Configure the channel pool to open a new channel when all of the currently open channels reach 100 subscriptions each (#3380)

Author: steveluscher

packages/rpc-subscriptions/README.md

@@ -27,7 +27,7 @@ Creates a function that returns new subscription channels when called.
 A config object with the following properties:
 
 -   `intervalMs`: The number of milliseconds to wait since the last message sent or received over the channel before sending a ping message to keep the channel open.
--   `maxSubscriptionsPerChannel`: The number of subscribers that may share a channel before a new channel must be created. Set this to the maximum number of subscriptions that your RPC provider recommends making over a single connection.
+-   `maxSubscriptionsPerChannel`: The number of subscribers that may share a channel before a new channel must be created (default: 100). It is important that you set this to the maximum number of subscriptions that your RPC provider recommends making over a single connection; the default is set deliberately low, so as to comply with the restrictive limits of the public mainnet RPC node.
 -   `minChannels`: The number of channels to create before reusing a channel for a new subscription.
 -   `sendBufferHighWatermark`: The number of bytes of data to admint into the `WebSocket` buffer before buffering data on the client. -`url`: The URL of the web socket server. Must use the `ws` or `wss` protocols.
 

packages/rpc-subscriptions/src/rpc-subscriptions-channel.ts

@@ -48,8 +48,15 @@ export function createDefaultRpcSubscriptionsChannelCreator<TClusterUrl extends
     return getChannelPoolingChannelCreator(createDefaultRpcSubscriptionsChannel, {
         maxSubscriptionsPerChannel:
             config.maxSubscriptionsPerChannel ??
-            // TODO: Determine this experimentally
-            1_000,
+            /**
+             * A note about this default. The idea here is that, because some RPC providers impose
+             * an upper limit on the number of subscriptions you can make per channel, we must
+             * choose a number low enough to avoid hitting that limit. Without knowing what provider
+             * a given person is using, or what their limit is, we have to choose the lowest of all
+             * known limits. As of this writing (October 2024) that is the public mainnet RPC node
+             * (api.mainnet-beta.solana.com) at 100 subscriptions.
+             */
+            100,
         minChannels: config.minChannels ?? 1,
     });
 }