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Golang modules

Organization of the etcd project’s golang modules

The etcd project (since version 3.5) is organized into multiple golang modules hosted in a single repository.

There are following modules:

go.etcd.io/etcd/api/v3 - contains API definitions (like protos & proto-generated libraries) that defines communication protocol between etcd clients and server.
go.etcd.io/etcd/pkg/v3 - collection of utility packages used by etcd without being specific to etcd itself. A package belongs here only if it could possibly be moved out into its own repository in the future. Please avoid adding here code that has a lot of dependencies on its own, as they automatically becoming dependencies of the client library (that we want to keep lightweight).
go.etcd.io/etcd/client/v3 - client library used to contact etcd over the network (grpc). Recommended for all new usage of etcd.
go.etcd.io/etcd/client/v2 - legacy client library used to contact etcd over HTTP protocol. Deprecated. All new usage should depend on /v3 library.
go.etcd.io/etcd/raft/v3 - implementation of distributed consensus protocol. Should have no etcd specific code.
go.etcd.io/etcd/server/v3 - etcd implementation. The code in this package is etcd internal and should not be consumed by external projects. The package layout and API can change within the minor versions.
go.etcd.io/etcd/etcdctl/v3 - a command line tool to access and manage etcd.
go.etcd.io/etcd/tests/v3 - a module that contains all integration tests of etcd. Notice: All unit-tests (fast and not requiring cross-module dependencies) should be kept in the local modules to the code under the test.
go.etcd.io/bbolt - implementation of persistent b-tree. Hosted in a separate repository: https://github.com/etcd-io/bbolt.

Operations

All etcd modules should be released in the same versions, e.g. go.etcd.io/etcd/client/[email protected] must depend on go.etcd.io/etcd/api/[email protected].

The consistent updating of versions can by performed using:
```
% DRY_RUN=false TARGET_VERSION="v3.5.10" ./scripts/release_mod.sh update_versions
```
The released modules should be tagged according to https://golang.org/ref/mod#vcs-version rules, i.e. each module should get its own tag. The tagging can be performed using:
```
% DRY_RUN=false REMOTE_REPO="origin" ./scripts/release_mod.sh push_mod_tags
```
All etcd modules should depend on the same versions of underlying dependencies. This can be verified using:
```
% PASSES="dep" ./test.sh
```
The go.mod files must not contain dependencies not being used and must conform to go mod tidy format. This is being verified by:
```
% PASSES="mod_tidy" ./test.sh
```
To trigger actions across all modules (e.g. auto-format all files), please use/expand the following script:
```
% ./scripts/fix.sh
```

Future

As a North Star, we would like to evaluate etcd modules towards following model:

This assumes:

Splitting etcdmigrate/etcdadm out of etcdctl binary. Thanks to this etcdctl would become clearly a command-line wrapper around network client API, while etcdmigrate/etcdadm would support direct physical operations on the etcd storage files.
Splitting etcd-proxy out of ./etcd binary, as it contains more experimental code so carries additional risk & dependencies.
Deprecation of support for v2 protocol.

Last modified June 14, 2021: Renaming content/en/next folder to content/en/v3.5. Updating redirects, links, and config as needed. (#363) (138926b)