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Service Broker Communication Protocols

Service Broker uses a broker-specific protocol to communicate with remote brokers. The broker manages connections separately from the normal pool of client connections. In order for two SQL Server instances to exchange Service Broker messages, each instance must be able to send TCP/IP traffic to the port that the other instance uses for Service Broker communications. By convention, Service Broker often uses port 4022 for broker-to-broker communication. However, the exact port is specified when the endpoint is created.

Service Broker takes a layered approach to communication. Each layer builds on the underlying layer to help ensure reliable delivery. This approach allows an application to operate without knowledge of the location of the remote service or the physical transport that the broker uses to communicate. In most cases, these protocols are transparent to an application. However, understanding the role each protocol layer plays may help in troubleshooting problems with an application.

The highest-level protocol that the broker uses is the dialog protocol. The dialog protocol layer handles reliable, sequenced message transmission. The dialog protocol layer generates sequence numbers for messages, generates acknowledgement messages, delivers messages to the proper queues, and fragments and reassembles messages. The dialog protocol handles authentication and encryption for a dialog.

The dialog protocol uses the adjacent broker protocol to transmit message fragments. The adjacent broker protocol handles the network transmissions exchanged between two broker instances.

The adjacent broker protocol uses a transport protocol, such as TCP/IP, to move messages from broker to broker.

Dialog Protocol

The dialog protocol manages the exactly-once-in-order (EOIO) delivery pattern for messages on a conversation. This protocol does not describe the format that Service Broker messages use on the network. Instead, the protocol specifies the logical steps required for a reliable conversation. The dialog protocol handles the tasks required for reliable delivery, including generating and processing acknowledgement messages.

Each side of a conversation is an endpoint in the dialog protocol layer The catalog view sys.conversation_endpoints shows information about dialog protocol endpoints. A conversation endpoint exists for the lifetime of the conversation.

Adjacent Broker Protocol

The adjacent broker protocol layer handles the mechanics of communication between two SQL Server instances. This layer encodes each message fragment into a standard format suitable for transmission over the network. Unlike the dialog protocol layer, the adjacent protocol layer is aware of the network transport used, and formats the message fragments appropriately. In effect, the adjacent broker protocol layer provides an abstraction layer between the dialog protocol layer and the transport protocol layer.

Each Service Broker network connection is an endpoint in the adjacent protocol layer. The dynamic management view sys.dm_broker_connections shows information about Service Broker network connections. Service Broker maintains the network connection while messages are actively exchanged. Service Broker closes the network connection when no messages have been sent or received over the network connection for a short period of time.

Transport Protocol

The transport protocol layer handles the actual network transmission. This layer is outside of Service Broker. For example, messages to a broker running in a different instance of SQL Server use TCP/IP as the transport protocol layer.

Service Broker endpoints set options for the transport protocol. SQL Server does not contain Service Broker endpoints by default. For more information on creating a Service Broker endpoint, see How to: Activate Service Broker Networking (Transact-SQL).

Service Broker uses two distinct categories of message. A sequenced message is a message that must be delivered to an application exactly once, in order. An unsequenced message is a message that can be processed immediately, regardless of the sequence in which the message arrives.

Service Broker uses sequenced messages for all user-defined message types, end dialog messages, and error messages created by an application. Each sequenced message has a sequence number. The instance that originates the message creates the message sequence number and assigns the sequence number to the message. The receiving broker uses the message sequence number to order the messages it provides to an application. For a given dialog, the application always receives the message with the lowest sequence number first. Service Broker also uses the message sequence number to detect duplicate messages. When the dialog protocol layer receives two messages on the same dialog with the same sequence number, the dialog protocol layer considers the messages to be duplicates and discards one.

Service Broker uses unsequenced messages for dedicated acknowledgement messages and error messages created by Service Broker. Service Broker takes no special precautions to deliver an unsequenced message. Notice, however, that Service Broker creates unsequenced messages in response to incoming messages. Therefore, if the unsequenced message is lost, the sender will retry the original message; the recipient then generates another unsequenced message.

Message Fragmentation

Service Broker splits outgoing messages into fragments, and combines incoming fragments into the original message. For small messages, the entire message is contained in one fragment. For large messages, Service Broker creates many fragments.

Fragmenting messages has several advantages. Sending a large message in small fragments improves overall speed and reliability when communicating over relatively slow and unreliable networks such as Wide-Area Networks (WANs). Should a fragment of the message be lost, the protocol retransmits only one fragment rather than the entire message. Fragmenting large messages can also reduce the amount of time required for a small message to reach the destination. Service Broker can send a fragment that contains an entire small message between fragments of a large message. This slows down the large message slightly in order to reduce the amount of time that the small message waits to be transmitted.

While a message is in the process of being reassembled, the partial message is stored in the destination queue, or the transmission queue for the database that hosts the destination queue if the destination queue is unavailable. A partial message cannot be received by an application. The status column for a partial message is set to 2 (Disabled). This value is also used for messages received out of order.

Message Acknowledgement

Service Broker acknowledges each message received. An acknowledgement can acknowledge one or several message fragments. If possible, an acknowledgement is included in the header of a message returned on the same conversation. If no other messages are ready to be sent, Service Broker returns a dedicated acknowledgement message. Message acknowledgement is handled entirely by Service Broker; an application that uses Service Broker does not receive these messages.

A sender retains message fragments that the receiver has not acknowledged. If no acknowledgement is received within a system-defined wait time, the sender sends the message fragment again. If no acknowledgement is received during the wait time, Service Broker exponentially increases the amount of time before the next retry, up to a maximum wait time. The initial wait time for a retry is a few seconds. The maximum wait time is approximately one minute. Notice that the wait time is not intended to be precise; depending on network traffic and the other activity in the SQL Server instance, a message fragment may not be retried for a few seconds after the wait time expires.

If an acknowledgement is lost or delayed, the recipient may receive duplicate messages. In this case, the recipient acknowledges receipt of the duplicate message, but does not deliver the duplicate message to the queue.

Service Broker uses message acknowledgement to provide reliable messaging without distributed transactions. A recipient sends an acknowledgement only after adding the message or message fragment to the queue. The sender holds the message in the transmission queue until the acknowledgement for that message arrives. Although the sender and the recipient never share a transaction, the protocol guarantees that the sender does not remove the message from the transmission queue until the recipient has successfully received the message.

Message Integrity Check

The format that Service Broker uses to transmit messages includes a message integrity check to determine whether a given message has been altered or corrupted during transport.

The message integrity check is an MD5 signature for the contents of the message. SQL Server encrypts the signature with the session key for the message, and includes the signature in the message headers.

The destination for the message decrypts the message, and then compares the signature in the message to a new signature computed over the actual contents received. If the signatures do not match, the message has been damaged or tampered with during transmission. The message fails the message integrity check. SQL Server discards the message and does not acknowledge the message to the sender. The Broker:Corrupted Message event class reports information when a message fails the message integrity check.

The illustration below presents a high-level view of Service Broker network communication between two SQL Server instances.

Broker network communication between two instances

Notice that the conversation is a persistent, logical connection. The conversation may take place over any period of time, and during that period of time, the conversation may use any number of network connections.

Network connections occur between two Service Broker endpoints. These connections use TCP/IP. If the connection is inactive for a short period of time, SQL Server closes the network connection.

To deliver a message, Service Broker holds the message in the transmission queue for the database that sent the message. The recipient delivers the message directly to the queue for the destination service. If that queue is OFF, the message is held temporarily in the transmission queue for the receiving database. Notice that the queue for the sending service is not involved in the operation, and the transmission queue for the database that hosts the receiving service is only involved if the destination queue is OFF.

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