Twilio ConversationRelay Concurrency Limits and Error 64109 Explained
Question
What are the rate limits and concurrency limits for Twilio ConversationRelay on Programmable Voice, and under what conditions is Error 64109 triggered? How can applications scale beyond single-host limits for high call volumes?
Product
Programmable Voice
Environment
Twilio Console
Answer
Twilio ConversationRelay does not enforce a rate limit on the number of new sessions established per second. However, a per-host concurrency limit is active to prevent host overloading, which triggers Error 64109 (ConversationRelay: Concurrency limit reached) if a single host reaches its maximum session capacity.
Key details regarding ConversationRelay concurrency and scaling include:
Per-Host Concurrency Limit: Error
64109is enforced by an in-memory limiter on an individual host, allocating a maximum of 900 concurrent sessions per account on a single host (out of approximately 1,000 total sessions per host). It is not a fleet-wide or account-level global limit.Autoscaling: Twilio automatically scales host instances as call traffic grows. Error
64109typically only occurs during sudden, unexpected traffic bursts that outpace the time required to spin up new compute instances.High-Volume Scaling: If you anticipate massive, sudden traffic increases (such as high-volume events or large deployments), notify Twilio Support in advance so host capacity can be pre-scaled to accommodate your traffic.
Additional Information
For more details on error codes and handling within ConversationRelay, refer to the official Twilio Error Code Reference documentation.