You can use WebSockets to create a persistent connection from a client (such as a mobile device or a computer) to an App Engine instance. The open connection allows two-way data exchange between the client and the server at any time, resulting in lower latency and better use of resources.
WebSockets
The WebSockets protocol, defined in RFC 6455, provides a full-duplex communication channel between a client and a server. The channel is initiated from an HTTP(S) request with an "upgrade" header.
Typical use cases for WebSockets include:
- Real time event updates, such as social media feeds, sports scores, news, or stock market prices
- User notifications, such as software or content updates
- Chatting applications
- Collaborative editing tools
- Multiplayer games
WebSockets are always available to your application without any additional setup. Once a WebSockets connection is established, it will time out after one hour. Usage of the WebSocket is billed by connection usage until timeout or socket termination.
Running a sample application with WebSockets
The code samples in this document describe how to run a sample application with Websockets.
Prerequisites and setup
Follow the instructions in Setting Up Your Development Environment to set up your environment and project, and to understand how apps are structured.
Clone the sample app
Copy the sample apps to your local machine, and navigate to the websockets
directory:
Run the sample locally
Deploy and run the sample on App Engine
Session affinity
Not all clients support WebSockets. To work around this, many applications use libraries such as socket.io that fall back on http long polling with clients that don't support WebSockets.
App Engine typically distributes requests evenly among available instances. However, when using http long polling, multiple sequential requests from a given user need to reach the same instance.
To allow App Engine to send requests by the same user to the same instance, you can enable session affinity. App Engine then identifies which requests are sent by the same users by inspecting a cookie and routes those requests to the same instance.
Session affinity in App Engine is implemented on a best-effort basis. When developing your app, you should always assume that session affinity is not guaranteed. A client can lose affinity with the target instance in the following scenarios:
- The App Engine autoscaler can add or remove instances that serve your application. The application might reallocate the load, and the target instance might move. To minimize this risk, ensure that you have set the minimum number of instances to handle the expected load.
- If the target instance fails health checks, App Engine moves the session to a healthy instance. For more information about health checks and their customization options, see Split health checks.
- Session affinity is lost when an instance is rebooted for maintenance or software updates. App Engine flexible environment VM instances are restarted on a weekly basis.
Because session affinity isn't guaranteed, you should only use it to take
advantage of the ability of socket.io and other libraries to fall back
on HTTP long polling in cases where the connection is broken. You should never
use session affinity to build stateful applications.
Enabling and disabling session affinity
By default, session affinity is disabled for all App Engine applications. Session affinity is set at the version level of your application and can be enabled or disabled on deployment.
To enable session affinity for your App Engine version, add the following
entry to your app.yaml file:
network:
session_affinity: true
Once the version is deployed with the updated app.yaml, new requests will start serving from the same instance as long as that instance is available.
To turn off session affinity, remove the entry from your app.yaml file,
or set the value to false:
network:
session_affinity: false