HADR_TRANSPORT_FLOW_CONTROL SQL Server Wait Type

Wait Type HADR_TRANSPORT_FLOW_CONTROL

The wait type HADR_TRANSPORT_FLOW_CONTROL is ranked #262 by Stedman Solutions and Database Health Monitor.

Wait statistics, in the context of SQL Server, refer to the amount of time that a query spends waiting to access data in the database. When a client application requests data from the database, the request is placed in a queue and the client application must wait for its turn to access the data. The time that the query spends waiting is called a "wait" and is tracked by SQL Server. This information can be used to identify potential performance bottlenecks and optimize the performance of the database. Wait statistics are commonly used by database administrators to diagnose and troubleshoot performance issues in SQL Server.


SQL Server Always On Availability Groups rely on efficient data transport to keep replicas synchronized. During this process, you might encounter the HADR_TRANSPORT_FLOW_CONTROL wait type. Understanding this wait can help you troubleshoot synchronization issues and ensure your Always On setup operates smoothly.

What is HADR_TRANSPORT_FLOW_CONTROL?

The HADR_TRANSPORT_FLOW_CONTROL wait type occurs when SQL Server is managing the flow of data between the primary replica and secondary replicas. It specifically relates to situations where SQL Server temporarily pauses data transmission to avoid overloading the receiving replica or network. This wait type helps maintain balance and prevent excessive resource usage.

In simpler terms, this wait type indicates that SQL Server is pacing the flow of data to ensure stability and avoid congestion. While some waits are expected during normal operations, excessive or prolonged waits could signal underlying performance or configuration issues.

Why Does HADR_TRANSPORT_FLOW_CONTROL Happen?

Several factors can lead to HADR_TRANSPORT_FLOW_CONTROL waits, including:

  • Network bandwidth limitations causing delays in data transmission between replicas.
  • Resource constraints (CPU, memory, or disk I/O) on the secondary replica, slowing down log processing.
  • High transaction log activity on the primary replica, generating more data than the system can handle efficiently.
  • Suboptimal Always On configurations, such as improper synchronization modes.
  • Contention or bottlenecks in the network infrastructure.

Addressing these factors can help reduce these waits and improve synchronization performance.

How to Monitor HADR_TRANSPORT_FLOW_CONTROL Waits

Monitoring HADR_TRANSPORT_FLOW_CONTROL waits is essential for identifying patterns and pinpointing issues in Always On Availability Groups. The Database Health Monitor provides an excellent way to track this wait type. Its Historic Waits Monitoring feature allows you to observe when these waits occur, their frequency, and their correlation with system events.

With Database Health Monitor, you can identify whether these waits stem from network issues, resource limitations, or workload imbalances and take corrective action to optimize your environment.

What Can You Do About HADR_TRANSPORT_FLOW_CONTROL Waits?

If you notice frequent or prolonged HADR_TRANSPORT_FLOW_CONTROL waits, consider taking these actions:

  • Ensure your network infrastructure has sufficient bandwidth and low latency to handle Always On data transport.
  • Allocate adequate CPU, memory, and disk I/O resources on both primary and secondary replicas to process data efficiently.
  • Optimize workloads on the primary replica to reduce transaction log activity.
  • Review and adjust Always On synchronization modes to match your workload needs (e.g., asynchronous for non-critical secondary replicas).
  • Analyze network traffic to identify and resolve bottlenecks or contention.

These steps can help you reduce transport flow control waits and improve the overall performance of your Always On setup.

Why Use Database Health Monitor?

The Database Health Monitor is a comprehensive tool for tracking SQL Server wait types, including HADR_TRANSPORT_FLOW_CONTROL. Its Historic Waits Monitoring feature provides actionable insights into wait trends, helping you identify and address performance bottlenecks in your Always On Availability Groups. With Database Health Monitor, you can keep your SQL Server environment running efficiently and reliably.

Start using Database Health Monitor today to monitor and optimize your SQL Server’s performance, ensuring a resilient and high-performing database system!


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Applies to

    Related Waits

    HADR_AG_MUTEX
    HADR_AR_CRITICAL_SECTION_ENTRY
    HADR_AR_MANAGER_MUTEX
    HADR_AR_UNLOAD_COMPLETED
    HADR_ARCONTROLLER_NOTIFICATIONS_SUBSCRIBER_LIST
    HADR_BACKUP_BULK_LOCK
    HADR_BACKUP_QUEUE
    HADR_CLUSAPI_CALL
    HADR_COMPRESSED_CACHE_SYNC
    HADR_CONNECTIVITY_INFO
    HADR_DATABASE_FLOW_CONTROL
    HADR_DATABASE_VERSIONING_STATE
    HADR_DATABASE_WAIT_FOR_RESTART
    HADR_DATABASE_WAIT_FOR_TRANSITION_TO_VERSIONING
    HADR_DB_COMMAND
    HADR_DB_OP_COMPLETION_SYNC
    HADR_DB_OP_START_SYNC
    HADR_DBR_SUBSCRIBER
    HADR_DBR_SUBSCRIBER_FILTER_LIST
    HADR_DBSEEDING
    HADR_DBSEEDING_LIST
    HADR_DBSTATECHANGE_SYNC
    HADR_FABRIC_CALLBACK
    HADR_FILESTREAM_BLOCK_FLUSH
    HADR_FILESTREAM_FILE_CLOSE
    HADR_FILESTREAM_FILE_REQUEST
    HADR_FILESTREAM_IOMGR
    HADR_FILESTREAM_MANAGER
    HADR_GROUP_COMMIT
    HADR_LOGCAPTURE_SYNC
    HADR_LOGCAPTURE_WAIT
    HADR_LOGPROGRESS_SYNC
    HADR_NOTIFICATION_DEQUEUE
    HADR_NOTIFICATION_WORKER_EXCLUSIVE_ACCESS
    HADR_NOTIFICATION_WORKER_STARTUP_SYNC
    HADR_NOTIFICATION_WORKER_TERMINATION_SYNC
    HADR_PARTNER_SYNC
    HADR_READ_ALL_NETWORKS
    HADR_RECOVERY_WAIT_FOR_CONNECTION
    HADR_RECOVERY_WAIT_FOR_UNDO
    HADR_REPLICAINFO_SYNC
    HADR_SYNC_COMMIT
    HADR_SYNCHRONIZING_THROTTLE
    HADR_TDS_LISTENER_SYNC
    HADR_TDS_LISTENER_SYNC_PROCESSING
    HADR_TIMER_TASK
    HADR_TRANSPORT_DBRLIST
    HADR_TRANSPORT_SESSION
    HADR_WORK_POOL
    HADR_WORK_QUEUE
    HADR_XRF_STACK_ACCESS

    See Also


    All Wait Types
    HADR_TRANSPORT_FLOW_CONTROL SQL Server Wait Type