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HTTP_STATUS_SERVICE_UNAVAIL
The server cannot handle the request (because it is overloaded or down for maintenance). Generally, this is a temporary state.
The HTTP server response code for a 502 Bad Gateway error
indicates that the server, acting as a gateway or proxy, received an invalid
response from the upstream server.
The network gateway can refer to different things on the network and a 502 error is usually not something you can fix but needs to be fixed by the webserver or proxy you are trying to access.
A network gateway provides interoperability between networks and contains devices, such as protocol translators, impedance matchers, rate converters, fault isolators, or signal translators. A network gateway requires the establishment of mutually acceptable administrative procedures between the networks using the gateway. Network gateways, known as protocol translation gateways or mapping gateways, can perform protocol conversions to connect networks with different network protocol technologies. For example, a network gateway connects an office or home intranet to the Internet. If an office or home computer user wants to load a web page, at least two network gateways are accessed—one to get from the office or home network to the Internet and one to get from the Internet to the computer that serves the web page.
In enterprise networks, a network gateway usually also acts as a proxy server and a firewall.
On Microsoft Windows, the Internet Connection Sharing feature allows a computer to act as a gateway by offering a connection between the Internet and an internal network.
Cloud storage gateway
A cloud storage gateway is a network appliance or server which translates cloud storage APIs such as SOAP or REST to block-based storage protocols such as iSCSI, Fiber Channel or file-based interfaces such as NFS or CIFS. Cloud storage gateways enable companies to integrate private cloud storage into applications without moving the applications into a public cloud, thereby simplifying data protection.
IoT gateway
An Internet of things (IoT) gateway provides the bridge (protocol converter) between IoT devices in the field, the cloud, and user equipment such as smartphones. The IoT gateway provides a communication link between the field and the cloud, and may provide offline services and real-time control of devices in the field.
To achieve sustainable interoperability in the Internet of things ecosystem, two dominant architectures for data exchange protocols are used: bus-based (DDS, REST, XMPP) and broker-based (AMQP, CoAP, MQTT, JMI). Protocols that support information exchange between interoperable domains are classified as message-centric (AMQP, MQTT, JMS, REST) or data-centric (DDS, CoAP, XMPP). Interconnected devices communicate using lightweight protocols that don't require extensive CPU resources. C, Java, Python and some scripting languages are the preferred choices of IoT application developers. IoT nodes use separate IoT gateways to handle protocol conversion, database storage or decision making (e.g. collision handling), in order to supplement the low intelligence of devices.