siva: add package documentation

siva/doc.go

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+/*
+
+Package siva implements a go-borges library that uses siva files as its
+storage backend.
+
+More information about siva files: https://github.com/src-d/go-siva
+
+Basics
+
+In this storage each location contains a single bare git repository that can
+contain the objects from several other logical repositories. These logical
+repositories are stored as remotes in the configuration file. Its ID is the
+remote name but it can also search its URLs. All repositories returned from a
+location have the same objects and references. The function AddLocation creates
+an empty location that is initialized when using Location.Init. It's
+initialization consists of initializing the repository if it's not already
+created and adding a remote with its name and URLs to the provided ID.
+
+For example:
+
+	r1, _ := library.Get("github.com/src-d/go-borges")
+	println(r1.Name()) # "0168e2c7-eedc-7358-0a09-39ba833bdd54"
+	r2, _ := library.Get("0168e2c7-eedc-7358-0a09-39ba833bdd54")
+	println(r1.Name()) # "0168e2c7-eedc-7358-0a09-39ba833bdd54"
+
+	loc, _ := library.AddLocation("test")
+	r1, _ := loc.Init("repo1") # the first repo initializes the git repository
+	r1.Commit()
+	r2, := loc.Init("repos2) # the second just adds a new remote
+	r2.Commit()
+	loc.Has("repo1") # true
+	loc.Has("repo2") # true
+
+Transactions
+
+The storage supports transactions and has location lock on transaction when
+using the same library. Transactional writes are done directly to the siva file
+performing appends and a checkpoint file is created with the size of the file
+before starting the transaction. This file is used to recover broken siva files
+to the last known good state. Locations can be accessed in read only mode while
+the repository is performing a transaction and its content remain stable.
+
+Committing a transaction finishes the writes to the siva file, closes it and
+deletes the checkpoint file. Rollback truncates the siva file to the last good
+size and deletes the checkpoint file.
+
+Only one repository can be opened in read write mode in the same location when
+the library is transactional. When a second repository wants to be opened in RW
+mode in the same location the library will wait a grace period for the previous
+repository to close. By default is 1 minute but it can be configured when
+creating the library.
+
+For example:
+
+	loc, _ := library.Location("foo")
+	r1, _ := loc.Get("github.com/src-d/go-borges", borges.ReadOnlyMode)
+	r2, _ := loc.Get("github.com/src-d/go-borges", borges.RWMode)
+	r2.R().CreateTag("tag", plumbing.ZeroHash, nil)
+
+	r1.R().Tag("tag") # not found
+	r3, _ := loc.Get("github.com/src-d/go-borges", borges.ReadOnlyMode)
+	r3.R().Tag("tag") # not found
+
+	# errors after configured timeout as r2 transaction is not completed
+	r4, _ := loc.Get("github.com/src-d/go-borges", borges.RWMode)
+
+	r2.Commit()
+	r1.R().Tag("tag") # not found
+	r5, _ := loc.Get("github.com/src-d/go-borges", borges.ReadOnlyMode)
+	r5.R().Tag("tag") # found
+
+Note: When using repositories in non transactional mode you should call Close
+after finishing, otherwise the siva file will be corrupted.
+*/
+package siva