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Use multiple native viewers

You can run multiple Native Viewer windows simultaneously, each displaying different data or views of the same data.

How it works

Every Native Viewer binds to a gRPC port on startup. By default, this is port 9876. When you run rerun, it checks if a viewer is already listening on that port:

  • If yes: it connects to the existing viewer (or sends data to it)
  • If no: it starts a new viewer on that port

To open multiple viewer windows, use different ports with the --port flag.

Examples 

# Start a viewer on the default port (9876)
$ rerun &

# This does nothing—a viewer is already running on :9876
$ rerun &

# Start a second viewer on port 6789
$ rerun --port 6789 &

# Log an image to the first viewer (port 9876)
$ rerun image.jpg

# Log an image to the second viewer (port 6789)
$ rerun --port 6789 image.jpg

From the SDK

When using connect_grpc() from the SDK, specify the port to target a specific viewer:

import rerun as rr

# Connect to viewer on default port
rr.init("rerun_example_demo")
rr.connect_grpc()

# Or connect to a specific port
rr.connect_grpc("rerun+http://127.0.0.1:6789")

Tips

  • Use spawn() to automatically start a new viewer if needed—it will reuse an existing viewer on the default port if one is running
  • Each viewer maintains its own Chunk Store, so data sent to different viewers is independent
  • The Web Viewer doesn't use gRPC ports the same way—it connects via WebSocket when served locally