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First decide what must be shared and protected
Several requests from one browser can arrive at once—for example, a page request and an API request. They may read or update the same server-side session. The right design depends on both the state’s scope and the operation’s consistency needs.
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| Need | Suitable mechanism | Scope and caution |
|---|---|---|
| Read-only or worker-local data | Lua module-level state | Module state persists within a worker, not across Nginx workers. Mutable values are risky when an operation can yield to the event loop mid-update. |
| Shared values or atomic counters | lua_shared_dict |
Shared among workers in one Nginx server instance. Dictionary operations such as incr are atomic, but the dictionary is not a multi-host store. |
| One read-modify-write session update must not race | lua-resty-lock plus shared state |
Lock a stable key derived from the session identity, then re-read and update state while holding the lock. The lock coordinates workers in one instance; it does not store the session. |
| Session state shared by several Nginx hosts | An external session store with documented cross-host behavior | Local shared memory and its locks cannot provide correctness across hosts. Choose storage and coordination semantics appropriate to the application. |
| Limit simultaneous requests for load control | resty.limit.conn or NGINX limit_conn |
These limit concurrency according to a configured key and scope; they do not make a session read-modify-write sequence safe. |
These distinctions are documented in the OpenResty Official Blog’s “How to Share Data Between Requests in OpenResty” (originally posted December 1, 2020; updated July 7, 2026), the ngx_lua documentation, and the OpenResty package documentation for lua-resty-lock and lua-resty-limit-traffic. The exact API behavior and supported Lua phases depend on the OpenResty/ngx_lua release you deploy.
Configure shared state and a lock zone
The following is a compact OpenResty example for a single server instance. Put shared-memory zones in the Nginx http context. The sizes are example values, not universal recommendations: validate memory use, capacity, and eviction behavior against your session volume and state size.
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http {
lua_shared_dict sessions 20m;
lua_shared_dict session_locks 1m;
server {
listen 8080;
location = /session-counter {
content_by_lua_file /etc/nginx/lua/session_counter.lua;
}
}
}Install and load a version of lua-resty-lock compatible with your OpenResty build. The Lua example below uses an X-Session-ID header only to keep the concurrency example focused; a real application should derive identity from its established, validated session mechanism and apply its own authentication, cookie, CSRF, and authorization rules. Do not log raw session identifiers or other secret session material.
Serialize a session read-modify-write operation
This example increments a per-session counter. It acquires the lock, reads the value only after acquiring it, writes the updated value, and then releases the lock. Re-reading after lock acquisition matters: another request may have changed the session while this request was waiting.
-- /etc/nginx/lua/session_counter.lua
local sessions = ngx.shared.sessions
local resty_lock = require "resty.lock"
local session_id = ngx.req.get_headers()["X-Session-ID"]
if type(session_id) ~= "string"
or #session_id < 16
or #session_id > 256
or not session_id:match("^[%w_-]+$") then
ngx.status = ngx.HTTP_BAD_REQUEST
ngx.say("Invalid session identifier")
return
end
-- Do not expose the raw session identifier in the dictionary key or logs.
local state_key = ngx.md5(session_id)
local lock, err = resty_lock:new("session_locks", {
timeout = 0.2,
exptime = 5,
})
if not lock then
ngx.log(ngx.ERR, "could not create session lock: ", err)
ngx.status = ngx.HTTP_INTERNAL_SERVER_ERROR
ngx.say("Session operation unavailable")
return
end
local elapsed, lock_err = lock:lock("session:" .. state_key)
if not elapsed then
if lock_err == "timeout" then
ngx.status = ngx.HTTP_SERVICE_UNAVAILABLE
ngx.header["Retry-After"] = "1"
ngx.say("Session is busy; retry the request")
else
ngx.log(ngx.ERR, "could not acquire session lock: ", lock_err)
ngx.status = ngx.HTTP_INTERNAL_SERVER_ERROR
ngx.say("Session operation unavailable")
end
return
end
local ok, new_value = pcall(function()
local current = sessions:get(state_key)
local count = tonumber(current) or 0
local updated = count + 1
local stored, set_err = sessions:set(state_key, tostring(updated), 3600)
if not stored then
error("shared dictionary write failed: " .. (set_err or "unknown error"))
end
return updated
end)
local unlocked, unlock_err = lock:unlock()
if not unlocked then
ngx.log(ngx.ERR, "could not release session lock: ", unlock_err)
end
if not ok then
ngx.log(ngx.ERR, "session update failed: ", new_value)
ngx.status = ngx.HTTP_INTERNAL_SERVER_ERROR
ngx.say("Session operation unavailable")
return
end
ngx.header.content_type = "application/json"
ngx.say('{"count":', new_value, '}')The example sets a one-hour TTL on the dictionary entry; that is application state expiry, not a recommendation for browser-session lifetime. Its 0.2-second lock wait and five-second lock-entry expiry are illustrative settings, not workload guidance. Measure the actual critical-section duration and contention, and choose bounded wait and expiry values with operational margin. The lua-resty-lock documentation gives defaults of a five-second wait timeout and a thirty-second lock-entry expiry, and says the timeout must not exceed the expiry. Avoid copying defaults without considering your own workload.
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This is an instructional pattern, not a tested drop-in session implementation. In production, review logging and error handling, session serialization and expiration, dictionary capacity, and behavior if a write succeeds but lock release fails. A separate stateful lock object should be created for each simultaneous lock in different Lua light threads.
Choose the lock lifecycle and failure response deliberately
- Validate the identity. Use the same stable session identity for all requests that must coordinate. Validate its format and avoid putting secret material in logs.
- Acquire a bounded lock. If acquisition times out or errors, do not continue as if the lock were held. Return a deliberate response or use an application-defined retry policy.
- Re-read current state. Do not rely on a value read before waiting for the lock; a concurrent request may have changed it.
- Keep the critical section short. Perform only the state read, necessary computation, and write while holding the lock. Avoid slow network calls inside it.
- Release on every path. Unlock promptly, including on failures and early exits. The lock-entry expiry is a recovery backstop, not a substitute for cleanup.
lua-resty-lock uses shared memory and cooperative sleeps while waiting, so waiting does not block an OS thread. Its documentation also cautions that yielding APIs are not available in every ngx_lua phase; examples include init_by_lua*, header and body filters, balancer, and log contexts. Keep lock usage in a supported request phase and check the constraints for your deployed version.
Know where local coordination stops
A lua_shared_dict is shared by workers in one Nginx server instance. The lock library likewise describes its lock as spanning the worker processes in the current instance. If a load balancer can send requests for one session to different OpenResty hosts, local dictionaries may diverge and local locks will not coordinate those hosts. Use a session backend or coordination mechanism whose documented semantics cover every instance that can process the session.
Worker-local module variables are narrower still. Required Lua modules persist within a worker, but another worker has its own state. OpenResty’s data-sharing guidance notes that worker-local mutable data is safe only when the calculation cannot yield during the operation; it is not a substitute for shared session storage.
Use concurrency limits for admission, not session correctness
Use resty.limit.conn from OpenResty’s lua-resty-limit-traffic package or NGINX’s limit_conn when the policy is to cap simultaneous requests, such as reducing load per configured client key. Decide whether that key represents an IP, account, session, or another identity, and ensure the limiter’s scope matches the policy. A request limit may reduce pressure, but it does not serialize a read-modify-write operation or prevent lost updates by itself.
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Plan for capacity and failure
- Shared dictionary sizing: Estimate the number and size of live entries, then validate memory behavior under representative load. No universal dictionary size follows from the APIs; a full zone or evictions can affect application behavior.
- Contention: A lock makes requests for the same key wait in sequence. Keep protected work short, set a bounded timeout, and decide what clients should do after a timeout rather than allowing indefinite waits.
- Worker crash or restart: In-memory state is not durable session storage. Treat restarts and deployment behavior as part of the session design; use a suitable external store where persistence or sharing across instances is required.
- External store outage: If a multi-host design depends on a remote backend, define how requests fail or retry when that backend is unavailable. The correct policy depends on the application’s consistency and availability requirements.
- Lock expiry: An expiry can allow recovery from a stale lock entry, but if it is too short relative to a legitimate critical section, another worker may proceed before the first operation is finished. Tune it from observed operation duration and verify semantics for the deployed library version.
Troubleshoot common symptoms
| Symptom | Likely cause | What to check or change |
|---|---|---|
| Different workers see different values | State is in a Lua module variable rather than shared storage, or requests are served by different hosts. | Use a lua_shared_dict for one-instance sharing; choose a cross-instance store if requests can land on multiple hosts. |
| Updates occasionally disappear or overwrite each other | A read-modify-write sequence is not atomic and is not protected by a per-session lock. | Acquire a stable per-session lock, re-read after acquisition, then update and release. For simple counters, use an atomic dictionary operation such as incr when its semantics fit. |
Lock acquisition returns timeout |
Another request holds the same key beyond the wait limit, or contention is high. | Keep the critical section short; inspect slow work under the lock; tune a bounded timeout and define a retry or response policy. |
| Lock creation or shared-memory access fails | The named shared dictionary is missing, configuration is not loaded, or the zone has capacity/configuration issues. | Confirm the lua_shared_dict declarations are in the http context and names match the Lua code; inspect Nginx error logs without logging session secrets. |
| Lock code errors in a particular directive | The Lua API yields in a phase where yielding is unsupported, or the deployed module version differs from assumptions. | Move the operation to a supported request phase and verify phase/API constraints against the deployed OpenResty/ngx_lua version. |
| A concurrency cap still permits lost session updates | A limiter controls admission/load; it is not a critical-section mutex. | Keep the limiter if needed for traffic policy, and add correct state serialization separately. |
Or skip the browser setup
If the debugging task is to capture a visual page snapshot rather than implement session coordination, ScreenshotNeo provides a screenshot API. Its one-request example is below; see the ScreenshotNeo API documentation for parameters and response details.
curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://stripe.com -o shot.webpScreenshotNeo accepts cookie and consent banners like a visitor and removes 60+ known consent platforms, newsletter popups, and chat widgets before capture; those steps can be turned off. Bot checks/CAPTCHAs, blank pages, timeouts, failed loads, and cache hits are not billed, and each response identifies the page verdict and billing status in headers. It also offers an MCP server for AI agents, with tools including take_screenshot, get_page_info, and capture_pdf. The free plan includes 1,000 screenshots per month with no card; paid plans start at $5 for 3,000 shots.
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Does a lock need to last as long as the browser session?
No. The lock expiry concerns recovery from a held lock entry; it is separate from the lifetime of the browser’s session state. Set session expiration according to your application’s policy.
Can I use a shared dictionary for a cache-fill lock pattern too?
Yes. The lock documentation describes checking the cache, locking after a miss, checking again, fetching only if still missing, writing before unlock, and releasing promptly on errors. That pattern prevents duplicate cache fills; it is not by itself a complete browser-session design.
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