Our system was having a problem with WCF connections being limited, which was solved by this answer. We added this setting to the client's web.config, and the limit of two concurrent connections went away:
Outside of the obvious impacts (e.g. overloading the server), are there any downsides to setting this limit to a number (possibly much) higher than the default "2"? Any source on the reasoning for having the default so low to begin with?
In general, it's OK to raise the client connection limit, with a few caveats:
- If you don't own the server, then be careful because your client app might be confused with a DoS attack which might lead to your client IP address being blocked by the server. Even if you own the server, this is sometimes a risk-- for example, we've had cases where a bug in our app's login page caused multiple requests to be issued when the user held down the Enter key. This caused these users to get blocked from our app because of our firewall's DoS protection!
- Connections aren't free. They take up RAM, CPU, and other scarce resources. Having 5 or 10 client connections isn't a problem, but when you have hundreds of open client connections then you risk running out of resources on the client.
- Proxies or edge servers between client and server may impose their own limits. So you may try to open 1,000 simultaneous connections only to have #5 and later refused by the proxy.
- Sometimes, adding more client connections is a workaround for an architectural problem. Consider fixing the architectural problem instead. For example, if you're opening so many connections because each request takes 10 minutes to return results, then you really should look at a more loosely-coupled solution (e.g. post requests to a server queue and come back later to pick up results) because long-lived connections are vulnerable to network disruption, transient client or server outages, etc.
- Being able to open many simultaneous connections can make it risky to restart your client or server app, because even if your "normal" load is only X requests/sec, if either client or server has been offline for a while, then the client may try to catch up on pending requests by issuing hundreds or thousands of requests all at once. Many servers have a non-linear response to overload conditions, where an extra 10% of load may reduce response time by 100%, creating a runaway overload condition.
The solution to all these risks is to carefully load-test both client and server with the maximum # of connections you want to support... and don't set your connection limit higher than what you've tested. Don't just crank the connection limit to 1,000,000 just because you can!
To answer the other part of your question, the default limit of 2 connections goes back to a very old version of the HTTP specification which limited clients to 2 connections per domain, so that web browsers wouldn't swamp servers with a lot of simultaneous connections. For more details, see this answer: https://stackoverflow.com/a/39520881/126352
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