rpc protocol

Client

This section describes the protocol that's used to send a RPC request from a client to a deepstream server and to send a rpc request from a deepstream server

The RPC protocol is a bit trickier - due to the messaging behind deepstream being anonymous pub-sub whereas a remote procedure call requires direct communication between two parties.

Every deepstream server has two internal topics: C.TOPIC.RPC_QUERY is used to send out rpc-query-requests, asking which other nodes would be able to provide a specific RPC. rpc-query-requests are structured like this

{
		topic: C.TOPIC.RPC_CTRL,
		action: C.ACTIONS.QUERY,
		data: [{
			rpcName: <rpcName>
		}]
}

Every deepstream server that can provide this rpc replies with

{
		topic: TOPIC.RPC_CTRL,
		action: C.ACTIONS.PROVIDER_UPDATE,
		data:[{
			rpcName: <rpcName>,
			serverName: <serverName>
			numberOfProviders: <numberOfProviders>
		}]
}

this response is received by all connected nodes, not only by the one that issued it. It will be cached for options.rpcProviderCacheTime, so that deepstream doesn't have to ask about providers every time a RPC comes in. Deepstream will then pick a random provider. The likelihood of a specific provider to be picked is <number of providers for that node> / <total number of providers across all nodes> Every deepstream node also listens on a private rpc topic, composed as C.TOPIC.RPC + '_' + <server name> The issuing node will then send a message to the providing nodes private rpc topic that's structured like this

{
		topic: C.TOPIC.RPC_<server name>
		action: C.ACTIONS.RPC_INVOKE
		data: [{
			correlationId: <uid>
			rpcName: <rpc name>
			data: <data>
		}]
}

and expects an immediate ack respone

{
		topic: C.TOPIC.RPC_<server name>
		action: C.ACTIONS.ACK
		data: [{
			correlationId: <uid>
			rpcName: <rpc name>
		}]
}

and subsequently the following response once the request is completed

{
		topic: C.TOPIC.RPC_<server name>
		action: C.ACTIONS.RESPONSE
		data: [{
			correlationId: <uid>
			rpcName: <rpc name>
			data: [<data>]
		}]
}

If the ack isn't received within options.rpcAckTimeout the rpc is sent to another provider and the rpc call will be removed from the provider cache to force a new RPC_QUERY for the next one. If no providers are left an error will be send to the client If the ack is received, but the response is not returned within options.rpcTimeout an error will be sent to the client.