timeout covers establishing the origin connection. first_byte_timeout covers the wait for the first response byte, while between_bytes_timeout and last_byte_timeout protect against a response that stalls partway through. max_connections prevents one backend from consuming unlimited Varnish connections. A timeout that is shorter than a normal application response creates false failures; one that is too long ties up workers during an outage. Measure normal and worst-case response times, then leave enough margin for legitimate slow requests.
When to use hash routing
Use a hash director when a stable request key should tend to the same origin, such as a cache key or a safely isolated tenant identifier. This can improve locality, but it can also concentrate load if the key distribution is uneven. Hashing is not a substitute for session replication, and it should not expose user-specific data through a shared cache key.
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Configure probes and automatic failover
A Varnish probe specifies the URL, expected response, timeout, polling interval, sample window, and threshold. The documented default probe values are a five-second interval, two-second timeout, eight-sample window, two successful initial polls, and three successes required for a healthy state. The example makes those values explicit so an operator can see the intended policy.
- Deploy
/healthon every nginx node. - Request the endpoint from the Varnish host and verify an HTTP 200 response within the probe timeout.
- Set the probe on every backend, not only on the first server in the list.
- Reload the VCL and inspect each backend’s probe state.
- Stop or isolate one origin and watch it transition to sick; new cache misses should use the remaining healthy nodes.
- Restore the origin and wait for the required successful samples before declaring it back in rotation.
Probe failure affects new backend selection. An object already cached can still be delivered. Configure grace behavior so Varnish can serve stale content while all origins are unavailable, but define a limit that matches the freshness requirements of the application. Stale delivery is useful for static game assets and news pages; it may be inappropriate for inventory, account, payment, or other state-changing data.
Do not confuse health with correctness
A 200 response proves that nginx answered the probe. It does not prove that a deployment is serving the correct build, that an application is returning valid JSON, or that a database write succeeded. Add deeper synthetic checks only when their cost and failure semantics are understood. Keep the basic probe fast and independent so it remains useful during partial outages.
Define what may be cached
Varnish’s cache policy belongs in VCL. Decide in vcl_recv which requests can be looked up, and in vcl_backend_response which responses may be stored.
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- Cache public
GETandHEADresponses when their content is identical for all users. - Pass requests with
Authorization, private session cookies, or other user-specific credentials unless you have explicitly designed a safe cache key. - Pass non-idempotent methods such as
POST,PUT,PATCH, andDELETEunless the application and invalidation policy support another behavior. - Normalize irrelevant query parameters and headers only after confirming they cannot change the representation.
- Honor origin cache headers where possible; override them only with a documented application requirement.
- Mark private, personalized, or error responses uncacheable.
sub vcl_recv {
if (req.method != "GET" && req.method != "HEAD") {
return (pass);
}
if (req.http.Authorization) {
return (pass);
}
if (req.http.Cookie ~ "(^|; )(session|logged_in)=") {
return (pass);
}
set req.backend_hint = vdir.backend();
}
sub vcl_backend_response {
if (beresp.http.Set-Cookie || beresp.http.Surrogate-control ~ "no-store" || beresp.http.Cache-Control ~ "private|no-store") {
set beresp.uncacheable = true;
set beresp.ttl = 0s;
return (deliver);
}
set beresp.grace = 5m;
}Adapt cookie names and headers to the application. A game portal may safely cache versioned JavaScript, images, and public patch notes while bypassing account pages, entitlement responses, and personalized dashboards.
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Make the Varnish tier highly available
Independent nodes behind a load balancer
Deploy at least two Varnish nodes and send client traffic to them through a health-aware load balancer. The balancer’s check should test the Varnish listener, while Varnish’s own probes test nginx origins. This creates two distinct health layers: cache-node availability and origin availability.
Independent nodes are easy to understand and generally easier to operate. Their cost is duplicated cache storage and cold-cache misses after traffic moves to a node that has not seen an object. You must also send purges or bans to every node, or stale content will remain available on whichever node was not updated.
Varnish Enterprise Cluster
An Enterprise Cluster coordinates node membership and cache ownership, performs health probes between cluster nodes, supports TLS for node-to-node traffic, and can be deployed with Helm. For troubleshooting, its documentation recommends a synthetic health response in vcl_recv, checking membership with varnishadm backend.list -p, testing a cacheable object with curl -I, and enabling X-Cluster-Trace to inspect cluster paths. Treat the cluster control plane and the origin probes as separate checks: a node can be a healthy cluster member while an nginx backend is sick.
Choose the Enterprise design when coordinated cache ownership and inter-node health are more valuable than the simplicity of independent nodes. It adds licensing, TLS key management, cluster membership configuration, and more failure modes to monitor.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Coordinate invalidation after deployments
A deployment can update nginx content while an old representation remains in one or more Varnish caches. Plan invalidation before the first production release.
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Ban matching objects
Varnish CLI bans mark matching objects obsolete so subsequent requests fetch fresh content from the backend. A controlled example is:
varnishadm ban 'req.url ~ ^/news/'Use a narrow expression that matches the release’s affected URLs. Broad bans can create a thundering herd of origin requests. In a multi-node cache tier, execute the ban on every independent Varnish node or use an authenticated purge workflow that fans out to all nodes. In a coordinated cluster, use the cluster’s documented invalidation mechanism and verify that the ban reached each member.
Prefer versioned static assets
For JavaScript, CSS, images, and downloadable game files, include a content hash or release version in the filename. New URLs avoid ambiguity and allow long cache lifetimes; retain the previous files until all clients have moved to the new release. Use bans for mutable URLs such as a news index or API response that cannot be versioned.
Validate the complete setup
- Run
varnishadm backend.list -pand confirm every backend is present with the expected probe state. - Request a known cacheable object twice:
curl -I http://cache.example/asset.js. Confirm the first request is fetched from an origin and the second is a cache hit using the response headers and access logs available in your deployment. - Stop or firewall one nginx node. Wait for probe samples to mark it sick, then request an uncached URL and confirm it reaches a healthy node.
- Restore the node and verify that successful probe samples return it to rotation.
- Stop one Varnish node behind the front-end load balancer. Confirm the balancer removes it while clients continue to receive responses from another cache node.
- Issue a controlled ban after a test deployment and verify that each cache node fetches the updated representation.
- If using Enterprise Cluster, enable
X-Cluster-Trace, request an uncached object, and inspect the node path. - Repeat tests with cookies, authorization headers, non-GET methods, backend 5xx responses, and a slow origin to ensure cache policy and timeout behavior match the design.
Troubleshoot common failure modes
| Symptom | Likely cause | Action |
|---|---|---|
| All origins show sick | Probe URL is wrong, blocked by a firewall, redirects unexpectedly, or exceeds the timeout | Request the exact health URL from the Varnish host; return a direct 200 and adjust network rules or probe timing. |
| Traffic still reaches a failed node | The object was already cached, or the director was not assigned in vcl_recv |
Distinguish a cache hit from a new miss; verify req.backend_hint and probe state. |
| Users see old content after a release | Only one cache node was purged, or the URL remained cacheable | Fan out the ban to every independent node and inspect cache-control and cookie rules. |
| Origin connections are exhausted | max_connections is too high, timeout values hold sockets too long, or a request storm followed a purge |
Measure origin capacity, lower concurrency where appropriate, lengthen only legitimate response timeouts, and stagger invalidations. |
| Personalized data appears to another user | Cookies or authorization were included in a shared cache path | Pass credentialed requests and responses; audit the cache key before re-enabling caching. |
| Cache nodes disagree | Independent caches received different invalidations or clients were routed unevenly | Check load-balancer health, purge fan-out, and cache headers; use coordinated ownership if duplication is unacceptable. |
| Hash routing overloads one origin | A small number of keys receive most traffic | Inspect key distribution, add capacity, or return to round-robin for that workload. |
Security and operations checklist
- Encrypt client traffic at the edge and use TLS for inter-node links when the deployment crosses untrusted networks.
- Restrict Varnish administration and purge commands to an authenticated management network.
- Do not expose
/healthdetails that reveal credentials, stack traces, or internal topology. - Monitor cache hit ratio, backend response time, probe transitions, open connections, bans, stale deliveries, and 4xx/5xx rates.
- Keep VCL in version control and validate it before reloads.
- Document which URLs are public, private, stale-tolerant, and purgeable.
- Exercise origin, cache-node, load-balancer, and invalidation failures separately; a green end-to-end request alone does not prove failover works.
Decision guide
Start with one Varnish node and multiple probed nginx backends when your immediate requirement is origin load balancing and failover. Add a front-end load balancer and independent Varnish nodes when cache-host failure must be tolerated and duplicated objects are acceptable. Select Varnish Enterprise Cluster when cache ownership, inter-node health, encrypted cluster traffic, and coordinated operations justify the added commercial and configuration complexity. In every design, keep nginx health checks lightweight, define cache rules explicitly, and test invalidation across the entire request path.
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