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When multiple LangSmith deployments share the same custom Redis instance without separate database numbers, streaming requests may fail silently—returning only heartbeats without any data.
This occurs because deployments without explicit database isolation can consume each other's streaming output from the shared Redis instance, causing data to be lost or routed to the wrong deployment.
Root Cause
When you configure REDIS_URI_CUSTOM without specifying a database number (e.g., redis://hostname:port instead of redis://hostname:port/1), all deployments default to Redis database 0. Shared use of the same Redis database causes:
Deployments to intercept each other's streamed messages
Cache key collisions between deployment instances
Undefined behavior in message routing and state management
Silent failures where streaming appears to work but no data flows back
How to Fix
Assign each deployment its own Redis database number by specifying the database index in your REDIS_URI_CUSTOM URI:
Deployment A:
REDIS_URI_CUSTOM=redis://hostname:port/1
Deployment B:
REDIS_URI_CUSTOM=redis://hostname:port/2
Deployment C:
REDIS_URI_CUSTOM=redis://hostname:port/3The number after the final / is the Redis database number. Multiple deployments can share the same Redis instance (same hostname and port), but each must use a different database number (0-15 are typically available, though this depends on your Redis configuration).
Verification Steps
Confirm your
REDIS_URI_CUSTOMincludes a unique database number for each deploymentRestart all affected deployments after updating the environment variable
Test streaming with
POST /runs/streamto verify data flows correctly
Important Note About Other Shared Infrastructure
The same isolation principle applies to custom PostgreSQL instances (POSTGRES_URI_CUSTOM):
Deployment A:
POSTGRES_URI_CUSTOM=postgresql://user:password@hostname/deployment_a
Deployment B:
POSTGRES_URI_CUSTOM=postgresql://user:password@hostname/deployment_bEach deployment must have its own database schema. You can share the same PostgreSQL instance (server), but not the same database.
For Redis, separate database numbers are preferred over sharing a single database with key prefixes. For PostgreSQL, you must use separate databases.