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This topic explains how Pivotal Application Service (PAS) uses WebSockets, why developers use WebSockets in their apps, and how operators can configure their load balancer to support WebSockets.
The WebSocket protocol provides full-duplex communication over a single TCP connection. Apps can use WebSockets to perform real-time data exchange between a client and a server more efficiently than HTTP.
PAS uses WebSockets for the following metrics and logging purposes:
To stream app logs from the Traffic Controller to developers using the Cloud Foundry Command Line Interface (cf CLI) or Apps Manager
- To stream all app event data and component metrics from the Traffic Controller over the Firehose endpoint to external apps or services
For more information about these Loggregator components, see Loggregator Architecture.
To form a WebSocket connection, the client sends an HTTP request that contains an
Upgrade header and other headers required to complete the WebSocket handshake. You must configure your load balancer to not upgrade the HTTP request, but rather to pass the
Upgrade header through to the PAS router. The procedures required to configure your load balancer depends on your IaaS and load balancer. The following list includes several possible approaches:
Some load balancers can recognize the
Upgradeheader and pass these requests through to the PAS router without returning the WebSocket handshake response. This may or may not be default behavior, and may require additional configuration.
Some load balancers do not support passing WebSocket handshake requests containing the
Upgradeheader to the PAS router. For instance, the Amazon Web Services (AWS) Elastic Load Balancer (ELB) does not support this behavior. In this scenario, you must configure your load balancer to forward TCP traffic to your PAS router to support WebSockets. If your load balancer does not support TCP pass-through of WebSocket requests on the same port as other HTTP requests, you can do one of the following:
- Configure your load balancer to listen on a non-standard port (the built-in PAS load balancer listens on 8443 by default for this purpose), and forward requests for this port in TCP mode. App clients must make WebSockets upgrade requests to this port. If you choose this strategy, you must follow the steps in Set Your Loggregator Port.
- If a non-standard port is not acceptable, add a load balancer that will handle WebSocket traffic (or another IP on an existing load balancer) and configure it to listen on standard ports 80 and 443 in TCP mode. Configure DNS with a new hostname, such as
ws.cf.example.com, to be used for WebSockets. This hostname should resolve to the new load balancer interface.
Note: Regardless of your IaaS and configuration, you must configure your load balancer to send the X-Forwarded-For and X-Forwarded-Proto headers for non-WebSocket HTTP requests on ports 80 and 443. For more information, see Securing Traffic into PAS.
By default, PCF assigns port 443 for TCP/WebSocket communications. If you have configured your load balancer to use a port other than 443 for TCP/WebSocket traffic, you must edit the Loggregator Port field in the Networking pane of the PAS tile.