Timeout on fetch while loading 841 pmtiles sources – Proposed fix by adjusting undici dispatcher

[marcioecom]

When using a configuration file (config.json) containing 841 pmtiles sources, tileserver-gl (v5.1.3) encounters a timeout error when fetching the sources. Initially, the log only displayed "fetch failed", but after adding a console.log(err) at line 768 in src/server.js, I obtained more detailed error information:

Using specified config file from config.json
Starting server
Listening at http://[::]:8080/
TypeError: fetch failed
    at Object.fetch (node:internal/deps/undici/undici:11730:11)
    at processTicksAndRejections (node:internal/process/task_queues:95:5)
    at runNextTicks (node:internal/process/task_queues:64:3)
    at process.processImmediate (node:internal/timers:449:9) {
  cause: ConnectTimeoutError: Connect Timeout Error
      at onConnectTimeout (node:internal/deps/undici/undici:6869:28)
      at node:internal/deps/undici/undici:6825:50
      at Immediate._onImmediate (node:internal/deps/undici/undici:6857:13)
      at process.processImmediate (node:internal/timers:478:21) {
    code: 'UND_ERR_CONNECT_TIMEOUT'
  }
}
fetch failed

Steps to Reproduce:

1. Create a config.json file containing 841 pmtiles sources.

{
  "options": {},
  "data": {
    "sampleSource1": {
      "pmtiles": "https://example.com/tiles/sample1.pmtiles"
    },
    "sampleSource2": {
      "pmtiles": "https://example.com/tiles/sample2.pmtiles"
    }
    // ... additional sources
  }
}

2. Start tileserver-gl using the following Docker command:

   docker run --name tileserver -v $(pwd)/config.json:/data/config.json --restart unless-stopped -p 8080:8080 -d maptiler/tileserver-gl

3. Check the logs and observe the timeout error as described.

Environment:

• Tileserver-gl: Version 5.1.3
• Platform: Docker (using the maptiler/tileserver-gl image)
• Configuration: config.json with 841 pmtiles sources (attachment available if needed)

Reference:

• Related issue in the undici library: [#1531](fetch times out in under 5 seconds nodejs/undici#1531)

Proposed Solution:
I tested a solution suggested in the undici issue, which involves setting a custom global dispatcher for undici by increasing the number of connections in the pool. The code used was:

import { Agent, Pool, setGlobalDispatcher } from 'undici';

setGlobalDispatcher(
  new Agent({
    factory: (origin) => new Pool(origin, { connections: 128 }),
  }),
);

With this modification, the server is able to load all the sources (even when using a pool with just 1 connection, albeit with reduced performance). This approach can be considered either a temporary workaround or the starting point for making this configuration customizable via environment variables or configuration parameters.

Final Considerations:
I would like to discuss if this change is appropriate for the project and, if so, I can open a pull request with the implementation and necessary documentation updates. I believe this change will help handle scenarios where a large number of sources are loaded simultaneously.

Thank you for your attention, and I'm available for any further questions or adjustments.

---