Insulating Service Bus Applications Against Service Bus Outages and Disasters
Mission-critical applications must operate continuously, even in the presence of unplanned outages or disasters. This topic describes techniques you can use to protect applications against a potential Windows Azure Service Bus outage or disaster.
An outage is defined as the temporary unavailability of a Service Bus scale unit. After the problem has been fixed, the scale unit becomes available again. Typically, an outage does not cause loss of messages or other data. Examples of outages are a power failure of the datacenter or a faulty datacenter network switch. An outage can last from a few minutes to a few days.
A disaster is defined as the permanent loss of a Service Bus scale unit or datacenter. The datacenter may or may not become available again. Typically a disaster causes loss of some or all messages or other data. Examples of disasters are fire, flooding, or earthquakes.
All Service Bus messaging entities reside in a service namespace. The same is true for Windows Azure Active Directory Access Control (also known as Access Control Service or ACS), which supplies the tokens that are required to access Service Bus entities. A service namespace is affiliated with a datacenter. The Service Bus and ACS do not enable automatic geo-replication of data, nor do they allow a service namespace to span multiple datacenters.
To allow for a failover between two datacenters, you can create one Service Bus and one ACS service namespace in each datacenter. For example, the Service Bus service namespace contosoPrimary.servicebus.windows.net might be located in the United States (North/Central) region, and contosoSecondary.servicebus.windows.net might be located in the United States (South/Central) region. If a Service Bus messaging entity must remain accessible in the presence of a datacenter outage, you can create that entity in both service namespaces.
Geo-replication of relay endpoints allows a service that exposes a relay endpoint to be reachable in the presence of Service Bus outages. To achieve geo-replication, the service must create two relay endpoints in different service namespaces. Both service namespaces must reside in different datacenters and both endpoints must have different names. For example, a primary endpoint can be reached under contosoPrimary.servicebus.windows.net/myPrimaryService, while its secondary counterpart can be reached under contosoSecondary.servicebus.windows.net/mySecondaryService.
The service then listens on both endpoints, and a client can invoke the service via either endpoint. A client application randomly picks one of the relays as the primary endpoint, and sends its request to the active endpoint. If the operation fails with an error code, this failure indicates that the relay endpoint is not available. The application opens a channel to the backup endpoint and reissues the request. At that point the active and the backup endpoints switch roles: the client application considers the old active endpoint to be the new backup endpoint, and the old backup endpoint to be the new active endpoint. If both send operations fail, the roles of the two entities remain unchanged and an error is returned.
The Geo-replication with Service Bus Relayed Messages sample demonstrates how to replicate relays.
To achieve resilience against datacenter outages when using brokered messaging, the Service Bus supports two approaches: active and passive replication. For each approach, if a given queue or topic must remain accessible in the presence of a datacenter outage, you can create it in both service namespace. Both entities can have the same name. For example, a primary queue can be reached under contosoPrimary.servicebus.windows.net/myQueue, while its secondary counterpart can be reached under contosoSecondary.servicebus.windows.net/myQueue.
If the application does not require permanent sender-to-receiver communication, the application can implement a durable client-side queue to prevent message loss and to shield the sender from any transient Service Bus errors.
Active replication uses entities in both service namespaces for every operation. Any client that sends a message sends two copies of the same message. The first copy is sent to the primary entity (for example, contosoPrimary.servicebus.windows.net/sales), and the second copy of the message is sent to the secondary entity (for example, contosoSecondary.servicebus.windows.net/sales).
A client receives messages from both queues. The receiver processes the first copy of a message, and the second copy is suppressed. To suppress duplicate messages, the sender must tag each message with a unique identifier. Both copies of the message must be tagged with the same identifier. You can use MessageId, Label, or a custom property to tag the message. The receiver must maintain a list of messages that it has already received.
The Geo-replication with Service Bus Brokered Messages sample demonstrates active replication of messaging entities.
|The active replication approach doubles the number of operations, therefore this approach can lead to higher cost.|
In the fault-free case, passive replication uses only one of the two messaging entities. A client sends the message to the active entity. If the operation on the active entity fails with an error code that indicates the datacenter that hosts the active entity might be unavailable, the client sends a copy of the message to the backup entity. At that point the active and the backup entities switch roles: the sending client considers the old active entity to be the new backup entity, and the old backup entity is the new active entity. If both send operations fail, the roles of the two entities remain unchanged and an error is returned.
A client receives messages from both queues. Because there is a chance that the receiver receives two copies of the same message, the receiver must suppress duplicate messages. You can suppress duplicates in the same way as described for active replication.
In general, passive replication is more economical than active replication because in most cases only one operation is performed. Latency, throughput, and monetary cost are identical to the non-replicated scenario.
When using passive replication, in the following scenarios messages can be lost or received twice:
Message delay or loss: Assume that the sender successfully sent a message m1 to the primary queue, and then the queue becomes unavailable before the receiver receives m1. The sender sends a subsequent message m2 to the secondary queue. If the primary queue is temporarily unavailable, the receiver receives m1 after the queue becomes available again. In case of a disaster, the receiver may never receive m1.
Duplicate reception: Assume that the sender sends a message m to the primary queue. The Service Bus successfully processes m but fails to send a response. After the send operation times out, the sender sends an identical copy of m to the secondary queue. If the receiver is able to receive the first copy of m before the primary queue becomes unavailable, the receiver receives both copies of m at approximately the same time. If the receiver is not able to receive the first copy of m before the primary queue becomes unavailable, the receiver initially receives only the second copy of m, but then receives a second copy of m when the primary queue becomes available.
The Geo-replication with Service Bus Brokered Messages sample demonstrates passive replication of messaging entities.
Durable Client-side Queue
If the application can tolerate a Service Bus entity being unavailable, but must not lose messages, the sender can employ a durable client-side queue that locally stores all messages that cannot be sent to Service Bus. Once the Service Bus entity becomes available again, all buffered messages are sent to that entity. The Durable Message Sender sample implements such a queue with the help of MSMQ. Alternatively, the messages can be written to the local disk.
A durable client-side queue preserves message order and shields the client application from exceptions in case the Service Bus entity is unavailable. It can be used with simple and distributed transactions.