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Add concept of a 'Backend', but still one to one proxying
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -1,24 +1,102 @@ | ||
| // Copyright 2017 Michal Witkowski. All Rights Reserved. | ||
| // Copyright 2019 Andrey Smirnov. All Rights Reserved. | ||
| // See LICENSE for licensing terms. | ||
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| package proxy | ||
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| import ( | ||
| "golang.org/x/net/context" | ||
| "context" | ||
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| "google.golang.org/grpc" | ||
| ) | ||
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| // StreamDirector returns a gRPC ClientConn to be used to forward the call to. | ||
| // Backend wraps information about upstream connection. | ||
| // | ||
| // For simple one-to-one proxying, not much should be done in the Backend, simply | ||
| // providing a connection is enough. | ||
| // | ||
| // When proxying one-to-many and aggregating results, Backend might be used to | ||
| // append additional fields to upstream response to support more complicated | ||
| // proxying. | ||
| type Backend interface { | ||
| // String provides backend name for logging and errors. | ||
| String() string | ||
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| // GetConnection returns a grpc connection to the backend. | ||
| // | ||
| // The context returned from this function should be the context for the *outgoing* (to backend) call. In case you want | ||
| // to forward any Metadata between the inbound request and outbound requests, you should do it manually. However, you | ||
| // *must* propagate the cancel function (`context.WithCancel`) of the inbound context to the one returned. | ||
| GetConnection(ctx context.Context) (context.Context, *grpc.ClientConn, error) | ||
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| // AppendInfo is called to enhance response from the backend with additional data. | ||
| // | ||
| // Usecase might be appending backend endpoint (or name) to the protobuf serialized response, so that response is enhanced | ||
| // with source information. This is particularly important for one to many calls, when it is required to identify | ||
| // response from each of the backends participating in the proxying. | ||
| // | ||
| // If not additional proxying is required, simply returning the buffer without changes works fine. | ||
| AppendInfo(resp []byte) ([]byte, error) | ||
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| // BuildError is called to convert error from upstream into response field. | ||
| // | ||
| // BuildError is never called for one to one proxying, in that case all the errors are returned back to the caller | ||
| // as grpc errors. | ||
| // | ||
| // When proxying one to many, if one the requests fails or upstream returns an error, it is undesirable to fail the whole | ||
| // request and discard responses from other backends. BuildError converts (marshals) error from backend into protobuf encoded | ||
| // response which is analyzed by the caller, so that caller reaching out to N upstreams receives N1 successful responses and | ||
| // N2 error responses so that N1 + N2 == N. | ||
| // | ||
| // If BuildError returns nil, error is returned as grpc error (failing whole request). | ||
| BuildError(err error) ([]byte, error) | ||
| } | ||
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| // SingleBackend implements a simple wrapper around get connection function of one to one proxying. | ||
| // | ||
| // SingleBackend implements Backend interface and might be used as an easy wrapper for one to one proxying. | ||
| type SingleBackend struct { | ||
| // GetConn returns a grpc connection to the backend. | ||
| // | ||
| // The context returned from this function should be the context for the *outgoing* (to backend) call. In case you want | ||
| // to forward any Metadata between the inbound request and outbound requests, you should do it manually. However, you | ||
| // *must* propagate the cancel function (`context.WithCancel`) of the inbound context to the one returned. | ||
| GetConn func(ctx context.Context) (context.Context, *grpc.ClientConn, error) | ||
| } | ||
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| func (sb *SingleBackend) String() string { | ||
| return "backend" | ||
| } | ||
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| // GetConnection returns a grpc connection to the backend. | ||
| func (sb *SingleBackend) GetConnection(ctx context.Context) (context.Context, *grpc.ClientConn, error) { | ||
| return sb.GetConn(ctx) | ||
| } | ||
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| // AppendInfo is called to enhance response from the backend with additional data. | ||
| func (sb *SingleBackend) AppendInfo(resp []byte) ([]byte, error) { | ||
| return resp, nil | ||
| } | ||
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| // BuildError is called to convert error from upstream into response field. | ||
| func (sb *SingleBackend) BuildError(err error) ([]byte, error) { | ||
| return nil, nil | ||
| } | ||
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| // StreamDirector returns a list of Backend objects to forward the call to. | ||
| // | ||
| // There are two proxying modes: | ||
| // 1. one to one: StreamDirector returns a single Backend object - proxying is done verbatim, Backend.AppendInfo might | ||
| // be used to enhance response with source information (or it might be skipped). | ||
| // 2. one to many: StreamDirector returns more than one Backend object - for unary calls responses from Backend objects | ||
| // are aggregated by concatenating protobuf responses (requires top-level `repeated` protobuf definition) and errors | ||
| // are wrapped as responses via BuildError. Responses are potentially enhanced via AppendInfo. | ||
| // | ||
| // The presence of the `Context` allows for rich filtering, e.g. based on Metadata (headers). | ||
| // If no handling is meant to be done, a `codes.NotImplemented` gRPC error should be returned. | ||
| // | ||
| // The context returned from this function should be the context for the *outgoing* (to backend) call. In case you want | ||
| // to forward any Metadata between the inbound request and outbound requests, you should do it manually. However, you | ||
| // *must* propagate the cancel function (`context.WithCancel`) of the inbound context to the one returned. | ||
| // | ||
| // It is worth noting that the StreamDirector will be fired *after* all server-side stream interceptors | ||
| // are invoked. So decisions around authorization, monitoring etc. are better to be handled there. | ||
| // | ||
| // See the rather rich example. | ||
| type StreamDirector func(ctx context.Context, fullMethodName string) (context.Context, *grpc.ClientConn, error) | ||
| type StreamDirector func(ctx context.Context, fullMethodName string) ([]Backend, error) |
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