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116 lines
5.0 KiB
Go
116 lines
5.0 KiB
Go
// Package rpc implements zrepl daemon-to-daemon RPC protocol
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// on top of a transport provided by package transport.
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// The zrepl documentation refers to the client as the
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// `active side` and to the server as the `passive side`.
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//
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// # Design Considerations
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//
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// zrepl has non-standard requirements to remote procedure calls (RPC):
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// whereas the coordination of replication (the planning phase) mostly
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// consists of regular unary calls, the actual data transfer requires
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// a high-throughput, low-overhead solution.
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//
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// Specifically, the requirements for data transfer is to perform
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// a copy of an io.Reader over the wire, such that an io.EOF of the original
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// reader corresponds to an io.EOF on the receiving side.
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// If any other error occurs while reading from the original io.Reader
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// on the sender, the receiver should receive the contents of that error
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// in some form (usually as a trailer message)
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// A common implementation technique for above data transfer is chunking,
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// for example in HTTP:
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// https://tools.ietf.org/html/rfc2616#section-3.6.1
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//
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// With regards to regular RPCs, gRPC is a popular implementation based
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// on protocol buffers and thus code generation.
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// gRPC also supports bi-directional streaming RPC, and it is possible
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// to implement chunked transfer through the use of streams.
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//
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// For zrepl however, both HTTP and manually implemented chunked transfer
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// using gRPC were found to have significant CPU overhead at transfer
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// speeds to be expected even with hobbyist users.
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//
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// However, it is nice to piggyback on the code generation provided
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// by protobuf / gRPC, in particular since the majority of call types
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// are regular unary RPCs for which the higher overhead of gRPC is acceptable.
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//
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// Hence, this package attempts to combine the best of both worlds:
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//
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// # GRPC for Coordination and Dataconn for Bulk Data Transfer
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//
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// This package's Client uses its transport.Connecter to maintain
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// separate control and data connections to the Server.
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// The control connection is used by an instance of pdu.ReplicationClient
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// whose 'regular' unary RPC calls are re-exported.
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// The data connection is used by an instance of dataconn.Client and
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// is used for bulk data transfer, namely `Send` and `Receive`.
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//
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// The following ASCII diagram gives an overview of how the individual
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// building blocks are glued together:
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//
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// +------------+
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// | rpc.Client |
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// +------------+
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// | |
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// +--------+ +------------+
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// | |
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// +---------v-----------+ +--------v------+
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// |pdu.ReplicationClient| |dataconn.Client|
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// +---------------------+ +--------v------+
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// | label: label: |
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// | zrepl_control zrepl_data |
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// +--------+ +------------+
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// | |
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// +--v---------v---+
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// | transportmux |
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// +-------+--------+
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// | uses
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// +-------v--------+
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// |versionhandshake|
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// +-------+--------+
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// | uses
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// +------v------+
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// | transport |
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// +------+------+
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// |
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// NETWORK
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// |
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// +------+------+
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// | transport |
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// +------^------+
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// | uses
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// +-------+--------+
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// |versionhandshake|
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// +-------^--------+
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// | uses
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// +-------+--------+
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// | transportmux |
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// +--^--------^----+
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// | |
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// +--------+ --------------+ ---
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// | | |
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// | label: label: | |
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// | zrepl_control zrepl_data | |
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// +-----+----+ +-----------+---+ |
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// |netadaptor| |dataconn.Server| | rpc.Server
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// | + | +------+--------+ |
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// |grpcclient| | |
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// |identity | | |
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// +-----+----+ | |
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// | | |
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// +---------v-----------+ | |
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// |pdu.ReplicationServer| | |
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// +---------+-----------+ | |
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// | | ---
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// +----------+ +------------+
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// | |
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// +---v--v-----+
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// | Handler |
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// +------------+
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// (usually endpoint.{Sender,Receiver})
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package rpc
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// edit trick for the ASCII art above:
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// - remove the comments //
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// - vim: set virtualedit+=all
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// - vim: set ft=text
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