1Load Balancing in gRPC 2====================== 3 4# Scope 5 6This document explains the design for load balancing within gRPC. 7 8# Background 9 10## Per-Call Load Balancing 11 12It is worth noting that load-balancing within gRPC happens on a per-call 13basis, not a per-connection basis. In other words, even if all requests 14come from a single client, we still want them to be load-balanced across 15all servers. 16 17## Approaches to Load Balancing 18 19Prior to any gRPC specifics, we explore some usual ways to approach load 20balancing. 21 22### Proxy Model 23 24Using a proxy provides a solid trustable client that can report load to the load 25balancing system. Proxies typically require more resources to operate since they 26have temporary copies of the RPC request and response. This model also increases 27latency to the RPCs. 28 29The proxy model was deemed inefficient when considering request heavy services 30like storage. 31 32### Balancing-aware Client 33 34This thicker client places more of the load balancing logic in the client. For 35example, the client could contain many load balancing policies (Round Robin, 36Random, etc) used to select servers from a list. In this model, a list of 37servers would be either statically configured in the client, provided by the 38name resolution system, an external load balancer, etc. In any case, the client 39is responsible for choosing the preferred server from the list. 40 41One of the drawbacks of this approach is writing and maintaining the load 42balancing policies in multiple languages and/or versions of the clients. These 43policies can be fairly complicated. Some of the algorithms also require client 44to server communication so the client would need to get thicker to support 45additional RPCs to get health or load information in addition to sending RPCs 46for user requests. 47 48It would also significantly complicate the client's code: the new design hides 49the load balancing complexity of multiple layers and presents it as a simple 50list of servers to the client. 51 52### External Load Balancing Service 53 54The client load balancing code is kept simple and portable, implementing 55well-known algorithms (e.g., Round Robin) for server selection. 56Complex load balancing algorithms are instead provided by the load 57balancer. The client relies on the load balancer to provide _load 58balancing configuration_ and _the list of servers_ to which the client 59should send requests. The balancer updates the server list as needed 60to balance the load as well as handle server unavailability or health 61issues. The load balancer will make any necessary complex decisions and 62inform the client. The load balancer may communicate with the backend 63servers to collect load and health information. 64 65# Requirements 66 67## Simple API and client 68 69The gRPC client load balancing code must be simple and portable. The 70client should only contain simple algorithms (e.g., Round Robin) for 71server selection. For complex algorithms, the client should rely on 72a load balancer to provide load balancing configuration and the list of 73servers to which the client should send requests. The balancer will update 74the server list as needed to balance the load as well as handle server 75unavailability or health issues. The load balancer will make any necessary 76complex decisions and inform the client. The load balancer may communicate 77with the backend servers to collect load and health information. 78 79## Security 80 81The load balancer may be separate from the actual server backends and a 82compromise of the load balancer should only lead to a compromise of the 83loadbalancing functionality. In other words, a compromised load balancer should 84not be able to cause a client to trust a (potentially malicious) backend server 85any more than in a comparable situation without loadbalancing. 86 87# Architecture 88 89## Overview 90 91The primary mechanism for load-balancing in gRPC is external 92load-balancing, where an external load balancer provides simple clients 93with an up-to-date list of servers. 94 95The gRPC client does support an API for built-in load balancing policies. 96However, there are only a small number of these (one of which is the 97`grpclb` policy, which implements external load balancing), and users 98are discouraged from trying to extend gRPC by adding more. Instead, new 99load balancing policies should be implemented in external load balancers. 100 101## Workflow 102 103Load-balancing policies fit into the gRPC client workflow in between 104name resolution and the connection to the server. Here's how it all 105works: 106 107 108 1091. On startup, the gRPC client issues a [name resolution](naming.md) request 110 for the server name. The name will resolve to one or more IP addresses, 111 each of which will indicate whether it is a server address or 112 a load balancer address, and a [service config](service_config.md) 113 that indicates which client-side load-balancing policy to use (e.g., 114 `round_robin` or `grpclb`). 1152. The client instantiates the load balancing policy. 116 - Note: If any one of the addresses returned by the resolver is a balancer 117 address, then the client will use the `grpclb` policy, regardless 118 of what load-balancing policy was requested by the service config. 119 Otherwise, the client will use the load-balancing policy requested 120 by the service config. If no load-balancing policy is requested 121 by the service config, then the client will default to a policy 122 that picks the first available server address. 1233. The load balancing policy creates a subchannel to each server address. 124 - For all policies *except* `grpclb`, this means one subchannel for each 125 address returned by the resolver. Note that these policies 126 ignore any balancer addresses returned by the resolver. 127 - In the case of the `grpclb` policy, the workflow is as follows: 128 1. The policy opens a stream to one of the balancer addresses returned 129 by the resolver. It asks the balancer for the server addresses to 130 use for the server name originally requested by the client (i.e., 131 the same one originally passed to the name resolver). 132 - Note: In the `grpclb` policy, the non-balancer addresses returned 133 by the resolver are used as a fallback in case no balancers can be 134 contacted when the LB policy is started. 135 2. The gRPC servers to which the load balancer is directing the client 136 may report load to the load balancers, if that information is needed 137 by the load balancer's configuration. 138 3. The load balancer returns a server list to the gRPC client's `grpclb` 139 policy. The `grpclb` policy will then create a subchannel to each of 140 server in the list. 1414. For each RPC sent, the load balancing policy decides which 142 subchannel (i.e., which server) the RPC should be sent to. 143 - In the case of the `grpclb` policy, the client will send requests 144 to the servers in the order in which they were returned by the load 145 balancer. If the server list is empty, the call will block until a 146 non-empty one is received. 147
