# Socket Hang-Up / ECONNRESET

**Symptom:** `socket hang up` or `ECONNRESET` errors, often intermittent.

**Root cause:** The server or load balancer closes the Keep-Alive connection before the client detects it and stops reusing the socket.

**Quick triage:**

- Errors on every request → likely dangling stream (Step 1–2)
- Errors only after idle periods → Keep-Alive timeout mismatch (Step 3)
- Errors on long-running queries (INSERT FROM SELECT, etc.) → load balancer idle timeout (Step 4)
- Can't diagnose → disable Keep-Alive as a last resort (Step 5)

## Step 1 — Enable WARN-level logging to find dangling streams

> **Requires:** `>= 0.2.0` (logging support with `log.level` config option). In `>= 1.18.1`, the default log level changed from `OFF` to `WARN`, so this step may already be active. In `>= 1.18.2`, the client auto-emits a WARN log with Keep-Alive troubleshooting hints when an `ECONNRESET` is detected. In `>= 1.12.0`, a warning is logged when a socket is closed without fully consuming the stream.

```js
import { createClient, ClickHouseLogLevel } from "@clickhouse/client";

const client = createClient({
  log: { level: ClickHouseLogLevel.WARN },
});
```

Look for log lines about unconsumed or dangling streams — these are a common hidden cause. A **dangling stream** is a query response stream that was never fully consumed or explicitly closed with `ResultSet.close()`. Because the Node.js client reuses sockets (Keep-Alive), leaving a stream open corrupts the socket and causes the _next_ request to fail with `ECONNRESET`. Errors on **every request** strongly suggest dangling streams rather than a Keep-Alive timeout mismatch.

**Common dangling stream patterns:**

```js
// ❌ Wrong — result stream never consumed; socket is left open
const resultSet = await client.query({ query: "SELECT ..." });
// result is abandoned without calling .json(), .text(), .stream(), or .close()

// ❌ Wrong — stream created but not fully piped/iterated
const resultSet = await client.query({
  query: "SELECT ...",
  format: "JSONEachRow",
});
const stream = resultSet.stream();
// stream is never iterated and resultSet is never closed

// ✓ Correct — consume via .json()
const resultSet = await client.query({ query: "SELECT ..." });
const data = await resultSet.json();

// ✓ Correct — consume via async iteration
const resultSet = await client.query({
  query: "SELECT ...",
  format: "JSONEachRow",
});
for await (const rows of resultSet.stream()) {
  // process rows
}

// ✓ Correct — explicitly close; this destroys the underlying socket immediately
const resultSet = await client.query({ query: "SELECT ..." });
resultSet.close();
```

## Step 2 — Check your ESLint setup

Add the [`no-floating-promises`](https://typescript-eslint.io/rules/no-floating-promises/) ESLint rule. Unhandled promises leave streams dangling, which can cause the server to close the socket.

Even with `await`, if the returned `ResultSet` is not consumed (no `.json()`, `.text()`, `.close()`, or full stream iteration), the socket is left open. The ESLint rule catches the promise case; code review is needed for the "awaited but unconsumed result" case.

## Step 3 — Find the server's Keep-Alive timeout

```bash
curl -v --data-binary "SELECT 1" <your_clickhouse_url>
```

Check the response headers:

```
< Connection: Keep-Alive
< Keep-Alive: timeout=10
```

> **Requires:** `>= 0.3.0` (`keep_alive.idle_socket_ttl` was introduced in 0.3.0 with a default of 2500 ms, replacing the older `keep_alive.socket_ttl` from 0.1.1 which was removed in 0.3.0).

The default `idle_socket_ttl` in the client is **2500 ms**, which is safe for servers with a 3 s timeout (common in ClickHouse < 23.11). If your server has a higher timeout (e.g., 10 s), you can safely increase:

```js
const client = createClient({
  keep_alive: {
    idle_socket_ttl: 9000, // stay ~500ms below the server's timeout
  },
});
```

> ⚠️ If you still get errors after increasing, **lower** the value, not raise it.

> **Tip (`>= 1.18.3`):** Enable `keep_alive.eagerly_destroy_stale_sockets: true` to proactively destroy sockets that have been idle longer than `idle_socket_ttl` before each request. This helps when event loop delays prevent the idle timeout callback from firing on time.

## Step 4 — Long-running queries with no data in/out (INSERT FROM SELECT, etc.)

> **Requires:** `>= 1.0.0` (`request_timeout` default was fixed to 30 000 ms in 0.3.0; `url`-based configuration including `request_timeout` via URL params available since 1.0.0).

Load balancers may close idle connections mid-query. Force periodic progress headers:

```js
const client = createClient({
  request_timeout: 400_000, // e.g. 400s for long queries
  clickhouse_settings: {
    send_progress_in_http_headers: 1,
    http_headers_progress_interval_ms: "110000", // string — UInt64 type; set ~10s below LB idle timeout
  },
});
```

### ⚠️ Critical: 16 KB Node.js Header Size Limit

**Node.js defaults to a total received HTTP header limit of approximately 16 KB (this can be increased via the `--max-http-header-size` CLI flag[^max-header-size]).** ClickHouse sends a new progress header with each interval (~200 bytes), and after ~75 progress headers accumulate, Node.js will throw an exception and terminate the request unless that limit is raised.

[^max-header-size]: Since `>= 1.18.5`, the ClickHouse JS Node client forwards a per-client header limit via the `max_response_headers_size` (bytes) option on `createClient` (it maps to Node's `http(s).request({ maxHeaderSize })`). On older versions, the practical workarounds are the `--max-http-header-size` CLI flag / `NODE_OPTIONS` (process-wide) or supplying a custom `http.Agent` configured with `maxHeaderSize`.

**Maximum safe query duration formula:**

```
Max duration (seconds) ≈ http_headers_progress_interval_ms × 75 ÷ 1000
```

**Examples:**

- `http_headers_progress_interval_ms: '10000'` (10s) → **~12.5 minutes** max safe duration
- `http_headers_progress_interval_ms: '60000'` (60s) → **~75 minutes** max safe duration
- `http_headers_progress_interval_ms: '120000'` (120s) → **~2.5 hours** max safe duration

> **Note:** `http_headers_progress_interval_ms` is a `UInt64` ClickHouse setting, so it must be passed as a **string** (e.g., `'10000'`).

**Raising the Node.js header limit (e.g., to 64 KB):**

If you need a longer max safe duration without lengthening the progress interval, raise Node's HTTP header limit. For example, increasing it from the default 16 KB to **64 KB** quadruples the max safe duration (≈300 progress headers instead of ≈75).

```ts
// Option 1 (recommended, since `>= 1.18.5`) — per-client, no process-wide flag needed
const client = createClient({
  request_timeout: 400_000,
  max_response_headers_size: 65536, // 64 KB; lifts the per-request header cap
  clickhouse_settings: {
    send_progress_in_http_headers: 1,
    http_headers_progress_interval_ms: "110000",
  },
});
```

```bash
# Option 2 — CLI flag when launching your app (process-wide; older client versions)
node --max-http-header-size=65536 app.js

# Option 3 — environment variable (works with any Node entry point, including npm/ts-node)
NODE_OPTIONS="--max-http-header-size=65536" node app.js
```

With `maxHeaderSize = 65536` (64 KB), the formula becomes:
Max duration (seconds) ≈ http_headers_progress_interval_ms × 300 ÷ 1000

```
Max duration ≈ http_headers_progress_interval_ms ÷ 1000 × 300
```

Examples at 64 KB:

- `http_headers_progress_interval_ms: '10000'` (10s) → **~50 minutes** max safe duration
- `http_headers_progress_interval_ms: '60000'` (60s) → **~5 hours** max safe duration
- `http_headers_progress_interval_ms: '120000'` (120s) → **~10 hours** max safe duration

**Guidelines for choosing the interval** (subject to your load balancer's idle timeout — see trade-offs below):

1. **For queries under 12 minutes:** Use `'10000'` ms (10s) intervals, if your LB idle timeout allows
2. **For queries 12 min – 1 hour:** Use `'60000'` ms (60s) intervals, if your LB idle timeout allows
3. **For queries 1–2 hours:** Use `'120000'` ms (120s) intervals, if your LB idle timeout allows
4. **For mutations over 2 hours:** Use the fire-and-forget pattern (see below)
5. **For SELECT queries over 2 hours:** Increase `http_headers_progress_interval_ms` to extend the safe duration, while keeping it below your LB idle timeout and within Node.js header-limit constraints

Use this command to experiment and debug:

```bash
curl -v "http://localhost:8123/?function_sleep_max_microseconds_per_block=10000000&wait_end_of_query=1&send_progress_in_http_headers=1&max_block_size=1&query=select+sum(sleepEachRow(1))+from+numbers(10)+FORMAT+JSONEachRow"
```

Experimenting with the exact load balancer stack might be required.

**Important trade-offs:**

- **Shorter intervals** = better load balancer keep-alive (prevents idle timeout) but **lower max duration**
- **Longer intervals** = higher max duration but **higher risk of LB idle timeout**

As a rule of thumb, set the interval slightly **below** your load balancer's idle timeout—typically by a few seconds (for example, often around 5–20 seconds), depending on your load balancer, proxies, and network behavior—while staying under the header limit for your expected query duration.

**Alternatively — fire-and-forget (mutations only):** Mutations (`INSERT ... SELECT`, `OPTIMIZE`, `ALTER`) are not cancelled on the server when the client connection is lost. You can send the mutation and immediately close the connection, then poll `system.query_log` or `system.mutations` for status. This bypasses both the load balancer idle timeout and the Node.js header limit. See the [client repo examples](https://github.com/ClickHouse/clickhouse-js/tree/main/examples) for a concrete implementation.

## Step 5 — Disable Keep-Alive entirely (last resort)

> **Requires:** `>= 0.1.1` (Keep-Alive disable option introduced in 0.1.1).

Adds overhead (new TCP connection per request) but eliminates all Keep-Alive issues:

```js
const client = createClient({
  keep_alive: { enabled: false },
});
```