http.Agent keep-alive pooling maxTotalSockets change in Node.js 18+

Question

Moving an HTTP client that relies on Node.js http.Agent keep-alive pooling across a Node.js major version boundary raises a configuration decision about socket limits.

The affected feature is keep-alive socket reuse governed by maxSockets per origin and, in newer releases, maxTotalSockets for the default agent. agent.destroy() does not guarantee immediate closure of sockets attached to in-flight requests, and socket.setTimeout emits 'timeout' without destroying the socket, so stalled keep-alive connections can remain open.

With keepAlive disabled, each request can open a new socket, while with keepAlive enabled reuse depends on pool size and idle timeout behavior. The interaction between per-host and total limits is version-sensitive and affects how bursts are handled.

What is the effective concurrent socket cap for the default agent in Node.js 18+ when maxSockets is unspecified? Does keepAlive false prevent socket reuse and increase creation rate under bursts? How does maxTotalSockets interact with per-host maxSockets when multiple hosts share the default agent?

Tasadduq Burney · Answer

Effective concurrent socket cap
When maxSockets is left unspecified (defaults to Infinity) on the default http.globalAgent in Node.js 18+, the agent is limited by maxTotalSockets, which defaults to 50. Thus the effective concurrent socket cap is 50 sockets across all origins.
Confirmed facts

http.globalAgent.maxSockets defaults to Infinity in Node 18+.
http.globalAgent.maxTotalSockets defaults to 50.
keepAlive: false only omits the HTTP Keep‑Alive header; the agent may still hold sockets briefly for pipelining, but it does not reuse them across requests.
Under a burst, new sockets are created up to the maxTotalSockets limit; excess requests queue until a socket becomes free.
maxTotalSockets is a hard global ceiling. If multiple hosts share the default agent, the sum of active sockets for all hosts cannot exceed this value. A per‑host maxSockets (if set) acts as a secondary cap, but the global limit wins when the two conflict.

Practical verification

Run node -e "console.log(require('http').globalAgent.maxSockets, require('http').globalAgent.maxTotalSockets)" to confirm the defaults.
Create a test script that issues concurrent http.request calls to several hosts using the default agent and count active sockets via process._getActiveHandles().filter(h => h.constructor.name === 'Socket').length.

http.Agent keep-alive pooling maxTotalSockets change in Node.js 18+

1 answer

Effective concurrent socket cap

Confirmed facts

Practical verification

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