diff --git a/docs/hints.md b/docs/hints.md index f6ca79aba..0b37237c6 100644 --- a/docs/hints.md +++ b/docs/hints.md @@ -5,6 +5,94 @@ in go-ceph work together. This is not meant to cover every possible use case but are recorded here as a quick way to get familiar with these calls. + +## General + +### Finding an API + +The go-ceph project wraps existing APIs that are part of Ceph. There are two +kinds of APIs that are wrapped. The first style of API is based on functions +Ceph exports as client libraries in C. The three packages in go-ceph `cephfs`, +`rados`, and `rbd` map to `libcephfs`, `librados`, and `librbd` respectively. + +The go-ceph packages that wrap Ceph C APIs follow a documentation convention +that aims to make it easier to map between the APIs. In functions that are +implemented using a certain C API function a line with the term `Implements:` +will be followed by the C function's declaration - matching what can be found +in the C library's header file. For example, if you knew you wanted to wrap the +rbd function to get an image's metadata, `rbd_metadata_get`, you could search +within the source code or https://pkg.go.dev/github.com/ceph/go-ceph/rbd for +`rbd_metadata_get` which would lead you to the `GetMetadata` method of the +`Image` type. + +The second style of API is based on functions implemented within Ceph services +based on Ceph's "command" system. These functions are primarily accessed using +the `ceph` command. Many of the functions within the ceph command +are implemented by sending a structured JSON message to either the Ceph MON or +MGR. Packages in go-ceph that wrap these sorts of APIs are found in +`cephfs/admin`, `rbd/admin`, and `common/admin/manager` for example. + +The command/JSON based API packages follow a different, but similar +documentation convention to the C based APIs. Functions that roughly map to a +particular `ceph` CLI command will contain a line with the term `Similar To:` +followed by the `ceph` command it is similar to. For example, if +you knew you wanted to create a CephFS subvolume group and would normally use +the command `ceph fs subvolumegroup create` to do so, you could search within +the source code or https://pkg.go.dev/github.com/ceph/go-ceph/cephfs/admin for +`ceph fs subvolumegroup create` which would lead you to the +`CreateSubVolumeGroup` property of the `FSAdmin` type. + +#### Can't find the API you want? + +The go-ceph project is maintained separately from Ceph and it is common for +APIs to be added to Ceph that are not present in go-ceph. Sometimes we resolve +the differences quickly but not always. + +Generally, there is no simple way to access a C based API from Go without +updates to the code. If there's an API that you need that doesn't appear to be +wrapped by go-ceph, please [file an +issue](https://github.com/ceph/go-ceph/issues). If you are comfortable writing +Go and would like to try writing a wrapper function we're more than happy to +[welcome contributions](./development.md#contribution-guidelines) as well. + +The command/JSON based APIs can be accessed without directly wrapping them. +The large majority of these functions are based on either +[rados.MgrCommand](https://pkg.go.dev/github.com/ceph/go-ceph@v0.21.0/rados#Conn.MgrCommand) +or +[rados.MonCommand](https://pkg.go.dev/github.com/ceph/go-ceph@v0.21.0/rados#Conn.MonCommand). +Both these functions accept a formatted JSON object that maps to a command line +constant prefix and the variable argument values. Determining the JSON prefix +and accepted arguments can be done using a special JSON-command: `{"prefix": +"get_command_descriptions"}`. This will return a dump of all commands the +service knows about. + +You can then use this information to construct your own JSON to send to +the server. For example the prefix `fs subvolumegroup ls` takes an argument `vol_name` +which is annotated as a CephString. Thus you can send the JSON +`{"prefix": "fs subvolumegroup ls", "vol_name": "foobar":, "format": "json"}`. +The last parameter `format` is a special general argument that *suggests* to the +server that you want the reply data to be JSON formatted. + +That all said, while you can directly interact with the command/JSON based APIs we +are also very happy to consider feature requests as well as contributions to make +working with these API more Go-idiomatic, convenient, and common. + +#### Preview APIs + +When a new API is added to go-ceph we consider the API to be a "preview" API. +This means that while we think the API is good enough to distribute we do not +promise not to change it. We assume that most consumers of go-ceph want a +stable API and so the preview APIs are "hidden" behind a go build tag. This +tag, `ceph_preview`, can be passed to the Go build command such that when you +import go-ceph packages the preview APIs will become "visible" to your code. +Do be aware that if you use preview APIs in your code there is the chance +they'll change between go-ceph releases. + +Preview APIs do not show up on pkg.go.dev but we do list all of them in +our [API status document](./api-status.md). We track when each API was +added and when the API is expected to become stable. + + ## rados Package ### Connecting to a cluster